Organic geochemistry of oil and source rock strata of the ...geoscience.unlv.edu/files/Hanson...

AUTHORS

Andrew D. Hanson � Department of Geoscience,University of Nevada Las Vegas, 4505 S. MarylandParkway, Box 454010, Las Vegas, Nevada 89154-4010; [email protected]

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; [email protected]

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;[email protected]

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.


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 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 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


mudstone

39j02.580 106j15.700 2.89 0.01 0.01 0.41 404££ 0.11 0 14 0 0.50 f


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 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 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 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



1278

Ordos

Oils

andSource

RockGeochem

istry



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


Carboniferous

Coaly

mudstone

39j10.620 106j37.100 3.58 0.01 0.20 0.81 535££ 2.47 6 23 0 0.05 f


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


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


Carboniferous

Coal 39j10.620 106j37.100 6.91 0.11 7.91 0.27 584 3.35 114 4 2 0.01 c htS2p


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


Jurassic

Mudstone 36j40.960 109j09.650 1.03 0.11 2.30 0.84 435 0.67 223 82 11 0.05 n


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 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


Carboniferous

Mudstone 37j33.460 110j53.900 1.86 0.27 1.01 0.04 468 1.26 54 2 15 0.21 n


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


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

£


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


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


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


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


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


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.


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).


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.


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

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