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Annual Report
DEPOSITIONAL SETTING, STRUCTURAL STYLE, AND SANDSTONE DISTRIBUTION IN THREE
GEOPRESSURED GEOTHERMAL AREAS, TEXAS GULF COAST
DOE/ET /27111-3
C. D. Winker, R. A. Morton, T. E. Ewing, and D. D. Garcia
Prepared for the U.S. Department of Energy
Division of Geothermal Energy
Under Contract No. DE-AC08-79ET27111
Bureau of Economic Geology W. L. Fisher, Director
The University of Texas at Austin University Station, Box X
Austin, Texas 78712
October 1981
DISCLAIMER
"This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof."
This report has been reproduced directly from the best available copy.
Available from the National Technical Information Service, U.S. Department of Commerce, Springfield, Virginia 22161.
Price: Printed Copy AD? Microfiche AD 1
Codes are used for pricing all publications. The code is determined by the number of pages in the publication. Information pertaining to the pricing codes can be found in the current issues of the following publications, which are generally available in most libraries: Energy Research Abstracts, (ERA); Government Reports Announcements and Index (GRA and I); Scientific and Technical Abstract Re orts (STAR); and publication NTIS-PR-36D available from NTIS at the above address. --
DEPOSITIONAL SETTING, STRUCTURAL STYLE, AND SANDSTONE DISTRIBUTION IN THREE
GEOPRESSURED GEOTHERMAL AREAS, TEXAS GULF COAST
C. D. Winker, R. A. Morton, T. E. Ewing, and D. D. Garcia
Assisted by
J. H. Han, J. L. Lawton, R. J. Padilla y Sanchez, J. J. Palmer, C. L. Ping, and R. D. Rasco
Prepared for the U.S. Department of Energy
Division of Geothermal Energy Under Contract No. DE-AC08-79ET27111
Bureau of Economic Geology W. L. Fisher, Director
The University of Texas at Austin University Station, Box X
Austin, Texas 78712
October 1981
TABLE OF CONTENTS
ABSTRACT •
INTRODUCTION.
PLEASANT BAYOU AREA.
Stratigraphic and Paleogeographic Setting
Operational Stratigraphy
Structural Setting
Local Structure •
Structural Style •
Structural Evolution
Sandstone Facies
Pressure Distribution
Seismic Studies .
MobiJ CB-78-l
Teledyne 6-F •
Western W -4 .
Western W-3 •
Western W-l •
Western W-5 •
Velocity Distribution
Interval Velocities •
Time-Depth Conversion.
BLESSING AREA
Stratigraphic and Paleogeographic Setting
iii
1
3
5
5
8
· 12
· 12
.13
· 18
· 21
• 30
· 35
38
• 41
• 41
· 43
43
· 43
• 47
• 48
• 50
• 52
• 52
a • t
Operational Stratigraphy · 52
Structural Setting · 55
Local Structure • · 55
Structural Style • · 55
Structural Evolution 58
Sandstone Facies · 60
Pressure Distribution · 60
Seismic Studies • · 67
Amoco BCH-27, BCH-29, BCH-30 • · 69
Coastal States CS-2; Mobil F • 69
Velocity Distribution 72
Interval Velocities · 72
Time-Depth Conversion · 72
CUERO AREA · 75
Stratigraphic and Paleogeographic Setting 75
Operational Stratigraphy 80
Structural Setting · 83
Local Structure • · 83
Structural Style . 84
Structural Evolution · 88
Sandstone Facies 88
Pressure Distribution · 94
Seismic Studies • 98
Proprietary Data · 104
New Data. .104
14W-F. .108
iv
..
7F-2 · 113
UT-4 . .114
UT -2 and UT-3 .117
UT -1 and UT -IA • .117
Velocity Distribution .122
Interval Velocities .122
Stacking and rms Velocities .122
Time-Depth Conversion. · 122
"Wipe-out" Zones .125
COMPARISON OF AREAS . .127
ACKNOWLEDGMENTS. · 128
REFERENCES .130
Figures
1. Location of Pleasant Bayou, Blessing, and Cuero study areas in relation to regional shelf-margin trends, northwestern Gulf of Mexico Basin. 4
2. Location of Pleasant Bayou and Blessing study areas in relation to lower Frio depositional systems in Texas. 7
3. Data base and locations of sections in Pleasant Bayou area, Brazoria County. 9
4. Structural dip section through Chocolate Bayou field, illustrating log character of stratigraphic units in Pleasant Bayou area · 10
5. Dip sections of four geopressured geothermal study areas . · 14
6. Structure map of T A marker · 15
7. Structure map of T2 marker · 16 4,
8. Structure map of T 5 marker · 17
9. Sequential isopach maps illustrating structural evolution of Pleasant Bayou study area . · 19
v
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
Seismic sections of modern continental slope, illustrating modern structural features analogous to Frio structures in Texas
Stratigraphic dip section of lower Frio
Stratigraphic strike section of lower Frio.
Log-pattern map of 'A' sand, lower Frio, with gas-well completions .
Log-pattern map of 'C' sand (geothermal reservoir sand), lower Frio, with gas-well completions .
Log-pattern map of '0' sand, lower Frio, with gas-well completions
Distribution of optimum sandstone facies as inferred from electric-log patterns in three geothermal study areas .
Distribution of pressure gradients across Chocolate Bayou field
Static bottom-hole pressures versus depth for a number of wells, Chocolate Bayou field .
Distribution of pressure gradients across Chocolate Bayou field
Mud-weight data for wells in cross section from figure 19
Base map of seismic data
Mobil line CB-78-1 after reprocessing and migration
Teledyne line 6F, after reprocessing and migration.
Interpreted version of line W -4, after reprocessing and migration by H.P.I. .
Line W - 3, after reprocessing and migration by H.P.r.
Western line W -1, as delivered.
Western line W-5, as delivered.
Interval velocities as determined from velocity surveys in five wells in Brazoria County
Range of time-depth curves obtained from six velocity surveys in Brazoria County.
Base map showing locations of available deep well control, seismic sections, and velocity surveys in the Blessing study area .
Structure map of B 1 correlation marker .
vi
20
23
24
25
26
• 27
· 28
31
· 32
· 33
34
• 36
39
• 40
• 42
44
• 45
• 46
• 49
· 51
• 54
• 56
32. Structure map of B5 correlation marker • . 57
33. Sequential isopach maps illustrating structural evolution of the Blessing study area . . 59
34. Stratigraphic dip sections of lower Frio . 62
35. Log-pattern map of 'B' sand, lower Frio . 63
36. Log-pattern map of 'C' sand, lower Frio . . 64
37. Static bottom-hole pressures versus depth for Matagorda County wells • . 65
38. Shale resistivity versus depth, Blessing area . . 66
39. Seismic dip section BCH-27, BCH-29 and BCH-30 . 70
40. Seismic dip section CS-2 and F. . 71
41. Interval velocities as determined from velocity surveys from six wells in the Blessing area . 73
42. Range of time-depth curves obtained from six velocity surveys in the Blessing area. . 74
43. Location of Cuero study area in relation to Rockdale Delta system and barrier/strandplain facies . . 77
44. Location of Cuero study area in relation to net-sandstone distribution and upper limit of geopressure (based on mud weights) in the lower Wilcox • . 78
45. Location of Cuero study area in relation to regional Wilcox growth faults in Texas 79
46. Data base and locations of well-log sections, Cuero study area 81
47. Structural dip section of Cuero study area, illustrating log character of stratigraphic units . 82
48. Structure map of C1 correlation marker • . 85
49. Structure map of TW correlation marker . . 86
50. Structure map of D4' correlation marker • . 87
51. Sequential isopach maps illustrating structural evolution of Cuero study area. • 89
52. Stratigraphic dip section of lower Wilcox. • 91
53. Stratigraphic strike section of lower Wilcox . . 92
vii
...
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
73.
74.
75.
76.
Lower Wilcox paleogeography of Cuero study area .
Log patterns of three lower Wilcox sandstones in the South Cook fault block
Electric log with permeability data and environmental interpretations for lower Wilcox section, Atlantic 111 Schorre .
Plot of bottom-hole shut-in pressures versus depth from gas wells in De Witt County . • • . • • . . . • . . . •
Shale-resistivity plot for Atlantic 111 Schorre used to estimate pressure gradients
Shale-resistivity plot for Gulf 111 Mueller well
Parameter plots for wells in section in figure 47, illustrating top of geopressure as picked from shale resistivities
Base map of seismic data in Cuero area .
Interpreted version of migrated line 14 W-F .
Interpreted version of migrated line 7F-2
Originally proposed layout of shotpoints with staggered offsets to provide cross-dip control
Map of shotholes, geophone stations, and shot-receiver midpoints for a segment of line UT -4 •
Structural dip sections constructed before acquisition of UT L W - 3 or reprocessing of proprietary lines
Structural dip sections from figure 66.
Wavefront and raypath diagram, Atlantic III Hartman well
Migrated line UT -4 with interpretations .
Line UT -4 restacked with velocities approximately 1.4 times optimum stacking velocities
Migrated line UT -2, with interpretations.
Migrated line UT -3, with interpretations.
Composi te strike section UT -1 and UT -1 A
Interval velocities based on velocity surveys from five wells .
Range of rms velocity curves •
Time-depth conversions for the Cuero area •
viii
• 93
• 95
.96
• 97
· 99
.100
· 101
.103
· 105
.106
.107
.109
.110
· 111
· 112
.115
.11&
.119
.120
.121
• 123
.124
.126
." ..
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Tables
Characteristics of operational stratigraphic units in the Pleasant Bayou area
Approximate stratigraphic position of log- and seismic-correlation markers and functional stratigraphic units in four geothermal areas
Electric-log facies in the Anomalina bilateralis zone, lower Frio, Pleasant Bayou area.
Reservoir parameters for three areas in the Texas Gulf Coast.
Seismic lines acquired for evaluation of the Pleasant Bayou area
Characteristics of operational stratigraphic units in the Blessing area
Electric-log facies in the Anomalina bilateralis zone, lower Frio, Blessing area •
Seismic lines acquired for evaluation of the Blessing area •
Characteristics of operational stratigraphic units in the Cuero area
Electric-log facies in the lower Wilcox used for paleogeographic mapping in the Cuero area .
Seismic lines acquired for evaluation of the Cuero area
ix
6
.11
· 22
29
• 37
· 53
61
· 68
76
· 90
.102
ABSTRACT
Three areas in the Texas Gulf Coast region with different depositional settings,
structural styles, and sandstone distribution were studied with well log and seismic
data to evaluate some of the controls on subsurface conditions in geopressured
aquifers. Major differences in these subsurface properties make some areas more
favorable than others for exploration and. development of unconventional gas.
Structural and stratigraphic interpretations were made primarily on the basis of
well log correlations. Seismic data confirm the log interpretations but also are useful
in structure mapping at depths below well control. Selected seismic sections were
reprocessed to improve signal-to-noise ratio, enhance deep reflectors, and suppress
multiples; however, not all of the lines benefited from reprocessing. Those lines that
benefited the least typically had poor signal-to-noise ratio, problems with statics,
multiples, or oblique dip orientation that caused further deterioration of the seismic
image. Migration proved to be beneficial for fault interpretations. For the most part,
velocity variations in these areas are stratigraphically controlled. As a result, the top
of geopressure cannot be mapped on the basis of velocity inversion alone.
The Pleasant Bayou area is characterized by a complex structure resulting from
salt tectonics superimposed on a greatly expanded and growth-faulted section of the
lower Frio Formation. The thickest and most laterally continuous geopressured
sandstones were deposited in delta-front or distributary-mouth bar and channel
environments of a delta system. The fault block area is large (::: 25 mi2) and nearly
equidimensional in both strike and dip directions. Sandstone distribution within the
fault block varies from patchy to continuous. Some of these sandstones exhibit high
permeabilities and appear to be excellent aquifers.
1
In the Blessing area, shale tectonics combined with regional growth faulting to
produce sinuous fault patterns and fault blocks of varying size in the lower Frio
Formation. The primary fault block, nearly 36 mi2 in extent, is elongate parallel to
strike and is about twice as long as it is wide. Optimum sandstone distribution roughly
parallels depositional and structural strike and occurs along the central axis of the
fault block. These strike-oriented sandstone bodies were probably deposited in
barr ier-strandplain environments.
In contrast to the Pleasant Bayou and Blessing areas, the structural evolution of
the Cuero area did not involve salt mobilization or shale diapirism. Instead, relatively
straight and closely spaced growth faults having minimal displacement and minor
rollover are typical in the lower W iIcox Group. The resulting fault blocks are narrow
and highly elongate. The fault block of interest covers nearly 10 mi2 and is about 10
times longer than it is wide. Optimum sandstone distribution in the primary fault
block is strongly dip aligned. These relatively narrow sandstone bodies exhibit blocky
SP patterns and most likely are distributary channel fills. Although the channels have
good reservoir properties, they have substantially smaller areas and volumes than Frio
reservoirs.
Features that are similar to the three areas include (1) progradation and deposi
tion near the contemporaneous shelf edge, and (2) structural and stratigraphic factors
that control the distribution of geopressure. In these examples, geopressured
sandstones are isolated on the updip side of a fault block by downfaulting against
shelf-slope shales, and on the downdip side by upfaulting against transgressive marine
shales. Moreover, they are isolated above and below by thick sequences of marine
shale or interbedded sandstone and shale.
2
5i
...
INTRODUCTION
Structural and stratigraphic interpretation of geopressured geothermal areas on
a site-specific scale is necessary to predict reservoir size and the potential volume of
fluids that can be withdrawn from geo·pressured sandstones. In addition, geological
interpretations enable models explaining the genesis and distribution of geopressured
geothermal reservoirs to be formulated and refined.
Comparative structural interpretation on a regional scale is important (1) to
understand why excess fluid pressure accumulates, (2) to identify factors that distin
guish good prospects from poor prospects, and (3) to formulate structural models to aid
exploration for and evaluation of areas where subsurface control is poor.
A genetic or depositional-systems approach was employed to gain understanding
of the relationships among stratigraphy, structure, and paleogeography in the Pleasant
Bayou, Blessing, and Cuero areas (fig. 1). Geological and geophysical studies had to be
conducted simultaneously and interactively, with well data being used to refine the
seismic interpretations and vice versa.
The history of syndepositional structures that create geopressured geothermal
reservoirs can be reconstructed by means of sequential isopach maps. In the case of
shallow-water deposition, it can usually be assumed that the amount of contem
poraneous structural growth is much greater than the relief of depositional topog
raphy, so that an isopach map is essentially a paleostructural map. By removing the
horizontal components of post-depositional fault displacement from the isopach map,
an approximately palinspastic reconstruction can be obtained. Relative rates of fault
movement are quantified in terms of the growth ratio (or expansion index), defined as
the ratio of downthrown thickness to upthrown thickness. This type of analysis is
particularly useful for structurally complex areas such as Pleasant Bayou. Whereas
deep structure maps may show the superimposed effects of different tectonic regimes
3
..
~
1::::1.('''1 CLASTIC :U'i/" SHELF MARGIN
r/5l CARBONATE ~ SHELF MARGIN
~ SUBMARINE ~ CANYONS
r:::l SALT ~ STRUCTURES
26°
.96
+ +
+
+'
+ +""
92° 9
?j~~_L~~_ -,_. LA. I
J (
'. + + +
+ + +
a
+
50
a 50 100
Ii>
+ + + + + 94° 92° 90°
+
Figure 1. Location of Pleasant Bayou, Blessing, and Cuero study areas in relation to regional shelf-margin trends, northwestern Gulf of Mexico Basin. After Winker and Edwards (in press).
!
+
+
+
;::i,,. cn\'r en,?"
100 MI ,
150 KM
+
88°
+ r
+
+ 28°
+
+2
+ 88°
~.
and deformational styles, individual isopach maps usuaUy show a single style and are
therefore easier to interpret.
PLEASANT BA YOU AREA
Stratigraphic and Paleogeographic Setting
Prospective geopressured aquifers in the Pleasant Bayou study area occur in the
Anomalina bilateralis zone in the lower part of the Oligocene Frio Formation. Their
absolute age is not weB known because of a lack of datable material in the sequence
and because paleontologic correlation in the Gulf Coast Basin has been based on
benthic foraminifera, which have not been tied into global stratigraphic nomenclature
based on planktonic microfauna. The lower Frio represents the maximum progradation
of sandy facies in southeastern Texas during the Oligocene (table O.
Both the Frio and the AnomaHna bilateralis zone are informally defined
stratigraphic units. For this study, the top of Frio (TO) picked by Bebout and others
(J 978) was not used. Instead, correlation markers were picked at the tops of major
upward-coarsening progradational cycles within the Frio; these were assigned to
benthic foraminiferal zones, according to an average of several wells in the area with
micropaleontological data. In this report, the term "Frio" will refer to the interval
from T2 to T6 of Bebout and others (1978).
The Pleasant Bayou area is situated within the Houston delta system (Galloway
and others, in press), the lesser of two Frio delta systems in Texas (fig. 2). The larger
Norias delta system to the south has previously been excluded from intensive
geothermal exploration because the volcanicaUy derived sandstones are diagenetically
unstable, resulting in severe reduction of porosity and permeability by cementation
(Loucks and others, 1979).
5
0>
Unit Numbers for Isopach Maps & Structural Sections
I
2
3
II
5
6
7
Log Correlation Marker
TA
T2
T3
TIl
T5
~- -
Table I. Characteristics of operational stratigraphic units in the Pleasant Bayou area.
Micropaleo Zone
Discorbis, Het.-Marg.
Cibicides hazzardi
Nodosaria blan~iedi
Textularia mississi~~iensis
Anomalina bilateralis
Textularia warreni
-- --_._-
Stratigraphic Unit
Undifferentiated Miocene and younger
Anahuac
Upper Frio
Middle Frio
Lower Frio
Vicksburg
-----.~.---
Lithology
High sandstone some "hard streaks" (high-carbonate beds) in lower part of section
Shale (poorly indurated)
Interbedded sand-stone and shale; sandstone percent-age generally de-creases up section
Relative Velocity Seismic Signature
High; approximately Variable amplitude and linear increase low continuity high with depth in section, grading
down into high ampli-tude and high continuity
Low Virtually transparent (coherent signals may be multiples)
High; general Moderate amplitude increase with depth. and continuity. On No apparent relation- many lines, obscured ship with geopressure by multiples from distribution Miocene section
Overall Transgressi vel Regressive Character
Regressive
Transgressive
Overall transgressive (stacked regressive cycles)
Regressive
-
i
Figure 2. Location of Pleasant Bayou and Blessing study areas in relation to netsandstone thicknesses mapped for the lower Frio depositional systems in Texas. After Galloway and others (in press).
7
The Pleasant Bayou area is located within a zone of syndepositional down-to-the
basin faults that were most active during early Frio time. This fault zone also marks a
rapid change in lithofacies, both in terms of sandstone percentage and in log
character. Winker and Edwards (in press) interpret this relationship to mean that the
Pleasant Bayou reservoir sandstones were deposited near the contemporaneous shelf
edge (fig. 1).
Operational Stratigraphy
Site-specific description of the Pleasant Bayou area (fig. 3) began with the
correlation of distinctive electric-log markers (fig. 4). Where possible, these were
chosen at the tops of major progradational cycles and major lithologic breaks.
Markers T2, T4, and T5 were used in earlier reports on the Pleasant Bayou area
(Bebout and others, 1978; Flanigan, 1980). Operational stratigraphic units bounded by
these markers were numbered sequentially from youngest to oldest (tables 1 and 2) to
simplify labeling of maps and sections.
Log markers were transferred to nearby seismic sections using velocity-survey
data from the Humble III Skrabanek weU for conversion from depth to time. Markers
were carried through the seismic grid as far as possible to check for internal
consistency. Mis-ties of up to 100 ms were noted, but these did not show a clear
regional trend.
Correlation was mostly straightforward. The only serious difficulties were
encountered with the sub-T2 markers across the South Chocolate Bayou fault zone,
where growth faults are closely spaced and exhibit large displacement and expansion
of section. A tenuous marker was picked at the apparent base of the sandy Frio
section for use in isopach mapping of interval 6 (table 1).
8
A 39E 40E 37E 38E .0 •• • .. - • • . ............ -• • • • :!:: .
• •
•
~ . l • . ~
•
• • 65 •
·I/.: V' . __ :>::::::/:.F::· • •
•
• •
• •
/
~
///
-'-'-'-'~l-'-'-'-'_/ •
•
75
• • • •
• o 2 4mi
" o 2 3 4 5 6 km
~
Figure 3. Data base and locations of sections in Pleasant Bayou area, Brazoria County.
~
A A'
~ .., ~ .., " " W " ~ ~ ~ e e 0 ~
1: ::; ~~ :>
~ ~ ~ 0::, ~ U)~(I) <: t<: u.~
~., (l)t~ .g ","'''' ~j §r:- :~- ;i~, ~'" -::' ~~w 3~~ 11, ..,w
~~* "'~ ,
~'" ..,
~~ wjW o:::::::'1.6J "W
at( :::!Q::'( ...J'T~ ~ ~~ ~7 ~ ~ ~~~ 6'" :::!. I
'" -, , o , z:r::'(
'(J) :X:'''' :x:"'''' :x:'''' :x: , '''' -'", w,'" :>,'" *,If) -6000ft 0.*,,,, a..'" • 0.*'" 0.11: *,,,, ::e ..... " ..... "' ..... TA
TA
T2
6000 '2000 18000
1000
2000
Figure 4. Structural dip section through Chocolate Bayou field, illustrating log character of stratigraphic units in Pleasant Bayou area (table 1).
10
Table 2. Approximate stratigraphic position of log- and seismiccorrelation markers (TW, 01, TA, Bl, etc.) and functional stratigraphic
units (I, 2, 3, etc.) in four geothermal areas.
Regional str a tigraphic units, NW Gulf Basin
Pleasant Bayou, Brazoria County
Blessing, Matagorda Co.
Cuero, DeWitt Co.
McAllen Ranch, Hidalgo County
PostAnahuac
Anahuac Formation
Frio Formation
Vicksburg Formation
Jackson Group
Claiborne Group
Wilcox Group
Midway Group
1
TA 2
T2
3 T3' 4 T4 5 T5 6
7
11
1
B1 2
B2
3
4 B3 5 B4 6 B5
7
1
C1 2
TW
3 01 4 03 5 04, 6
7
1
2
3 4 5
6
..
Structural Setting
The location of the Pleasant Bayou area along the Frio growth-fault zone llmits
the potential size of Frio geothermal reservoirs. These growth faults resulted from
large-scale basinward sliding of the continental slope, which created an extensional
regime along the shelf margin (Winker and Edwards, in press). Frio growth faults tend
to be more sinuous and widely spaced, with larger rollover anticlines, than growth
faults in the W i1cox shelf margin.
The study area also lies within the Houston salt-structure province. Salt
tectonics has superimposed a pattern of domes and withdrawal basins on the regional
growth-fault structures, resulting in a complicated, multiphase structural fabric.
Furthermore, well control is very uneven because most wells are drilled on or near the
salt domes. For these reasons, seismic information is particularly important for
structural interpretation away from structural highs.
Local Structure
Structure on five correlation markers was mapped using both well logs (fig. 3)
and seismic data. A series of isopach maps was also constructed to interpret the
structural growth history of the area. Well control is dense and was therefore the
primary basis for structural mapping except in the salt-withdrawal basin between
Danbury Dome and the Chocolate Bayou field where wel1 control is sparse and
structural mapping was based primarily on seismic data. In general, seismic data were
used to verify the structural style, to check the direction and angle of fault dip, and to
extend the structural interpretation below and downdip of the area of dense well
control. The resulting structure map (fig. 8) on the zone of interest (T 5) is a
substantial departure from those maps obtained in earlier studies of this area (Bebout
and others, 1978; Flanigan, 1980).
12
Structural Style
The structural style in the Pleasant Bayou area (figs. 5-&) is typical of the Frio
trend in the southeast Texas salt basin. Its early structural history also has elements
in common with that of the Frio trend to the southwest, such as in the Blessing study
area (fig. 1), which is dominated by shale tectonics. Characteristics of the Frio trend
include highly sinuous growth faults with widely variable spacing «1 mi to >5 mi),
predominantly down-to-the-south offsets, large displacements (thousands of feet) and
growth ratios (up to 10) at depth, and substantial rollover anticlines (landward dips as
great as - 200). Displacements and growth ratios vary greatly along major faults.
Where cut by wells, fault dips average around 450 • Most major growth faults flatten
with depth, although no decollement surface is seen on seismic sections, unlike the
Vicksburg trend (fig. 50). Decollement is considerably deeper than in the South Texas
Vicksburg trend, but the listric nature of these faults is evidenced by the substantial
rollover.
In the Pleasant Bayou area, three sets of down-to-the-basin faults cut the lower
Frio (fig. g). (1) A single fault of large displacement separates the East and West
Chocolate Bayou fields and cuts through the Danbury Dome field. This fault is
difficult to map properly except by use of sequential isopach maps. Its displacement
decreases and probably dies out to the southwest. (2) Updip of this fault is a set of
faults with closer spacing and less displacement. (3) In the South Chocolate Bayou
field is a set of closely spaced faults of substantial displacement.
In addition to these major down-to-the-basin faults, small faults cut obliquely
across the Chocolate Bayou fault block (fig. g). The small fault in the Chocolate
Bayou field is known from both well and seismic data; those near the test well are
known only from seismic data. The origin of these faults is not well understood, but
they probably resulted from minor distortions of the large fault block during
movement of its bounding faults.
13
.-~
1'.
A. CUERO Stuart City
Trend Cook Field
~~J~~~~~il:~~:'~_5=:; ~~~~~~~~~~-\-~--
B. BLESSING Miltields Field Tideha ven Field
Cottonwood Creek Field
------ ------ --- ------- --------
E. Blessing Field
4 ~~:: ... ~:'.' ::: .... ;; .... : .. ,. ........ . ... ~ '. \:. . . ......... ' ' .......... " .. ·····~'·2········:···'···:··::··::·::::::;:':::::::i::::.:::,,::;::::;:::;;:'0'.,"::;::::
56
6
C. PLEASANT BAYOU
0
0
2 3
2 3 4 5
NO VERTICAL EXAGGERATION
:f§{\%E!&
4 5 MI
6 KM 15000
10,000 2
13 -j 15,000
4 J 20,000 5
6 25,000
KM FT
MAJOR SAND DEVELOPMENT
TOP OF GEOPRESSURE
Danbury Dome Field (salt withdrawal basin)
l~!;~~z:f;;::~~~\W~fl~~i\ff~lrj,~JaJo/?" ____ CORRELATION
MARKER
SEISMIC REFLECTOR
D. McALLEN RANCH EI Benadito Field
1
:.::"'-:::'::::'~~""'"':':-''' -:-:. '-:.
McAllen Ranch Field
... -:\~:;.:. :':'=:-:;-:::';':~.:.::.~. :::.' ... :.::-:.::.-... : .. :-: .. :: ........ :.:~.
Monte Christo Field
::':.:.':::."0:::::.::. ... -.:
_:~·::~:t'.;.:.;!:·~.;:·;::··:··:·::::::.:::= ---~-~~~~--/-------~~~:..~.=.':.~ ... 3
~.~. -------------- -- --Figure 5. Dip sections of four geopressured geothermal study areas. Based on seismic sections and well control. After Winker and Edwards (in press).
Contour interval: 200 ft
37E 38E 39E 40E
Figure 6. Structure contour map of TA marker (top of Anahuac).
15
65
75
37E
~IJ.OOO
·. . ..
1/
CHOCOLATE
BA';o"U
• FIELD
fJ /:iT
/"1
'f /J /1
39£
o I o
4 ml I I" I, i I
6 km
Conlour inlerval • 500 II
Figure 7. Structure contour map of T2 marker (top of first Frio sand and Cibiddes hazzardi zone).
16
..
75
o 4 mi 1-1 --,--L'~, ~,-'-' ~-"~, ----.,--' o 6 km
Contour intervol· 500 It
37E 38E 39E 40E
Figure S. Structure contour map of T5 marker (top of Anomalina bilateralis zone). Structural relief is the greatest of any geothermal study area in Texas.
17 ..
Salt mobilization caused by the deposition of clastic sediments over salt led to
the growth of Danbury Dome and an extensive salt-withdrawal syncline to the east and
south. Arching of the strata over the growing dome resulted in the formation of radial
faults (fig. 6) that are largely unrelated to the down-to-the-basin growth faults and do
not cut the lower Frio.
Structural Evolution
Sequential isopach maps (fig. 9) allow recognition of three stages of structural
evolution. (1) During early Frio time (intervals 5 and 6), a major down-to-the-basin
growth fault formed. Growth ratios were as high as 6, and substantial rollover
anticlines developed. This fault isolated downdip sandstones by juxtaposing them
against older (Vicksburg?) slope shales, thus creating the seal necessary for formation
of geopressure. The South Chocolate Bayou fault system was less active during this
time. (2) During later Frio time (intervals 3 and 4), growth of the main reservoir
bounding fault declined substantially, while the South Chocolate Bayou fault system
became very active. This basinward shift in the locus of major fault growth was
probably caused by basinward progradation of the shelf margin and thus of the
extensional regime. (3) In post-Frio time (intervals 1 and 2), structural growth was
dominated by deep-seated salt intrusion and related radial faults. Some down-to-the
basin faults continued to move, but at a greatly diminished rate.
Chocolate Bayou field apparently did not experience either salt withdrawal or
uplift during the time interval covered by the isopach maps. A likely explanation is
that this area experienced salt withdrawal early in its history, probably pre-Frio, while
still in a continental-slope environment. Early salt-withdrawal basins (fig. 10) are
common on the modern slope of the northwestern Gulf of Mexico (Lehner, 1969).
Thus, Chocolate Bayou field may be analogous to "turtle structures" in the East Texas
Basin and the North Sea, where broad structural closure is ultimately created in areas
18
6 Anomolino btlolero/is ZONE (FRIO) + + + +-+ + + +
+
N
! EXP.LANATION
/GROWTH FAULT ., j "-c;ROWTH RATIO
(Expansion index)
Isopach contour interval- iOO ft
o 4mi , ,', " .', I o 6km
Figure 9. Sequential isopach maps illustrating structural evolution of Pleasant Bayou study area. Distortion of grid is caused by removal of horizontal component of postdepositional faulting. Early movement was dominated by down-to-the-basin faulting with substantial rollover. Late movement was dominated by salt withdrawal and growth of Danbury Dome.
19
Q -
,
N o
o
A. GOLDEN LANE SLOPE
SHELF. B~EAK I !
10 MI 10 KM VERTICAL EXAGGERATION = 4-7 X
1 ;;¥;~':~
2·
3
SEC
S
THRUST-FAULTED _ANrICLlNE~ _
Figure 10. Seismic sections of modern continental slope, illustrating modern structural features analogous to Frio structures in Texas. Early salt-withdrawal basins may later form persistent broad structural highs like Chocolate Bayou field.
of initial salt withdrawal. In the Chocolate Bayou field, rollover associated with
growth faulting also helped to accentuate structural closure.
Sandstone Facies
Sandstone facies in the Anomalina bilateralis zone were classified on the basis of
SP patterns and on continuity of individual sandstones. Four facies were recognized
(table 3); these may represent different environments, but some of the differences in
log character may be attributable to the large differences in subsidence rates (fig. 11).
Spatial relationships among these facies are summarized by two stratigraphic sections
(figs. 11 and 12) and by maps of log patterns for individual prospective sandstones
(figs. 13, 14, and 15).
For reservoir evaluation, two kinds of facies variability that can be deduced
from log patterns are considered significant: maximum thickness of an individual
sandstone, and upward-fining, blocky, or upward-coarsening vertical trends. Upward
fining sandstones are generally thought to be channel or distributary facies, and tend
to be thicker than upward-coarsening sandstones, although this relationship is not as
well defined as in the Cuero study area. Aquifer permeabiIities tend to be high,
commonly greater than one darcy in the 'A' and 'C' sandstones; however, there is no
obvious relationship between log pattern and maximum permeability. Nonetheless, we
regard the thicker upward-fining or blocky sandstones to represent the best geother
mal reservoir facies. These optimum sandstones are extensive (fig. 16); this, in
combination with consistently high permeabilities, makes the Pleasant Bayou area an
excellent geothermal prospect (table 4). The large salt-withdrawal syncline (fig. 8)
places a large area of the 'C' sandstone off structure and away from current gas
production.
21
...
Table 3. Electric-log facies in the Anomalina bilateralis zone, lower Frio, Pleasant Bayou area.
Environmental I nterpreta tion
Fluvial/deltaic plain
Distributary channel and channel-mouth bar
Delta front
Delta front in rapid-expansion zone
Char acter istics
High sandstone; blocky patterns; laterally discontinuous; total section thin
Blocky to upward-fining pattern. Thick sandstones with few shale breaks. Laterally continuous with delta-front facies
Thin to thick, upwardcoarsening sandstones. Laterally continuous
Many thin sands interbedded with shale; difficult to correlate; very thick section
22
Geographic Distribution
Updip of main growth fault
Downdip of main growth fault; patchy distribution
Widely distributed downdip of main growth fault
Near main growth fault on downthrown side
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TEXACO #I£~rs 6S-39E-1 0_
'A'SAND 6S
7S
38E + 39E
o 5~ I ,', " , " ,', I o 8km
Figure 13. Log-pattern map of 'A' sand, lower Frio, with gas-well completions.
25
38E + 39E
'C'SAND
6$
40E
m fI
100 50 200
0
10
100 300
o 5~ 11----,.,--"-.-, -+1 I -.---'r'1 --r", '-, r-, -41 o 8km
Figure 14. Log-pattern map of lei sand (geothermal reservoir sand), lower Frio, with gas-well completions.
26
38E
'D'SAND
6S
7S
+ 39E
11 ~ 11~ ~ ~/~
. ~ .
--f-40£
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1~
m It
°1° 50 :,
100 300
?I----r, -L' -,-, --,,-'-' -,-".,-, ---.,-"-' -,--, --iT mi
° 8km
Figure 15. Log-pattern map of 'D' sand, lower Frio, with gas-well completions.
27
CUERO
DB Sand OCSand DOSand
BLESSING
N
/
PLEASANT BAYOU
IIIill A Sand DCSand DOSond
/ /
/ /'
/'
/
. .
/
I /
I
I I
I
/ / 0 I 2 3 4mi
I I I I I , I I I I'
/ 0 2 4 6km
Figure 16. Distribution of optimum sandstone facies as inferred from electric-log patterns in three geothermal study areas. Areal extent and permeabHities of geopressured sandstones are best in the Pleasant Bayou area.
28
Table 4. Reservoir parameters for three geothermal areas in the Texas Gulf Coast.
Pleasant Bayou Blessing Cuero
Maximum fault-block width (mi) 5 9 10
Fa~t-block area 25 36 12 (mi )
Depth range of reservoir sandstones in type wel1* (ft) 14,000 - 15,200 10,650 - 11,300 10,800 - 11,100
Area of optimum res~voir sandstones A - 15 B - 10 B - 2.3 (mi ) C - 25 C - 9 C - 1.5
D - 16 D - 0.4
Aggregate reservoir thickness in type well* (ft) 400 170 200
Permeability range of reservoir sandstones 0.01 - 515 0.01 - 56 0.01 - 250 (md)
A verage formation pressure (psi) 11,000 8,500 8,250
Average formation temperature (OF) 300 245 285
*For Pleasant Bayou area, based on GCO-DOE 112 Pleasant Bayou.
29
Pressure Distribution
Pressure distribution in the Pleasant Bayou area has been described in three
independent reports using different criteria for estimating formation pressures.
Fowler (1970) plotted isobaric gradient lines on cross sections on the basis of
proprietary test and production data (fig. 17). Bebout and others (1978) estimated an
approximate average top of geopressure for the study area by plotting bottom-hole
shut-in pressures (BHSIP) against depth (fig. 18). Flanigan (1980) estimated over
pressure distribution in the study area by plotting shale resistivity, conductivity, and
transit time against depth for individual wells and comparing these plots with type
curves for normal compaction. The three parameters used by Flanig~n indirectly
measure shale consolidation and, by inference, overpressure (fig. 19).
Flanigan's (1980) interpretation of pressure distribution differs significantly from
that of the other two in that he predicts the shallowest geopressure within the
Anahuac Formation. Log parameters in the Anahuac deviate substantially from the
normal compaction curve, suggesting fluid pressures above hydrostatic. However, mud
weights indicate that the Anahuac can be drilled with the same low-weight muds
«13 lb/gal) used for drilling the normally pressured upper Frio (fig. 20). In drilling the
General Crude-Department of Energy Pleasant Bayou test well, geopressure was not
encountered until deep within the Frio section. Shale resistivities, conductivities,
transit times, and interval velocities are apparently influenced strongly by factors
other than fluid pressure, possibly including formation-water salinity, depositional
environment, organic content, mineralogy,diagenesis, and flocculation behavior during
deposi tion.
Except for the Anahuac, the various criteria provide a fairly consistent picture
of pressure distribution. All criteria indicate that, updip of the reservoir fault block,
Frio sandstones are normally pressured. Within the reservoir fault block, overpressure
indicated by BHSIP's and mud weights is first encountered within the Frio section;
30
PHILLIPS NW 2M HOUSTON S£
SOUTH CHOCOLATE
BAYOU CHOCOLATE CHOCOLATE
WEST I EAST
-8 BAYOU BAYOU
-9
-10
-12
-13
-14
-I~
-16 o 10,000 tt
6 3600m
---- 0.5PSI/FT PRESSURE GRADIENT - - 0.8PSI/FT PRESSURE GRADIENT
A ANOMALINA BILATERALIS MARKER
Figure 17. Distribution of pressure gradients across Chocolate Bayou field. After Fowler (1970).
31
2
4
~
f-6
w W lL.
lL. 0
8 C/) 0 z <l: C/) ::::>
~ 10 f-
I f-a... w 12 0
14
16
18
0.465
o 2 4
\\\"" 0.6 07 0.8 0.9 1.0 ~ GRADIENT (PSI PER FOOT)
\\~"" . \ \ \~" \. \, ~ '"
\ \.\. "" \ \ \ '" \ .. \ .\ .. ~,
\ .. \. '{ "" \ \ ~ " \ \ '\ ""
6 8 10 12 14 16
BOTTOM-HOLE PRESSURE (THOUSANDS OF PSIA)
Figure 18. Static bottom-hole pressures versus depth for a number of wells, Chocolate Bayou field. After Bebout and others (1978).
32
o 0 1
10 25 30 37 41 44 If
5000mmmillITRmmmmrnmmlmmTImmmrnfutrmITTITIBrnTITITmmIT03smmmmrmmmsmmmwTImTImmmrzn5000 It
10000~1E~ 10000
Figure 19. Distribution of pressure gradients across Chocolate Bayou field, according to Flanigan (980).
33
..
A AI It 10 25 30 37 41 II
5000 5000
A
A -10.2
-9.8
9.9 -10.7 T2
T4
T5 10000 10000
-12.1 -13.9 -16.5 -14.8
-16.5 T2 -14.8
-14.9 -14.9 -16.9 -15.1 16.7--17.5 -17.5
-17.4 -17.2
-17.5 T4
-17.2 0 5 mi
15000 I I -17.5 15000
T5 -17.4
-17.9 IS.8-
17.S 17.5
17.8 17.3
Figure 20. Mud-weight data for wells in cross section from figure 19. After Flanigan (1980).
34
..
pressure gradient is transitional from normal pressure to geopressure over a distance
of about 1,000 ft. Downdip of the reservoir fault block, all of the Frio sandstones are
overpressured. As a general rule, isograds follow bedding planes within fault blocks
but may jump a few hundred feet across major faults (fig. 17).
Seismic Studies
Structural and stratigraphic interpretation of the Pleasant Bayou area was
enhanced by 129 mi of proprietary seismic data (fig. 21) that ranged from >10 yr to
<3 yr old and was subject to a variety of shooting parameters and processing techniques
(table 5). In this area, the most serious problem in interpretation was the presence of
strong multiples at the depth of interest, which could have been largely suppressed by
use of a long shot-receiver spread. Therefore, the General Crude-Western data set,
which employed the longest maximum offset, exhibited by far the best data quality.
Because of the structural complexity of the prospect area and because of the
wide variety of stratal and fault orientations, it is difficult to characterize the seismic
sections in terms of strike and dip orientation. Nonetheless, some lines proved to have
an optimal orientation, whereas others were located so as to be of little use.
Selected lines were reprocessed to ascertain the amount of potential improve
ment. Routine reprocessing of some of the General Crude and Mobil lines near the
test well was subcontracted to Houston Processors Incorporated (H.P .I.). Reprocessing
of Teledyne line 5-F was attempted with the Phoenix system at The University of
Texas at Austin. From this reprocessing it was concluded that the recently shot and
processed Western and Mobil lines do not benefit substantially from standard repro
cessing, whereas the Teledyne lines benefit greatly. Consequently, standard produc
tion reprocessing (i.e., automatic residual statics, slower AGC, reworked stacking
velocities, and migration) should be reserved for fairly old data (-10 yr or more),
35
•
6S38[
EXPLANATION • Well control
• Wei I with velocity survey
- Seismic secrion
6539£ 6S40[
7S4()(
0,-, ___ I_o.~?O_0 ___ 2~0,OOOfl
O~, -'_--'---'----'~~'m
Figure 21. Base map of seismic data. Three Brazoria wells with velocity surveys (Humble I-Angleton, Humble I-Williamson, and Humble I-Ward) are outside the area of this map.
36
Table 5. Seismic lines acquired for evaluation of the Pleasant Bayou area.
Data Maximum Quality at
Fold Offset Length Depth of Reprocessed Line Shot by Date (Nominal) (ft) (mi) Interest by H.P.I. Migrated Orientation
4F Teledyne 5/69 6 8,250 19.9 Fair Dip 5F Teledyne 4/69 6 8,250 16.2 Poor x Dip 6F Teledyne 8/69 6 8,250 18.0 Fair Strike GUS-11 G.U.S. 6/72 12 9,000 6.3 Fair x Dip
I.J.) GUS-18 " 8/72 12 9,000 4.4 Poor x Strike '-I W-1 Western (for Gen.
Crude) 10/74 12 15,840 5.9 Good x x Oblique W-2 11/74 12 15,840 8.4 Good x x Oblique W-3 11/74 12 15,840 6.6 Good Oblique W-4 11/74 12 15,840 6.6 Good x x Oblique W-5 (EN) 12/74 12 15,840 8.4 Good x x Oblique W-5 (NS) 12/74 12 15,840 3.6 Fair Dip W-6 12/74 12 15,840 4.4 Poor Strike CB-78-1 P.G.I. (for Mobil) 11/78 12 10,780 11.0 Fair x x Dip CB-78-2 " 11/78 12 10,780 3.1 Fair x Dip CB-78-3 " 11/78 12 10,780 2.7 Fair x Oblique CB-78-18 " 9/78 12 10,780 3.7 Good x x Oblique
..
whereas more recent data should be reprocessed only for a specific purpose such as
waveform studies or for comparison with synthetic seismograms.
MobiJ CB-78-1
This line best demonstrates the structural style of the bounding faults of the
reservoir fault block (fig. 22). On the northwestern (updip) end of the section is a
single fault with large displacement and rollover. Unfortunately, the seismic line does
not extend far enough updip to show the upward extension of this fault. To the
southeast (downdip) is a series of faults of large displacement but with relatively little
rollover; traps created by these faults constitute the South Chocolate Bayou field.
Between the updip and downdip faults, this seismic line traverses the Chocolate Bayou
field, a broad low-relief dome. A small fault crosses the Chocolate Bayou field nearly
parallel to the seismic line and intersects the line within the Frio interval. As a
result, the Frio is difficult to interpret on this line even after migration (fig. 22).
Data quality deteriorates severely at Frio depths in the South Chocolate Bayou
fault zone. The reason for this is not evident, but may be related to the problem of
"wipe-out" zones beneath major faults as seen in the Cuero area. At depth in the
South Chocolate Bayou area, migration does not improve the image and may even
worsen it.
Nonetheless, migration resulted in considerable improvement of the rest of the
seismic image. Most evident was the improved clarity and sharpness of the South
Chocolate Bayou faults where they cut the Miocene section (fig. 23). Migration also
resulted in a more accurate representation of the rollover geometry. Migration did
not help the seismic image where the small Chocolate Bayou fault cuts the seismic
line in a near-strike orientation. The two-dimensional migration algorithm assumes a
true-dip orientation for all dipping reflectors, and deviation from this assumption can
result in deterioration rather than improvement of the image by migration. Migration
38
NW TIE TIE TIE
CB-78-2 W-3 CB-78-3 ,l
TIE 6F
TIE GUS-IS
SF
Figure 22. Mobil line CB-78-1, after reprocessing and migration. This line best demonstrates the styles of regional down-to-basin growth faults and the structural setting of the Chocolate Bayou and South Chocolate Bayou fields.
39
•
~ o
TIE 4;F BEND TIE
_ _",,'" _ W-5 BEND " """",,,,lj'I" , i' _ '" '" ,J TIE TIE ., ',. .', .""'" " ""I"'" . G ~~::;;;;:;.~~i''''''''"''~:'~!!~:'~~:!::' ;:::::,-",' . W;'. TIE TEIEO~~t~MAL ,~~Z:='~".=::::"'-'""'" .," .'. ,LlI',' W·' I TIE ~~~~~:~!i;i;l: ",*,~,~~.:<l ~= Ie ~~"ir---':"':;~ ~;l~~ ~" .• ",.".--.. ~~ .....:::-;.--"",., .df:<'''~~ ~~"'i;;;-~""""'d-" .==~~~ ~~_~=~-""" ~_ .. ~~ ... , ~. i -=-----~' ... -~~ ... ~--."..~~_.:~ __ ..:s-;- ~~.:::-:-.... , ... =?~ --' ~ ~_~_~~'?;!l .... ~"'.,~~' -.... -;?L~",~~.o=-." .~~.;) .. -,"' --~ ~~ ... '" ~~~ . --- p=W.,
"-~~~ -:..~_~of:&6',_;;.:: ~~~ .... -::;:;.~" ___ --E"""~~
sw NE
~
~...;;;:
·-?~:!fS(.t:S:~~;:1 ... ~1~~
~-~~~.... ..~
r.'~~~ .~3.0 ~~"~
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,"'
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~ ~.-~-:It~-~- ...... , ~'<"!~"':!t:-..._._. __ . ~
~
~~.::;?'"~~~ ,..-.z;;;, ~~~;;-'~~ ~ ~~-.... ~~~~~-..:;..:.:..~.~ ~'.-..l~~~ ~~~~~ ~~:;.r.- -.-0;-:;;~~~., ... ,:,<-:,,-;;(/. ~~~-.~~.----~- ,~~,~~
"...;. ~, ~~~~ ... ~~~ ...... ~...--:: '~ ....: ..... ~~ .. ~ .-..... ~~~~~~~~ ... ...:s;~ .. ~"""'~.~ _ ....... Y'"~ - ........ -"'<V~..::;::.~ . ';:::'~"~ ._~~ .." .• ~. ~ .. .' .,..,. •• ;;.::~~~..,....... :.:;.o~~'"''';;: .~~. ~-'-~'~.' ' ...... "'.""'...,,'~~~ .• )~~-'--""-;:-~;.,r;::; .... ~~ _Yo, ~ .... , ;~':-<.o~ ~~ .... _"" ~. ~. " ...... ~~. ;-"'8~~~ .-~~ ..... ~""""""1
- ;".:i"l''''~'''-;'''''-I>I .... ..J"' ..... "-"!.~"o:--• ,,- ~~"'I ;;;;,-.;.c.J_ ~~ :::";~= ~"r.A. .. J>:'<,;. ~~ ... ~<=-~~~~~ ~~~~~~''::~~~ qg~~~~~~?~~~ e=.~~~i"~~~~~~:...~ .. _ , ..... ~'-.:%:, ....... ~..,..-... ~~.,;;:: .. ~" .. ~. , ~~~,' . .....,A~ ..... " ....... , ... " .•..
~~~>V~~~~_~~ ;;;0:::
...... ~ ~
- ... ,; ~~ ~
., . ..... ~
;:;,
~~ ,,~~,
~
.--......~
;..,-
~ '. _ ... -~ ----# ~~~
~;e -'
W.i~';;;~~· ~= ._-...-""""-~ :;0:, .....
.~~:f') -~....-:~~~~ ..... .;,;.- .. .. .,.,.,.,~~ ..... ,,~
~ =.~=:;:~~
- -',.,,~ .::
~ .....:I . ....,
.. ..;."".-~ . ..::::J.r..~~""~:"1:-~ '"":"''' ~-.:~~!~~~~ ~=--~-""..."." '.-"'i'~~~~ - z-o-"" ~ - '" '. -s~~<'"'_
~~ ~~~~ ---" ..... ~~. ~
.,- :'--'-'1" ~ __ ~~~~
-~~"~~ ~""-'::::::-':~~""'--~~""'~~'·"II""~~"".-
'. ,~,~ . ~':'-'" ':.-~"-~ ~~~j~_CA~~ !Jo-i.4L~~~~.~~"':':'".M
~ ... ~ ~~
.. '~.' :;":,,,--:.;- --Figure 23. Teledyne line 6F, after reprocessing and migration. This is a strike line approximately down the middle of the Chocolate Bayou field. To the west, dominance of salt withdrawal is apparent.
commonly creates the impression that the faults are listric near the base of the
section. In the Frio, this inference is often a valid one, but may also be an artifact
created by migrating noise.
Reprocessing of CB-78-1 was probably unnecessary. Application of automatic
residual statics and reworked stacking velocities did not significantly improve the
image in the South Chocolate Bayou fault zone as was hoped. Minor improvements in
coherence and continuity were observed, but not enough to improve the interpret
abiJi ty of the section.
Teledyne 6-F
This is essentiaBy a strike line running down the center of the reservoir fault
block; it ties the Chocolate Bayou field on the structural high -.yith the geothermal
weB on the flank of the syncline (fig. 23). It shows structures resulting primarily from
post-Frio salt mobilization rather than from growth faulting during Frio time. It is
also properly oriented to show the smaB down-to-the-west fault that cuts the Frio in
the Chocolate Bayou field. Seismic signature in the Frio suggests high continuity, but
some of this may be due to multiples of the Miocene section.
Western W-4
This is an example of a seismic line that does not follow the regional dip but is
nevertheless ideaUy oriented to show certain structures. It ties the Pleasant Bayou 112
geothermal test weU on th~ east end with the updip-bounding growth fault on its west
end (fig. 24). Most significantly, it shows the small faults that cut the lower Frio near
the test well. These small faults are not immediately apparent on the section, and
their exact position is speculative. Their presence was detected by tying the T -4 and
T - 5 reflectors to Humble III Skrabanek and the geothermal test well. Although the
exact position of these faults is not known, their displacement is clearly down-to-the
west.
41
w
TIE W-I
TIE TIE W-3 6F
TIE C8-78-18
Figure 24. Interpreted version of line W-4, after reprocessing and migration by H.P.I. Deep faults near the geothermal well were inferred by tying loops in the Western grid.
42
As with CB-78-l8, reprocessing resulted in only minor changes in the seismic
image of W -4. Migration improved the accuracy of the rollover anticline near
Danbury Dome, but had relatively little effect on the interpretability of the smaU
faults near the geothermal well.
Overall, this line is one of the best in the area in terms of data quality at the
depth of interest. This high quality can be attributed primarily to the long offset,
resulting in adequate suppression of multiples.
Western W-3
This line nearly ties the geothermal welJ and runs obliquely to the updip-bounding
growth fault without meeting it. Data quality is slightly poorer than that of W -4, and
reflector continuity in the Frio suffers somewhat (fig. 25). Reprocessing resulted in
slight improvement in reflector continuity. Migration did not help this line because
there is significant dip across the line rather than with it.
Western W-l
This line paraUels W-3 to the east (fig. 21); unlike W-3 it crosses the updip
bounding growth fault of the reservoir fault block. This bounding fault is seen on this
line at nearly its maximum displacement and growth ratio; expansion of the Frio
section into the fault is readiJy evident (fig. 26). The anticlinal closure was created by
post-Frio salt withdrawal that tilted the area of the line southward.
Western W-5
This line also shows the updip-bounding growth fault to advantage (fig. 27), but
at a point somewhat to the south of its maximum displacement. Expansion of the
individual lower Frio units across the fault is evident. The fault is also seen to die out
upward in the undifferentiated Miocene section. As a general rule, this fault dies out
either in the Miocene section or within the Anahuac.
43
..
N
TIE CB-78-2
TIE CB-78-1
TIE CB-78-3
TIE W-2
s
Figure 25. Line W-3, after reprocessing and migration by H.P.I. Line does not benefit from migration because most of the dip is across the line.
44
N TIE TIE
W-5S W-2 " .... : ,: .. : l;::;;';"; ;;!,.,., .1". ,. p' ,. I. " •• '.' ., -. ,I, .~ •.
TIE W-4
I
TIE 6F I.
TIE W-5
I
s
-0.0
n'
5
Figure 26. Western line W-l, as delivered. This line best shows the major down-tothe-basin growth fault near Danbury Dome.
45
w TIE
4 -F TIE 6F
Figure 27. Western line W-5, as delivered, shows the difference in fault styles with depth. Deep structure is dominated by large down-to-the-basin growth faults. Shallow structure is dominated by radial crestal faults associated with Danbury Dome. These crestal faults die out with depth.
46
•
Also seen on this line are two faults that cut the Miocene section but which die t
out downward before reaching the lower Frio. These are interpreted as radial faults
from Danbury Dome that were formed in post-Frio time. Expansion of sections across
these faults is slight.
Velocity Distribution
Knowledge of velocity distribution in three dimensions is essential for data
processing. One objective of this study was to determine the geological controls on
velocity in the study areas.
Velocity data are obtained in three ways: acoustic logging of weUs, velocity
surveys, and velocity analyses. The acoustic logging tool measures transit time and,
hence, average velocity over a short distance (several feet). Average interval
velocities and time-depth conversions can be obtained by integration. Acoustic logs
are prone to systematic errors and tend to underestimate velocities; integrated
acoustic logs are therefore prone to cumulative errors and thus are unreJiable for
time-depth conversion.
Velocity surveys are obtained by measuring down-hole arrival times from surface
shots near a well, and by making appropriate geometric corrections. Errors are due
primarily to difficulties in picking the first breaks consistently; this problem becomes
more serious as the depth increases. Interval velocities are obtained essentiaUy by
discrete differentiation; any errors in measuring arrival times result in substantial
anomalies in interval velocities. Nonetheless, velocity surveys are generally the most
reliable source of time-depth conversions and average formation velocities.
Velocity analyses attempt to determine the optimum stacking velocities for
multi-channel seismic reflection data, using a variety of displays to show peak
ampJitudes of the stacked signal as a function of stacking velocity and arrival time.
47
Ideally, for flat-lying reflectors, the optimum stacking velocity is equal to the root
mean square (rms) velocity. As with velocity surveys, obtaining interval velocities is
essentially a process of differentiation, and time-depth conversions are obtained by
reintegration. In practice, optimum stacking velocities are not a very precise
estimator of rms velocities, and interpretation of velocity analyses is highly subjec
tive. Consequently, interval velocities obtained by this method are generally only a
crude estimator of true interval velocities.
Interval Velocities
As determined from velocity surveys (fig. 28), velocity distribution in the
Pleasant Bayou area is controlled primarily by stratigraphy. In the Miocene section,
interval velocities increase fairly steadily with depth, reaching a maximum near the
base of the Miocene. High interval velocities in the lowest Miocene may be caused in
part by "hard streaks," high-carbonate zones evidenced by high log resistivities
(Flanigan, 1980). The very high velocity interval showing up in the geothermal test
well at the base of the Miocene is probably due to an inconsistency between adjacent
shots in picking the first breaks; the calculated interval velocity of 17,000 ft/sec is
unreasonably high.
The top of the Anahuac is marked by a sharp velocity inversion; all velocity
surveys show the Anahuac to be a low-velocity zone. Flanigan (1980) cites this as
evidence of geopressure within the Anahuac, although, as previously discussed, the
relationship between formation velocity and pressure is probably not as straightfor
ward as he suggests.
Within the Frio section, interval velocities again increase fairly steadily, with
the exception of the Phillips II2-M Houston well. This well penetrated a major growth
fault and continued to considerable depth through pre-Frio shale. Consequently, the
low-velocity zone encountered in the lowest part of this well is not representative of
the Frio.
48
5.000 7,000
6,000
5,000
4,000
3,000
2,000
.;:
~ 1,000
:::> ti 0 0
~ -1,000
o ~ -2,000
I-0-3,000
W I -4.000
-5,000 .
-6,000
-7,000
-8.000
-9.000
INTERVAL VELOCITY (ft/sec)
! , ~---l
c----l--:=-------:~~------l
~_l_--: L i --j J_-~.,----] i
~-tTll I~--- :-: ,1- ±
Figure 28. Interval velocities as determined from velocity surveys in five wells in Brazoria County (three shown in fig. 21). Datum is velocity inversion at top of the Anahuac. Extremely high velocity zone in Pleasant Bayou well is probably an artifact.
49
Otherwise, the data do not indicate that a significant velocity inversion occurs
consistently within the Frio as would be suggested by the pressure distribution
(fig. 17). This further suggests that a simple general relationship between interval
velocities and formation pressures does not exist. This observation casts doubt on the
validity of using velocity inversions observed on velocity analyses to predict the top of
geopressure ahead of the drill. This relationship is known to work in some areas of
offshore Louisiana, but it does not appear to be universally applicable.
Two factors may invalidate the pressure-interval velocity relationship. First,
there may be controls on shale velocities other than physical compaction, such as were
suggested in the discussion on pressure distribution. In the Pleasant Bayou test well,
the consolidation state of clays increases steadily from the Anahuac, to the normally
pressured Frio, to the overpressured Frio (Milliken and others, 19&1).
Second, a high percentage of well-cemented sandstones in the geopressured zone
could overshadow any effect of low shale velocities, and no inversion of interval
velocities would be apparent. This effect is expected to be particularly significant in
geothermal prospect areas where a high sandstone percentage in the geopressured zone
is a prerequisite.
Time-Depth Conversion
In spite of major stratigraphically controlled velocity variations and substantial
structural relief, lateral velocity variations do not appear to create serious problems
for time-to-depth conversion (fig. 29), particularly when compared with the Cuero
study area. Apparently, stratigraphically controlled velocity variations tend to cancel
out when integrated to obtain the time-depth curves.
As a result, conversion to depth of all migrated sections is recommended if
further structural analysis is to be undertaken. Not only is this a simple process when
performed automatically, but it yields a more accurate picture of the structural
geometry.
50
to
4000
6000
8000
10,000
12,000
14,000
18,000
TWO-WAY TRAVEL TIME (SEC) 2
--- HUMBLE 1- SKRABANEK
RANGE FOR SIX WELLS
w,OOOL-________ ~ __________ _L __________ ~ __________ L_ __________ L_ ________ _L __________ ~ ________ ~
Figure 29. Range of time-depth curves obtained from six velocity surveys in Brazoria County.
51
BLESSING AREA
Stratigraphic and Paleogeographic Setting
Sandstones containing potential1y useful geopressure geothermal energy in the
Blessing study area occur in the Anomalina bilateralis zone of the lower part of the
Oligocene Frio Formation. The lower Frio represents the maximum progradation of
sandy facies in this area during the Oligocene (table 6).
Both the Frio and the Anomalina bilateralis zone are informally defined
stratigraphic units. Correlation markers B1-B6 (Weise and others, 1981) were picked
on distinctive breaks in the log pattern; these are roughly assigned to foraminiferal
zones based on several wells with paleontological data (table 6). Note that the top of
Frio as used in this report; the Cibicides hazzardi marker is between B2 and B3.
The Blessing reservoirs are situated within the Greta-Carancahua strandplain
system of the Frio (Gal1oway and others, in press), near the southwestern edge of the
Houston delta system (fig. 2). The study area is located within a trend of contem
poraneous down-to-the-south faults that were most active in early and middle Frio
time. This trend also marks a rapid downdip change in lithofacies, both in sandstone
percentage and log character implying deposition near the contemporaneous shelf edge
(Winker and Edwards, in press).
Operational Stratigraphy
Detailed study of the Blessing area (fig. 30) began with the correlation of
distinctive electric Jog markers. Where possible, these were chosen at the tops of
major facies packages or major lithologic breaks. Stratigraphic units bounded by these
markers were numbered sequentially from youngest to oldest (table 6) to simplify
labeling of maps and sections. Log markers were transferred to nearby seismic
52
01 (.,)
Unit Numbers for Isopach Maps &: Structural Sections
I
2
3
4
5
6
7
Log Correlation Marker
BI
B2
B3
B4
B5
B6
--_._-
Table 6. Characteristics of operational stratigraphic units in the Blessing area.
Micropaleo Zone
Discorbis
Het-Marg Cibicides hazzardi
Nodosaria blanl!iedi
Textularia seligi/ T. miss.
Anomalina bilateralis
Textularia warreni
-- -- '---
Stratigraphic Unit
Undifferentiated Miocene and younger
Anahuac
Upper Frio
Middle Frio
Lower Frio
Vicksburg
Lithology
Thin sandstones, in shale; net thickness increases downdip
Shale
Shale; thin sands towards base
Sand and shale, mostly thin sands
Sand and shale, moderate amounts of sand
Sand and shale, moderate amounts of sand
Shale
-
Relative Velocity Seismic Signature
Fairly high; linear Moderate to high increase with depth amplitude and
continuity/highest at base of unit
Low to moderate, Virtually no increase with transparent depth
Moderate, increasing Transparent at top, with depth high amplitude at
base
Moderate to high, Moderate to high increasing with amplitude and depth continuity
Fairly high, slight Moderate to high increase with depth amplitude and
continuity
Fairly high, slight Low to moderate increase with depth amplitude and
continuity
Probably low Generally low to moderate amplitude
.----~- ---
Overall Transgressive/ Regressive Character
Regressive
Transgressive
Overall transgressive (stacked regressive cycles)
Regressive
Transgressive?
-- --
I
I I
30 E 31 £
E
95
88
105! /
V.: I · I
" 5 I •
o 1 o
• Well centro!
32E
2 3 4km , 1 i '
2mi
. . •
EXPLANATION
--Slismicsec:1ion
• Type we" ........- SlructUft _ion • We" wilh velocily uvey
33£
+
Figure 30. Base map showing locations of available deep well control, seismic sections, and velocity surveys in the Blessing study area.
54 ~ . ..
sections, and velocity survey data from five weJJs in the area were used for conversion
from depth to time.
Structural Setting
The location of the Blessing area along the Frio growth-fault trend limits the
potential size of Frio geothermal reservoirs. These growth faults are interpreted to
result from large-scale basinward sliding of the continental slope, creating extension
along the shelf margin. The area lies southwest of the Houston salt dome basin; salt in
the Blessing area was thin, and no salt structures are present. Shale diapirism was,
however, active in the area during Vicksburg and lower Frio time.
Local Structure
Structural Style
The structural style 9f the Blessing study area (figs. 5B, 31, and 32) is typical of
the Frio trend in the middle Gulf Coast area. Characteristics of the Frio trend include
highly sinuous growth faults of variable spacing (generally less than 1 mi), pre
dominantly down-to-the-basin with thousands of feet of displacement, and growth
ratios up to 10 with substantial rolJover on the downthrown block. Displacements and
growth ratios typicaUy vary greatly along the major faults. Fault dips at the depths of
interest are typicalJy 450 to 500• Faults flatten slightly with depth, but the
decollement has not been seen on seismic sections. Decollement must be considerably
deeper than in the South Texas Vicksburg trend (fig. 5D), but the listric nature of these
faults is evidenced by the substantial amount of rollover.
In the Blessing area, two major sets of growth faults cut the lower Frio (fig. 32).
The updip set consists of several faults enclosing lesser fault compartments. This set
bounds the Blessing block on the north, separating it from Tidehaven and Francitas
55
..
~OE
1 N
9$
II
lOS
liS 0 I 5 mi rl-'~'~'--~-'~T---~--~' o 5 km
~IE 32£
Contour Inter\lol = 100 II
• Wen Control
(I) Type Well
Figure 31. Structure contour map of Bl correlation marker (top of Anahuac).
56
.to
30E
V>
?
32E
Contour interval = 100 ft
• Well control
o Type well
33 E
----
Figure 32. Structure contour map of B5 correlation marker (top of Anomalina bilateralis zone).
57
fields. The downdip fault appears to consist of a single large growth fault, bounding
the Blessing block on the south and separating it from the Trull field. Both sets of
faults have a subdued expression in the Miocene (fig. 31).
In addition to these faults, small cross faults cut across the Blessing fault block.
A zone of small faults bisects the Blessing block east of the Blessing field and the area
where geopressured sandstones are best developed. The nature of this zone is not well
defined by the available data, although it appears to break continuity in unit 6 (sub-B5;
fig. 32). A small cross fault is inferred to bound the Blessing block on the southwest.
Structural Evolution
Structural evolution of the Blessing area was studied by means of sequential
isopach maps (fig. 33). In this area distinct stages of structural evolution have not
been recognized. Instead, structural evolution was characterized by a steady decline
in growth fault activity and late southward tilting. During earliest Frio time (inter
val 6), the updip growth fault underwent large-scale movement, growth ratios being
from q. to 8, and large rotation into the fault. This updip growth-fault movement
continued through unit 5 time and into unit q. (middle Frio). '[he downdip fault
probably began to move during interval 6 time as well, but the data are inadequate to
define this stage. The downdip fault was the primary growth fault during early and
middle Frio time (units q. and 5), and continued into unit 3. In late- to post-Frio time
(units 1-3), fault activity was relatively minor, growth ratios being generally less than
1.2.
The Blessing fault block underwent large-scale rotation and thickening of section
into the updip growth fault from unit 6 through unit 4 time. This activity has resulted
in predominantly landward dips in these units, and in sealing of the sands in these units
by pre-Frio slope shales to the north and northwest.
58
6 Allomo/illa bilo/llro/is zone
+
4 Nodosaria b/OIIpil1di zone
+ +
N
f
+
3 Hllf-Marg Cibicidl1s horrardi
-+
+
-+
2 Upper Anahuac / (Discorm zonel '
-+ '+. + ' +' +'''''' , ~ + ~~' /,'" \ /
'" \."
+
+ \,+ ,+(,>,:-\."""'~"" ',+' , ..... +, / + ,",' ,0','", /
I' ." 'ffIJ ' ' , ' ,I , ' I ". '.. /
,;' , +- I~:t"~~-~+ ..... --~:+ . ,~/ '-" -..... ~ "gr:fJ -
+ § ,~r--;-:- + : + +' +
liS
EXPLANATION
/GROWTH FAULT ., I ..... ~ROWTH RATIO
(Expansion index)
I ,). + ,
Isopach contour interval: 100 ft
o 4mi I ;', " i'i I'
o Skm
Figure 33. Sequential isopach maps illustrating structural evolution of the Blessing study area. Distortion of grid is caused by removal of horizontal component of postdepositional faulting. Early large-scale growth faulting and rollover ended by late Frio time.
59
•
..
Sandstone Facies
Sandstone facies in the Anomalina bilateralis zone were classified on the basis of
SP patterns and on continuity of individual sandstones. Three facies, representing two
different environments and the effect of growth faulting, were recognized (table 7).
The first facies represents barrier or strandplain sands stacked one atop another by the
slower rates of subsidence northwest of the updip growth fault. The spatial
relationships of the other two facies are summarized for the Blessing fault block by
stratigraphic sections (fig. 34) and by maps of log patterns for individual sandstones
(figs. 35 and 36).
For reservoir evaluation, two kinds of facies variability that can be deduced
from log patterns are considered significant: maximum thickness of sandstone, and
upward-fining (or blocky) versus upward-coarsening vertical trend. Most of the sands
in the Blessing area exhibit upward-coarsening, or blocky, vertical trends, representing
prograding barrier settings. The areas of thickest sand free from shale breaks are
strike aligned (fig. 16), as is typical of bars or strandplain sands. Permeabilities of the
sand bodies are not high (sidewall cores show about 25 md), but do show some increase
upward within the individual sand bodies. The area of optimum sandstone is fairly
extensive and is favorably aligned with the elongation of the Blessing fault block
(fig. 16), leading to moderately high reserve estimates.
Pressure Distribution
Pressure distribution in the Blessing area has been estimated by three different
criteria: bottom-hole shut-in pressures (fig. 37), weights of drilling mud used, and
shale resistivity plots. The mud weights used in the type well (Texaco 1116 Thomas)
indicate a deeper top of geopressure than is estimated by the other two techniques
(9,800 ft vs. about 8,500 ft, fig. 38). Relatively low-weight muds (11.8 lb/gal) were
60
Table 7. Electric-log facies in the Anomalina bilateralis zone (unit 6), lower Frio, Blessing area.
Environmental Interpreta tion
Stacked barrier/ strandplain sands
Barrier barf strandplain
Shelf/slope
Characteristics
Thin section; high percent sand in blocky patterns, individual sands not correlatable
Blocky SP sand, largely in upward-coarsening sequences with sharp tops; thin shale breaks appear in areas of rapid subsidence
Shale, occasional thin sands
61
Geographic Distribution
Northwest of block of interest
In block of interest; forming str ike-elonga te sand bodies
In downdip block, and separating sands in block of interest
B
c
D
BROWN '" Dowdy 105-)1[-2
B
A
B
UNION III Po!lhOSl IOS-30[-6
I~
A
B
\
RUTHERFORD " Trull 105-31£-2
SUPERIOR HALBOUTY HALBOVTv III NtflSOn ;,2 McDonald IN McDonald 105-31[-5 105-31[·5 105-31[-5
BROWN GETTy MOSBACH #1 Ev/trsI"n 112 Sullivan #1 VIgus
105-31[-5 10$-31[-5 105-3IE'5
OATUM • B5
I Mite
0 2000 4000 fl I I
"m
t' 100
50 200
100 300
400
" Verllcol UQQQ'Holion • 10.
c
c
UNION N2Morfifl IOS-31E.·4
~
c' DATUM'BS
C6K ~7}~~~ "I fbff"osl IOS-30E _ 7 CBK IOS-3ClE-6 TENNECO TEXKAN MITCHELL
112 ?alll/osl ,A2 Trull IIIC Sloan ~I Sloan 10$-30[-6
I If
SOCONV "I Harrison
105-30[-7 t D' OATUM' 85 \
I \
?,1
~ f '
Figure 34. Stratigraphic dip sections of lower Frio (Anomalina bilateralis zone) within the prospective fault block, showing changes in sand character in the northern growth fault.
62
,""f
/ ~ilt· ~11.j~~ ~-~ jilk I { @lr·"'\·1111, ~. {~~!!'~
o •• ... . .• m ~llf ~'II,~Ik-
'~
~irr " f;(I~ ~r
Figure 35. Lo o 'B' s g-pattern map f
63
and, lower F . no.
lOS -31E
Figure 36. Lo g-pattern map of leI s and, lowe F . r riO.
64
..
2
4
6
I-w w IJ..
(3
88 3 I I-a. W 0
10
12
14
16
BOTTOM HOLE SHUT-IN PRESSURE (x 1000) kPo 20 40 60 80
~--------~~--------~r---------~r-----------~------~O
ZONE
A
0
0
B
t C
1
0 2 4
BOTTOM 6
MATAGORDA CO ,TEXAS BHSIP vs DEPTH Data from 122 wells
.60
\ \
\ \0
8 10
HOLE SHUT-IN PRESSURE (xIOOO) PSI
2(f) a:: w I-w ~
0 0 Q. )(
I
b.:: w
3 0
4
1.0
12 14
Figure 37. Static bottom-hole pressures versus depth for Matagorda County wells. From Weise and others (I 981).
65
6
f- B w w ... 0 0 Q ~
~ 1O 0.. W 0
12
Figure 38. (1981).
1.0
Rsh,ohm-m 10.0
Shale resistivity versus depth, Blessing area.
66
2
CI) ex: w f-w :E (3 0 Q .!
3J: f-0.. W 0
4
From Weise and others
used in this well until the pressure gradient reached nearly 0.7 psi/ft, at which point
casing was set and heavier mud used.
Except for this slight discrepancy between pressure-gradient estimates, the
various criteria provide a consistent picture of pressure distribution. Updip of the
Blessing fault block, all of the Frio sandstones are normally pressured (except for a
few of the lowest sands in the small blocks within the updip growth fault). Within the
reservoir block, overpressure is first encountered within the Frio section, between
markers B3 and B4, with a transition zone of several hundred feet. Downdip of the
reservoir fault block, nearly all the Frio is overpressured. As noted elsewhere, isograd
surfaces generally follow bedding planes within the fault blocks, although this
correspondence is not necessarily exact.
Seismic Studies
Structural and stratigraphic interpretation of the Blessing area was enhanced by
132 mi of proprietary seismic data. The lines acquired were shot between 1957 and
1973 (table &), and employed a great variety of shooting parameters and processing
techniques. Many of the critical lines passing through the area of interest are single
fold lines, which are not suited for stratigraphic work. In contrast to the Pleasant
Bayou area, strong mUltiples were not a serious problem in the multifold Blessing lines.
The location of seismic lines is shown in figure 30. Most of them are dip lines,
but a few cross more than two major faults. A complete picture of the reservoir block
can be obtained only by piecing together different lines, commonly from different
companies.
The Blessing seismic data was not reprocessed. The lack of a serious multiple
problem and the relative simplicity of the structure have made "as received" copies
adequate for interpreting the basic structure. The simple velocity structure of the
67
Table 8. Seismic lines acquired for evaluation of the Blessing area.
Data Maximum Quality at
Fold Offset Length Depth of Line Shot by Date (Nominal) (ft) (mi) Interest Migrated Orientation
IF Teledyne /69 6 8,250 24.1 Fair Dip to oblique BCH-8 Amoco /60 1 2,835 4.5 Poor Dip BCH-9 II /60 1 8.3 Poor Dip BCH-20 II /58 1 1,420 3.7 Fair Dip BCH-27 II /60 1 2,940 3.0 Poor Dip BCH-29 II /57 1 1,420 5.1 Poor Dip BCH-30 II /57 1 1,420 4.8 Poor Dip 0-20 Mobil 12/73 24 3.7 Fair Dip
0'\ 0-21 II 12/73 24 6.9 Fair Dip 00
F II 5/71 6 7,500 2.0 Good Dip G " 4/71 6 7,500 9.2 Fair Strike J " 10/72 6 7.5 Good Dip L-507 Conoco 5/66 5 3,300 5.2 Fair x Oblique L-850 (855) II 7/66 5 3,300 3.5 Good x Dip L-205 II /66 10 3,300 6.7 Poor x Strike L-838 II 6/66 5 3,300 6.8 Good x Dip 0-1 Getty 10/70 6 7,500 8.3 Fair Dip 0-2 II 3/71 6 7,500 9.1 Good Oblique 0-4 II /71 6 7,500 6.8 Fair Dip CS-1 Coastal States 7/73 12 11 ,000 4.2 Fair Strike CS-2 II 7/73 12 7,130 5.1 Good Dip A80-A Sun /73 12 8,250 4.2 Good Oblique A80-B II /73 12 8,250 4.0 Good Oblique
..
area also has made the original processing more useful. Reprocessing would assist
largely in migrating the data and determining fault locations more accurately.
Experience elsewhere suggests that Teledyne line IF would presumably benefit from
reprocessing, but this line is located too far from the main fault block to be useful in
reservoir evaluation.
Amoco BCH-27, BCH-29, BCH-30
This set of lines (the western set marked on fig. 30) passes directly over the area
of interest and best shows the structural configuration of that area (fig. 39). A large
growth fault is updip of the area and a smaller fault block with minor rollover lies
updip of the large fault. Another large fault, is located downdip. This fault caused
substantial rollover that extends into strata younger than the main fault block.
Rollover within the main fault block (figs. 34 and 39) has created a thin area in the
Frio at the southern edge of the block (south of the type well).
Data quality on these lines is good for single-fold data, although little detailed
information on the stratigraphy and reflection character can be deduced. Sub-B5
reflectors, however, are prominent in the Blessing and Trull blocks.
Coastal States CS-2; Mobil F
This set of lines (fig. 40) runs north-south and is located just west of the zone of
small normal faults (fig. 30, line F -F'). The large updip and downdip growth faults, as
well as the broad rollover and generally landward dip of strata can be seen. Data
quality is good enough to show the varying reflector continuity through the section,
and the presence of strong sub-B5 reflectors is evident. Poor data quality at the north
end of the Mobil F line (updip of the southern major growth fault) may be due to the
effects of the zone of minor faulting mentioned previously. The thin area in the. Frio
at the south end of the Blessing block shows up well. No indication of shale diapirism
69
§ ~ ~ 0 I I I
I 8 I I I I I I I I I 0 8 8 8 0 8 8 8 § ~ ~ Q 0 ~ 8 0 (\J ... '" U) .... CD '" -w
(&puo~as) 3l1jll
70
8 E 0; ... :rg I I I h:§ 8. ~ :: ~~
£! !2 !!
c o c ~ o .c III
o
"" I
:c U cO
" C III ~
0\ N
I
:c U cO ~
"N
I
:c U cO III C o .;:: U dI III
0.. ..... " .~ E III ..... dI
V)
• !IIIi.
"-..I .....
,.
.. "'0
6 u .. ~ .... ::It ~
F 0-+~~~~~~~tmffi
1.5-
o 2 ~ I i I' o 2 3 km
Figure 40. Seismic dip sections CS-2 and F. Location shown on figure 30.
~
F' o
1000
2000
3000
4000
m
can be detected on this or other available seismic lines that cross the Blessing block;
however, in the updip block, sharply landward-dipping reflectors suggest possible late
Vicksburg diapirism.
Velocity Distribution
Interval Velocities
As determined from six velocity surveys in the area, velocity distribution is
controlled primarily by stratigraphy (fig. 41). In the Miocene section above the
Anahuac, interval velocities increase steadily with depth, reaching a maximum of
about 8,800 ft/sec near the base of the Miocene. The top of the Anahuac is marked by
a velocity inversion in four of the six surveys, but this inversion is not as pronounced
as that at Pleasant Bayou. As discussed in the Pleasant Bayou section, this inversion is
probably not a marker of geopressure in the Anahuac. Within the Frio, velocities vary
widely, from 8,000 ft/sec to over 10,000 ft/sec; velocities generally increase down
ward, but the trend is irregular. Local velocity inversions do exist, but no correlation
with the top of geopressure is evident. Local zones of high velocities are noted in one
velocity survey, but these may be artifacts, as at the Pleasant Bayou well.
Time-Depth Conversion
The wide scatter of interval velocities mentioned above is, however, not
reflected in the time-to-depth curves (fig. 42), which show a relatively narrow range
of variation for the velocity surveys and exceUent agreement with velocity analyses.
This narrow range of variation indicates that computer conversion of time sections to
depth sections would be fairly accurate in this area and could be used for further
structural analysis.
72
6.000
5.000
4.000
3.000
2POO
~ 1.000
::::l J-g ~ -1.000
o ~ -2POO
J-
0-3.000 W
I '4.000
'5.000
-6.000
-7.000
-8.000
INTERVAL VELOCITY (fl/sec) 6.000 7,000 8,000 9,000 10.000 11.000 12.000 13.000 14,000 15.000 16.000 17.000
Figure 41. Interval velocities as determined from velocity surveys from six wells in the Blessing area. Datum is the top of the Anahuac, which in some wells marks a velocity inversion.
73
~.
'I 4:"
2000
4000
6000
I-~ ::r 8000 I-a.. w o
10,000
12,000
14,000
TWO-WAY TRAVEL TIME (SEC) 2 3
RANGE OF 6 VELOCITY SURVEYS
16,0001
"
Figure 42. Range of time-depth curves obtained from six velocity surveys in the Blessing area.
4
CUERO AREA
Stratigraphic and Paleogeographic Setting
Geopressured sandstones in the Cuero study area (fig. 1) occur in the lower
Wilcox Group of late Paleocene to early Eocene age. Their absolute age is uncertain
because of a lack of datable material in the sequence. The lower Wilcox is the first
major influx of sand into the northwestern Gulf Basin during Tertiary time (table 9).
As a stratigraphic unit, the lower Wilcox is informally defined and subdivided.
Its lower boundary with the Midway Group is gradational from sandstone above to
shale below and is probably time-transgressive throughout the region. In this report,
the top of the lower Wilcox is defined as the beginning of the middle Wilcox
transgression, marked by a decrease in sandstone percentage (table 9). As with the
lower boundary, the upper boundary is probably time-transgressive along strike and dip
on a regional scale and does not correspond with the top of lower Wilcox used in
earlier regional studies (Fisher and McGowen, 1967; Bebout and others, in press).
Updip of the study area, the entire Wilcox section takes on a uniform character, and
subdivision into upper, middle, and lower units becomes difficult.
The Cuero study area is situated on the southwestern fringe of the Rockdale
delta system (Fisher and McGowen, 1967) in the area of transition from fluvial-deltaic
facies to barrier-strandplain facies (fig. 43). This transition is marked by a decrease in
net-sandstone thickness from northeast to southwest and from a lobate pattern to the
northeast to 'a more linear, strike-oriented pattern to the southwest (fig. 44).
The Cuero study area is located along a trend of syndepositional down-to-the-
south faults that were most active during Wilcox time (fig. 45). This trend also marks
a rapid downdip change in lithofacies, both in sandstone percentage and in log
character.
75
_&
...... Ol
Unit Numbers for Isopach Maps & Structural Sections
1
2
3
4
5
6
7
Log-Correlation Marker
Cl
TW
01
03
04'
(boundary transitional)
Table 9. Characteristics of operational stratigraphic units in the Cuero area.
Stratigraphic Unit
Claiborne Group
Upper Wilcox Group
Middle Wilcox Group
Lower Wilcox Group
Midway Group
----
Lithology
Mostly shale; thin sandstones (Yegua) near top
High sandstone
Shale, with sand-stone decreasing upward; shales out downdip within study area
High sandstone, shales out downdip within study area
Virtually all shale (progradational slope sequence)
---- _________ L_
Relative Velocity Seismic Signature
Low; approximately Moderate amplitude, linear increase with high continuity depth
High, with sharp, Strong reflector at strong discontinuity top dominates at top signature
Lower than upper Low amplitude, low Wilcox continuity updip.
High frequency, high continuity downdip
High updip; declines Low amplitude, low substantially downdip continuity updip. due to lithofacies Some increase in am-change and geopres- plitude and continuity sure downdip, but data
quality deteriorates
Low Moderate amplitude; well-developed oblique clinoforms
-
Overall Transgressivel Regressive Character
T ransgressi ve (regressive in Yegua)
Regressive
Transgressive
Regressive
Regressive
o u x w :E
)
o 25 mi f----'I PRODELTA FACIES o 40km
Figure 43. Location of Cuero study area in relation to Rockdale Delta system and barrier/strandplain facies. After Fisher and McGowen (1967).
77
o BID]
• u ~ ~
CZJ
• [8]
~
EXPLANATION
<100ft
100-400
400-800
800-1200
1200-1600
1600-2000
> 2000 ft
Wilcox oulcrop
Updip limil of geopressure
CUERO STUDY AREA
o 2p 4p 6,0 ~Omi
b 20 4b 60 e'o IOOkm Conlour inler\lol- 400ft,with supplemento(y 100f, conlour in downdip oreo
Figure 44. Location of Cuero study area in relation to net-sandstone distribution and upper limit of geopressure (based on mud weights) in the lower Wilcox. After Bebout and others (in press).
7&
f
o I o
-N-
II
20 40 I, "
60 80mi ! I
EXPLANATION
~ Wilcox outcrop
~ Stuort City shelf morgin
[tf::?:] Sligo shelf morgin
I---~I Faults
o Salt domes
Figure 45. Location of Cuero study area in relation to regional Wilcox growth faults in Texas. After Bebout and others (in press). Fault map courtesy of Geomap, Inc.
79
As a delta system comparable in scale to the Quaternary Mississippi delta, the
Rockdale delta was probably fed by a large river system having a drainage basin of
continental scale feeding into southeastern Texas. Petrographic study of subsurface
Wilcox sandstones (Loucks and others, 1979) indicates a predominantly plutonic,
metamorphic, and siliciclastic source area for the Wilcox in southeast Texas and only
minor volcanic and carbonate contributions. The volcanic contribution increases
toward the south; the Cuero study area is located between these two areas.
On the basis of high-grade metamorphic minerals in outcropping Wilcox sand
stones, Todd and Folk (1957) proposed that the southern Appalachians provided the
major source for Eocene sandstones in Texas, as opposed to the more conventional idea
of the southern Rockies (Storm, 1945; Murray, 1955). We consider the southern
Rockies as the more plausible source area because (1) they are much closer; (2) high
grade metamorphites crop out extensively in the southern Rockies, as well as in the
Appalachians; (3) Eocene rocks in southern Georgia and Florida are predominantly
clean carbonates (Toulmin, 1955), indicating that the southern Appalachians were not
shedding much clastic detritus at the time; and (4) intermontane basins in New Mexico
show a major increase in grain size about 55 m.y. (S. M. Cather, personal communica
tion, 1981), indicating renewed activity of the southern Rockies about the same time
as the lower Wilcox clastic influx. Low-grade metamorphic rock fragments are
probably derived primarily from the Ouachitas, as Todd and Folk (1957) suggested;
Operational Stratigraphy
Site-specific description of the Cuero area (fig. 46) began with the correlation of
distinctive electric-log markers (fig. 47). Where possible, these were chosen to
correspond to boundaries of named stratigraphic units or major lithologic breaks
(tables 2 and 9); markers TW, 01, and 03 were used in an earlier report on the Cuero
area (Bebout and others, in press). Stratigraphic units bounded by these markers were
80
•
------ ---- --------------------m
,as ••
BUCHEL FIELD
i
i
i
----j---------J I
Figure 46. Data base and locations of well-log sections, Cuero study area.
81
..
A A' ll:
" W , ~N .~
..J
'" en ... 0"1 0'1 ~U) V~'" 0:
~'? ~~ '3 '<. '" ~ ;:~<p w~' "
,", ' \j~ ~~~ ~w z~w ~w
l~ ~ ~~ ~~ a::1J~ ~~ V> ~ ~~ 8~
,v, ..iiN
~~~ E. 0GN O~N g~N VjN Clti)N ~~~ ~~~ 0-,' ~i~ ~*~ ~.~ .... ' -I ..... '
.~ 11 <Xl ~.~ .~ .~ ~.:g 9i:g ~i~ It m 1 \ ')
-3000
f ~
1.
-1000 1 ~ ~ CI ! r
CI
2 -5000
---- ---r-r----r'n
-2000
-7000
-9000
-3000
-11,000
l ~ 1
0 I 2 3mi f----r
, 2 J
, 4km
,
\ 0 I
-13,000
Figure 47. Structural dip section of Cuero study area, illustrating log character of stratigraphic units (table 9).
82
numbered sequentially from youngest to oldest (tables 2 and 9) to simplify labeling of
maps and sections. Log markers were also transferred to seismic sections where tied
by wells, using appropriate velocity surveys for the conversion from depth to time.
These markers were then traced through the seismic grid to check for internal
consistency.
Correlation was mostly straightforward; the only serious difficulties were
encountered with D3 and D4' downdip of the fault block of interest. The base of the
Wilcox is difficult to correlate in that it is transitional and time-transgressive and
probably represents a change from topset to foreset stratification. Therefore, no
correlatable log marker could be found to clearly mark an operational base of Wilcox.
Structural Setting
The location of the Cuero study area along the Wilcox growth-fault trend limits
the potential size of Wilcox geothermal reservoirs. These growth faults resulted from
large-scale basinward sliding of the continental slope, which created an extensional
regime along the shelf margin. Wilcox growth faults tend to be subparallel as well as
closely,and evenly spaced, particularly when compared with Vicksburg and Frio growth
faults. Shale diapirism has apparently not been a major factor in the Wilcox structural
style, also unlike the VickSburg and Frio. The relatively simple structural style of the
Wilcox appears to be typical of the initial style of growth faulting that forms as a
shelf margin progrades into a basin and becomes increasingly unstable (Winker and
Edwards, in press).
Local Structure
Structure and interval isopach maps were constructed on all five correlation
markers using well logs (fig. 46) and seismic data. Because of the density of well
control, final mapping was based primarily on log correlation. Seismic lines were used
83
...
primarily to verify the structural style, to check the direction and angle of fault dip,
and to extend the structural interpretation below and downdip of the area of dense
well control. The resulting structure map on the zone of interest (04') is a moderate
departure from that obtained in the initial study of this area (Bebout and others, in
press).
Structural Style
The structural style in the Cuero area (figs. 5, 45, 48, 49, and 50) is typical of
the lower Wilcox trend. Growth faults are virtually all down-to-the-south, with
relatively small throw and minor rollover, which is nonetheless sufficient to create
anticlinal closures within fault blocks. Fault traces are subparallel and fairly evenly
and closely spaced, and faults dip at an average of approximately 450 (fig. 5). The
overall result is long, narrow fault blocks with very low structural relief; this style
changes little from one mapping horizon to the next (figs. 48, 49, and 50). The small
amount of rollover indicates that the faults flatten at considerable depth into a
decollement surface or zone. This decol1ement is probably below the Lower
Cretaceous Edwards Limestone and may be in thin, non-diapiric Jurassic salt.
The beginning of growth-faulting corresponds to the Midway-Wilcox stratigraphic
boundary. A number of possible explanations for this timing include the following:
(1) Growth faulting began once the shelf margin prograded beyond the buried Lower
Cretaceous shelf edge; this may have allowed the shale section to attain the critical
thickness (Crans and others, 1980) for gravity sliding. (2) Growth faulting began with a
major influx of sand; therefore, growth faulting could have been caused by loading of
sand on shale (Bruce, 1973). (3) If decollement took place in non-diapiric salt, growth
faulting may have begun once the shelf margin reached the updip limit of salt, which
permitted large-scale gravity sliding. It should be noted that the presence of salt has
not been documented in this part of the Gulf Basin; its presence is nearly impossible to
detect with present technology.
84
..
8S
21E 22E
o I o
I 2
2 4ml I' I '. It
4 6 km
23E
Figure 4-8. Structure contour map of Cl correlation marker (top of Yegua).
85
..
8S
2 o 4m, II--........... ~-+-,--'-r-,,-', I
I I I I I
I
9S o 6km 2 4
21E 22E 23E
Figure 49. Structure contour map of TW correlation marker (top of Wilcox).
86
85 ••
21E 22E 23E L-________________________ ~ __________________________ ~ ______________________ ___
Figure 50. Structure contour map of 04' correlation marker (local top of lower Wilcox). First fault is post-depositional; second fault marks the updip limit of geopressure. Gas is produced from gentle rollover anticlines.
87
Structural Evolution
Structural evolution of the Cuero area was studied by means of sequential
isopach maps (fig. 51) in the same manner as was the Pleasant Bayou and Blessing
areas. Reconstruction of the earliest structural growth is hampered by the difficulty
in correlating any marker below 04'; the Wilcox-Midway boundary is transitional and
probably time transgressive. From stratigraphic dip sections hung on 04', it appears
that faults bounding the South Cook field were active during early Wilcox time and had
growth ratios of about 2. In interval 5 (middle Wilcox), these same faults continued to
move, but growth ratios decreased below 2. This early, relatively rapid growth
isolated lower Wilcox sandstones and permitted geopressure to build up. The fault
farthest updip did not begin to move until the time of deposition of interval 4 (upper
middle Wilcox); this fault did not create a geopressure trap. The growth ratios of
other faults declined with time; by the time of deposition of interval 2 (Claiborne),
growth ratios had dropped below 1.1. By this time, the shelf margin had prograded
approximately 20 to 30 mi basinward of its position in early Wilcox time. In post
Claiborne time (representing approximately 40 m.y.), the main structural activity was
regional southeastward tilting. In spite of the tilting, closure of rollover anticlines
was maintained.
Sandstone Facies
Sandstone facies in the lower Wilcox were classified on the basis of SP pattern
and on continuity of individual sandstones; five environments were inferred from these
log facies (table 10). Spatial relationships among these facies are summarized by two
stratigraphic sections (figs. 52 and 53) and a paleogeographic map (fig. 54). The strati
graphic dip section (fig. 52) shows the basic progression of facies associated with
progradation: slope shales (characterized by clinoform reflectors on seismic sections)
88
I.
N
I
5
EXPLANATION
/GROWTH FAULT ~
I "'GROWTH RATIO (Expansion index)
Isopach contour interval = 100 ft
o 4mi Ii" ,'i', " o 6km
LOWER MIDDLE WILCOX .
:3 UPPER WILCOX
2 CLAIBORNE
zw:P
. + +
-
+
+
4 / UPPER MIDDLE WILCOX of
;/t- .+ +
Figure 51. Sequential isopach maps illustrating structural evolution of Cuero study area. Distortion of grid is caused by removal of horizontal component of postdeposi tional faulting.
89
Table 10. Electric-log facies in the lower Wilcox (unit 6), used for paleogeographic mapping in the Cuero area.
Environmental I nterpreta tion
Fluvial/deltaic plain
Strandplain
Delta front
Distributary channel
Prodelta/slope
Log Character is tics
High sandstone, blocky patterns; laterally discontinuous (individual sands cannot be correlated)
Thin sandstones, mostly upward-coarsening, high lateral continuity
Similar to strandplain facies*
Thick sandstones, blocky to upward-fining, few shale breaks
Low sandstone; greatly expanded section; correlation virtually impossible
Geographic Distribution
Updip of growth faults; to northeast; toward Rockdale Delta
Mostly updip of growth faults; to southwest of fluvial/ deltaic-plain facies
Along growthfault trend, basinward of fluvial/ del taic-plain facies
Within growthfault trend, laterally continuous with delta-front sandstones
Downdip of main fault block
*Similar pattern also observed below fluvial/deltaic-plain facies in Midway-Wilcox transition.
90 •
"-..J ::>
"
8
FLUVIAL/ DELTA PLAIN
~ 12:l", 0 ....
~~~ a. • ::>'VI VI"",
Il: .. ' w., ..J~
\:5' z,:; _-l~ >-,
"" ..
... &'" .,.
lj~w 'j,gw ~a~ ~~~ ::>-VI i~~ " .. '"
r-+--- DELTA ----++J>------+-+....s.--~~ n---FRONT---tf;=,--. __ --.:.~~ _ _W_
17:-------_--1lt_::----__ -- --- --------o 2mi >-1 ----~--~~------~. • o 30m
Vertical EJ,09gerotion: lOX
TIE CoCO 8'
I ~~~ ~~ ... <w ~~~ 8~ O,N "V,N (.?ctl~ ..J •
12:- 0:- VI >- -VI .... .... '" .... <Xl "
04'
or I
PRO- 100J DELTA
I SLOPE ~ 500
ZOO
300 1000
400
1500
500
600 2000
700
2500
Figure 52. Stratigraphic dip section of lower Wilcox, illustrating changes in log character and expansion of section across second and third growth faults.
91
C TIE 8-8'
.~ I ~ ~
a:~ " ~~~ ~~O? "'co ~a~ Vl"W ~-<::w
W~N Z"N co ·N z~cy :3t/)~ ::E~CY g~:2 ~"""I/)
«'II:co ::> ...... 1/)
I 'II: CO
o I o
~ I: ~co
~~w !D'S N ::Et/) CY ::> ...... Vl I'll: CO
~ ·':;co
~~yj o~~ Vl ,
~*~
, 2
Verlicol Elowerolion = lOX
~ Distributary facies
c'
t,... §,... w~ I wI: , .J.,W ..Jtw !D~~ !Dt>N ::E , ::E~ CY ::> ...... Vl :r'll:co
::> ...... Vl I'll: CO
m It
o 0
100
500
2
300 1000
400
1500 2mi . 500
3 km
600 2000
700
2500
800
900
1000
3500
Figure 53. Stratigraphic strike section of lower Wilcox in South Cook fault block, illustrating distributary sandstone facies laterally continuous with delta-front facies.
92
..
FLUVIAL!
• Well control
0,-, ___ 'O • ..L'F ___ 20....)900 II
/' Updlp limil of geopressure
'" in lower Wilcox o 5km '-' --'--'--~---'--"
Figure 54. Lower Wilcox paleogeography of Cuero study area, based on electric-log facies (table 10).
93
..
overlain by delta-front sandstones, in turn, overlain by the fluvial/deltaic-plain facies
of sandstones and shales. A similar progression can be seen in a basinward direction on
the same section, although the pattern is complicated somewhat by expansion of the
section across growth faults. On a strike section -within the main fault block (fig. 53),
delta-front sandstones are seen to grade lateraUy into thick, blocky or upward-fining
sandstones, interpreted as distributary channel-fill or channel-mouth bars (fig. 55).
These distributary sandstones exhibit high permeabilities (>100 md)in the Atlantic
Schorre III well (fig. 56).
In plan view (fig. 54), the distribution of sandstone facies reflects the paleogeo
graphic relationships of the Rockdale delta system as a whole (fig. 43). The Cuero
area lies in a transition from predominantly deltaic systems, with fluvial/deltaic-plain
facies grading downdip into delta-front and distributary facies, to predominantly
strike-fed barrier/strandplain facies characterized by high continuity along strike and
dip.
Within the geopressured part of the lower Wilcox, distributary sandstones exhibit
the best potential as geothermal reservoirs. However, the small areal extent of these
sandstones (figs. 16, 54, and 55) places severe restrictions on the possible location and
producing volume of a geothermal well. Ideally, a well site should be chosen to
intersect two or more over lapping sandstones of high potential. In the case of the
Cuero area, there is insufficient latitude in well-site location to avoid interference
with existing conventional gas production from the South Cook field (fig. 55).
Pressure Distribution
Pressure distribution in the Cuero area is easily delineated by a combination of
drill-stem test, mud weight, and shale resistivity data (fig. 57). As a general rule,
drill-stem tests (BHSIP) tend to underestimate true formation pressures because time
94
's' SAND
'e' SAND
. . . .
. . :. m It
'0' SAND ~·'·'llf·~llt ':: ~ ;
~ II~'/ ~ ~ ~ .~ @~ III
Ii' Ti;, II ~ .... ~llr~ ~II?
ala 100
50 200
a I 2m; i I! I I ! I a I 2 3 4 km
Figure 55. Log patterns of three lower Wilcox sandstones in the South Cook fault block, illustrating facies changes from distributary to delta front, and gas-well completions.
95
•
MUD WT. (#/90 1)
11.1
IS.7
LOG DATA BOTTOM-HOLE
TEMPERATURE (OF) (uncorrected) (corrected)
222 254
252 285
Datum for .trati9roPhic sections -and structure map (04')
IS.7 2S0 293
15.7 280 313
AI/antic ,I Scl10rre
~S_22E>: )
J. '<
~: ~ <: ;: ~
.£. .
c:
WHOLE -CORE DATA DEPOSITIONAL PERMEABILITY ENVIRONMENT RANGE (md) (interpreted 'rom
sedimentary structures)
DeeM ... in shale resl,tivity· transition into QeOprauure
25· 242
Oello liontm#1 EJ
Figure 56. Electric log with permeability data and environmental interpretations (based on whole core) for lower W i1cox section, Atlantic 111 Schorre (type well).
96
u
..
-.c;
a. ., 0
20.000
4000
0 0
8000 0
co 0 0
o 0 0 0
00
o o
o
12.000
o o
(kPo) 40.000
o
CD
60.000 80.000 0
l). Yegua - normally pressured
0 Wilcox- normally pressured
• Lower Wilcox - overpressured
a Edwards - normally pressured 10 sl ighll y overpressured 1000
2000
3000
TOP GP
•
4000
o 0 o o
�6.o00o!-------2o.i..o-o------4..J.000-------6000...J....----J...--SOOO.l.-------�-0 . ..J.000-----'~-,-2 . ..J000
Soltom-hole shut-in press .... e (PSI)
E
Figure 57. Plot of bottom-hole shut-in pressures (BHSIP) versus depth from gas wells in DeWitt County. Note well-defined top of geopressure.
97
limitations may prevent well bore-pressures from equilibrating with the formation
during the test. Mud weights, on the other hand, tend to overestimate true formation
pressures when a simple conversion of units (lb/gal to psi/ft) is made because mud
systems are generally overbalanced. Dickinson (1953) recommends a correction factor
when converting mud weights to pressure gradients. Between mud weights and
bottom-hole shut-in pressures, it is possible to bracket the true formation pressure.
Shale-resistivity plots generally give results consistent with those obtained in the
preceding fashion (fig. 58). An exception occurs in the case of the Midway shale updip
of the growth-fault trend (figs. 59 and 60). Shale resistivities in the Midway fall well
below the normal compaction curve, suggesting fluid pressures above hydrostatic
(Bebout and others, in press). However, mud weights indicate that the Midway can be
drilled with the same low-weight muds «13 Ib/gaJ) used for drilling the normally
pressured Wilcox. Drilling experience indicates that full geopressure is not encoun
tered until the Sligo is reached (D. G. Bebout, personal communication, 1979). Shale
resistivities, conductivities, transit times, and interval velocities are apparently
influenced strongly by factors other than fluid pressure.
Except for the Midway section, the various sources of data provide a fairly
consistent picture of pressure distribution. Pressures within the lower Wilcox
sandstones are hydrostatic updip of the second growth fault. Downdip of this fault,
formation pressure gradients in the lower Wilcox are in excess of 0.7 psi/ft. Vertical
transition from hydrostatic pressure to full geopressure, as indicated by shale
resistivity, is fairly rapid, taking place within a few hundred feet (fig. 56).
Seismic Studies
Examination of structure and stratigraphy in the Cuero area was enhanced by
35.9 mi of proprietary seismic lines and 25.1 additional miles of seismic lines shot for
this study (table 11, fig. 61).
98
'" '"
l·
2-
4
8 4;
'" u.
8 0
~IO .c a. '" 0
12
14
16 .I
-;--r
N w ,. til 01 ..J Q) I) - N til • UI~..JQlC_ N w ,. C. G ..., at" 0
2
II>
,- , I .. '" I lli/i Q;
c. ::E ,II", ;
-~~ :,1
1111 8 , I., , Ii, I I 'j'
I! 1:11
13~ ," 111i!
!llUi !!I'li IlnUIT 1I:li
,.1'1 ' I, j ~ <I loli Ii'
,'I~ JIIc' , .c
Illd i ,! 111111 1I1111lI1II1! I" ill. a. ' :, '" ~ GoI"'il 'i":1 I 11'1
~' I!!::!:::j ''-ll.li LWllllBlHI11III1111111 I III 1lI1l11111 Iii I I II Hill 11I11':ilIIII1I1IU.O
'I I" I'll!' I 1111i 1:111i il",:Jrnl: II-il', ,: I I !Ii 1,'1', I!:I ,11!~11I! I
;1.1 , "Iii 1'1 I',:" ", 'r: IIIITnIU I
'i! I i:"I' j 1'1' I 'I 11'1: :",1'1' ,I IlIlmlll
1111 " , 11'11' ~JJmm
R sh. OHM - Meters
Figure 58. Shale-resistivity plot for Atlantic III Schorre (type well) used to estimate pressure gradients. Shale resistivity and mud weights yield consistent estimates and, place top of geopressure just above 04' marker. After Bebout and others (in press).
4
2-
c; If a 8 X
::1 ..... a. 0 G
0 0
.,
" " .. w ~ UI 01 ...., (II C) 0
, Ii'
Iii- ''','' I , ,I" ,1,'1
" i,I,1 :Ii 'i!'1 I i i ,!iI' , "'" I"
'1,1.' 'i., , , II!"I I 1"lIi ':,1', ',I. !l11j ", I i' I i
I I I!, I
I·: , ~,
'I ': lii'i l 1:1 I,!I ri'l! Iii, ,I :1,
i,ll'i II :,11 I,
10 100
Rsh. OHM-Meters
Figure 59. Shale-resistivity plot for Gulf III Mueller well, Stuart City trend, updip of Wilcox growth faults. Shale resistivities indicate top of geopressure in Midway shale, whereas mud weights indicate normal pressure above the Edwards carbonates. After Bebout and others (in press) .
on L-Q> Q; ~
II ~ i" 0
0 Q
3?$ .c Ci. Q> 0
4
>-o >-
..
o GULF
.#1 Mueller 85-2IE-1
INTERNATIONAL NUCLEAR COR~ TANA
#1 Weber itl Albert White 85-2IE-6 85-22E-9
ATLANTIC .itl 5chorre
85-22E-9
KILROY #1 Mueller
95-22E-3
TEXACO #1 Angerstein
95-22E-5
0'
'" RSh,A-m Rsh,J\-m Rsh.A-m SHALE Rsh,J\.-m Rsh,J\ -m R,h,A-m ~
t;; 2 ~ .~ ~3 .5~4 RESISTIVITY .~ .~6 .5ri.:-M ~i f;j ~ !~----~:----------------------t.------ -- _: . ..ANO GEOPRESSUR¥' __________ .?r: ----------------------f''- -- -- - ! ~ g g i'I TOP WILCOX, GRADIENTS .~: if 2000F(930C) ,:f g g o 0 •• 0 Q 8 ~ r- - - - - - - 1-:: TOP GEOPRESS RE:r - .: :.:.. I 8 O~ ::t: I 3::t: " ::t: >-:;:1:;: !! 8~~~~11'lH' :t-= ~ :!. ~ 12~ ~ ~ 0 0
o 2 Miles I • o 2 Kilometers
Figure 60. Parameter plots for wells in section in figure 47, illustrating top of geopressure as picked from shale resistivities. After Bebout and others (in press) .
Table 11. Seismic lines acquired for evaluation of the Cuero area.
Data Maximum Quality at
Fold Offset Length I?epth of Reprocessed Line Shot by Date (Nominal) (ft) (mi) Interest by H.P.I. Migrated Orientation
7F-2 Arco 7/68 6 5,500 15.4 Fair x x Dip to oblique 14W-F Teledyne 9/70 6 8,250 20.5 Fair x x Dip UT-I Geosource (for UT) 5/80 24 10,800 3.1 Poor Strike UT-IA " 5/80 24 10,800 2.3 Poor Strike UT-2 " 11/79 24 10,800 6.0 Good x Dip UT-3 " 3/80 24 10,800 7.4 Fair x Dip UT-4 " 4/80 24 10,800 6.3 Fair x Dip
>-0 N
• Well with velocity survey -Seismic section
"--__ :::.'O!J:..\lOO=--_-=20::;).900 ft o 5km '-' ---'--'----'--'----',
Figure 61. Base map of seismic data in Cuero study area. UT lines, acquired by Geosource, Inc., for this project, were laid out in true-dip orientation.
103
Proprietary Data
The proprietary lines (figs. 62 and 63) are six-fold and were shot in the late
1960's; the data are of good quality for lines of that age. The processing quality of the
lines as delivered was reasonable for 10 years ago but is inadequate by modern
standards. Therefore, standard production reprocessing was performed on these two
lines (table 11). The major areas of difference between originaJ and reprocessed
displays, in order of processing steps, are (1) automatic residual statics (not a serious
problem in this area), (2) slower automatic gain control, which significantly improved
the useful amplitude information, (3) reworked stacking velocities, (4) migration, and
(5) display of all sections at approximately the same horizontal scale. In this area,
migration is especially necessary for a proper structural interpretation. However,
migration in areas of oblique-dip orientation, poor signal-to-noise ratio, bad statics
problems, or mUltiples may only cause further deterioration of the seismic image.
New Data
New data in the study area were obtained in order to compensate for inadequa
cies in the existing data. The major changes in specifications from previous data were
(1) change of orientation from oblique to true dip, necessary for proper two
dimensional migration; (2) increase of multiplicity from 6- to 24-fold to increase
signal-to-noise ratio; (3) increase of maximum offset from 5,500 ft (Arco) and 8,250 ft
(Teledyne) to 10,800 ft to improve multiple suppression and to obtain more precise
estimates of rms velocities at depth; (4) true-amplitude recording rather than record
ing with field AGe; and (5) use of a staggered-offset layout for shot holes (fig. 64).
The staggered-shothole configuration was designed to produce four closely
spaced, 6-fold lines, which could be either processed separately to provide cross-dip
control or stacked together to produce a single 24-fold line. In practice, the desired
configuration was impossible to achieve for several reasons. The offsets greatly
104
•
,
>-a V1
NW
Figure 62. Interpreted version of migrated line lit W-F. Unusual fault patterns result from non-true-dip orientation of seismic line; these faults could not be interpreted correctly from a single seismic line.
SE
>-o 0'\
TIE UT-4
TIE TIE UT-IA UT-I ,
Figure 63. Interpreted version of migrated line 7F -2. Deep projection of faults is speculative; they are drawn to suggest decollement in deep non-diapiric salt. Note "wipe-out" zone of poor signal-to-noise ratio and apparent velocity pulldown under faults cutting top of Wilcox.
TIE UT-3
ASP 0 o o o o o o o o
ACDP ============================== ~ACTIVE SP
B SP \Q; 0 0 0 0 0 0 0
o
o o B COP ==================================0
I 6 12 18 24 30 RECORDING LINE ••••••••••••••••••••••••••••••••• -C COP = = = = = = = = = = = = = = = = = = = = = = = = = = = = CONTINUE TO COMPLETE C SP 0 0 0 0 0 0 0 0 48 ACTIVE RECORDING
o COP ======= =============== ==== GROUPS
o SP o o o o o o o o o
Figure 64. Originally proposed layout of shotpoints with staggered offsets to provide cross-dip control. From M. M. Backus (personal communication, 1979). Intention was to obtain four closely spaced parallel 6-fold lines.
107
increase the amount of land that needs to be permitted, and they more than double the
total length of line that must be cleared for access by surveyors, drillers, and
recording crews. In many cases, it was impossible to place the shotholes where
specified. The resulting deviations were so severe that in no area was it possible to
obtain four 6-fold lines of acceptable quality (fig. 65), although the combined 24-fold
stacks are of good quality. It would be safe to conclude that such a shooting plan is
impractical in areas similar to that of Cuero, which has numerous small land holdings,
many buildings and structures, and dense brush.
To take full advantage of modern migration processing, we attempted to make
use of fault-plane reflectors as a criterion for fault recognition. On the basis of
preliminary structural interpretations from well logs and two proprietary lines, three
sections were constructed, both in depth and in time with the fault planes in their
predicted correct position (fig. 66). Then, these fault planes were graphically "de
migrated" to their predicted position on un-migrated sections (fig. 67) by use of a ray
path and wave-front chart derived from a velocity survey and Snell's law (fig. 68).
This demonstrated that the fault-plane reflectors would be detected at depth points as
much as 3 mi downdip from the correct migrated position (fig. 68). For this reason, it
was considered desirable to extend the coverage of dip lines substantially downdip of
the fault block of interest, which required relatively long lines (fig. 61).
Permit problems further limited the coverage of the new seismic data. No
permit areas, which precluded both geophones and shotholes, restricted the updip limit
of line UT -4 and the western end of line UT -1. No-shot areas created serious gaps in
full-fold coverage near the area of interest on UT -4 and on lines UT -1 and lA, and
necessitated the unusual configuration of the strike lines (fig. 61).
14W-F
This line provides the best overview of the structural style of the prospect area,
although it passes to the east of the primary area of interest and has an oblique-dip
108
•
x x x x x x x x x x
Ox x x x
ox x x x
XOX 84X X X X XOX x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x ox x x x XOX x x
0 x x x X 74 XO X X X
0 X X X X X X X X X X X X X X X X X X X X
Figure 65. Map of shotholes, geophone stations, and shot-receiver midpoints for a segment of line UT -4. Deviations from proposed shothole layout result in low multiplicity for outboard sections. Shot locations are represented by circles.
109
ED, 15 10 5
1-
Tw
2- Twl
3- Ke ----- ...................
4-
MD I 15 10 5
I
1-
2-----~L_------~
4-o I
Imi I
0 5
0 5
o I
10
10
5000 I
15 20 I
15 20
Tw - Wilcox
Twl- Lower Wi Icox
Ke - Edwards
Tw
10000 " I
25 I
sec +
-I
-2
-3
-4 Vertical E",aggeratian
25 I 1.5 2 I
-I
-3
Figure 66. Structural dip sections (vertical scale in travel time) constructed before acquisition of UT L W -3 or reprocessing of proprietary lines. These sections were designed to predict positions of unmigrated fault-plane reflectors (fig. 67).
110
•
ED 15 10 5 0 5 10 15 20 25
I f
1- -I
2- -2
-----
3- -3
---
4- -4
15 10 5 0 5 10 15 20 25 MD----~t---~----~~----~----~I------r-----~----~-----T-
1-
-2-
3-__ _
4-
-o I
---I mi o
I 5000 10000 ft
I I
-I
--2
-3
-4
sec i
Figure 67. Structural dip sections from figure 66 with fault-plane reflectors moved to their predicted unmigrated positions using graphical methods. In order to record faultplane reflectors, lines must be extended downdip from area of interest.
111
...
0
Figure 68. Snell's law, well. This to 67.
Velocity Depth
4 ~
11 ~ 4
- 2,000
- 4,000
8684 - 6,000
9538 - 8,000
11900
10035 -10,000
9842 -12,000
11133 -14,000
12489
-16,000
-18,000
- 20,000
Wavefront and raypath diagram, constructed graphically according to based on interval velocities from velocity survey, Atlantic til Hartman diagram was used to "de-migrate" fault-plane reflectors from figures 66
112
•
orientation. The interpreted line (fig. 62) shows the Lower Cretaceous carbonate
Stuart City and Sligo shelf edges, the progradation of the Paleocene Midway slope
(clinoforms), the initiation of growth faulting as the Wilcox shelf edge prograded
basinward from the Lower Cretaceous shelf-edge positions, and the continued move-
ment of the faults in post-Wilcox time.
Reprocessing greatly improved the amplitude content of the display, emphasizing
the high amplitude of the Stuart City, Sligo, and Wilcox tops. It also shows
surprisingly high amplitudes in the all-shale Midway and lower Claiborne sections. In
contrast, the updip Wilcox is characterized by low amplitude and continuity. Data
quali ty in the downdip lower Wilcox, however, did not benefit from reprocessing as
much as was anticipated.
Migration was of great benefit in picking faults for the final interpretation
(fig. 62). In the post-Wilcox section in particular, migration increased the sharpness of
bedding-plane terminations and discontinuities. Within the Wilcox section correct
interpretation of faults was more ambiguous. The most confusing section occurs
within the reservoir fault block because of the substantial departure from true-dip
orientation. Faults appear to flatten with depth as the line departs farther from a
true-dip orientation. These fault traces cannot be properly interpreted directly from a
single line, either un-migrated or migrated. It was necessary first to interpret the
other lines, then tie those lines to 14 W -F.
7F-2
This line (fig. 63) is more useful than 14 W -F from the standpoint of structural
evaluation, in that it more nearly maintains a true-dip orientation through the fault
block of interest. It shows basically the same structural features as 14W -F, except
that it does not extend as far downdip. In the section as delivered, the continuity of
reflectors and general data quality, particularly in the growth-faulted area, are not as
good as in 14W -F.
113
I
\
Much of this difference in data quality was eliminated by reprocessing. Re
flector continuity was improved by application of automatic residual statics. Rework
ing of stacking and velocities contributed to improved data quality in the Wilcox fault
zone.
As with 14W-F, migration significantly improved the fault interpretation
(fig. 63) with the added advantage that this line maintains a nearly constant orienta
tion. However, fault picks were complicated by zones of greatly reduced signal-to
noise ratio beneath the faults cutting the top of Wilcox. These "wipe-out" zones
appear on the migrated section to be velocity pull-downs, although their exact cause
has not been determined. Ties with wells have demonstrated that these zones do not
indicate discontinuities of bedding-plane reflectors representing the true fault posi
tions, as was originally thought. Instead, they occur significantly below and updip of
the fault trace. The velocity pull-down effect results in apparent structure on the top
of lower Wilcox, which conflicts with that mapped from well control.
UT-4
This dip section (fig. 69) ties the type well and the primary area of interest. The
improved data quality over line 7F -2, which ties the same weJls, can be attributed
primarily to the increased multiplicity resulting in improved signal-to-noise ratio.
Multiples are apparently not a serious problem in this area, as the increased spread on
line UT -4 did not result in any significant mUltiple suppression when compared with
7F -2 (fig. 63).
Continuity of reflectors, frequency content, and clarity of faults is very good in
the shallow part of the section, down to the top of Wilcox. Within the Wilcox section,
reflector continuity and amplitude are not as good as was hoped, but they still
represent an improvement over 7F -2. Large no-permit areas, particularly the one
around the type well, significantly reduced in data quality because only far-offset
114
..
TIE 75-2
--!
TIE
Figure 69. Migrated line UT -4 with interpretations. Note "wipe-out" zone similar to that in figure 63.
115
traces could be recorded. This is particularly true in the shallow part of the section,
because far-offset traces are contaminated with early ground roll, usually requiring
muting.
The most serious problem with interpreting the lower Wilcox on this section is
the "wipe-out" of data quality below the major faults where they cut the top of
Wilcox. The consistency of this phenomenon among all the UT dip lines and 7F-2
suggests that it is inherent in the geometry and velocity structure of the area and is
not attributable to incorrect shooting parameters or improper data processing. As
with 7F-2, migration results in apparent troughs in the structure on the top of lower
Wilcox under the growth faults (fig. 69); these apparent troughs are contradicted by
closely spaced well control.
Fault-plane reflectors show up better on UT -4 than on 7F-2 or 14W-F, perhaps
because of the increased multiplicity and resultant improvement in signal-to-noise
ratio. As predicted (fig. 67), the unmigrated reflectors appear far downdip of their
correct position, justifying the downdip extent of coverage. Migration unfortunately
does not preserve the full amplitude of fault-plane reflectors, which tends to reduce
their usefulness. Upward concavity of fault-plane reflectors at depth may be an
artifact of migration. The migration algorithm "smears" noise along estimated
wavefronts; often these crescentic features appear to be continuous with shallower
fault-plane reflectors. As mentioned earlier, the small amount of rollover at depth
suggests that the faults probably become listric beneath the Cretaceous section.
A further attempt was made to utilize fault-plane reflectors by restacking the
section at the optimum velocity for these steeply dipping events. Theoretically, the
optimum stacking velocity of a reflector of dip 15 is:
Crmsfcos 15
(M. M. Backus, personal communication, 1980). Because most of these faults dip
approximately 450, the original stacking velocities were multiplied by a factor of 1.4.
116
The result of this high-velocity stack was enhancement of fault-plane reflectors and
suppression of bedding-plane reflectors (fig. 70). Migration of the enhanced fault
plane reflectors should be performed at the true rms velocity rather than at the
enhanced velocity. Unfortunately, the contractor used the higher velocity, with the
result that the fault planes were migrated too far and lost much of their coherence.
The error was not discovered until after the completion of the contract.
UT -2 and UT-3
Most of the remarks on line UT -4 are equally applicable to dip lines UT -2 and
UT -3 (figs. 71 and 72). The main differences among the lines are in the gains applied
to the final displays. All these sections show the same "wipe-out" zone on the
unmigrated sections and apparent velocity pull-down on the migrated sections. High
velocity stacks were also performed on UT -2 and UT -3, with similar results.
UT -1 and UT -lA
Originally, it was planned to shoot a single strike line within the fault block of
interest to tie the three dip lines. However, permit problems prevented tying with
UT -2 and required putting a substantial dogleg between UT -3 and UT -4, causing it to
be shot as two separate lines (fig. 61). Permit problems also prevented the use of the
staggered-offset shothole configuration.
The main problem encountered with the strike line, however, was that it lies
entirely within the "wipe-out" zone; consequently, the section is virtually uninterpret
able (fig. 73). Even without this phenomenon, as a general rule strike sections are
adversely affected by diffractions off faults which, unlike dip lines, cannot be
eliminated by migration. It can be concluded that strike lines are commonly not worth
shooting if the zone of interest lies directly underneath a nearby fault or if the faults
are closely spaced.
117
•
TIE 7F-2
TIE TIE UT-I UT-IA
Figure 70. Line UT -4 restacked with velocities approximately 1.4 times optimum stacking velocities, designed to enhance fault-plane reflectors over bedding-plane reflectors. Results were moderately successful on this line and UT -3, less successful on UT -2.
118
TIE 14W-F
----__ -_________________________ ._1 ____________________________ _ 0.0
Figure 71. Migrated line UT -2, with interpretations. Again, "wipe-out" zones show up under faults cutting top of Wilcox. Correct interpretation of faults requires well control, especially when faults are closely spaced.
119
TIE 14W-F
. ___ ~., __ L __
TIE UT-IA
I
TIE UT-I
I
TIE 7F-2
I ~- ~---~---~ -- -0.0
Figure 72. Migrated line UT -3, with interpretations. "Wipe-out" zone is consistent with other lines.
-,
120
..
w TIE
UT-4 BEND TIE
UT-3
NE
Figure 73. Composite strike section UT -1 and UT -lAo Entire line is within "wipe-out" zone, accounting for poor quality of deep data.
121
..
V eloci ty D istr ibution
Interval Velocities
To a first approximation, interval velocities in the Cuero area (fig. 74) are a
function of stratigraphy (table 9). The most notable aspect of interval velocities in
this area is a large discontinuity at the top of Wilcox, resulting in a high-amplitude
reflector. Major velocity inversions are created by (1) a transition from Wilcox sand
stone to Midway shale (updip area); (2) a transition from a more sandy upper Wilcox to
a more shaly middle Wilcox; and (3) a low-velocity lower Wilcox downdip of these.
Only the last inversion can probably be attributed to a transition from normal pressure
(normal consolidation of shale) to overpressure (underconsolidation of shale). Conse
quently, it would be unreasonable in this area to map the top of geopressure from the
first major velocity inversion.
Stacking and rms Velocities
Velocity-survey data were used to bracket the range of rms-velocities in the
study area (fig. 75) and to compare these with stacking velocities obtained from
velocity analyses from the UT lines. From these comparisons the following observa
tions can be made. (1) Stacking velocities generally fall within the range of rms
velocities. However, some stacking velocities fall above the range, for unknown
reasons. In addition, the range of uncertainty caused by interpreter subjectivity may
be greater than the rms range. (2) The skilled interpreter can usualJy pick the
inflections in the rms curve, and therefore pick major breaks in interval velocity.
(3) Stacking velocities do not show the systematic downdip decrease that is seen in rms
veloci ties.
Time-Depth Conversion
Time-depth conversion derived from integrated acoustic logs and from velocity
analyses were compared with those obtained from velocity in two nearby welJ surveys
122
INTERVAL VELOCITY (fflsee) 8,00~rOO---,7''-r0OO~----,8:.:.:c,0,c.00,--------=-9:.:,.,o0:..;:0--1.::.:0,0:;..::OO-=---_--,-IIc.:;'0.::..:00=--'-----�.::.:2,o:;..::o--=-o_---.:.:�3c.:;,OO--=-o=--_�:..;:4,c:;.:oo:..::o_----.:.:�5:::;,OO--=-o=---~1~6,0:;.:0::::0---.:..17:.::;,000
7,000
6,000
5000
4,000
3,000
2,000 .:=
:2: 1000
::J g 0
~ -1,000 o OJ <{ -2,000
l-I S2 -3,000
W
I -4,000
-5,000
-6,000
-7,000
-8,000
-9,000
Figure 74. Interval velocities based on velocity surveys from five wells (fig. 61) which are widely enough distributed to bracket velocities in the study area. Note large velocity discontinuity at top of Wilcox.
123
..
2000
4000
6000
8000
12,000
14,000
16,000
18,000
TWO-WAY TRAVEL TIME (SEC) 2
Downd'ip wellVelaci ty survey
Updip wellVelocity survey
Downdip well-
Updip wellVelocily analysis
RANGE OF 5 VELOCITY SURVEYS
Updip wellIntegrated sanic
Dawndip wellIntegrated sonic
20,0001--____ ...1-____ -1-____ --'-_____ 1--____ ...1-____ -1-____ --'-____ --1
Figure 75. Range of rms velocity curves derived from interval velocities in figure 74, compared with stacking velocities picked from Velscans (trial stacks with a range of stacking-velocity functions) and contour plots of velocity analyses.
124
that were assumed to be the most accurate (fig. 76). Integrated acoustic logs
systematically overestimate the travel time by underestimating velocities. The
magnitude of this error is difficult to predict, but depends on the quality of the
acoustic log and on the amount of prior editing for obvious cycle skips. Neither log
used in figure 76 was edited before integration.
Velocity analyses tend to give fairly good time-depth estimates at shallow depths
but deteriorate as depth increases. This is a result of both decrease of maximum
offset-to-depth ratio, causing a decrease of normal moveout, and decrease of
amplitude and signal-to-noise ratio of reflectors. Interpreters tend to pick stacking
velocities on the high side to avoid stacking of multiples. Therefore, interval
velocities are generally overestimated from velocity analyses, and travel times are
underestimated, in contrast with estimates from integrated acoustic logs.
Conversion to true depth and removal of vertical exaggeration is easy to perform
digitally with seismic data and is valuable for structural interpretation. The major
problem is proper handling of lateral velocity variations caused by stratigraphically
controlled velocity variations in structurally complex areas. If velocity increases
steadily with depth, this will not be a major problem. However, large velocity
discontinuities in this area make proper depth conversion difficult to achieve. Lateral
velocity variations caused by variations in facies, diagenesis, and consolidation state
(related to geopressure) are potentially a problem as well, but are more easily handled
by lateral linear interpolation.
"Wipe-Out" Zones
The data quality in the area of greatest interest is poor and deserves further
study. Because the "wipe-out" zones occur just below the faults cutting the top of
Wilcox, we suspect that the velocity discontinuity at the top of Wilcox is one cause.
Common-depth-point (COP) traces at different offsets represent different ray paths
125
OPT 40 .,10(., I Y ~ II /lte)
OPT 60 .,loC.lylll/Hd
OPT 60 .,Ioc·lylii/ltc)
OPT 100 .. loc'I,IIt/uc)
6000 1
00-
eooo 1000 12000 6000 1
8000 10000 12000 6000 8000 10000 12000 6000 8000 10000 12000
,-
, 0-
20-
'0-
,,-
<0-
1 1
-------- ....
'* "--------~ >---<
\ OPT 120
w,LOC11, [II/we)
\ \
OPT 140 \'floc",III/StC)
OPT 160 .,IOC"J (Il/ltc)
........
, 1 , 1
00-
\ \
,-
'0-
, ,-
2.0-
B-
3.0-
,,-
<0-
OPT 160 ¥'locil,I!I/nel
.000 1
8000 10000 12000 6000 8000 10000 12000 6000 8000 10000 12000 .000
"" 10000 12000
I I 1 I 1
00- \
,-
10-
10-
3.0-
40-
\ \
I I
- \ \
I I
.......... --< '-1 .
\ \ \
EXPLANATION
__ - (ieosoufce
-------- Comovr Velslodl display
t---i V.lacon
"-~ MinimUm and Moxlmum """'-.... """ valu., from uphole times
I 1 I 00-
\ \
,-f
'0-
t>-
20-
,,-
'0-
t--< ,,-
'0-
Figure 76. Time-depth conversions for the Cuero area based on different sources of data. Integrated logs tend to underestimate velocities and overestimate travel times, but to an extent that is difficult to predict. Velocity analyses tend to overestimate deep velocities and underestimate travel times, particularly where velocity inversions occur.
126
I
through the top of Wilcox to reach lower reflectors. A large velocity discontinuity on
a non-flat surface could cause significant static shifts from one CDP trace to the
next. This hypothesis could be tested by applying an automatic residual static shift to
that portion of the data below the top of Wilcox.
COMPARISON OF AREAS
Among the three areas in Texas chosen for intensive site-specific evaluation, the
most striking differences are in structural style (fig. 5) and sandstone distribution
(fig. 16). Structure in the Pleasant Bayou area is the result of two independent
structural mechanisms and styles superimposed to produce a complex structure of high
relief. Growth faulting and salt mobilizati.on operated in concert to produce the
Chocolate Bayou anticline, Danbury Dome, and the intervening salt-withdrawal basin.
Structural relief is also high in the Blessing area, but it resulted from a relatively
simple sequence of growth faults with large rollover and counter-regional dip.
Structural highs on the up thrown side of major growth faults may be due partly to
shale masses beneath the fault planes. In contrast to the high-relief structure in the
two areas along the Frio trend (Pleasant Bayou and Blessing), the Cuero area is
characterized by low-relief, virtually flat structure, with very little rollover. In plan,
the Wilcox faults at Cuero are subparallel, of low sinuosity, and fairly uniformly
spaced 0-2 mi), as opposed to the sinuous, arcuate faults and the wide range of fault
spacing in the Frio areas.
The two Frio fault blocks are not only larger than the Wilcox fault block, but the
relationship of optimum reservoir facies to fault block morphology is better suited for
water production in the Pleasant Bayou and Blessing areas than in the Cuero area
(fig. 16). In the Pleasant Bayou area the thickest sandstones with blocky SP patterns
occur in an area that is nearly equidimensional and that comprises about 25 rni2 of the
127
large fault block. At Blessing the axes of optimum sand distribution paraUel the strike
of the fault block and are central1y located within it. These arrangements provide a
wide radius, free of permeability boundaries. In contrast, the optimum reservoir
facies of the Cuero area are much more limited in areal extent than those of the Frio
sandstones (fig. 16). This results from the relatively narrow, dip-oriented sandstone
trends perpendicular to the long axis of the fault block. This not only limits the
potential reservoir volume but severely constrains possible test-well location in
distributary channels.
In spite of the higher geothermal gradient of the Wilcox as a whole (Bebout and
others, in press), potential geothermal aquifers in the Cuero area have lower
temperatures than those in the Pleasant Bayou area because of shaUower depths, but
aquifer temperatures are slightly higher than temperatures in the Blessing area
(table 4). The Cuero and Blessing areas have similar pressures, but these are
substantial1y lower than in the deeper Pleasant Bayou area (table 4). In general, the
stratigraphic and structural relationships that result in geopressure are similar in all
three areas. Geopressured sandstone reservoirs are limited on the updip side by
downfaulting against shelf-slope shales and on the downdip side by upfaulting against
transgressive shales (Anahuac and middle Wilcox). They are overlain by thick
sequences of marine shale (fig. 5).
ACKNOWLEDGMENTS
This work was made possible through funding by the U.S. Department of Energy,
Division of Geothermal Energy, under Contracts DE-AS05-76ET28461 and DE-AC08-
79ET27111. Geological maps and interpretations in the Blessing area were initially
prepared by Scott Hamlin and Bonnie Weise as part of a study for the Gas Research
Institute. Seismic studies greatly benefited from discussions with Milo M. Backus, who
128
designed the shooting plan for the Cuero area and provided advice on data processing
and interpretation. We are particularly grateful to personnel at the Bureau of
Economic Geology who assisted in preparing this report. The manuscript was typed by
Charlotte J. Frere under the direction of Lucille C. Harrell; illustrations were drafted
under the direction of Dan F. Scranton.
129
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