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AUTHORS
Andrew D. Hanson � Department of Geoscience,University of Nevada Las Vegas, 4505 S. MarylandParkway, Box 454010, Las Vegas, Nevada 89154-4010; andrew.hanson@unlv.edu
Andrew Hanson is an associate professor at the Uni-versity of Nevada Las Vegas. He conducts researchrelated to oil and source rock organic geochemistry,extensional basin analyses in the southwesternUnited States, and hydrocarbon-migration issuesassociated with salt structures. He has a Ph.D. ingeological sciences from Stanford University.
Bradley D. Ritts � Chevron Energy TechnologyCompany, 6001 Bollinger Canyon Road, SanRamon, California 94583; britts@indiana.edu
Bradley Ritts received his Ph.D. in geological sciencesfrom Stanford University in 1998. He worked as anexploration geologist for Chevron Overseas Petro-leum in 1998 and 1999 and then moved to Utah StateUniversity as an assistant professor. In 2005, Rittswas appointed the Robert R. Shrock Professor ofGeological Sciences at Indiana University. Beginningin September 2007, Ritts rejoined Chevron EnergyTechnology Company in San Ramon, California. Hisresearch expertise is in regional interpretation ofsedimentary basins, continental tectonics, and clasticsedimentology.
J. Michael Moldowan � Department of Geo-logical and Environmental Sciences, StanfordUniversity, Stanford, California 94305-2115;moldowan@pangea.stanford.edu
Mike Moldowan attained a B.S. degree in chemistryfrom Wayne State University (1968) and a Ph.D. inchemistry from the University of Michigan (1972).After a postdoctoral fellowship at Stanford Univer-sity, he joined Chevron in 1974, where he devel-oped technology related to biomarkers. Since 1993,Moldowan has been a research professor in Stan-ford University’s Department of Geological and En-vironmental Sciences.
ACKNOWLEDGEMENTS
Financial support for this research was provided bya University of Nevada Las Vegas New InvestigatorAward (to A. D. Hanson), the National Science Foun-dation under Grant 0604443 (to B. D. Ritts), andthe Donors of the American Chemical Society Petro-leum Fund (PRF 38900-B8 to B. D. Ritts). DavidZinniker and Fred Fago provided support in theStanford Molecular Organic Geochemistry Labora-tory. Reviews by Les Magoon, Ben Dattilo, andtwo anonymous reviewers greatly improved themanuscript.
Organic geochemistry of oil andsource rock strata of the OrdosBasin, north-central ChinaAndrew D. Hanson, Bradley D. Ritts, andJ. Michael Moldowan
ABSTRACT
Paleozoic and Mesozoic strata and a suite of oil samples from wells
in the Ordos Basin were studied to determine which strata are source
rocks for oil produced in the basin. Analyses included total organic
carbon, Rock-Eval pyrolysis, vitrinite reflectance, and conventional
biomarker analyses on source rock extracts.
Results reveal that Carboniferous coal and organic-rich fluvial-
deltaic mudstone samples appear to be gas prone and mature to
overmature. Both Upper Triassic and Middle Jurassic lacustrine
mudstone samples contain organic matter of sufficient quantity and
good quality to be slightly immature or to have low thermal ma-
turity. Oil-oil correlations result in the establishment of one genetic
family that can be divided into subfamilies based on degree of oxi-
city in the source environment, differences in thermal maturity,
and differences in clay versus carbonate content of the source rock.
An oil-source rock correlation is established between produced oil
and Upper Triassic source rock strata. Vitrinite data indicate that
the source rock is more thermally mature in the western part of the
basin than in the east. These results should drive future exploration
strategies for the basin.
A bitumen vein is classified as pre-oil solid bitumen using bio-
marker data. Age-related biomarkers suggest it is derived from a
pre-Jurassic source rock. Similar veins in other basins globally are
linked to very rich source rocks.
INTRODUCTION
China’s first oil discovery (in 1907) was made in the Ordos Basin,
and modern oil exploration and production (including use of seis-
mic data and rotary drilling) began in the basin in the 1950s (Li et al.,
AAPG Bulletin, v. 91, no. 9 (September 2007), pp. 1273–1293 1273
Copyright #2007. The American Association of Petroleum Geologists. All rights reserved.
Manuscript received November 18, 2004; provisional acceptance February 2, 2005; revised manuscriptreceived January 16, 2007; final acceptance May 4, 2007.
DOI:10.1306/05040704131
1992, Yang et al., 2005). By 1992, 7500 wells had been
drilled in several parts of the basin (Yang et al., 1992).
More recently, large gas fields (e.g., the Sulige gas field
with proven reserves of 220 billion m3 [7.7 tcf]) have
been discovered in the central Ordos Basin (Xie, 2004).
In the past decade, there has been interest in, and ac-
tivity related to, coalbed methane in the Ordos Basin
(Jenkins et al., 1999).
Despite the long history of oil, gas, and coalbed
methane exploration and production, several impor-
tant issues regarding aspects of the Ordos Basin pe-
troleum system remain unanswered. For example, no
oil-source rock correlations have been published, and
only minimal organic geochemical analyses have been
reported from oil produced in the basin (Yang et al.,
1992). Additional unconstrained issues in the basin are
related to which strata serve as the source rock for the
produced hydrocarbons and the thermal maturity of
various potential source rock strata.
Results of analyses conducted for this study are
presented, which bear on some of these issues as they
relate to oil. Namely, this study reports the results of
geochemical analyses of strata that are possible source
rocks for oil. Also presented are molecular organic geo-
chemical results fromoil and source rock samples, which
allow for oil-oil correlations and an oil-source rock cor-
relation. Other new data (vitrinite reflectance, thermal
alteration indices, etc.) also help to constrain the ther-
mal history of different strata in the basin and, thus,
additionally bear on issues related to gas and coalbed
methane exploration.
Geologic Background
The Ordos Basin sits in north-central China and is one
part of the North China block (Figure 1) (Yang et al.,
1992). The basin is floored by Archean and Proterozoic
continental crust, which is overlain by Cambrian and
Ordovician carbonates deposited in shallow-marine set-
tings (Yang et al., 1986). A significant regional uncon-
formity overlies the Ordovician section such that no
Silurian or Devonian strata are present (Yang et al.,
1992; Liu et al., 1997) (Figures 2, 3). Carboniferous
strata consist mainly of thin shallow-marine limestone
and thick fluvial-deltaic deposits that are overlain by
fluvial Permian strata. Triassic and Jurassic strata con-
sist of fluvial and lacustrine deposits (Li et al., 1995;
Liu, 1998). Cretaceous strata are fluvial and eolian
redbeds (Li et al., 1995). The only Tertiary strata that
exist in the Ordos Basin occur within grabens that en-
circle the Ordos Basin (Zhang et al., 1998) and Quater-
nary loess and alluvium (Ding et al., 2001). The maxi-
mum thickness of the stratigraphic section in the Ordos
Basin is in excess of 10 km (6 mi) (Yang et al., 2005).
Structurally, the central part of the Ordos Basin
has been relatively stable throughout the Phanero-
zoic despite persistent deformation around the mar-
gins (Zhang, 1989; Liu, 1998; Darby and Ritts, 2002).
Strata in the eastern part of the basin are relatively
flat lying or dip gently to the west (Figure 3); there-
fore, the most complete outcrop sections are exposed
along the eastern side of the basin near the YellowRiver
(Figure 1).
Several structural elements surround the central
part of the Ordos Basin. To the south and east, older
strata are covered by Cenozoic fill of theWeihe-Shanxi
grabens (Figure 1) (Zhang et al., 1998). Farther south
are the Qinling Shan (Shan means mountain in Chi-
nese), which are partially the product of the Triassic
collision of the North and South China blocks (Yang
et al., 1991; Enkin et al., 1992; Meng and Zhang, 1999).
Along thewest side of the basin are theYinchuan graben,
Helan Shan, Zhuozi Shan, and Liupan Shan (Zhang
et al., 1991; Zhang et al., 1998; Darby and Ritts, 2002)
(Figure 1). North of the Ordos Basin is the Hetao Basin
(Zhang et al., 1998), and the Daqing Shan (Figure 1)
(Darby et al., 2001; Ritts et al., 2001). To the east, Ar-
chean and Proterozoic metamorphic rocks crop out.
Previous Analyses
According to Li et al. (1992), several source rock in-
tervals, reservoir rocks, and regional seals are present
in the basin (Figure 2). Yang et al. (1992a) identified
as many as nine potential source rock intervals with-
in Proterozoic to lower Paleozoic marine carbonate,
Carboniferous and Permian coal deposited in paralic
sequences, and Mesozoic lacustrine strata. However,
reported total organic carbon (TOC) values for Pro-
terozoic to lower Paleozoic marine carbonates have
all been low (Dai and Xia, 1990), indicating that they
lack source rock potential. Upper Paleozoic strata de-
scribed as coals have low TOC values, and the petro-
leum generation potential is very low (Yang et al.,
1992). Li (1990) and Yang et al. (1992a) pointed to two
Mesozoic intervals as source rocks: Upper Triassic black
lacustrine shale of the Yanchang Formation (TOC =
1.56–1.87 wt.%) as an oil source and Lower Jurassic
coal and mudstone of the Yanan Formation (TOC =
2.32–2.5 wt.%) as a gas source rock. Despite the higher
1274 Ordos Oils and Source Rock Geochemistry
TOCvalues, Li et al. (1992) recognized that the organic
content of Upper Triassic lacustrine strata is much bet-
ter than Jurassic lacustrine strata, and that Jurassic strata
are in the low-mature to mature stage. Hence, Li et al.
(1992) considered the Jurassic strata to be a poor oil
source rock.
Song (1988, p. 371), based on a rather crude cor-
relation, presumed that the oil source rock in the basin
is Triassic lacustrine strata. Jiang (1988) reported recov-
ery of actual Late Triassic spores and pollen typical of
lacustrine environments in oil samples recovered from
Jurassic reservoir rock and stated that the source for the
oil was probably Late Triassic strata, but that the Juras-
sic Yanan Formation might be a secondary source rock.
However, no geochemical correlations have shown
which strata generated produced oils in the basin.
Figure 1. Geologicmap of the Ordos Basin(modified from Li et al.,1992); inset map (fromWatson et al., 1987)shows the location of theOrdos Basin in the NorthChina block (NCB), north-central China. Much ofthe central Ordos Basin iscovered by Pleistocene–Quaternary loess, which isnot shown on this map.Oil sample sites related tothis study are shown bytriangles. Major citiesare indicated with solidcircles.
Hanson et al. 1275
Thermal-maturity data from potential Ordos Basin
source horizons are limited. Yang et al. (1992a) re-
ported vitrinite reflectance (Ro) values of 0.57–0.93%
for Triassic samples, but did not report where the sam-
ples came from.
Solid bitumen veins have been overlooked in the
Ordos Basin. However, during the course of this study,
a solid bitumen vein was found in one of the Juras-
sic stratigraphic sections in the southeastern Ordos
Basin. This finding is significant with regard to oil po-
tential because the best documented solid bitumen
veins occur in the highly petroliferous, Green River
and Uinta basins of North America (Curiale, 1985).
The Green River and Uinta basins contain laminat-
ed lacustrine strata with very elevated TOC values
and are the source rocks for large accumulations in
those basins (Cross and Wood, 1976; Palacas et al.,
1989).
Figure 2. Stratigraphy of the Ordos Basin showing potential source rock intervals and previously obtained TOC values.
Figure 3. Cross section (east-west) of the central part of the Ordos Basin (from Yang et al., 2005; used with permission from theAAPG whose permission is required for further use). Note that immature and marginally mature rocks with source rock quality in theeastern Ordos Basin dip westward where they may be more thermally mature.
1276 Ordos Oils and Source Rock Geochemistry
METHODS
Source Rock Screening
Rock samples suspected of having source potential,
based on color or sedimentologic indicators, were col-
lected from outcrops in the field when encountered.
Fresh samples were sent to Humble Geochemical Ser-
vices for initial screening, which consisted of TOC (Leco
TOC, in wt.%) content measurements and Rock-Eval
analyses. Based on the initial screening, potential source
rocks were chosen for more detailed molecular geo-
chemical analyses. Vitrinite reflectance analyses were
also performed at Humble Geochemical Services on
12 samples.
Molecular Organic Geochemical Methods
Suspected source rocks and the solid bitumen vein
sample were crushed using a mortar and pestle. Bitu-
men within the samples was extracted using a Soxhlet
apparatus and a mixture of methanol (66%) and tol-
uene (34%) for 4 hr. Weighed fractions of source rock
extract and whole oil were diluted 100� with hexane
and then analyzed via standard (n-C12 and higher) gas
chromatography (GC) on a Hewlett-Packard 5890A
gas chromatograph. The column was a 22m DB-1 col-
umn with an i.d. of 0.20 mm coated with a 0.33 mmmethyl silicone film. A splitless injection was usedwith
the purge valve off for 2 min. The carrier gas was hy-
drogen with a 20 psi head pressure. The initial start-
ing temperature was 80jC for 0.5 min, followed by a
programmed temperature ramp of 10jC/min until a
final temperature of 320jC was reached and held for
15.5 min.
The remaining fractions of the source rock and
bitumen vein extracts, as well as oil samples, were sub-
sequently separated using glass columns with a 10 mL
inner diameter filled with silica gel that was flushed
with hexane to remove the saturate fraction, followed
by a methylene chloride flush to remove the aromatic
fraction. Saturate fractions were then treated with high
Si/Al ZSM-5 zeolite (‘‘silicalite’’) to remove normal
alkanes. All saturate and aromatic fractions were ana-
lyzed on a Hewlett-Packard GC–mass selective detector
(GC-MSD). Sulfur precipitates that were present in
extracts of Triassic source rock were removed using ac-
tivated copper prior to the analyses. Selected ion moni-
toring of them/z 191, 217, 218, 231, 259, 245, and 253
was performed. All of the above analyses were com-
pleted in the Molecular Organic Geochemistry Labo-
ratory at StanfordUniversity. A small subset of samples
was run on the Stanford Autospec in the metastable
reaction monitoring GC–mass spectrometry (MRM-
GCMS) mode to determine if C30 steranes were pre-
sent in the samples and also to be able to calculate tet-
racyclic polyprenoid (TPP) ratios as defined by Holba
et al. (2000). Diamondoid analyses were run follow-
ing the same GC-MS procedure used by Dahl et al.
(1999) using deuterated diamondoid internal standards
to provide accurate concentrationmeasurements at sub-
ppm levels. A well-characterized standard routinely em-
ployed in the lab was run with samples in this study,
thus allowing compound determinations by comparing
the results to the standard. All calculated biomarker
ratios are based on peak height measurements.
RESULTS AND DISCUSSION
Potential Source Rocks
All of the lower Paleozoic carbonates examined in the
field during this study appeared to be organically lean
based on visual assessment, and none were sampled for
this study. Instead, Carboniferous, Permian, Triassic,
and Jurassic outcrop samples from the eastern and west-
ern margins of the Ordos Basin were collected and ana-
lyzed to assess their potential as hydrocarbon source
rocks. Source rocks included in this study are indi-
cated in Figure 2 and listed in Table 1. Table 2 sum-
marizes organic petrographic analyses for a subset of
the source rock samples, including degree of thermal
alteration, kerogen type, and palynofacies. Calculated
biomarker ratios for the source rocks, the bitumen vein
sample, and the oils are provided in Table 3.
Source Rock TOC and Rock-Eval Pyrolysis
Total organic carbon content andRock-Eval analysiswere
performedon59potential source rock samples (Table 1).
Most samples are mudstone deposited in lacustrine set-
tings. Other samples include mudstone and coal that
were deposited in fluvial or deltaic environments and two
limestone samples (one lacustrine, one shallow marine).
When using TOC as a discriminator of source
rock–generating potential as defined by Peters (1986),
25 samples had TOC values more than 2 wt.% and in-
dicate very good generative potential. The highest TOC
values (24.3 and 43.1 wt.%) were measured on Upper
Triassic black, laminated lacustrine mudstone samples
Hanson et al. 1277
Table 1. List of Potential Source Rock Samples Included in This Study along with TOC, Rock-Eval, and Vitrinite Reflectance Analyses Results
Location Notes
Region
Sample
Name Age Lithology Latitude Longitude TOC* S1** S2y S3
yyTmax
z
(jC)Cal.
Ro (%)zz
Measured
Ro (%) HIx OIxx S1/TOC PI{ Checks{{ Pyrogram£
Western 01RU1 Upper Triassic Mudstone 39j01.970 106j00.910 1.29 0.02 0.00 0.14 �1££ �1.00 0 11 2 1.00 f
Western 01RU2 Upper Triassic Mudstone 39j02.030 106j00.870 2.58 0.03 0.02 0.30 300££ �1.00 1 12 1 0.60 f
Western 01RU3 Upper Triassic Mudstone 39j02.080 106j00.840 2.36 0.01 0.02 0.40 447££ 0.89 1 17 0 0.33 f
Western 01RU4 Upper Triassic Mudstone 39j02.130 106j00.810 3.24 0.02 0.08 0.63 476££ 1.41 2 19 1 0.20 f
Western 01RU5 Upper Triassic Mudstone 39j02.180 106j00.790 1.25 0.00 0.02 0.45 401££ 0.06 2 36 0 0.00 f
Western 01RU6 Upper Triassic Mudstone 39j02.230 106j00.760 2.27 0.02 0.10 0.31 582££ 3.32 4 14 1 0.17 f
Western 01RU7 Upper Triassic Mudstone 39j02.290 106j00.720 2.02 0.01 0.02 1.38 420££ 0.40 1 68 0 0.33 f
Western 01RU8 Upper Triassic Mudstone 39j02.340 106j00.690 3.63 0.02 0.14 0.97 580££ 3.28 4 27 1 0.13 f
Western 01RU9 Upper Triassic Mudstone 39j02.390 106j00.640 5.64 0.03 0.08 2.17 484££ 1.55 1 38 1 0.27 f
Western 01RU10 Upper Triassic Mudstone 39j02.450 106j00.610 0.55 0.14 0.03 0.06 335££ �1.00 5 11 25 0.82 c f
Western 01RU11 Lower Jurassic Coal 39j02.560 106j00.550 15.76 0.38 12.62 0.31 547 2.69 1.89 80 2 2 0.03 c htS2p/cont.
Western 01RU12 Lower Jurassic Mudstone 39j02.530 106j00.510 2.51 0.02 0.08 0.28 578££ 3.24 3 11 1 0.20 f
Western 01RU13 Lower Jurassic Mudstone 39j02.580 106j00.480 0.76 0.01 0.01 0.00 384££ �1.00 1 0 1 0.50 f
Western 01RU14 Lower Jurassic Mudstone 39j02.600 106j00.460 1.39 0.02 0.02 0.09 423££ 0.45 1 6 1 0.50 f
Western 01RU15 Lower Jurassic Mudstone 39j02.610 106j00.450 2.41 0.02 0.04 0.35 513££ 2.07 2 15 1 0.33 f
Western 01RU111 Upper Triassic Coal 39j02.390 105j55.190 7.17 0.01 0.48 2.45 579££ 3.26 1.11 7 34 0 0.02 f
Western 01DK83 Upper Triassic Silty
mudstone
39j02.580 106j15.700 0.58 0.02 0.01 0.02 359££ �1.00 2 3 3 0.67 c f
Western 01DK84 Upper Triassic Coal 39j02.580 106j15.700 15.81 0.02 0.33 9.94 549££ 2.72 1.15 2 63 0 0.06 c f
Western 01DK85 Upper Triassic Silty
mudstone
39j02.580 106j15.700 2.89 0.01 0.01 0.41 404££ 0.11 0 14 0 0.50 f
Western 01DK86 Upper Triassic Silty
mudstone
39j02.580 106j15.700 0.58 0.02 0.12 0.17 481££ 1.50 21 29 3 0.14 c f
Western 01DK88 Upper Triassic Mudstone 39j02.580 106j15.700 2.98 0.01 0.00 0.81 �1££ �1.00 0 27 0 1.00 f
Western 01DK89 Upper Triassic Coaly
mudstone
39j02.580 106j15.700 0.40 0.02 0.15 0.16 547££ 2.69 0.95 38 40 5 0.12 c f
Western 01DK91 Upper Triassic Mudstone 39j02.580 106j15.700 1.08 0.01 0.00 0.00 �1££ �1.00 0 0 1 1.00 f
Western 01DK92 Upper Triassic Mudstone 39j02.580 106j15.700 0.85 0.05 0.10 0.08 469££ 1.28 12 9 6 0.33 c f
Western 01DK93 Upper Triassic Mudstone 39j02.580 106j15.700 0.89 0.01 0.02 0.00 300££ �1.00 2 0 1 0.33 c f
Western 01DK94 Upper Triassic Mudstone 39j02.580 106j15.700 1.99 0.02 0.00 0.27 �1££ �1.00 0 14 1 1.00 f
Western 01DK96 Upper Triassic Mudstone 39j02.580 106j15.700 4.31 0.02 0.05 0.90 417££ 0.35 1 21 0 0.29 f
Western 01DK97 Upper Triassic Mudstone 39j02.580 106j15.700 1.92 0.02 0.03 0.16 356££ �1.00 2 8 1 0.40 f
Western 01DK98 Upper Triassic Mudstone 39j02.580 106j15.700 1.82 0.02 0.03 0.12 359££ �1.00 2 7 1 0.40 f
Western 01DK99 Upper Triassic Limestone 39j02.580 106j15.700 0.24 0.02 0.07 0.00 300££ �1.00 29 0 8 0.22 f
Western 01DK100 Upper Triassic Mudstone 39j02.580 106j15.700 2.01 0.03 0.06 0.12 424££ 0.47 3 6 1 0.33 f
Western 01DK101 Upper Triassic Mudstone 39j02.580 106j15.700 4.25 0.04 0.10 0.37 453££ 0.99 2 9 1 0.29 f
1278
Ordos
Oils
andSource
RockGeochem
istry
Western 01DK103 Upper Triassic Mudstone 39j02.580 106j15.700 2.17 0.01 0.07 0.18 363££ �1.00 3 8 0 0.13 f
Western 01DK104 Upper Triassic Mudstone 39j02.580 106j15.700 3.30 0.02 0.02 0.45 499££ 1.82 1 14 1 0.50 f
Western 01ND105 Middle
Carboniferous
Mudstone 39j10.620 106j37.100 3.08 0.00 0.33 1.42 480££ 1.48 11 46 0 0.00 f
Western 01ND106 Middle
Carboniferous
Coaly
mudstone
39j10.620 106j37.100 3.58 0.01 0.20 0.81 535££ 2.47 6 23 0 0.05 f
Western 01ND107 Middle
Carboniferous
Coal 39j10.620 106j37.100 12.35 0.08 8.39 0.09 518 2.16 2.08 68 1 1 0.01 n
Western 01ND108 Middle
Carboniferous
Coal 39j10.620 106j37.100 1.46 0.01 0.10 0.04 495££ 1.75 2.25 7 3 1 0.09 n
Western 01ND109 Middle
Carboniferous
Coal 39j10.620 106j37.100 6.91 0.11 7.91 0.27 584 3.35 114 4 2 0.01 c htS2p
Western 01ND110 Middle
Carboniferous
Coal 39j10.620 106j37.100 3.67 0.07 0.44 0.01 582££ 3.32 12 0 2 0.14 n
Southeast 00TC40 Middle Triassic Coal 36j03.060 110j07.380 10.63 0.04 0.52 4.77 523 2.25 0.60 5 45 0 0.07 n
Southeast 01JP116 Middle Jurassic Coal 35j18.500 108j54.820 47.32 3.33 102.83 3.59 426 0.51 0.51 217 8 7 0.03 c n
Southeast 01JP117 Middle Jurassic Bitumen 35j18.870 108j54.770 43.10 1.20 118.20 4.32 412 0.26 274 10 3 0.01 c n
Southeast 01TC119 Upper Triassic Mudstone 35j15.540 108j58.940 24.29 10.33 96.91 2.40 436 0.69 399 10 43 0.10 c n
Southeast 01TC120 Upper Triassic Mudstone 35j15.540 108j58.940 46.94 18.25 206.52 1.92 436 0.69 440 4 39 0.08 c n
East-central 01YA130 Middle–Upper
Jurassic
Mudstone 36j40.960 109j09.650 2.18 0.17 9.06 0.24 437 0.71 416 11 8 0.02 c n
East-central 01YA131 Middle–Upper
Jurassic
Mudstone 36j40.960 109j09.650 1.03 0.11 2.30 0.84 435 0.67 223 82 11 0.05 n
East-central 01YA132 Middle–Upper
Jurassic
Mudstone 36j40.960 109j09.650 1.21 0.03 1.05 0.39 435 0.67 0.49 87 32 2 0.03 n
Eastern 01YI137 Upper Triassic Mudstone 36j03.060 110j07.380 2.07 0.02 0.55 1.13 444 0.83 27 55 1 0.04 n
Eastern 01YI138 Upper Triassic Coaly
mudstone
36j03.060 110j07.380 3.24 0.02 0.43 1.33 468££ 1.26 0.64 13 41 1 0.04 n
Eastern 01YI139 Upper Triassic Mudstone 36j04.120 110j11.990 3.12 0.03 0.93 1.66 444 0.83 30 53 1 0.03 n
Eastern 01YI140 Upper Triassic Mudstone 36j04.120 110j11.990 3.35 0.19 3.28 0.99 442 0.80 98 30 6 0.05 n
Eastern 01LL150 Middle
Carboniferous
Mudstone 37j33.470 110j53.900 0.72 0.05 0.10 0.01 468££ 1.26 14 1 7 0.33 n
Eastern 01LL151 Middle
Carboniferous
Mudstone 37j33.460 110j53.900 1.86 0.27 1.01 0.04 468 1.26 54 2 15 0.21 n
Eastern 01LL152 Middle
Carboniferous
Coal 37j33.470 110j53.790 55.28 2.99 80.73 1.85 482 1.52 1.37 146 3 5 0.04 c n
Eastern 01LL153 Middle
Carboniferous
Mudstone 37j33.450 110j53.710 1.48 0.03 0.16 0.65 500££ 1.84 11 44 2 0.16 n
Eastern 01LL154 Upper
Carboniferous
Limestone 37j33.440 110j53.680 0.18 0.01 0.03 0.05 539££ 2.54 17 28 6 0.25 c n
Hansonetal.
1279
of the Yanchang Formation from near Tongchuan in
southeastern Ordos Basin (Figures 1; 4A, B).
Geochemical parameters that indicate what type
of hydrocarbon would be generated (hydrogen index,
S2/S3) suggest that most source rock samples would
generate gas if they were to generate any hydrocarbons.
On a pseudo-Van Krevelen diagram (Figure 5), most
samples plot near the origin or along the x-axis, indi-cating that they are not oil source rocks.However, some
samples plot upward along the y-axis, indicating that
they could generate both oil and gas or just oil. Samples
with the best organic quality include the Upper Trias-
sic mudstone samples of the Yanchang Formationmen-
tioned earlier with highTOCvalues (samples 01TC119
and 01TC120) (Figure 4A, B) as well as one Middle–
Upper Jurassic mudstone from the lacustrine Anding
Formation fromeasternOrdos Basin (sample 01YA130)
(Figure 4C).
Vitrinite Reflectance Results
Vitrinite reflectance analyses were completed on 12 rock
samples (Table 2). TheRo values for six samples from the
western Ordos Basin (0.95–2.25) are, on average, more
mature than the six from the easternOrdos Basin (0.51–
1.37). The Ro values from Upper Triassic and Lower
Jurassic strata of the western Ordos Basin (0.95–1.89)
are in the oil-condensate range, and the Carboniferous
samples are in thedry-gas range of the oilwindow (2.08–
2.25). In contrast, the Ro results from the eastern Ordos
Basin indicate that the Middle Jurassic strata are at the
top of the oil window (0.49–0.51), the Triassic samples
are in the early-mature window (0.60–0.64), and the
Carboniferous strata are in the late mature to early con-
densate window (1.18–1.37). The Upper Triassic source
rock samples with the best potential for generating
liquid hydrocarbons are close to the oil-generating win-
dow whereas the Jurassic samples analyzed with good-
quality organic matter analyzed in this study may still
be slightly immature. However, these may be effective
source rocks that were buried deeper in the basin.
Source Rock Molecular Organic Geochemistry Results
Carboniferous strata yielded TOC and Rock-Eval re-
sults that indicate that they lack sufficient organic qual-
ity to be oil source rocks, and most samples have ker-
ogens that are dominated by coaly fragments. Although
these rocks are thermally overmature andmayhave had
higher TOC content earlier, the dominance of coaly
fragments suggests that they did not generate oil, andEastern
01LL155
Upper
Carboniferous
Coal
37j33.42
0110j53.590
47.80
2.80
101.85
2.16
471
1.32
1.18
213
56
0.03
cn
Eastern
01LL156
Lower
Perm
ian
Mudstone
37j33.42
0110j53.160
1.93
0.08
1.00
0.50
477
1.43
5226
40.07
n
*TOC=totalorganiccarbon.
**S 1
=milligramsof
hydrocarbons
that
canbe
thermallydistilled
from
1gof
rock.
y S2=milligramsof
hydrocarbons
generatedby
pyrolytic
degradationof
thekerogenin
1gof
rock.
yyS 3
=milligramsof
carbon
dioxidepergram
ofrock.
z Tmax=thetemperature
atwhich
themaximum
amount
ofS 2
hydrocarbons
aregenerated(injC
).zzCal.R o
(%)=calculated
vitrinite
reflectance
basedon
Tmax.
x HI=hydrogen
index=S 2
�100/TO
C.
xxOI=oxygen
index=S 3
�100/TO
C.
{ PI=productionindex=S 1/(S 1
+S 2).
{{c=analysischeckedandconfirmed.
£ n=norm
al,htS2p=high
temperature
S 2peak;f=flat;cont.=contam
inates.
££�1indicatesnotmeasuredor
meaningless
ratio.Tmaxdata
arenotreliablebecauseof
poor
S 2peak.
Table
1.Continued
Location
Notes
Region
Sample
Nam
eAge
Lithology
Latitude
Longitude
TOC*
S 1**
S 2y
S 3yy
Tmaxz
(jC)
Cal.
R o(%
)zz
Measured
R o(%
)HIx
OIxx
S 1/TOC
PI{
Checks{
{Pyrogram
£
1280 Ordos Oils and Source Rock Geochemistry
nomolecular geochemistry analyseswere performed on
them. These strata are likely sources for gas accumula-
tions in the basin.
Source Rock GC Results
Gas chromatography results for the Upper Triassic Yan-
chang Formation mudstone samples (samples 01TC119
and 01TC120) and Middle–Upper Jurassic Anding
Formation mudstone (01YA130) show that they con-
tain well-developed n-alkanes and exhibit a clear odd/
even preference (OEP) in their n-alkane distributions
(Figure 6A; Table 3) (Peters and Moldowan, 1993).
Carotane compounds are absent in these samples, which
is important in that carotanes are key biomarkers for a
lacustrine source rock in the Qaidam Basin of north-
western China (Ritts et al., 1999; Hanson et al., 2001)
and the Jianghan Basin in eastern China (Peters et al.,
1996). Themost striking difference between the Triassic
and the Jurassic samples relates to the pristane/phytane
ratio (Pr/Ph): the Triassic ratios are 0.87 and 0.92, which
is indicative of an anoxic depositional environment (Fu
et al., 1990), whereas the Pr/Ph ratio for the Jurassic
sample is 3.50 (Table 3).
Source Rock GC-MSD Results
Terpanes
Chromatograms (m/z 191) with peaks related to dif-
ferent terpane compounds are shown in Figure 7A,
and biomarker ratios calculated from these chromato-
grams are provided in Table 3. In all samples, the rel-
ative peak heights of tricyclic terpanes are small com-
pared to the pentacyclic terpanes. Most samples have
C30 hopane as the dominant peak, show low gamma-
cerane peak heights, and have some, although low, pres-
ervation of the higher homohopanes.
The ratio of tricyclic to pentacyclic terpanes in-
creases with increasing thermal maturity (Seifert and
Moldowan, 1978) and the calculated tricyclic/penta-
cyclic ratios were low for all three source rocks. All
three samples have 22,29,30, 18a-trisnorneohopane(Ts) peaks that are significantly smaller than the
22,29,30, 17a-trisnorhopane (Tm) peaks. The ratio
of Ts/(Ts + Tm) is controlled by both source rock input
and thermal maturity (Peters and Moldowan, 1993),
and the ratios for Triassic source rock samples in this
study are low (0.13–0.14) and even lower in the Ju-
rassic source rock sample (0.09) (Table 3). The results
suggest that the Triassic samples are slightly more ma-
ture than the Jurassic sample.
Them/z 191 (Figure 7A) for the Triassic and Juras-
sic source rock samples show poorly preserved higher
homohopanes. Well-preserved higher homohopanes
only occur when anoxic conditions and dissolved sulfate
are present in the depositional environment in which
the source rockswere deposited (Peters andMoldowan,
1993). The very low C35 homohopane indices in all
samples indicate a probable freshwater depositional en-
vironment. The C35 homohopanes are observed in all
of the oil samples, indicating that the source rock was
deposited in at least suboxic conditions, but calculated
Table 2. Thermal Alteration, Kerogen Type, and Palynofacies Data for a Subset of the Source Rock Samples
Hanson et al. 1281
Table 3. Calculated Biomarker Ratios of the Triassic and Jurassic Source Rocks, the Bitumen Vein Sample, and the Oil Samples
Source Rock Samples Bitumen
Sample Number* 01TC119 01TC120 01YA130 01JP117 00YP25 00YP26 00JB27 00JB28 00MD36 01DS114 01DS115 01YA128 01YA133 01YA134 01YA135 01MD136 01YA141 01YA142 01YA143 Mean St Dev
m/z 191 Terpanes
C29Ts/(C29Ts + C29 Hopane) 0.13 0.18 0.28 0.07 0.25 0.33 0.27 0.28 0.31 0.51 0.51 0.78 0.31 0.00 0.28 0.29 0.29 0.29 0.32 0.33 0.16
C23 Tricyclic**/(C23tricyclic + C30 hopane)
0.06 0.06 0.02 0.02 0.03 0.03 0.03 0.04 0.08 0.04 0.03 0.74 0.08 0.31 0.05 0.06 0.07 0.08 0.09 0.12 0.18
Ts/(Ts + Tm) 0.13 0.14 0.09 0.04 0.30 0.48 0.33 0.46 0.54 0.73 0.73 0.81 0.53 0.20 0.48 0.47 0.45 0.49 0.48 0.50 0.15
C31 22S/(C31 22S + 22R)
Hopane
0.57 0.57 0.59 0.34 0.51 0.61 0.56 0.56 0.56 0.56 0.58 1.00 0.55 0.51 0.58 0.53 0.57 0.55 0.56 0.59 0.11
C32 22S/(C32 22S + 22R)
Hopane
0.56 0.55 0.55 0.13 0.59 0.59 0.61 0.61 0.58 0.60 0.61 0.50 0.58 0.57 0.63 0.59 0.59 0.56 0.58 0.59 0.03
C35 Hopane/(C31 � C35Homohopanes)
0.03 0.04 0.02 0.00 0.04 0.04 0.03 0.03 0.03 0.03 0.03 0.00 0.03 0.05 0.04 0.03 0.03 0.03 0.03 0.03 0.01
Moretane/(Moretane +
Hopane)
0.08 0.11 0.23 0.26 0.07 0.08 0.07 0.07 0.06 0.07 0.07 0.18 0.07 0.17 0.07 0.07 0.08 0.07 0.07 0.08 0.04
C29Ts/(C29Ts + C29 Hopane) 0.13 0.18 0.28 0.07 0.25 0.33 0.27 0.28 0.31 0.51 0.51 0.78 0.31 0.00 0.28 0.29 0.29 0.29 0.32 0.33 0.16
Gammacerane/
(Gammacerane +
C30 Hopane)**10
0.16 0.16 0.61 0.00 0.24 0.31 0.24 0.24 0.40 0.40 0.32 1.25 0.32 2.05 0.24 0.32 0.32 0.32 0.40 0.49 0.48
C24 Tetracyclic/(C24Tetracyclic + C26 Tricyclic)
0.63 0.70 0.75 0.90 0.60 0.50 0.60 0.50 0.43 0.43 0.43 0.31 0.47 0.26 0.50 0.50 0.50 0.70 0.50 0.48 0.11
C24 Tetracyclic/(C24Tetracyclic + C30 Hopane)
0.04 0.05 0.02 0.07 0.02 0.02 0.02 0.03 0.05 0.02 0.02 0.46 0.06 0.11 0.03 0.04 0.05 0.05 0.06 0.07 0.11
Diahopane/(Diahopane +
C30 Hopane)
0.01 0.02 0.06 0.00 0.03 0.06 0.04 0.07 0.11 0.21 0.21 0.85 0.12 0.06 0.06 0.09 0.09 0.10 0.12 0.15 0.20
Diahopane/(Diahopane +
C29 Hopane)
0.01 0.03 0.10 0.00 0.06 0.15 0.08 0.14 0.19 0.48 0.48 0.84 0.19 0.07 0.13 0.16 0.16 0.17 0.19 0.23 0.20
C29 Hopane/(C29 Hopane +
C30 Hopane)
0.38 0.39 0.37 0.42 0.32 0.27 0.32 0.32 0.34 0.23 0.22 0.53 0.36 0.44 0.31 0.34 0.35 0.36 0.36 0.34 0.07
C23/(C23 + C29)Tricyclic 0.80 0.73 0.67 0.50 0.54 0.57 0.57 0.63 0.61 0.45 0.36 0.61 0.55 0.60 0.60 0.57 0.60 0.59 0.60 0.56 0.07
Tricyclic/(Tricyclic +
Hopane)**
0.09 0.08 0.02 0.10 0.05 0.06 0.05 0.07 0.12 0.07 0.07 0.38 0.13 0.39 0.08 0.10 0.11 0.11 0.13 0.13 0.10
C25/C26 Tricyclic 1.00 1.00 1.00 2.00 1.00 0.67 1.00 0.50 0.63 0.75 0.50 0.67 0.75 0.80 0.50 0.80 0.67 2.00 0.75 0.80 0.35
Diahopane/(Diahopane +
C29Ts)
0.08 0.11 0.22 0.00 0.17 0.26 0.19 0.29 0.35 0.46 0.47 0.59 0.35 1.00 0.28 0.32 0.32 0.34 0.33 0.38 0.20
(28 + 29)tricyclic/(28 +
29)tricyclic + hopanes
0.03 0.03 0.02 0.02 0.04 0.07 0.05 0.07 0.13 0.14 0.14 0.75 0.13 0.12 0.08 0.10 0.10 0.12 0.13 0.14 0.17
GC
Pristane/(Pristane + Phytane) 0.46 0.48 0.78 na 0.54 0.56 0.53 0.56 0.54 0.52 0.53 0.63 0.55 0.40 0.55 0.51 0.55 0.53 0.54 0.54 0.04
Pristane/n-C17 0.66 0.56 0.55 na 0.43 0.36 0.35 0.23 0.21 0.45 0.39 0.14 0.24 0.57 0.28 0.21 0.24 0.22 0.24 0.30 0.11
Phytane/n-C18 0.76 0.65 0.16 na 0.37 0.28 0.31 0.18 0.19 0.41 0.32 0.08 0.20 0.60 0.24 0.20 0.19 0.20 0.21 0.26 0.12
Pr/Ph 0.87 0.92 3.50 na 1.16 1.27 1.14 1.28 1.17 1.10 1.13 1.71 1.22 0.67 1.24 1.06 1.24 1.11 1.17 1.18 0.20
1282
Ordos
Oils
andSource
RockGeochem
istry
Carotanes present no no no no no no no no no no no no no no no no no no no n/a n/
Odd/even preference 1
(OEP1)**
1.06 1.04 1.26 na 1.16 1.08 1.10 1.06 1.02 1.08 1.11 1.06 1.07 1.11 1.08 1.04 0.99 1.05 1.03 1.07 0.04
Odd/even preference 2
(OEP2)**
0.41 0.38 0.55 na 0.41 0.42 0.41 0.42 0.39 0.41 0.41 0.40 0.40 na 0.40 0.40 0.39 0.40 0.39 n/a n/
Biodegraded no no no yes no no no no no no no no no no mostly no no no no n/a n/
n-alkane with maximum
peak height
19 19 23 na 20 19 19 19 19 19 19 15 19 21 20 20 15 19 15 19 1.88
m/z 217 Steranes
Total C27/Total (C27 +
C28 + C29)
0.29 0.25 0.44 0.19 0.28 0.29 0.28 0.30 0.30 0.31 0.33 0.30 0.30 0.10 0.31 0.29 0.29 0.30 0.30 0.29 0.05
Total C28/Total (C27 +
C28 + C29)
0.32 0.31 0.23 0.05 0.32 0.31 0.34 0.34 0.31 0.27 0.27 0.29 0.32 0.46 0.32 0.32 0.33 0.33 0.33 0.32 0.04
Total C29/Total (C27 +
C28 + C29)
0.40 0.44 0.32 0.76 0.39 0.40 0.38 0.36 0.39 0.42 0.40 0.41 0.38 0.44 0.38 0.39 0.37 0.37 0.36 0.39 0.02
C29abb 20S + R/(aaa(20S + R) +
abb (20S + R))
0.32 0.23 0.19 0.24 0.38 0.45 0.41 0.53 0.55 0.55 0.54 0.63 0.58 0.57 0.55 0.52 0.54 0.55 0.55 0.53 0.08
C29aaa 20S/(S + R) 0.44 0.33 0.23 0.06 0.34 0.42 0.37 0.47 0.51 0.47 0.47 0.48 0.47 0.56 0.48 0.50 0.49 0.49 0.49 0.47 0.05
C27 ba Diasterane 20R +
S/(C27 aaa (20S + R) +
abb (20S + R))
0.06 0.05 0.70 0.07 0.13 0.26 0.15 0.15 0.14 0.58 0.62 0.54 0.15 0.15 0.17 0.13 0.12 0.12 0.12 0.23 0.18
m/z 245 Dinosteroids
3/3 + 4 + 6 (standard
definition from
Moldowan et al., 1996)
0 0 0.75 0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 n/a 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
3/3 + 5 (standard
definition from
Moldowan et al., 1996)
0 0 0.82 0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 n/a 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
m/z 253 Monoaromatics
%27s 0.27 0.28 0.23 0.02 0.22 0.25 0.22 0.27 0.28 0.28 0.29 0.16 0.29 0.11 0.28 0.28 0.30 0.30 0.32 0.26 0.06
%28s 0.34 0.34 0.40 0.10 0.40 0.36 0.39 0.34 0.32 0.31 0.32 0.41 0.30 0.38 0.34 0.31 0.32 0.33 0.31 0.34 0.04
%29s 0.39 0.39 0.36 0.88 0.37 0.39 0.39 0.40 0.39 0.41 0.39 0.43 0.41 0.51 0.38 0.41 0.38 0.38 0.37 0.40 0.04
Sum of C21 to C22monoaromatic-steroids
(MA(I))
38 30 15 4 53 53 57 74 78 62 72 25 76 13 68 77 77 77 77 63 20
Sum of all C27 to C29monoaromatic-steroids
(MA(II))
392 357 257 95.5 254 259 249 229 109 226 216 69 111 285 249 107 97 98 71 175 80
MA(I)/MA(I) + MA(II) 0.09 0.08 0.06 0.04 0.17 0.17 0.19 0.24 0.42 0.22 0.25 0.27 0.41 0.04 0.21 0.42 0.44 0.44 0.52 0.29 0.14
m/z 231 Triaromatics
Sum of C20 to C21triaromatic-steroids (TA(I))
20 15 12 9 45 53 50 82 61 74 72 0 70 10 83 68 74 72 65 59 24
Hansonetal.
1283
ratios are low, indicating that the depositional environ-
ment was not strongly anoxic (Table 3).
Key differences occur between the Triassic and
the Jurassic source rock samples. Diahopane is present
in all of the samples, but the relative peak heights in the
Triassic samples are lower than in the Jurassic sample.
Elevated relative amounts of diahopane are linked to
oxic-suboxic, clay-rich depositional environments (Mol-
dowan et al., 1991). The Jurassic source rock diahopane
ratio in this study is 0.06; the same ratio in the Triassic
samples is much lower at 0.01–0.02 (Table 3), which
suggests that the depositional environment of the Tri-
assic source rocks was more reducing than that of the
Jurassic rock.
Another difference between the Triassic and Juras-
sic samples relates to the relative abundance of more-
tane. Moretane converts to C30 hopane with increasing
thermal maturity (Seifert and Moldowan, 1980), and
thus,moretane decreases as thermalmaturity increases.
The calculated value for the Jurassic sample is 0.23,
whereas the ratios for the Triassic samples are 0.08–
0.11 (Table 3).
Triassic source rocks are uncommon worldwide;
thus, a comparison to Triassic source rocks from other
regions is warranted. Holba et al. (2002) studied the
well-known Triassic Shublik Formation of the North
Slope of Alaska and derived an extended tricyclic ter-
pane ratio (ETR) that clearly discriminated between
Triassic and Jurassic source rocks. Holba et al. (2002)
found that ETRs for Shublik-derived oils exceed 2.0,
whereas Jurassic ETRs have lower ratios. Triassic source
rock ETRs from the Ordos Basin are 0.8–1, whereas
the Ordos Basin Jurassic source rock ETR is 0.05. Al-
though the calculated ratios in this study are lower than
those reported by Holba et al. (2002), Ordos Triassic
ETRs are more than an order of magnitude higher than
the Jurassic ETR.
Steranes
A second group of important biomarkers is the sterane
compounds. The sterane chromatograms (m/z 217) forsource rock samples in this study are shown in Figure 8A.
A common way to distinguish different source rocks
or organic facies of different facies of the same source
rock (Peters andMoldowan, 1993) is through the use of
C27-C28-C29 sterane ternary plots. C27-C28-C29 ster-
anes were measured from the m/z 217 chromatograms
(Figure 8A), and results are given in Table 3. As shown
on the ternary plot in Figure 8B, the Triassic source rock
samples plot relatively close to each other, whereas theSumofallm
ajor
C 26to
C 29triaromatic-steroids
(TA(II))
177
175
151
14185
174
182
164
31118
101
036
172
166
3837
2818
9772
TA(I)/TA(I)+TA(II)ratio
0.10
0.08
0.07
0.39
0.20
0.23
0.22
0.33
0.66
0.39
0.42
n/a
0.66
0.05
0.33
0.64
0.67
0.72
0.78
0.45
0.23
Diamondoids(ppm
)0.27
1.19
0.11
0.53
0.6
MRM-GCM
S
Calculated
tertracyclic
polyprenoid(TPP)ratio
0.60
0.26
0.71
0.42
C30steranes
present?
nono
nono
Sub-family
assignment
(P=primaryfamily)
n/a
n/a
n/a
n/a
AP
AP
PB
BC
PD
PP
PP
Pn/a
n/a
*Tricyclic/(tricyclic
+hopane)=
(20,23,24,25,26,28,and
29tricyclics)dividedby
thesametricyclictotalplusT s,T
m,C
29hopane,29T
s,diahopane,C 3
0hopane,m
oretane,andallofthe
homohopanes.T
s=22,29,30,18a-trisnorneohopane;T
m=22,29,30,17a-trisnorhopane.
**Asdefined
inPeters
andMoldowan
(1993).
Table
3.Continued
Source
Rock
Samples
Bitumen
SampleNum
ber*
01TC11901TC12001YA13001JP11700YP25
00YP26
00JB27
00JB28
00MD36
01DS114
01DS115
01YA12801YA13301YA13401YA13501MD13601YA14101YA14201YA143MeanStDev
1284 Ordos Oils and Source Rock Geochemistry
Jurassic source rock is enriched in C27 steranes and is
easily distinguished from the Triassic samples.
Calculated ratios for two different sterane thermal-
maturity parameters were conducted in this study: the
C29 aaa 20S/(S + R) ratio and the C29 aaa 20S + R/
((aaa 20S + R)+(abb 20S + R)) (Peters andMoldowan,
1993) (Table 3). All of the source rock samples yielded
values that are well below the end point for these pa-
rameters, suggesting that they are low in the oil window
(Table 3).
An important distinction between the Triassic sam-
ples and the Jurassic sample is the relative abundance of
theC27 diasteranes (Figure 8A).Diasteranes are thought
to form as a result of clay-catalyzed rearrangement of
regular steranes (Rubenstein et al., 1975). Therefore,
elevated diasterane/regular sterane ratios are an indica-
tion of active clays being present in the source rock and,
thus, are an indicator of a clastic source rock. The dia-
sterane ratios for the Jurassic source rock are elevated
compared to Triassic samples (Figure 8A; Table 3).
Aromatics
Triaromatic methylsteroid ratios (measured on the m/z245 chromatograms) (Moldowan et al., 1996) aremark-
edly different between the Triassic and Jurassic samples
(Table 3). Specifically, Triassic source rock samples are
completely lacking in dinosteroids, resulting in calcu-
lated dinosteroid ratios that equal zero. However, cal-
culated ratios for the Jurassic source rock are 0.75 and
0.82, depending on which ratio is used (Table 3).
Figure 4. Triassic and Jurassic outcrops in Ordos. (A) Triassic organic-rich, thinly laminated lacustrine mudstones of the YanchangFormation (backpack in foreground is approximately 35 cm [13.7 in.] wide). (B) Close-up view of thinly laminated Triassic mudstones(lens cap is 4 cm [1.5 in.] wide). (C) Fish fossil within Jurassic lacustrine mudstones of the Anding Formation. (D) Jurassic coalmeasures near Tongchuan.
Hanson et al. 1285
Monoaromatic (MA(I)/MA(I) + MA(II)) and tri-
aromatic (TA(I)/TA(I) + TA(II)) ratios were calculated
as defined by Peters and Moldowan (1993) (Table 3).
Both of these ratios increase with higher thermal ma-
turity. When oil and source rock samples were plotted
on a crossplot using these two parameters (Figure 9), a
well-developed linear trend is apparent, and the source
rocks all plot in the area of low maturity on the fig-
ure, in agreement with the other thermal-maturity
indicators.
Oil Results
Fifteen oil samples taken from wells scattered across
the basin (Figure 1) were collected and analyzed. Oil
samples were mostly collected at unattended oil wells,
and subsurface data are generally unknown. Most sam-
ples are waxy, dark-brown to black oils; a few are ligh-
ter in color. All of the samples were liquids at the time
that they were collected but several solidified at room
temperature.
Oil GC Results
Representative GC traces for the oil samples are
shown in Figure 6B. One of the analyzed oils (sample
01YA134) is biodegraded (it was collected from a
small pool of oil adjacent to the well) and, thus, lacks
the n-alkane fraction and displays the characteristic
‘‘humpogram’’ of biodegraded oils. Other oil samples
are not biodegraded and have well-preserved n-alkane
envelopes that maximize in the n-C15 to n-C21 range
(Figure 6B; Table 3), exhibit a strong odd/even prefer-
ence, and extend out to the n-C32 to n-C40 range. Based
on the GC data, beta-carotane and gamma-carotane are
absent. The average ratio of Pr/Ph is 1.18, but calcu-
lated values range from0.67 to 1.71 (Figure 6B; Table 3).
The calculated value of 0.67 may be misleading be-
cause the sample is biodegraded. These characteristics
match the Triassic source rock samples much better
than the Jurassic sample.
Oil GC-MSD Results
Terpanes
Sample 01YA128 has a m/z 191 chromatogram
(Figure 7B) that is distinct from the other oil sam-
ples and is discussed separately below. All other oil
samples havem/z 191 traces that are quite similar. This
main group of oil samples mostly has high tricyclic/
hopane ratios, and C30 hopane is the largest peak on
all of them/z 191 chromatograms (Figure 7B; Table 3).
All of the oil samples, except 01YA128, have peaks
corresponding to the entire homohopane series, includ-
ing C35 homohopane peaks (Figure 7B). Similar to the
source rocks, the relative height of the homohopane
peaks decreases systematically, and the calculated C35
homohopane ratio is low (Table 3).
Diahopane ratios are relatively low except for
01DS114 and 01DS115 (Figure 7B; Table 3). The
Figure 5. Pseudo–Van Krevelen diagram for samples fromOrdos Basin.
1286 Ordos Oils and Source Rock Geochemistry
calculated ratios for most samples are between 0.05
and 0.08, but the ratios for 01DS114 and 01DS115 are
0.20 and 0.23, respectively (Table 3).
Moretane ratios are relatively low except for
01YA128 and 01YA134. The average value (excluding
01YA128 and 01YA134) is 0.07 (Table 3). Calculated
values for 01YA128 and 01YA134 are 0.18 and 0.17,
respectively (Table 3).
Ts/(Ts + Tm) ratios reveal variability among the
oil samples. Most samples have ratios that are rela-
tively similar (average values of 0.52) (Figure 7B;
Table 3). However, three samples (00YP25, 00JB27,
Figure 6. (A) Gas chromatograms for extracts of source rocks included in this study. (B) Gas chromatography traces forrepresentative oil samples. Numbers along the top indicate the number of carbon atoms in the n-alkane; Pr = pristane; Ph =phytane.
Hanson et al. 1287
Figure 7. (A) m/z 191 chromatograms for source rock samples. (B) m/z 191 traces for representative oil samples. Numbered peakscorrespond to different terpane biomarker compounds as follows: 1 = C23 tricyclic; 2 = C24 tetracyclic; 3 = Ts; 4 = Tm; 5 = C29 hopane;6 = diahopane; 7 = C30 hopane; 8 = moretane; 9 = C31 homohopanes; 10 = gammacerane; 11 = C32 homohopanes; 12 = C33homohopanes; 13 = C34 homohopanes; 14 = C35 homohopanes. One section of the chromatogram for 01TC119 is expanded andshows the C28 and C29 tricyclics and Ts (peak 3), which were used for calculating extended tricyclic terpane ratios (ETRs).
1288 Ordos Oils and Source Rock Geochemistry
Figure 8. (A) m/z 217 chromatograms for source rock samples. (B) Sterane ternary plot for oils (open circles), Triassic source rocksamples (filled circles), Jurassic source rock sample (open triangle), and the bitumen vein sample (filled square). (C) m/z 217 tracesfor representative oil samples. Numbered peaks correspond to different sterane biomarker compounds as follows: 15 = C27diasterane 20S; 16 = C27 diasterane 20R; 17 = C27 aaa20S; 18 = C27 abb20R; 19 = C27 abb20S; 20 = C27 aaa20R; 21 = C28 aaa20S;22 = C28 abb20R; 23 = C28 abb20S; 24 = C28 aaa20R; 25 = C29 aaa20S; 26 = C29 abb20R; 27 = C29 abb20S; 28 = C29 aaa20R.
Hanson et al. 1289
and 01YA134) have lower ratios, and two samples
(01DS114 and 01DS115) have higher ratios (Table 3).
Although C35 homohopanes are observed in all of
the oil samples (Figure 7B), the calculated homohopane
indices are low (<0.05) (Table 3), indicating that the
source rock depositional environmentwasmildly anoxic.
TheC31 homohopane isomerization index is linked
to diagenetic burial conditions and increases with ther-
malmaturity, reaching a final ratio of about 3:2 22S:22R
at the beginning of the oil window and changes little
with further thermal maturation (Peters and Moldo-
wan, 1993). The average value of the main population
of oil samples is consistent with values for top oil-
window maturity.
The tricyclic/pentacyclic ratios for oil samples are
low (<0.09) (Table 3) except for sample 01YA128,
which has a value of 0.74 (Table 3).
Steranes
Representative results of the sterane analyses are shown
in Figure 8C, and calculated ratios are given in Table 3.
All oil samples except one (sample 01YA134) cluster
(Figure 8B) with the Triassic source rocks.
Calculated values for the two sterane maturity pa-
rameters (Table 3) indicate that the oil samples arewell
below the end point for these thermal-maturity param-
eters. These results suggest the source rock that gen-
erated them was low in the oil window.
Aromatics
All of the oil samples in this study lack dinosteroids
(Table 3). This finding is similar to the Triassic source rock
samples and dissimilar to the Jurassic source rock sample.
Samples that plot toward the upper right in Figure 9
are more thermally mature, whereas samples plotting
near the origin are of low thermal maturity. Sample
01YA128 is not plotted in this figure because the aro-
matic compounds are absent in this sample. This is inter-
preted as being the result of higher thermalmaturity than
the other samples shown in Figure 9, with attendant
loss of the aromatic biomarkers.
Diamondoids
Diamondoid concentrations of five of the oils in this
study were analyzed, and the results (in ppm) are in-
cluded in Table 3. Diamondoids are molecular com-
pounds whose concentrations increase with increasing
thermal destruction of the oil by cracking or by ther-
mal chemical sulfate reduction (Dahl et al., 1999). The
diamondoid results agreewith the aromatic resultswith
regard to thermalmaturity. The samplewith the lowest
concentration of diamondoids also plotted closest to
the origin in Figure 9. The sample with the highest con-
centration of diamondoids (01YA128) is the one that
lacks triaromatic compounds.
Oil MRM-GCMS Results
None of the oil samples contain C30 steranes (Table 3).
C30 steranes, when present, are indicators of marine
source rocks (Moldowan et al., 1985; Peters andMoldo-
wan, 1993). Therefore, the lack of C30 steranes in the
oil samples lends support to the inferred lacustrine
source rock setting interpretation.
The calculated TPP ratios (Holba et al., 2000) for
oil samples in this study (Table 3) range from 0.26 to
0.71. Holba et al. (2000) indicate that TPP ratios be-
tween 0.25 and 0.40 are mixed deltaic lacustrine, and
ratios greater than 0.40 are considered to be from la-
custrine source rocks. Of the four samples analyzed in
this study, three (00YP26, 01YA128, and 01YA134)
have TPP ratios in excess of 0.40, whereas one sample
(01DS114) has a calculated ratio of 0.26, indicative of
a mixed lacustrine-deltaic setting for the source rock.
Oil-Oil Correlation
Although there are differences in the oil samples based
on the biomarker results, all oils are grouped into one
genetic family. Four subfamilies were identified: A,
consisting of samples 00YP25 and 00JB27; B, consist-
ing of samples 01DS114 and 01DS115; C, consisting
Figure 9. Crossplot of monoaromatic ratios (MA(I)/(MA(I) +MA(II))) versus triaromatic ratios (TA(I)/(TA(I) + TA(II))). Symbolsare the same as those used in Figure 8B.
1290 Ordos Oils and Source Rock Geochemistry
of 01YA128; and D, consisting of 01YA134. All of the
oil samples contain mainly algal and terrestrial organic
matter. The oil data generally point to a source rock that
was deposited in a weakly reducing or suboxic setting.
However, the highest Pr/Ph ratio of any of the oil sam-
ples in the study was 1.71 for sample 01YA128 (sub-
family C) (Figure 6B), suggesting that the source rock
depositional environment may have been more oxic
than what is indicated by other oil samples. All oil sam-
ples except one have gammacerane ratios indicative
of nonhypersaline conditions without water column
stratification. The one exception is 01YA134 (subfam-
ily D) (Figure 7B) (Table 3). Oil samples in subfam-
ilies B and C have higher diasterane ratios than other
oil samples (Table 3), suggesting that catalytic clays
were more abundant in the source rock that generated
them. Subfamilies B and C also have elevated diaho-
pane ratios compared to other oils (Table 3), consis-
tent with a more oxidative early diagenesis (Moldowan
et al., 1991).With the exception of 01YA128 (subfam-
ily C), the oil samples have thermal maturity indica-
tors that show that they aremature in the early part of
the oil window. The oils have variable thermal ma-
turity, and subfamily A is partially defined by having
lower thermal-maturity indicators than the rest of the
samples.
Triaromatic dinosteroids and carotanes are lack-
ing in all of the oils. All of the oil samples, with the
exception of subfamily D (01YA134), cluster tightly
when the sterane ratios are plotted on a ternary dia-
gram (Figure 8B).
Oil-Source Rock Correlation
Overall, the oil data closely match the Triassic source
rock data and are different than the Jurassic source
rock data. Key factors include the Pr/Ph ratios and the
similar positions on the sterane ternary plot (Figure 8B).
Perhaps the most convincing data are the lack of tri-
aromatic methylsteroids in all Triassic source rock sam-
ples, whereas the Jurassic source rock has a high triaro-
matic methylsteroid ratio (Table 3). None of the oil
samples contain the triaromatic dinosteroids that are
unique to the Jurassic source rock sample. Instead, they
match the Triassic source rock samples (Table 3).
Solid Bitumen Vein
The solid bitumen vein extract is biodegraded and
lacks n-alkanes, as well as pristane and phytane. Rel-
evant biomarker ratios indicate low thermal maturity.
These results are characteristic of immature hydrocar-
bons, suggesting that the bitumen vein was probably a
‘‘pre-oil’’ solid bitumen using the criteria of Curiale
(1985). Curiale (1985) reported that such pre-oil solid
bitumens are ‘‘products of rich source rocks.’’ The C29
steranes dominate the ratio of C27-C28-C29 steranes
(Figure 8B),which implies an important terrestrial plant
input. The lack of triaromatic dinosteroids suggests a
pre-Jurassic source rock.
CONCLUSIONS
Data generated in this study indicate that there is one
oil family represented in the suite of analyzed samples.
These oil samples were derived from a source rock that
was deposited in mildly anoxic to suboxic conditions.
The source rock contained terrestrial and nonmarine
algal organic matter. Most of the oil samples are waxy
and were generated from a lacustrine source rock. The
thermal maturity of oil samples varies throughout the
basin, but most samples are indicative of an early
thermal-maturity stage that is within the oil window.
Only one oil sample was biodegraded.
The best source rocks in the basin, with regards
to liquid hydrocarbon generation, are lacustrine mud-
stone of the Upper Triassic Yanchang Formation and
the Middle–Upper Jurassic Anding Formation. These
two source rocks are near the early stages of the oil win-
dow. However, oil samples correlate with Upper Tri-
assic lacustrine mudstone source rock and not with
Jurassic source rock, so exploration strategies should
focus on the known location of Triassic lacustrine strata
for predicting the source kitchen. Oil wells that are
producing oil from Triassic source rocks are generally
located in areas where Triassic lacustrine and lower
delta-plain facies merge in the subsurface (Figure 10),
which may have implications for future exploration in
the basin. Carboniferous coal and mudstone are not oil
prone and are overmature, but are probable sources for
large gas fields. In general, outcrop samples from the
western Ordos Basin are more thermally mature than
samples from the eastern and southeasternOrdos Basin,
so gas fields may be more likely in the west, whereas
oil may be more common in the east.
The solid bitumen vein more closely resembles
the Triassic source rock, but age-related biomarkers
suggest that the solid bitumen is pre-Jurassic and might
logically be related to another Triassic source rock
facies.
Hanson et al. 1291
REFERENCES CITED
Cross, A., and G. Wood, 1976, Palynology and petrography of somesolid bitumens of the Uinta Basin, Utah: Brigham Young Uni-versity, Geology Studies, v. 22, no. 3, p. 157–173.
Curiale, J. A., 1985, Origin of solid bitumens, with emphasis onbiological marker results: Advances in Organic Geochemistry,v. 10, p. 559–580.
Dahl, J. E., J. M. Moldowan, K. E. Peters, G. E. Claypool, M. A.Rooney, G. E. Michael, M. R. Mello, and M. L. Kohnen, 1999,Diamondoid hydrocarbons as indicators of natural oil cracking:Nature, v. 399, p. 54–57.
Dai, J., and Y. Xia, 1990, Ordovician gas accumulations in easternChina: Journal of Petroleum Geology, v. 13, p. 79–86.
Darby, B., and B. Ritts, 2002, Mesozoic contractional deformation in
the middle of the Asian tectonic collage: The enigmatic west-ern Ordos fold-thrust belt: Earth and Planetary Science Letters,v. 205, p. 13–24.
Darby, B., G. Davis, and Y. Zheng, 2001, Structural evolution ofthe southwestern Daqing Shan, Yinshan belt, Inner Mongolia,China, in M. S. Hendrix and G. A. Davis, eds., Paleozoic andMesozoic tectonic evolution of central and easternAsia— Fromcontinental assembly to intracontinental deformation: Geolog-ical Society of America Memoir 194, p. 199–214.
Ding, Z., J. Sun, S. Yang, and T. Liu, 2001, Geochemistry of thePliocene red clay formation in the Chinese Loess Plateau andimplications for its origin, source provenance and paleocli-mate change:Geochimica etCosmochimicaActa, v. 65, p. 901–913.
Enkin, R., Z. Yang, Y. Chen, and V. Courtillot, 1992, Paleomag-netic constraints on the geodynamic history of the major blocks
Figure 10. Facies map (modi-fied from Li et al., 1995) ofUpper Triassic strata in thesubsurface of Ordos. Producingwells tend to be located abovethe interface between lacustrineand lower delta-plain facies.
1292 Ordos Oils and Source Rock Geochemistry
of China from the Permian to the present: Journal of Geo-physical Research, B, Solid Earth and Planets, v. 97, p. 13,953–13,989.
Fu, J., G. Sheng, J. Xu, G. Eglinton, A. Gowar, R. Jia, S. Fan, andP. Peng, 1990, Application of biological markers in the assess-ment of paleoenvironments of Chinese non-marine sediments:Organic Geochemistry, v. 16, p. 769–779.
Hanson, A. D., B. D. Ritts, D. Zinniker, J. M. Moldowan, and U.Biffi, 2001, Upper Oligocene lacustrine source rocks and petro-leum system of the northern Qaidam Basin, NWChina: AAPGBulletin, v. 85, p. 601–619.
Holba, A., E. Tegelaar, L. Ellis, M. S. Singletary, and P. Albrecht,2000, Tetracyclic polyprenoids: Indicators of freshwater (la-custrine) algal input: Geology, v. 28, p. 251–254.
Holba, A., W. D. Masterson, L. Ellis, and L. Dzou, 2002, Triassicsource facies in high paleo-latitude petroleum systems: AAPGBulletin, v. 86, p. 1146.
Jenkins, C., C. Boyer, II, R. Fisher, B.Gobran, J. Sheng, and S. Zhang,1999, Appraisal drilling focuses on Ordos Basin coal seams: Oil& Gas Journal, v. 97 (April 26, 1999), no. 17, p. 43–49.
Jiang, D., 1988, Spores and pollen in oils as indicators of lacustrinesource rocks, in A. J. Fleet, K. Kelts, and M. R. Talbot, eds.,Lacustrine petroleum source rocks: Geological Society (Lon-don) Special Publication 40, p. 159–169.
Li, D., 1990, Recent advances in the petroleum geology of China:Journal of Petroleum Geology, v. 13, p. 7–18.
Li, K., D. Zhang, S. Zhang, and S. Liu, 1992, Petroleum Geology ofChina (in Chinese): Beijing, Oil Industries Press, v. 12, 490 p.
Li, S., S. Yang, and T. Jerzykiewicz, 1995, Upper Triassic– Jurassicforeland sequences of the Ordos Basin in China, in S. L. Do-robek and G. M. Ross, eds., Stratigraphic evolution of forelandbasins: SEPM Special Publication 52, p. 233–241.
Liu, B., Y. Wang, and X. Qian, 1997, Two Ordovician unconfor-mities in northChina: Their origins and relationships to regionalcarbonate-reservoir characteristics: Carbonates and Evaporites,v. 12, p. 177–184.
Liu, S., 1998, The coupling mechanism of basin and orogen in thewestern Ordos Basin and adjacent regions of China: Journal ofAsian Earth Sciences, v. 16, p. 369–383.
Meng, Q., and G. Zhang, 1999, Timing of collision of the North andSouth China blocks: Controversy and reconciliation: Geology,v. 27, p. 123–126.
Moldowan, J. M., W. K. Seifert, and E. J. Gallegos, 1985, Relation-ship between petroleum composition and depositional envi-ronment of petroleum source rocks: AAPG Bulletin, v. 69,p. 1225–1268.
Moldowan, J. M., F. J. Fago, R. M. K. Carlson, D. C. Young, G. VanDuyne, J. Clardy, M. Schoell, C. T. Pillinger, and D. S. Watt,1991, Rearranged hopanes in sediments and petroleum: Geo-chimica et Cosmochimica Acta, v. 55, p. 3333–3353.
Moldowan, J. M., J. Dahl, S. R. Jacobson, B. J. Huizinga, F. J. Fago,R. Shetty, D. S. Watt, and K. E. Peters, 1996, Chemostrati-graphic reconstruction of biofacies: Molecular evidence linkingcyst-forming dinoflagellates with pre-Triassic ancestors: Geol-ogy, v. 24, p. 159–162.
Palacas, J. G., D. Anders, J. King, and C. Lubeck, 1989, Use ofbiological markers in determining thermal maturity of biode-graded heavy oils and solid bitumens, in R. Meyer and E.Wiggins, eds., The Fourth United Nations Institute for Train-ing and Research/United Nations Development ProgramInternational Conference on Heavy Crude and Tar Sand, v. 2,Geology, geochemistry: p. 575–592.
Peters, K. E., 1986, Guidelines for evaluating petroleum source rockusing programmed pyrolysis: AAPG Bulletin, v. 70, p. 318–329.
Peters, K. E., and J. M. Moldowan, 1993, The biomarker guide.Interpreting molecular fossils in petroleum and ancient sedi-ments: Englewood Cliffs, Prentice Hall, 363 p.
Peters, K. E., A. Cunningham, C.Walters, J. Jiang, and Z. Fan, 1996,Petroleum systems in the Jiangling-Dangyang area, JianghanBasin, China: Organic Geochemistry, v. 24, p. 1035–1060.
Ritts, B. D., A. D. Hanson, J. M. Moldowan, and U. Biffi, 1999,Early–Middle Jurassic source rocks and petroleum systems ofthe Qaidam Basin, NWChina: AAPG Bulletin, v. 83, p. 1980–2005.
Ritts, B. D., B. J. Darby, and T. Cope, 2001, Early Jurassic ex-tensional basin formation in the Daqing Shan of the Yinshanbelt, northern North China block, Inner Mongolia: Earth andPlanetary Science Letters, v. 339, p. 239–258.
Rubenstein, I., O. Sieskind, and P. Albrecht, 1975, Rearranged ster-anes in a shale: Occurrence and simulated formation: Journal ofChemical Society, Perkin Transaction I, p. 1833–1836.
Seifert, W. K., and J. M. Moldowan, 1978, Applications of steranes,terpanes, and monoaromatics to the maturation, migration,and source of crude oils: Geochimica et Cosmochimica Acta,v. 43, p. 77–95.
Seifert, W. K., and J. M. Moldowan, 1980, The effect of thermal stresson source-rock quality as measured by hopane stereochemistry:Physics and Chemistry of the Earth, v. 12, p. 229–237.
Song, G., 1988, Jurassic paleo-landform oil fields in the OrdosBasin, north China, in H. C. Wagner, L. C. Wagner, F. F. H.Wang, and F. L. Wong, eds., Petroleum resources of China andrelated subjects: Houston, Texas, Circum-Pacific Council forEnergy and Mineral Resources Earth Science Series 10, p. 371–386.
Watson, M. P., A. B. Hayward, D. N. Parkinson, and Z. M. Zhang,1987, Plate tectonic history, basin development and petroleumsource rock deposition onshore China: Marine and PetroleumGeology, v. 4, p. 203–225.
Xie, C., 2004, Recent significant gas discoveries in China: Influenceon national energy structure and future gas exploration (abs.):AAPG Annual Meeting Program, v. 13, p. A150.
Yang, J., K. Li, D. Zhang, S. Zhang, and S. Liu, 1992, Petroleumgeology of China, Changqing oil field: Beijing, PetroleumIndustry Publishing House, v. 12, 490 p.
Yang, Y.,W. Li, and L.Ma, 2005, Tectonic and stratigraphic controlson hydrocarbon systems in the Ordos Basin: A multicycle cra-tonic basin in central China: AAPGBulletin, v. 89, p. 255–269.
Yang, Z., Y. Cheng, and H. Wang, 1986, The geology of China:Oxford, Clarendon Press, 303 p.
Yang, Z., X. Ma, J. Besse, V. Courtillot, L. Xing, S. Xu, and J.Zhang, 1991, Paleomagnetic results from Triassic sections inthe Ordos Basin, north China: Earth and Planetary ScienceLetters, v. 104, p. 258–277.
Zhang, K., 1989, Tectonics and resources of Ordos fault-block:Beijing, Geological Publishing House, 394 p.
Zhang, P., B. Burchfiel, P. Molnar, W. Zhang, D. Jiao, Q. Deng, Y.Wang, L. Royden, and F. Song, 1991, Amount and style of lateCenozoic deformation in the Liupan Shan area, Ningxia autono-mous region, China: Tectonics, v. 10, p. 1111–1129.
Zhang, Q., J. Mercier, and P. Vergely, 1998, Extension in thegraben systems around the Ordos (China), and its contributionto the extrusion tectonics of south China with respect to Gobi-Mongolia: Tectonophysics, v. 285, p. 41–75.
Hanson et al. 1293
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