Upload
abrian-ade-setiawan
View
219
Download
0
Embed Size (px)
Citation preview
8/16/2019 sumatera basin.pdf
1/20
101
IPA05-G-156
PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION
Thirtieth Annual Convention & Exhibition, August 2005
TELISA SHALLOW MARINE SANDSTONE AS AN EMERGING EXPLORATION TARGET IN
PALEMBANG HIGH, SOUTH SUMATRA BASIN
R.M. Iman Argakoesoemah*
Maria Raharja*
Sonny Winardhi **
Rudhy Tarigan*
Tino Febriwan Maksum*
Amritzar Aimar*
ABSTRACT
The main productive reservoirs in the PalembangHigh, South Sumatra Basin, are sandstones of the
Talang Akar Formation and carbonates of the
Baturaja Formation. The largest oil field in the region
is the Kaji-Semoga Giant Oil Field discovered in
1996. To maintain the production rate, exploration
efforts continue with new ideas and concepts. It is
believed that one of the potential exploration
candidates is the Telisa Sandstone reservoir.
The Telisa Sandstone, which includes the sandstones
in the lower part of the Telisa Formation, consists of
very fine- to fine-grained sandstones with minor
shales, deposited in a shallow marine shoreface
setting during both sea level lowstand and
transgression. The acoustic impedance contrast
between the sandstones and the overlying and
underlying Telisa shales is very small because of
highly argillaceous content of the sandstone.
Consequently, the sandstone reservoir prediction
becomes very difficult. In addition, the geophysical
data available comprise only 2-D seismic in various
vintages and qualities.
The most crucial objective in this exploration effort is
to predict the sandstone distribution, its quality and
fluid content. This is required to generate a drillable
prospect, add a development well, a work-over, and
enhance field reserves calculation. The test-line work
results of the Extended Elastic Impedance (EEI)
Inversion technique have been applied and the
delineation of sandstone distribution, determination of
* Medco E&P Indonesia
** Institute Technology Bandung
sandstone quality, and fluid content potential have
been defined. The objective of this paper is to share
the Telisa Sandstone geological interpretation and theuse of the EEI inversion in exploration of low
acoustic impedance contrast of clastic reservoir in the
Palembang High region, South Sumatra Basin.
The hydrocarbon potential in this Telisa Sandstone
play remains unknown, but the results are
encouraging. Several successful tests have been
conducted through the hydraulic fracturing efforts.
Although most of the sandstones are relatively tight,
the reservoir flows oil. The petroleum system of this
Early Miocene play remains uncertain, but it is
believed that the source rocks are mature shales of the
Lemat and Talang Akar Formations in the Jemakur
Graben and Tamiang Lows immediately to the north.
It is possible that the oil has been migrating from the
fetch areas through the sandstones of the Talang Akar
Formation and porous limestones of the Baturaja
Formation. In the Kaji-Semoga Field, the Telisa
Sandstone onlaps onto the Baturaja Formation. Fault
conduits could also be an important role in oil
migration process as indicated in the Langkap Field.
The top and up-dip lateral seals are the thick, basin-
scale Telisa shales.
INTRODUCTION
The Telisa Sandstone exploration has been
intensively conducted very recently since the year
2004 though the hydraulic fracturing jobs have begun
since late 2002. Pre-2002 period and prior to the
success of the hydraulic fracturing work results, the
Telisa Sandstone reservoir potential was not viewed
attractive as a primary drilling objective. This despite
http://gobackdoc/http://gobackdoc/http://gobackdoc/
8/16/2019 sumatera basin.pdf
2/20
102
the fact, that oil shows were observed in the wells and
the hydrocarbon gas readings during drilling were
significantly high, and in some wells, the gas readings
are even higher than the targeted reservoir objectives.
The sandstones were considered thin, shaly, and tight.
At the time, the primary reservoir objectives were the
Talang Akar and Baturaja Formations. Consequently,a lack of geological and geophysical data has been
acquired for this Telisa Formation. Most of the logswere not properly run to cover the whole section and
a lack of conventional cores was available to evaluate
the reservoir deliverability. To maintain hydrocarbon
production of the existing oil and gas fields,
exploration efforts are obviously needed to searchother exploration plays. The Telisa Sandstone is one
of the potential candidates. Further geological data is
now beginning to be acquired, including conventional
cores and modern wireline logs.
One of the detailed exploration efforts being
undertaken is the study of EEI inversion using
reprocessed pre-stack time migration of 2-D seismic
lines of various vintages, mainly to cover the Kaji-
Semoga Oil Field. Some of the seismic lines included
in the study are those extending over the Old Rimau
Fields such as Langkap and Kerang Fields. The EEI
test-line project, involving several selected 2-D
seismic lines, was conducted to test the method in
predicting the presence and distribution of the Telisa
reservoir sandstone in the Palembang High region.
The main objective of this paper is to share the results
of the Telisa Sandstone interpretation in this region.
The Telisa Sandstone as defined in this paper includes
the sandstones in the lower part of the Telisa
Formation. The thick argillaceous sediments
overlying and underlying the Telisa Sandstone will
not be discussed in detail. The discussion will focus
only on the sandstone interval.
TELISA SANDSTONE
The Palembang High is located in the eastern part of
the South Sumatra Basin. Geographically, it is about
70 km to the northwest of Palembang City, Figure 1.
The Palembang High is one of the local paleo- basement highs formed as part of the easternmost
margin of the South Sumatra Basin. This margin is
obviously the westernmost end of the Sunda
Landmass and sediment source for most of the
Tertiary sediments. On this high, syn-rift deposits of
the Late Oligocene to Early Miocene Lemat and
Talang Akar Formations are very thin, except for
those deposited in the Jemakur Graben, partially
forming a low-angle angular unconformity and later
overlain by carbonate rocks of the Baturaja
Formation. These carbonates have been a major
producing reservoir for the Kaji-Semoga oil field. Inthis paleo-basement high region, the Telisa Formation
was deposited immediately above the Baturaja
Formation. Toward the basinal areas to the west, the
formation is also stratigraphically inter-fingering with
the Baturaja Formation, Figure 2.
The Telisa Formation is lithostratigraphically defined
as shallow-to-deep open marine, dark grey shales.
The shallow marine shales have been observed in the
Palembang High area. In this region, the lower part of
the formation usually contains thin reservoir
sandstones called “Telisa Sandstone” and hasthicknesses ranging from 20 to 80 feet. Immediately
to the northeast of the Iliran Fault, the sandstone
thicknesses could be in the range of 300-foot gross
with net sand thickness of up to 200 feet. Figure 3
shows the isochron of the Telisa Sandstone and the
shales below it across the Palembang High. Two
major paleo-lows, oriented northwest-southeast, are
the areas where the shaly sediments beneath the
Telisa Sandstone are well developed. The low
between Kaji-Semoga High and Iliran High is mainly
filled-in by shales and siltstones with minor
sandstones in the uppermost part of the sequence. Incontrast, the low immediately to the north of the Iliran
Fault consists of fine-grained sandstones with minor
shales. It is believed that various degrees of syn-
depositional normal faulting is responsible for the
varying thickness development of the argillaceous
sequence underlying the Telisa Sandstone in both
half-grabens.
Wireline log motifs of the Telisa Sandstone sequence
varies significantly from well to well. The sequence
in the Kaji-Semoga wells generally shows a blocky
motif, occasionally coarsening upward in the lower portion and fining upward in the upper part, with an
erosional surface in between, Figure 4. Based on this
log shape and core descriptions, the sandstones can be
subdivided into several more detailed facies lobes. Ingeneral, the sandstones are light olive gray, all fine
and very fine grained, calcareous, angular to sub-
rounded, very well sorted lithic arenites with feldspar
and substantial numbers of globigerinids, small benthic
foraminifers, small echinoid spines, and splinters of
8/16/2019 sumatera basin.pdf
3/20
103
vertebrate bone. It is a mix of sediment particles,
created in a fully marine environment (pelagic skeletal
materials) with detrital particles (fine sand and some
clay mud) wafted in from the nearby coast. Wavy
lamination and ripple bedding, and bioturbated to
various degrees, suggest deposition below the fair-
weather wave base but it could be close to the stormwave base.
The Telisa Sandstone in KS-138 well has a total
thickness of about 80 feet, consisting of three stacked
facies, Figure 4. In the lower lobe, the facies has the
highest gamma ray and lower resistivity values.
Shales with many green peloids and minor scatteredsilts and fine quartz sands possibly cause this high
gamma ray reading. Glauconite is present in the
whole sequence, but becomes a significant component
in this lower facies. This glauconite could be formed
in marine waters, still quite shallow, in the shelfdepths with continuous reworking and a slow rate of
clastic input. In contrast, the presence of glauconite in
the higher sandstone beds suggests the activity of
intensive erosion of the sediments below it. The
glauconite in this sandstone could therefore be
allochthonous rather than autochthonous origins.
In KS-138, the middle facies is represented by a
blocky log motif with a sharp basal contact to the
sandstone, suggesting a possible local erosional
surface. The sandstone consists of abundant very
angular fine quartz sands and foraminifers. The uppersandstone lobe occasionally shows a fining upward
log motif with a sharp basal contact. The sandstone is
separated from the middle lobe by less than 5-foot of
shale. The top of the sandstone sequence has a sharp
top contact and is immediately overlain by the deeper,
transgressive outer-neritic-to-bathyal marine shales.
These erosional surfaces are interpreted as the product
of the submarine incision during a transgression
period.
The conventional cores collected in KS-106 well
suggest that the sandstones were deposited in amarine middle-outer sublittoral environment.
Planktonic foraminifers are abundant indicating an
offshore environment strongly influenced by open-sea
currents. Hummocky cross-stratification is present onlyrarely suggesting possible minor storm influence.
Locally derived detritus is also abundant, mainly from
the granitic basement rocks interpreted to be derived
from the nearby Kaji-Semoga High. Less common
planktonic foraminifers in the lowermost part of the
section may be indicative of shallower water
conditions.
In Kerang-1, the sandstone interval has several
erosional surfaces that are difficult to directly
interpret from the logs, Figure 5, with the upper
sandstone bed was heavily burrowed. The sandstonesare very fine to fine grained and calcareous with
planar sedimentary structures. The shales
immediately below the sandstones consist of
abundant foraminifers, are calcareous, and glauconitic
with a streak of conglomeratic glauconitic sandstone
(2-foot thick).
AGE AND DEPOSITIONAL ENVIRONMENT
Faunal content in the shales immediately overlying
the Telisa Sandstone is abundant and has a high
diversity of deep marine faunas. The benthic faunasinclude Uvigerina sparsicostata, Uvigerina
peregrina, Uvigerina schwageri, Cibicides foxi,
Haplophragmoides compressa, and Bolivina sp
suggesting an outer neritic marine environment.
Several deeper marine fossils are also present:
Cassidulina, Gyroidina, and Buliminella, suggesting
an outer neritic to upper bathyal environment.
Globigerinoides diminutus, Globorotalia birnageae,
and Globorotalia peripheroronda are present
indicating that the sequence is N7 in age. The base of
Globorotalia birnageae in the shales just above the
Telisa Sandstone is used as the basal boundary of N7.The presence of Catapsydrax cf dissimilis and
Globigerinoides trilobus group suggests that the
Telisa Sandstone and shales between this sandstone
and the top of the Baturaja limestone is N5-N6 zones,
Early Miocene. This interpretation is supported by the
occurrence of nannofossils Sphenolithus belemnos
and Helicosphaera ampliaperta indicating NN2-NN3
zones.
The overall Telisa Sandstone sequence in the
Palembang High region was deposited during N5-N6
(middle Early Miocene). The water depth isincreasing from the top Baturaja to the shales
overlying the Telisa Sandstone: middle neritic to
upper bathyal. This is consistent with the major
transgression period during deposition of the TelisaFormation. Following this transgression, a series of
regional regressions occurred when the Palembang
Group (Lower, Middle, and Upper Palembang
Formations) was deposited across the South Sumatra
Basin.
8/16/2019 sumatera basin.pdf
4/20
104
The Telisa Sandstone was deposited immediately
above the Baturaja Formation. Thin shales of about
100-foot thick usually separate the Telisa Sandstone
from the Baturaja limestones. When the Baturaja is
absent, the Telisa Sandstone directly overlies the
Talang Akar Formation or Basement. The Baturaja
Formation consists of mainly carbonate rocks withoccasional stringers of calcareous shales. Deposition
of these carbonates range from reefal build-up tocarbonate bank. The carbonates are usually found on
the paleo-basement highs either on top or the gentle
flank of the highs. They were deposited in shallow
marine, clear water, inner neritic environment. The
limestones of the Baturaja Formation were exposed tothe atmosphere (Kalan et al, 1984). The extent of the
subaerial exposure remains unknown, but it is
possible that the upper part of the section is the
product of the highstand carbonate deposition.
Another indication is that the fine clasticsimmediately overlying the Baturaja limestone tend to
consistently show coarsening upward sequences,
Figure 4. Although it is difficult to recognize, the
depositional model for the Telisa Formation seems to
be in line with the possibility of a Baturaja highstand
model.
The preliminary regional interpretation results of the
Telisa Formation indicate that there is a lack of
evidence for the presence of lowstand deepwater
sediments in the Palembang High during deposition
of the Telisa Sandstone. However, the sedimentsshould be present in the basinal settings away from
the paleo-highs. In contrast, throughout most of the
Tertiary, the sediments in the paleo-high region tend
to be non-marine to shallow marine. Further detailed
interpretation of depositional environment of Telisa
Sandstone in the Palembang High is difficult to define
due to limited supporting data being available.
The regional parasequence chronostratigraphic
correlation, biostratigraphic analysis results,
conventional core, thin section, wireline log shape
analysis, and regional geological modeling, are
integrated together with the seismic interpretation to predict the depositional environment. It appears that
the sandstones were possibly deposited in a shallow
marine shoreface setting though this could still be
controversial, as is usual in defining the origin of theisolated shallow marine sandstone bodies.
DEPOSITIONAL MODEL
The biostratigraphic analysis results are insufficient to
describe the details of the depositional environment of
the Telisa Sandstone. The 2-D seismic interpretation
also failed to support the presence of the sandstone
reservoir due to limited seismic quality and low
impedance contrast between the sandstones and the
overlying and underlying shales. Regional geological
log correlation is very useful in verifying some of the
regional markers and parasequences. Figure 6 showssouthwest-northeast parasequence chronostratigraphic
correlation across the Kaji-Semoga, Langkap, andKerang areas. Several progradational parasequences
of 4th and/or 5th order occurred in the lower section of
the Telisa Formation. Datum of the correlation is
hung to the maximum flooding surface (MFS)
immediately above the Telisa Sandstone. Position ofthis MFS is defined by using wireline log responses
particularly gamma ray and deep resistivity curves
and roughly controlled by local biostratigraphy. This
chronostratigraphic correlation suggests that the
Telisa Sandstone comprises isolated shallow marinesandstone bodies encased in the Telisa marine shales.
The wireline log shape analysis utilized in this
correlation indicates that deposition of the Telisa
Sandstone is not simple. The log responses in each
well show inconsistent wireline log shape.
A geological model of the Telisa Sandstone
deposition is shown in Figure 7. In this model the
sandstones are deposited at a time when the shoreline
rapidly prograded basinward below fair-weather wave
base during a lowstand period, producing progradinglowstand shorefaces overlying the fine clastics of the
open-marine highstand shelf sequence
(Figure 7, stage 2). When relative sea level rises, the
lowstand shoreface sediments will be truncated by a
higher erosion surface as transgression continues. The
transgressively incised shoreface sandstones, lie on
the transgressive surface of erosion (ravinement) and
are related to the initial transgression across large
shelf areas. It is possible that, as shown in this model,
each transgressive shoreface is underlain by an
erosion surface of the initial transgression. This is
interpreted to be the back-stepping shoreface orerosion shoreface retreat. The ravinement surface is
not simple, but it could be amalgamated to be a
widespread erosional surface (Figure 7, stage 3). It is
an amalgamation of the ravinement surface and
lowstand sequence boundary surface. If this is the
case, the sandstones internally in “sand-to-sand”
contact are genetically discrete due to widespread
cannibalization or intense reworking of the earlier
prograding lowstand sandstones. The end product is
8/16/2019 sumatera basin.pdf
5/20
105
that the amalgamated sandstones of different origins
become an isolated shallow marine sandstone body,
Figure 6. Hence, the transgressive sequence sandstone
has the highest preservation potential.
This geological model of deposition is apparently
consistent with the preliminary interpretation of theregional high-resolution sequence stratigraphy. Using
the geological model discussed above, the mainsandstone reservoir encountered in the Kaji-Semoga
and Langkap wells is believed to be a product of the
depositional remnant of various lowstand shorefaces.
This remnant is thin with individual sandstone
thickness ranging from 20 to 50 feet but could bedeposited over widespread areas. In the case of the
Kaji-Semoga and Langkap, the remnant of the
shoreface facies sandstone could extend
approximately 15 kilometers. The overall interval of
the Telisa Sandstone deposition and erosion duringlowstand and transgression periods in the Kaji-
Semoga, Langkap, and Kerang areas is only about
200 feet, Figure 6. This evidence suggests that the
shelf dip and morphology during the Telisa Sandstone
deposition is likely to be very gentle. Small relative
sea level change could have direct and large impact to
the overall sedimentation on the Palembang shelf.
Hence, the 4th or 5th order of parasequences may be
sufficient to produce significant sandstone
development. The thin transgressively incised
shorefaces encountered in the wells are possibly
caused by rapid relative sea level rise.
The individual sandstone thickness of each
transgressively incised shoreface is usually very thin,
ranging from less than 5 feet to about 20 feet. The
lateral distribution of each transgressive shoreface
sandstone body seems to be independent and may not
be connected laterally and vertically. Each of the
sandstone units seems to have a different age of
deposition. The sandstone becomes younger toward
the west from Kerang to Langkap and Kaji-Semoga
as shown in Figure 6. Possible stacked transgressive
shoreface sandstones are likely not to occur due tolimited lateral extent of each sandstone body.
Several isolated shallow marine sandstone bodies in
other basins have been discussed by Burton andWalker (1999), George (2000), Hart and Plint (1993),
Leckie and Walker (1982), Leggitt et al (1990),
MacEachern et al (1998 and 1999), McBride et al
(2002), Martinsen (2003b), Pattison and Walker
(1992), Posamentier (2002), Posamentier and
Chamberlain (1993), Snedden and Bergman (1999),
Snedden and Dalrymple (1999), Tesson et al (2000),
and Walker and Eyles (1988). The origin of isolated
shallow marine sand bodies remains controversial
(Sutter and Clifton, 1999; Snedden and Bergman,
1999). It is undoubtedly true that estuarine parts of
incised valley fills, erosional remnants, lowstanddeltas, and other systems may form isolated marine
sand bodies (Nummedal, 2002).
SEDIMENT PROVENANCES
It is difficult to define the provenance for the Telisa
Sandstone and further study is still required.However, speculation is that the sediment supply is
likely sourced from more than one location. Dipmeter
and image logs are not available, and the preliminary
seismic stratigraphic interpretation in this paleo-
basement high region does not sufficiently show thedepositional direction. The test-line results of the EEI
inversion work carried out suggest that the nature of
the sandstones is different in each region. For
example, the sandstones in the Kaji-Semoga area
have different wireline log responses compared to
those in the Kerang and Langkap wells due to
possible differences in the mineralogic composition.
In the Kerang wells, the deep resistivity readings have
low contrast compared to those in the shales
overlying and underlying the sandstones, but flow oil.
The sandstone resistivity value is less than 1.5 times
the resistivity of the shale base line. Hence, a low-resistivity low-contrast pay (LRLC) is present. In the
Kaji-Semoga wells, LRLC seems not to develop
(Figure 4).
It is possible that at least two speculative sediment
provenances for the Telisa Sandstone sequence can be
predicted, Figure 3. The main sediment supply is
most likely from the Sunda Landmass further to the
east. Although the Kaji-Semoga High is only
considered to be a local high, the sediments supplied
are very significant to reservoir development in the
region. Thin sections of the sandstones in the Kaji-Semoga wells shows very angular to sub-rounded
quartz grains suggesting a short distance to the
sediment supply, Figure 4. The regional parasequence
chronostratigraphic correlation (Figure 6) and seismicinterpretation (Figure 8) are also consistent with the
interpretation of the Kaji-Semoga High as a local
high. The Iliran High might be the source of the
sediments, but the shallow wells drilled in this paleo-
high indicate that the Telisa Formation is present
8/16/2019 sumatera basin.pdf
6/20
106
directly on top of the basement suggesting that the
Iliran High might be valid for the lowermost part
only.
EXPLORATION OBJECTIVES
The primary objective of the Telisa Sandstone studyand EEI inversion work is to predict the distribution
of the sandstone reservoir. The present day timestructure map of the Upper Telisa Marker shows that
the structure is continuously and consistently
shallowing towards the Sunda Landmass to the east.
Figure 9 shows a west-east oriented seismic line
showing this continuous shallowing of structuretowards the east. On the other hand, hydrocarbons
have been encountered in the Kaji-Semoga, Langkap,
and Kerang Fields. The most acceptable explanation
for this geological trap is that the hydrocarbons are
trapped in a stratigraphic play, however, a possibleup-dip small fault seal could also occur. Location of
the up-dip lateral permeability barrier either as a
shale-out play of the transgressively incised
shorefaces or lowstand depositional remnant is
important to define. It should also be noted here that
remnants range in size from small to very large
(basin-scale), and the larger remnants commonly
contain several smaller remnants (Martinsen, 2003a
and 2003b). If this permeability barrier can be defined
and identified, perhaps some of the geological
uncertainties can be minimized in (1) refining existing
field reserves calculation, (2) designing work-over jobs, (3) better planning for production and in-fill
wells, and (4) the most critical to search for additional
new drillable exploration prospects.
Another critical objective is to identify and map the
presence of better quality within the sandstone
reservoir. It is possible that the location of better
reservoir quality should be in the areas close to the
sediment provenances. Given the parasequence
chronostratigraphic correlation in Figure 6 and the
geological model in Figure 7 are both valid, it appears
that the search for lowstand depositional remnants iscrucial. The lowstand sand body might not be in
communication with other lowstand sand bodies. The
sedimentary reservoir compartmentalization caused
by the sandstone body geometry, built from differentsediment provenances, could result in separate
stratigraphic traps. The orientation of the shoreface
deposition might still be in agreement to the related
shoreline orientation of the sediment provenance.
Similarly, as shown in Figure 6, each of the
transgressively incised shoreface sandstones is likely
not connected to other sandstones of the same genetic
depositional process.
Since the thick, basin-scale shales of the Telisa
Formation encase the sandstones, seal should not be
problem, except in the area where the sandstones arein direct contact with the Baturaja limestones and/or
basement. Source rocks are the mature shales of the
Lemat and Talang Akar Formations in the Jemakur
Graben and Tamiang Lows to the north. Hydrocarbon
migration is interpreted to occur mainly through the
normal faults present in the Jemakur and Langkap-
Kerang region. The importance of lateral carrier beds
of the Baturaja limestones and Talang Akar
sandstones for hydrocarbon migration may not be
crucial, because the sandstones are usually thin and
tight, Figures 8 and 9. The Plio-Pleistocene orogeny
resulted in the Palembang High re-orientation andtilting. Therefore, the migration pathways during the
post Plio-Pleistocene could be substantially different
and oil re-migration should be considered as one of
the important geological risk components.
EXPLORATION METHOD
An integrated approach of geophysical and geological
analyses is required to identify and examine the
Telisa Sandstone geometry and potential hydrocarbon
content in this complex. The work involves Extended
Elastic Impedance (EEI), Lambda(λ )-Mu(µ)-Rho(ρ)(LMR) inversions and S-wave prediction based on
other logs using generalized linear regression method.
The EEI approach is based on Whitcombe et al (2002)
who had modified the definition of Elastic Impedance
(EI) beyond the range of physically meaningful angle
by substituting tan χ for sin2 θ. The primary variable
now becomes χ rather than θ, and it is allowed to vary
between -900 and +900. This practically allows us to
define a single function, which is proportional to a
number of different elastic parameters, depending on
the value of χ used.
In the Telisa Sandstone, the EEI (χ = -410) ≈ µρ, EEI
(χ = 70) ≈ λρ, and EEI (χ = 50) ≈ ρ. The λ /µ parameter
can then be obtained by dividing µρ from λρ. λ , µ,
and ρ are Lamé’s parameters. µ which is related torock’s rigidity, gives information on lithology. µρ
can be used to distinguish between sandstones,
calcareous sandstones, and shales. Calcareous
sandstone has higher rigidity and incompressibility
than sandstone, while sandstone has higher rigidity
8/16/2019 sumatera basin.pdf
7/20
107
than shale and coal so it has a higher value of µρ. On
the other hand, λ represents the rock’s
incompressibility, which is sensitive to pore fluid
type. Therefore, λρ and λ /µ can be utilized to detect
the presence of fluid. Sandstones containing gas are
more compressive than wet sandstones so they have
low incompressibility values. Sandstones containinggas are also less dense than sandstones containing
water.
Extended Elastic Impedance Method
Connolly (1999) introduced Elastic Impedance (EI) as
a generalization of Acoustic Impedance (AI) for non-normal incidence angle, enabling the benefits of the
inversion approach to be applied for pre-stack/ AVO
data.
Reflection amplitude, as a function of angles based onthree-term Zoeppritz linearization’s (Aki and
Richards, 1980), can be expressed as:
θ θ θ 222 tansinsin)( C B A R ++= (1)
where θ is the average of the incidence and
transmission angles at a plane-reflecting interface.Connolly (1999) further showed that EI could be
expressed as a simple function of Vp,Vs , and density
(α,β, and ρ) :
cba EI ρ β α θ =)( (2)
where:
a = (1 + sin2 θ ),
b = -8K sin2 θ , (3)
c = (1-4K sin2 θ ),
and where K is a constant, usually set to the average
value of (β/α)2 over the log interval of interest.
The EI function [equation (2)] was then modified(Whitcombe et al, 2002) by introducing reference
constants α0, β0 and ρ0,, which remove the variabledimensionality of equation (2) and provide an EI
function which returns normalized impedance values
for all angles θ :
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎟ ⎠
⎞⎜⎜⎝
⎛ ⎟⎟ ⎠
⎞⎜⎜⎝
⎛ ⎟⎟ ⎠
⎞⎜⎜⎝
⎛ =
cba
EI 000
00)( ρ
ρ
β
β
α
α ρ α θ (4)
However, there are two difficulties in using the
current EI definition. There is a requirement for |sin2θ |
to exceed unity, and reflectivity values may exceed
unity as sin2θ increases; clearly, no impedance
contrast can give rise to a reflectivity value greater
than unity (unless negative impedance is allowed). In
practice, this will mean that as |sin2θ | approaches and passes unity, the EI log, by its current definition, will
become increasingly inaccurate. To compensate forthese difficulties, Whitcombe et al (2002) make two
changes to the current definition of EI. First, by
replacing sin2θ with tan χ such that the equation is
defined between ± ∞ rather than the 0–1 limit
imposed by sin2θ . The scaled version of reflectivity,
which corresponds to a specific elastic parameter
contrast, can then be defined as
χ tan B A R += (5)
from which they derive
χ
χ χ
cos
sincos B A R
+= (6)
Now introducing RS , or scaled reflectivity,
χ cos R RS = (7)
results in
χ χ sincos B A RS += (8)
The EI equivalent of equation (8) is then
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎟ ⎠
⎞⎜⎜⎝
⎛ ⎟⎟ ⎠
⎞⎜⎜⎝
⎛ ⎟⎟ ⎠
⎞⎜⎜⎝
⎛ =
r q p
EEI 000
00)( ρ
ρ
β
β
α
α ρ α χ (9)
where:
p = (cos χ + sin χ )
q =-8 K sin χ (10)
r = (cos χ - 4 K sin χ ):
Whitcomb et al (2002) called this Extended Elastic
Impedance, or EEI.
8/16/2019 sumatera basin.pdf
8/20
108
Data preparation and Extended Elastic Impedance
Inversion
Data used for the test-line work to predict the
sandstone distribution using the EEI technique,
consists of 15 selected 2-D seismic lines and
petrophysical analyses of 10 wells. Seven additionalwells have been utilized for validation of the test
results. The seismic balancing work for amplitude and phase is carried out first, since the seismic lines are of
various vintages. The maximum angle of 230 is used
to get the optimal results in using stacking data. The
seismic lines are selected to cross several exploration
wells and oil and gas fields: Kaji-Semoga, Langkap,west Iliran, Rumbi, Rimbabat, and Kerang. The Telisa
Sandstone penetrated in these wells displays various
wireline log responses caused by variations in
mineralogy and hydrocarbon content.
a. Cross-plots between well log properties
Logs of P-wave velocity, S-wave velocity, density,
and gamma ray are needed. Cross-plot of these log
properties is required to know whether the properties
are usable for distinguishing lithology and fluid
contents. Using the predicted S-wave sonic, KS-01,
KS-26, Langkap-1A, Langkap-4, and Kerang-10
wells will have the required logs for performing
cross-plot analysis. Cross-plots between density
versus gamma ray and P-impedance versus gamma
ray apparently cannot be used to distinguishsandstone from shale since both have almost similar
density value. However, cross-plot between P-
impedance and density shows that sandstone and
shale plot at somewhat different linear trends. Cross-
plot between λ /µ and density also shows a different
cluster trend between sandstone and shale. It is
observed that shale tends to cluster together with high
water-saturated sandstones, while hydrocarbon-
bearing sandstones tend to have low λ /µ values.
Based on the above cross-plot analysis, wet or shale
trend can then be used as a reference in calculating P-
impedance and λ /µ anomalies.
b. S-wave prediction
To generate S-wave velocities for wells, relationships between multi log data at KS-28 well and the S-wave
velocity derived from Castagna’s equation needs to be
established. To estimate the S-wave velocity, various
wireline logs such as the gamma ray, neutron
porosity, and density logs, were used. The P-wave
velocity log was not used to predict the S-wave
velocity log because the P-wave velocity is sensitive
to the changes in both pore fluids and rocks. The
introduction of small amount of gas into the pore
spaces of a rock can reduce the P-wave velocity. On
the other hand, the S-wave velocity is sensitive only
to the rocks and weakly dependent on pore fluidcontents (Mavko et al, 1988).
c. Extended Elastic Impedance Inversion
As mentioned above, µρ, λρ, and λ /µ can be
approached by EEI with specific χ value. Figure 10
illustrates the correlation between EEI for varyingangles (-900 to 900) with well log data at several wells
involved in the study. It shows that the best
correlation to predict lithology is to use µρ with the
angle of EEI at -410; to predict fluid, one should use a
combination of λρ and µρ to form λ /µ. Figure 11 shows the location map of the selected 2-D seismic
lines and wells with cross-plot between λ /µ and P-
impedance anomalies in various regions, and Figure
12 shows a seismic display across KS-01, KS-26, and
KS-28 wells. Results of the EEI inversion are shown
in Figure 13.
Mu-rho can be used to distinguish between shale and
Telisa Sandstone. A high value of mu-rho (greater
than 6 Gpa * g/cc) will mostly be comprised of
sandstone and calcareous sandstone while shale will
be mostly having a low value (less than 6 Gpa * g/cc).After separating sandstone from shale, the remaining
task is to separate good sandstone from the calcareous
sandstone. The latter is done using density. The lower
density value will be mostly comprised of good
sandstone while the higher value of density is an
indication of calcareous sandstone. The calcareous
sandstone will mostly have a higher value of P-wave
velocity than sandstone and shale. Cross-plots
between P-impedance and density can also be used to
exaggerate the separation between good sandstone
and calcareous sandstone. In order to detect the
presence of fluid, cross-plots between λ /µ and P-impedance anomalies are used to help in
distinguishing between the two, sandstone and shale,
as well as in distinguishing between hydrocarbon
bearing sandstone and shale or wet sandstone. Figure14 is a map of λ /µ anomaly that shows distribution of
the predicted sandstones. Good sandstones are
predicted to exist in the area surrounding Langkap,
Rimbabat, and Rumbi. The calcareous sandstones are
present around Kaji-Semoga area while sandstones
8/16/2019 sumatera basin.pdf
9/20
109
that are rather shaly in nature, are predicted to cluster
around Kerang area.
Hydraulic Fracturing
The Telisa Sandstone is present at shallow depths between 1700’ and 2500’. The sandstone generally
has low permeability causing low production. Toimprove the production performance, the stimulation
technique called “hydraulic fracturing” has been
employed. This technique uses hydraulic power to
create crack or opening at the reservoir, followed by
pumping sand to fill the crack to avoid closing.
Efforts to optimize the fracturing results have been
made using six sigma (statistical approach) and
analytical approaches. The goal is to focus on
increasing oil gain from the fracturing job. The firststep is to identify all factors that could impact oil
gain. The candidate selection criteria include area
(field), resistivity, and hydrocarbon pore thickness
(HPT). Fracturing design includes fracturing length,
fracturing conductivity, propant size, and propant
loads. Each of these factors is evaluated using
statistical analysis.
The second step is to evaluate data using a statistical
approach: hypothesis test, such as t-test and F-test.
The t-test is to compare the mean of the two data sets
while the F-test is to compare the standard deviationof the two data sets. By using these hypothesis tests
the impact of the factors to the fracturing results can
be predicted. The evaluation continues for all of the
factors and is combined with the analytical approach
using a fracturing simulator to determine the
candidate selection criteria and fracturing design.
The third step is the execution plan and field trials in
the Kaji-Semoga wells to confirm the work results.
The TSO fracturing design was applied in the Kaji
Field wells and the long hydraulic fracturing design
was applied in the Semoga wells.
The fourth step is monitoring work. Initially the oil
production, after the fracturing job, from the selected
Kaji-Semoga wells is high. There was significant oilgain improvement after the work. The mean oil gain
after the work was 220 BOPD (35 m3/day) compared
to the mean oil gain before the work of 168 BOPD
(26.7 m3/day). Standard deviation also decreased from
186 BOPD (26.9 m3/day) to 130 BOPD (20.7
m3/day). The performance of the wells will have to be
closely monitored.
There are some challenges in optimizing fracturing
job results such as sand flow back to the well bore,
production drop after production for 3-5 months, and
high water cut. Sand flow back to the well bore willnot only cause problems for the facilities such as pipe
line and pumps, but will also result in productivityreduction. Both the causes of the problems and how
to control them needs to be addressed. The
application of resin-coated sand may be one suitable
means of preventing this happening in the future. The
production drop after initially good productionappears to happen in those wells that have extremely
low permeability, as indicated by the low resistivity
values. To develop the wells that have resistivity less
than 5 ohm-m, will need further economic study will
be required.
CONCLUSIONS AND DISCUSSIONS
1. The existing 2-D seismic lines of various vintages
could not successfully define the reservoir
geometry and distribution. Hence, 3-D seismic
acquisition is recommended to better explore the
hydrocarbon potential in the Telisa Sandstone
stratigraphic play.
2. The Telisa Sandstone comprises isolated shallow
marine sandstone bodies encased in the Telisamarine shales. The origin remains uncertain, but
an integrated interpretation of the regional
parasequence chronostratigraphic correlation,
conventional core and thin section description,
and wireline log shape analysis suggests that the
sequence was possibly deposited in the lowstand
shoreface and transgressively incised shoreface
environments.
3. The lowstand shoreface sediments were truncated
by higher erosion surface as the transgression
continued resulting in depositional remnants. Thetransgressively incised shorefaces were deposited
on the ravinement surface and/or amalgamated
surface associated with the lowstand sequence
boundary. Consequently, the sandstones of theTelisa Sandstone, internally in “sand-to-sand”
contact, are genetically discrete.
4. The lowstand shoreface remnant is thin with
individual sandstone thickness up to about 50
8/16/2019 sumatera basin.pdf
10/20
110
feet, but it could extend laterally as much as 15
kilometers. The overall interval of the Telisa
Sandstone deposition and erosion during
lowstand and transgression periods is only about
200 feet suggesting that the shelf dip and
morphology is likely to be very gentle. Small
relative sea level change could have a direct andlarge impact on the overall sedimentation on the
Palembang shelf.
5. The EEI inversion work results have defined the
presence of the reservoir sandstone and
hydrocarbon. Lambda-per-Mu and P-impedance
anomalies have been used to distinguishsandstone from shale as well as to predict
distribution of the good sandstone which
potentially have low water-saturation values.
6. The hydraulic fracturing technique has beensuccessfully applied to flow oil and has
significantly improved in oil gain from the thin,
highly shaly, and tight reservoir sandstones of the
Telisa Sandstone.
ACKNOWLEDGEMENT
The authors wish to thank the Management of P.T.
Medco E&P Indonesia and BP Migas for their
permission to publish this paper. The authors wish to
thank Asril Kamal and Ukat Sukanta for their input,
review, and discussion. Anang Ismail prepared thefigures and presentation slides.
REFERENCES
Aki, K.I. and P.G. Richards, 1980. Quantitative
seismology: theory and methods, v. 1, W.H. Freeman
& Co.
Burton, J. and R.G. Walker, 1999. Linear
transgressive shoreface sandbodies controlled by
fluctuations of relative sea level: LowerCretaceous Viking Formation in the Joffre-
Mikwan-Fenn area, Alberta, Canada, SEPM Special
Publication No. 64, Isolated Shallow Marine Sand
Bodies: Sequence Stratigraphic Analysis and
Sedimentologic Interpretation, p.255-272.
Connolly, P., 1999. Elastic impedance, The Leading
Edge, v. 18, n. 4, p. 438-452.
George, G.T., 2000. Characterisation and high
resolution sequence stratigraphy of storm-dominated
braid delta and shoreface sequences from the Basal
Grit Group (Namurian) of the South Wales Variscan
peripheral foreland basin, Marine and Petroleum
Geology, v. 17, p. 445-475.
Goodway, B., T. Chen, and J. Downton, 1997.
Improved AVO fluid detection and lithology
discrimination using Lame petrophysical parameters;
“Lambda-Rho”, “Mu-Rho”, and “Lambda/Mu fluid
stack”, from P and S inversions, SEG Technical
Program Expanded Abstracts, v.16, p.183-186.
Hart, B.S. and A.G. Plint, 1993. Tectonic influence on
deposition and erosion in a ramp setting: Upper
Creataceous Cardium Formation, Alberta Foreland
Basin, v. 77, n. 12, p.2092-2107.
Kalan, T., R.J. Maxwell, and J.H. Calvert, 1984.
Ramba and Tanjung Laban oil discoveries, Corridor
Block, South Sumatra, Proceedings of the 13 th
Indonesian Petroleum Association Annual
Convention, p. 365-384.
Leckie, D.A. and R.G. Walker, 1982. Storm- and tide-
dominated shorelines in Cretaceous Moosebar-Lower
Gates Interval – Outcrop equivalents of deep basin
gas trap in western Canada, AAPG Bulletin, v. 66,
n. 2, p.138-157.
Leggitt, S.M., R.G. Walker, and C.H. Eyles, 1990.
Control of reservoir geometry and stratigraphic
trapping by erosion surface E5 in the Pembina-Carrot
Creek area, Upper Cretaceous Cardium Formation,
Alberta, Canada, AAPG, v. 74, n. 8, p. 1165-1182.
MacEachern, J.A., B.A. Zaitlin, and S.G. Pemberton,
1998. High-resolution sequence stratigraphy of early
transgressive deposits, Viking Formation, Joffre
Field, Alberta, Canada, AAPG Bulletin, v. 82, n.5A,
p.729-756.
MacEachern, J.A., B.A. Zaitlin, and S.G. Pemberton,
1999. Coarse-grained, shoreline-attached, marginal
marine parasequences of the Viking Formation, Joffre
Field, Alberta Canada, SEPM Special Publication No.
64, Isolated Shallow Marine Sand Bodies: Sequence
Stratigraphic Analysis and Sedimentologic
Interpretation, p. 273-296.
8/16/2019 sumatera basin.pdf
11/20
111
Martinsen, R.S., 2003a. Depositional remnants, part
1: Common components of the stratigraphic record
with important implications for hydrocarbon
exploration and production, AAPG Bulletin, v. 87,
n.12, p.1869-1882.
Martinsen, R.S., 2003b. Depositional remnants, part2: Examples from the Western Interior Cretaceous
basin of North America, AAPG Bulletin, v. 87, n. 12, p. 1883-1909.
Mavko, G., T. Mukerji, and J. Dvorkin, 1988. The
rock physics handbook – tools for seismic analysis in
porous media, Cambridge University Press.
McBride, R.A., H.H. Roberts, T.F. Moslow, and R.
Diecchio, 2002. Sedimentology and depositional
history of a major shelf sand sheet in the northeast
Gulf of Mexico: modern analog for ancient shallow-marine sandstones, AAPG Annual Meeting Abstract,
March 10-13, 2002, Houston, Texas.
Nummedal, D., 2002. Sequence stratigraphic
significance of continental shelf sand ridges (Gulf of
Mexico, East China Sea, North Sea), Abstract, 22nd
Annual GCSSEPM Foundation Bob F. Perkins
Research Conference, Sequence Stratigraphic Models
for Exploration and Production, 8-11 December,
2002, Houston, Texas.
Pattison, S.A.J. and R.G. Walker, 1992. Depositionaland interpretation of long, narrow sandbodies
underlain by a basinwide erosion surface: Cardium
Formation, Cretaceous Western Interior Seaway,
Alberta, Canada, Journal of Sedimentary Petrology,
v. 62, n. 2, p. 292-309.
Posamentier, H.W., 2002. Ancient shelf ridges – a
potentially significant component of the transgressive
systems tract: case study from offshore northwest
Java, AAPG Bulletin, v.86, n. 1, p. 75-106.
Posamentier, H.W. and C.J. Chamberlain, 1993.Sequence-stratigraphic analysis of Viking Formation
lowstand beach deposits at Joarcam Field, Alberta,
Canada, in H.W. Posamentier, C.P. Summerhayes,
B.U. Haq, and G.P. Allen (eds), Sequence
stratigraphy and facies associations: International
Association of Sedimentologists Special Publication
18, p. 469-485.
Snedden, J.W. and K.M. Bergman, 1999. Isolatedshallow marine sand bodies: deposits for all
interpretations, SEPM Special Publication No. 64,
Isolated Shallow Marine Sand Bodies: Sequence
Stratigraphic Analysis and Sedimentologic
Interpretation, p. 1-11.
Snedden, J.W. and R.,W. Dalrymple, 1999. Modern
shelf sand ridges: from historical perspective to a
unified hydrodynamic and evolutionary model, SEPM
Special Publication No. 64, Isolated Shallow Marine
Sand Bodies: Sequence Stratigraphic Analysis and
Sedimentologic Interpretation, p.13-28.
Sutter, J.R. and H.E. Clifton, 1999. The Shannon
sandstone and isolated linear sand bodies:
interpretations and realizations, SEPM Special
Publication No. 64, Isolated Shallow Marine Sand
Bodies: Sequence Stratigraphic Analysis and
Sedimentologic Interpretation, p. 321-356.
Tesson, M., H.W. Posamentier, and B. Gensous,
2000. Stratigraphic organization of Late Pleistocene
deposits of the western part of the Golfe du LionShelf (Languedoc Shelf), Western Mediterranean Sea,
using high-resolution seismic and core data, AAPG
Bulletin, v. 84, n. 1, p. 119-150.
Walker, R.G. and C.H. Eyles, 1988. Geometry and
facies of stacked shallow-marine sandier upward
sequences dissected by erosion surface, Cardium
Formation, Willesden Green, Alberta, AAPG
Bulletin, v. 72, n. 12, p. 1469-1494.
Whitcombe, D.N., P.A. Connolly, R.L. Reagan, and
T.C. Redshaw, 2002. Extended elastic impedance forfluid and lithology prediction, Geophysics, v.67, n.1,
p. 63-67.
8/16/2019 sumatera basin.pdf
12/20
112
F i g u r e 1
-
I n d e x
m a p o f t h e s t u d y a r e a a p p r o x i m a t
e l y a b o u t 7 0 k m t o t h e n o r t h w e s t
o f P a l e m b a n g C i t y . N o t e t h a t t h e
K a j i - S e m o g a
O i l F i e l d a n d s e v e r a l o t h e r s m a l l e r o i l a n d g a s f i e l d s h a v e b e e n d i s c o v e r e d i
n t h e P a l e m b a n g H i g h r e g i o n .
8/16/2019 sumatera basin.pdf
13/20
113
Figure 2 - Generalized stratigraphic column of the South Sumatra Basin. In the Palembang High region, the
Telisa Sandstone developed immediately above the Baturaja Formation.
8/16/2019 sumatera basin.pdf
14/20
114
F i g u r e 3
-
I s o c h r o n
m a p o f t h e T e l i s a S a n d s t o n e s e q u
e n c e s h o w i n g t w o p a l e o - l o w s b e t w
e e n t h e K a j i - S e m o g a H i g h a n d I l i r a n H i g h , a n d
b e t w e e n
I l i r a n H i g h a n d S u n d a L a n d m a s s . T h e s e t w o p a l e o - l o w s a r e b e l i e v e
d t o o c c u r d u e t o s y n - d e p o s i t i o n a l
n o r m a l f a u l t s
r e s u l t i n g
i n h a l f - g r a b e n f e a t u r e s .
8/16/2019 sumatera basin.pdf
15/20
115
Figure 4 - Typical Telisa Sandstone sequence in the Kaji-Semoga area in KS-138 well with thin sections of
conventional core from (a) 2859.4 feet (middle facies lobe) showing dominant, angular to sub-
rounded quartz grains of the blocky log motif section, and from (b) 2874.8 feet (lower facies lobe)
showing partially coarsening upward pattern with well development of the green peloids and pore-
filling ferroan calcite cement.
Figure 5 - Part of the conventional cores cut in Kerang-1 showing strong bioturbation with erosional surface
present in the Telisa Sandstone sequence.
8/16/2019 sumatera basin.pdf
16/20
116
Figure 6 - Southwest-northeast biostratigraphic controlled stratigraphic cross-section across the Kaji-Semoga,
Langkap, and Kerang with a datum along the highest gamma ray values. Note that the
transgressively incised shoreface sandstones may rest on the amalgamation surface of the
ravinement surface and lowstand sequence boundary. Part of the lowstand shoreface sediments has
been eroded resulting in lowstand depositional remnants. Line of cross-section is approximately 20
km long, and wells are not proportionally spaced.
Figure 7 - Geological model of the Telisa Sandstone deposition is apparently consistent to the preliminary
interpretation of the regional high-resolution sequence stratigraphy. The isolated shallow marine
sandstone bodies could be a mix of the lowstand shoreface remnants and transgressively incised
shoreface sediments.
8/16/2019 sumatera basin.pdf
17/20
117
Figure 8 - A southwest-northeast interpreted seismic line across the Kaji-Semoga High region. The seismic
line is flattened at the Upper Telisa Marker showing the possibility of the Kaji-Semoga High as a
local sediment provenance. The Telisa Sandstone is in direct contact to the limestones of the
Baturaja Formation and/or basement as hydrocarbon conduits.
Figure 9 - West-east section of the interpreted regional 2-D seismic line across the Palembang High region.
The structure is continuously and consistently shallowing towards the Sunda Landmass to the east.
The hydrocarbons are trapped in the Telisa Sandstone in stratigraphic play.
8/16/2019 sumatera basin.pdf
18/20
118
Figure 10 - Correlation between EEI for varying angle (-900 to 900) with well log data at KS-26.
Figure 11 - Location map of the selected 2-D seismic lines and wells with cross-plot between λ /µ (Lambda/Mu)
and P-impedance anomalies in various regions.
8/16/2019 sumatera basin.pdf
19/20
119
Figure 12 - Traditional seismic display across KS-01, KS-26, and KS-28 wells.
Figure 13 - EEI inversion display of µρ (Mu-Rho) across KS-01, KS-26, and KS-28 wells.
8/16/2019 sumatera basin.pdf
20/20
Figure 14 - EEI inversion map of λ /µ (Lambda/Mu) anomaly at the top Telisa Sandstone (20 ms window)across Kaji, Semoga, Langkap, and Kerang Fields. The map shows distribution of the predicted
sandstones.