01_0601 Upper Oligocene lacustrine source rocks and petroleum systems of the northern Qaidam basin,

Upper Oligocene lacustrine source rocks and petroleum systems of the northern Qaidam basin,northwest ChinaAndrew D.Hanson,Bradley D.Ritts,David Zinniker,J.Michael Moldowan,and Ulderico BiffiA B S T R A C TOur organic geochemical study of oils from the northern Qaidam basin defines a family of genetically related oils that contain bio-markers indicative of source rocks deposited in Tertiary hypersaline, anoxic lacustrine settings.Although Cenozoic outcrop samples from northern Qaidam are too organic lean to be of source quality, dark laminated upper Oligocene mudstones containing gypsum crystals and pyrite from the Shi28well yield total organic carbon (TOC)and Rock-Eval data indicative of fair to good source rocks. Organic matter is derived from algae and bacteria and there appar-ently was little contribution from terrestrial material.Biomarker data provide a good correlation between the produced oils and the upper Oligocene Shi28core samples.Hydrocarbons derived from these source rocks are contained in upper Oligocene,Miocene,and Pliocene reservoirs.Although eight of the oil samples are from the northwest corner of the basin,one sample in this genetic family of hypersaline oils comes from northeast Qaidam,an area previously believed to only produce oils derived from Jurassic freshwater la-custrine source rocks.This sample thus indicates the presence of an unidentified and undocumented source rock in the northeast part of the basin.Hypersaline oils and the associated source rocks have low biomarker maturity parameters.Thermal modeling indicates that hydrocarbon generation probably occurred in northwestern Qaidam within the last3million years.I N T R O D U C T I O NThe Qaidam basin is a nonmarine,petroliferous basin that lies in the northeastern corner of the Tibetan Plateau in northwest ChinaCopyright᭧2001.The American Association of Petroleum Geologists.All rights reserved. Manuscript received November17,1998;revised manuscript received June9,2000;final acceptance August30,2000.A U T H O R SAndrew D.HansonϳDepartment ofGeological and Environmental Sciences,Stanford University,Stanford,California,94305–2115;current address:Department of Geoscience,University of Nevada Las Vegas,4505MarylandParkway,Las Vegas,Nevada,89154;andrewh@Andrew Hanson attained a B.S.degree in nursingfrom Montana State University,an M.S.degree ingeological sciences from San Diego StateUniversity,and a Ph.D.in geological andenvironmental sciences from Stanford University.His dissertation focused on organic geochemistry,petroleum geology,and tectonics andsedimentation of the Tarim and Qaidam basins innorthwest China.After graduation he worked forTexaco Exploration investigating deep-water areasoffshore Nigeria.Hanson recently joined the facultyof the University of Nevada,Las Vegas and isestablishing research projects in China and thewestern United States.Bradley D.RittsϳDepartment of Geologicaland Environmental Sciences,Stanford University,Stanford,California,94305–2115;current address: Department of Geology,Utah State University,4505Old Main Hill,Logan,Utah,84322–4505;ritts@Bradley Ritts received his Ph.D.in geologicalsciences from Stanford University,where heworked on tectonics,sedimentology,andpetroleum systems of the Qaidam and Tarimbasins.He then moved to Chevron OverseasPetroleum,as an exploration geologist working onChina.Ritts is an assistant professor at Utah State University,where his research focuses on tectonicsand sedimentation and petroleum systems oftectonically active,nonmarine basins.His currentresearch projects are in China and the westernUnited States.David ZinnikerϳDepartment of Geologicaland Environmental Sciences,Stanford University,Stanford,California,94305–2115;zinniker@David Zinniker is a Ph.D.candidate in thedepartment of Geological and EnvironmentalSciences at Stanford University.His work focuseson the use of molecular organic geochemistry,stable isotope geochemistry,andmicropaleontology to understand ancient terrestrial ecosystems and petroleum that has significanthigher plant input.He received his B.S.degree ingeology at Stanford University in1996.Contents Search ResultsSearch IndexAAPG Bulletin,v.85,no.4(April2001),pp.601–619601602Petroleum Systems of the Northern Qaidam Basin (China)(Figure 1).The basin floor sits approximately 3–3.5km above sea level,and the basin is surrounded on all sides by high mountains.To the north are the Altun Mountains,the Qilian Mountains are along the northeast,and the eastern Kunlun Mountains are to the southwest (Figure 1).Each of these ranges has peaks in excess of 5–7km elevation.The basin sits in the center of Asia in the rain shadow of the Tibetan Plateau and the three surrounding ranges,has very low precipitation,and is internally drained.Palynological studies of Cenozoic strata of Qaidam indicate that flora were prin-cipally species that were tolerant to extremely dry and saline hab-itats and suggest that,during most of the Tertiary,the basin was characterized by a dry climate (T.Fouchs,1994,personal com-munication).The dry climate and lack of significant higher plants have had a direct impact on the type of organic matter available for preservation in source rock intervals in the basin.Facies relationships,provenance,sediment-dispersal paths (Hanson,1997,1999),isopach relationships (Anonymous,1990),and growth structures seen on seismic data (Song and Wang,1993)all indicate that the basin has been internally drained since at least the late Oligocene.As a result,the depocenter of the basin has been occupied by lakes that have elevated salinity throughout much of the Cenozoic (X.Huang and Shao,1993).In simplest terms,basin fill is characterized by stacked coarse alluvial fan and braided-stream deposits near the active basin margins that interfinger and grade laterally basinward into lacustrine deposits.The depocenter of the basin has progressively shifted southeastward during the Cenozoic from an initial position in the northwest part of the basin to its current depocenter at Dabushin Lake near the center of the basin (Lee,1984;X.Huang and Shao,1993).Small modern lakes in the basin center continue to be highly saline in the present arid climate and are currently the site of deposition of unusual potash evaporites (Lowenstein et al.,1989;Casas et al.,1992).Paleocene and Eocene strata occur in the subsurface and a few small outcrops in northern Qaidam but are not widespread in the rest of the basin.Oligocene strata are more widespread and include lacustrine rocks within the lower Ganchaigou Formation in north-western Qaidam (Anonymous,1990)(Figure 2).Lacustrine dep-osition was also widespread in Qaidam during the Miocene,whereas lesser amounts of lacustrine strata are present in lower Pli-ocene sections.Upper Pliocene and Quaternary sections are gen-erally coarser grained fluvial and alluvial deposits,which are quite thick and reflect high sedimentation rates.Where thickest,Oligocene–Miocene strata exceed 3km,whereas Pliocene and Qua-ternary sections in places exceed 5.5and 2.8km,respectively (Lee,1984).Oil was first discovered in the basin in the late 1950s.Initial successes were in the northeast around the town of Lenghu (Figure 1).Further exploration in the 1950s discovered additional fields in the northwest part of the basin around the town of Huatugou (Lee,1984;Ulmishek,1984)(Figure 1).Production and exploration are largely limited to shallow Tertiary reservoirs.In the Lenghu area,A C K N O W L E D G E M E N T SPrimary financial support for this article was pro-vided by the Stanford China Industrial Affiliates.Additional funding was provided by the Shell and McGee Funds of Stanford University and student grants from the AAPG Grants-in-Aid Program and the Geological Society of America.The Stanford Molecular Organic Geochemistry Industrial Affiliates supported biomarker studies.David Zinniker and J.Michael Moldowan acknowledge partial research support from ACS-PRF Grant #30245-AC2.Ching-Yen Chu provided excellent translation and field as-sistance.Edmund Chang arranged logistics and provided translation.Alan Carroll (formerly with Exxon Production Research [EPR]Corporation)ar-ranged for analytical work that was completed at Humble Geochemical with expenses being covered by EPR.J.Michael Moldowan ϳDepartment ofGeological and Environmental Sciences,Stanford University,Stanford,California,94305–2115;moldowan@J.Michael Moldowan attained a B.S.degree in chemistry from Wayne State University (1968)and a Ph.D.in chemistry from the University ofMichigan (1972).After a postdoctoral fellowship with Carl Djerassi at Stanford University,he joined Chevron in 1974where he developed fundamental and applied technology related to petroleum biomarkers.Since 1993,Moldowan has been professor (research)in Stanford University’s Department of Geological and Environmental Sciences.He has published more than 80journal articles and three books.Two articles,published in 1978and 1989,received best paper awards from the Organic Geochemistry Division of the Geological Society.Ulderico Biffi ϳAgip,20097San Donato,Milan,ItalyUlderico Biffigraduated from the University of Milan’s Geological Sciences department.He joined AGIP in 1977and studied Tertiary palynomorphs of the Niger Delta.He has conducted original palynological studies of the Mesozoic NubianSequence in Libya,the Mesozoic stratigraphy of the Tarim basin,the Tertiary Numidic basin in Sicily,and the Italian Paleogene and Neogene.Currently he is involved in the study of the Caspian Neogene.Figure1.Location map of the northern Qaidam basin showing its position on the northeast margin of the Tibetan Plateau(lower right inset).Hypersaline lacustrine–derived oils discussed in the text were collected at sites marked byfilled squares.Eight of these oils came from the northwest part of the basin,but one sample was collected near Lenghu in the northeast.many wells have experienced declining production and have been abandoned owing to increasing water cuts (C.Fan,1995,personal communication).As a result, efforts of the past decade have focused on the north-west area,and active infill drilling and exploration are currently centered around Huatugou.Previous geochemical studies of produced oils from Qaidam basin have indicated the presence of two distinct genetic groups of oils(Ritts et al.,1996,1999; Ritts,1998).Links between one group of oils and sa-line lacustrine source rocks within the Tertiary section have been made(Anonymous,1990)based on pris-tane/phytane(Pr/Ph)ratios,n-alkane distributions, and terpane and sterane attributes,although no source rock geochemical data were presented.D.Huang et al.(1991,1994)reported dominantly low-maturity oils in Qaidam and the presence of pregnane and C30ster-anes,which they related to thermal degradation and diagenetic effects.Philp et al.(1989)analyzed two samples from Qaidam and noted uncommonly heavy carbon isotopes(מ24.31%for saturate andמ23.55% for aromatic fractions)for one sample,which they linked to a saline lacustrine environment.Philp et al. (1991)analyzed13oils from Qaidam and reported similarfindings to those of Anonymous(1990),but no source rock geochemical data were reported in this ar-ticle either.In addition to the hypersaline lacustrine oils,on which this article focuses,the Qaidam basin has a ge-netically distinct group of freshwater lacustrine oilsHanson et al.603Figure2.Mesozoic–Cenozoic stratigraphy,depositional sys-tems,and potential source rock intervals of the northern Qaidam basin(reprinted from Ritts et al.,1999).that have been correlated to Middle Jurassic source rocks(Ritts,1998;Ritts et al.,1999).Although many basins in northwest China share similar organic matter characteristics during the Jurassic(e.g.,Qaidam,Ta-rim,Turpan,and southern Mongolia coals and lacus-trine shales),Cenozoic petroleum systems are not known to exist in northwest Chinese basins,other than in Qaidam.Recent discoveries in the Lunpola basin on the high Tibetan Plateau remain poorly documented but are believed to be derived from Tertiary lacustrine strata(Ai et al.,1998).In this article we describe and provide further documentation of Tertiary oils,as well as documenta-tion of an oil–source rock correlation with Tertiary source rocks.Additionally,we describe components of the Tertiary petroleum system within Qaidam that bear significantly on the prospectivity of the basin.T E R T I A R Y S O U R C E R O C K G E O L O G YQuaternary alluvium,eolian sand,lacustrine silt,and salt pans cover much of the Qaidam basin today.Ter-tiary strata crop out in fault-bound sections adjacent to the basin margins and in anticlinal sections that com-monly are fault cored.No Paleocene strata are exposed in the basin,and Eocene outcrops are limited to small outcrops in the northern part of the basin.Paleocene–Eocene strata have been penetrated by drilling in the northern part of the basin but do not extend into the southern part of the basin.Oligocene–Pliocene strata are beautifully exposed in outcrops along the northern margins of the basin. Ourfieldwork in Qaidam has included documentation of outcrop sections at Lenghu,Oboliang,Hongshan-han,and Ganchaigou(Hanson,1999)(Figure1).Qaidam has been in a compressional tectonic set-ting throughout the Cenozoic.Southwestward-verging thrust faults along the northeastern margin of the basin (the Qilian margin)(Anonymous,1990)and north-westward vergent thrusts along the southwestern mar-gin of the basin(the Kunlun margin)(Song and Wang, 1993),combined with strike-slip motion along the Al-tyn Tagh fault on the northern margin(CSBS,1992), provided topographic highs along the basin margins that subsequently shed thick sedimentary packages into the basin.Provenance analyses(sandstone petrographic anal-ysis,heavy-mineral analyses,and conglomerate counts) indicate that Cenozoic sediments adjacent to the basin margins can be directly tied to the adjacent mountain belts(Hanson,1997,1999).Documentation of Ce-nozoic sections indicates rapid lateral facies changes where thick alluvial conglomeratic sections may grade into laminated lacustrine mudstones over distances of a few kilometers.Profundal Lacustrine Source RocksProfundal lacustrine deposits are characterized by mas-sive to laminated mudstones.Mudstones may range from a few meters to several hundred meters in thick-ness.At the Ganchaigou section(Figure3A),we ob-served massive mudstones(Figure3B)whose total thickness was more than300m.Packaged within the massive mudstones were several tens of meters of lam-inated mudstones(Figure3C).These mudstones are generally devoid of obvious minated thin,fine-grained to medium-grained sandstones occur throughout the mudstone sections.The mudstone sec-tions are bounded above and below by medium-grained to coarse-grained tabular sandstones and minor pebble conglomerates that are texturally and compo-sitionally immature.We interpret the mudstones to be quiet,relatively deep-water lacustrine mudstones based on the lami-nated to massive character,and thin sandstones within the mudstones are turbidity-current deposits within604Petroleum Systems of the Northern Qaidam Basin(China)Hanson et al.605C.B.A.3.(A)Outcrop photo the Ganchaigou section il-the general outcrop within part of the mar-lacustrine section.(B)and (C)laminated within the lower Formation.606Petroleum Systems of the Northern Qaidam Basin (China)A.B.B.C.Figure 4.(A)Photo of laminated dark mudstone from the Shi 28core.Scale bar at the top is 5cm long.(B)Euhedral gypsum crystals are seen in part of the Shi 28core and (C)in the photomicrograph.The horizontal dimension of the photomicro-graph is 21mm.the lacustrine setting.The encasing coarser sandstones and conglomerates were deposited in braided-plain or alluvial settings that bordered the lake margins.Within the Oligocene section at Ganchaigou,the coarse sandstones and conglomerates are charged with hydrocarbons.Mudstones seen within the Shi 28well,which is a few kilometers farther from the basin margin than the Ganchaigou section,have better developed laminae and are much darker colored (Figure 4A).Contained within the mudstones are euhedral gypsum crystals(Figure 4B,C).These mudstones were deposited in a deeper lacustrine setting in which water depths were sufficiently deep to allow the development of a strati-fied water column that had anoxic bottom water.Paly-nological analysis and geochemical attributes discussed in following sections enhance this interpretation.Marginal Lacustrine StrataAdjacent to (both vertically and horizontally)the pro-fundal lacustrine mudstones described previously are marginal lacustrine mudstones and carbonates.The fa-cies consists of thin sandstones and carbonates con-tained within mudstones.Mudstones tend to be mas-sive and are shades of red,green,yellow,and brown.Thin (few decimeters to 3m)micritic carbonates were seen at the Ganchaigou section.The micrites appear to have been deposited as algal stromatolites and form thin crusts over sandstones.Uncommonly the micrites contain ostracods and pisoliths.Fine-grained to medium-grained sandstones are sharp based and lat-erally continuous for several hundreds of meters.In some places thicker sandstones contain large-scale pla-nar cross-stratification.Some mudstone layers contain mud cracks.The pisolitic carbonates indicate a marginal lacus-trine setting.Massive,variegated mudstones were de-posited in nearshore-lake settings that were in places subaerially exposed.Thinner sandstones are inter-preted as nearshore sand deposits,whereas thicker cross-stratified sandstones are interpreted as progra-dational fluvial sands.Nearly the entire Miocene sec-tion at the Ganchaigou,Hongshanhan,and Oboliang sections consists of this facies assemblage.Although these strata do not have good source rock quality in the outcrop,we were particularly interested in the carbon-ates because of molecular organic geochemical attrib-utes,which suggested a lack of shale within the source rocks,and an implied association with an evaporite-carbonate source rock.O L I G O C E N E S O U R C E R O C K S MethodsUnweathered source rock samples were collected from the most promising outcrop section.We attempted to diminish the effects of weathering by excavating and collecting the freshest samples possible,although we admit that we are uncertain how much an effectweathering of outcrop samples might have had on our results.Leco total organic carbon(TOC,wt.%)data generated by Global Geochemical Corporation and Leco TOC and vitrinite reflectance performed on some samples by Humble Geochemical Services Division compliments of Exxon Production Research Company are shown in Table1.Leco TOC,Rock-Eval pyrolysis, and elemental analysis conducted on source rocks by Core Laboratories are provided in Table2.Kerogen assessment and vitrinite reflectance conducted on two of the Shi28core samples by Core Laboratories are shown in Table3.Source Rock QualityOur initial investigations of source rocks were re-stricted to outcrop samples.These included samples from the Ganchaigou,Hongshanhan,and Oboliang areas.Tertiary strata at the Lenghu locality contained no apparent source rocks.The TOC values for these samples were uniformly low despite the presence of well-developed laminae and preservedfish-bone frag-ments in some samples within the moderately dark green-gray mudstones.Oligocene mudstones from Ganchaigou outcrops had TOC values of0.03–0.30%Table1.Outcrop Sample DataSample ID Locality Age TOC*(wt.%)R o**Lithology Data Source†95GG128Ganchaigou Pliocene0.08carbonate195GG127Ganchaigou Pliocene0.07carbonate195GG125Ganchaigou Miocene0.05carbonate195GG123Ganchaigou Miocene0.18carbonate195GG121Ganchaigou Miocene0.10mudstone195GG120Ganchaigou Oligocene0.10mudstone195GG119Ganchaigou Oligocene0.20mudstone195GG118Ganchaigou Oligocene0.05mudstone195GG117Ganchaigou Oligocene0.21carbonate195GG115Ganchaigou Oligocene0.28carbonate195GG113Ganchaigou Oligocene0.36carbonate195GG111Ganchaigou Oligocene0.03mudstone195GG110Ganchaigou Oligocene0.31carbonate195GG109Ganchaigou Oligocene0.18mudstone195GG108Ganchaigou Oligocene0.09mudstone194GG137Ganchaigou Oligocene0.30 1.06mudstone294GG191Ganchaigou Oligocene0.30mudstone294GG192Ganchaigou Oligocene0.28 1.03mudstone294GG193Ganchaigou Oligocene0.30mudstone294GG196Ganchaigou Oligocene0.22mudstone294GG202Ganchaigou Oligocene0.280.90mudstone295HS57Hongshanhan Miocene0.09mudstone195HS75Hongshanhan Miocene0.07mudstone195HS77Hongshanhan Miocene0.03mudstone195HS79Hongshanhan Miocene0.02mudstone195HS80Hongshanhan Miocene0.02mudstone195HS83Hongshanhan Miocene0.03mudstone195HS84Hongshanhan Miocene0.02mudstone195HS88Hongshanhan Miocene0.17mudstone195OB38Oboliang Miocene0.09mudstone195OB30Oboliang Miocene0.07mudstone1*TOCסtotal organic carbon(wt.%)**R oסvitrinite reflectance†1סHumble Geochemical Services Division;2סGlobal Geochemistry CorporationHanson et al.607608Petroleum Systems of the Northern Qaidam Basin (China)and an average value of 0.20%(n ס12);carbonate samples had only slightly higher TOC values (0.21–0.36%,average ס0.29%,n ס4)(Table 1).Miocene mudstone outcrop samples provided even lower TOC contents:at Ganchaigou,TOC values were 0.10%for mudstone (n ס1)and 0.12%for carbonates (n ס2)(Table 1).Miocene mudstones from Hongshanhan were also organic poor (0.02–0.17%,average ס0.05%,n ס8),as were those from Oboliang (0.08%,n ס2)(Table 1).Pliocene carbonates in the Ganchaigou sec-tion also had low TOC levels (0.08%,n ס2)(Table 1).Although the outcrop samples have been weath-ered,their close association within shallow lacustrine and fluvial strata suggests that their low TOC values are a true reflection of an original organic-poor state.We also analyzed core samples from two wells,one set of samples from Oboliang (Miocene mudstones that have TOC values of 0.19%,n ס2)and another set of samples from the Shi 28well within the Shizigou anticline near Huatugou (Table 2;Figure 1).Samples from the Shi 28well came from depths of 3990–4016.9m (the lower Ganchaigou Formation)and are middle-late Oligocene in age according to the Qinghai Petroleum Bureau.Samples generally consist of millimeter-scale laminated dark-gray mudstones (Fig-ure 4A),although some core segments contain fairly massive dark-gray mudstones that contain large (up to 2cm in length)euhedral gypsum crystals (Figure 4B,C).The TOC values for these mudstones range from 0.27to 1.22%and have an average of 0.82%(n ס11)(Table 2).The TOC values in this range are considered to have fair to good generative potential (Peters and Cassa,1994).Rock-Eval pyrolysis was carried out on Shi 28sam-ples whose TOC values exceed 0.8wt.%(Table 2).The S1and S2values indicate fair to good quantity of organic matter (Peters and Cassa,1994).Calculated hydrogen indices (mg HC/g TOC)for these samples range from 240to 361and S2/S3values are between 2.4and 9.7,having an average of 5.7(Table 3).These results suggest that,based on the organic material,they could generate either gas or oil (Peters,1986;Peters and Cassa,1994).A crossplot of hydrogen index (HI)vs.oxygen index (OI)for these samples is shown in Figure 5A and reveals that two of the samples plot along the type I pathway,four plot adjacent to the type II pathway,and the remaining two samples plot be-tween the type II and type III pathways.Because HI vs.OI plots can produce unreliable results for some organic-lean samples,we wanted to confirm these find-ings by plotting the data on a van Krevelen diagram.To derive the necessary data,we performed elemental analysis (H,C,N,O,and S)on the three samples that have the highest TOC values.Only one sample,how-ever,had sufficient elemental oxygen to yield a reliable measurement (Table 2).This sample (sample 96SI127),which is one of the two samples that plotted along the type I pathway on the HI vs.OI plot,is plot-Table 2.Total Organic Carbon,Rock-Eval Pyrolysis,and Elemental Analyses Results for Core Samples*wt.%Sample ID Type Locale TOC (wt.%)S1(mg/g)S2(mg/g)S3(mg/g)S2/S3T max (ЊC)HIOI Transformation RatioHCNOS96OB135CoreOboliang 0.1996OB136Core Oboliang 0.1996SI124Core Shizigou 0.4296SI125Core Shizigou 0.81 1.55 2.170.69 3.1424268850.4296SI126Core Shizigou 0.2796SI127Core Shizigou 1.16 1.40 4.060.429.7437350360.26 6.3159.750.96 4.4215.3696SI128Core Shizigou 1.22 1.08 4.210.498.6437345400.20 2.6026.970.3796SI129Core Shizigou 0.85 1.58 2.040.86 2.44212401010.4496SI130Core Shizigou 0.94 1.69 3.150.60 5.3436335640.35 2.5924.100.5096SI131Core Shizigou 1.15 1.46 3.950.60 6.6434343520.2796SI132Core Shizigou 0.5296SI133Core Shizigou 0.90 1.73 3.250.61 5.3437361680.3596SI134CoreShizigou0.821.932.600.544.8431317660.43*TOC סtotal organic carbon (wt.%);S1סfree hydrocarbons (mg/g);S2סpyrolyzable hydrocarbons (mg/g);S3סCO 2released during pyrolysis (mg/g);hydrogen index (HI)ס[(S2/%TOC)ן100];oxygen index (OI)ס[(S3/%TOC)ן100];transformation ratio סS1/(S1םS2).All data from Core Laboratories.Hanson et al.609ted on a van Krevelen diagram (Figure 5B)and shows that this sample again plots along the type I pathway.This suggests that the HI vs.OI plots are reliable and that at least some samples are highly oil prone.The Shi 28core sample that has the highest TOC value was analyzed to determine the kerogen maceral composition.This sample contained 20%fluorescing and 80%nonfluorescing amorphous kerogen and had trace amounts of liptinite,vitrinite,and inertite (Table 3).Thermal Maturity of Core SamplesVitrinite reflectance for the stratigraphically lowest of the Shi 28core samples (depth ס4016.9m)yielded a vitrinite reflectance value of 0.68%R o ,although very poor vitrinite recovery provided insufficient vitrinite for a reliable determination (total number of readings ס33,number of readings for the indigenous popula-tion ס8,mean reflectance ס0.68,standard deviation ס0.07).A thermal alteration index (TAI)of 2to 2םwas estimated for core samples based on the color and appearance of amorphinite.These values suggest an early maturity stage.The T max values range from 421to 437ЊC,which are below or just at the birthline value of Peters (1986).Thermal Maturity of Outcrop SamplesVitrinite reflectance values for three samples taken from a similar stratigraphic interval at the adjacent Ganchaigou outcrop are higher than the R o value of the core sample discussed in the previous section and range from 0.90to 1.06%(Table 1).Amorphous ma-terial within Ganchaigou Oligocene outcrop samples is light brown.Our measured section suggests that the outcrop samples were buried to a deeper depth prior to exhumation than the core samples,which would explain the higher reflectance values seen in the out-crop samples.Source Rock PotentialCalculated source potential index (SPI)values (De-maison and Huizinga,1994)based on the Shi 28core data yield low values (Ͻ1).We limited our source rock thickness to 26m,however,which is the total source rock thickness within the core from the Shi 28well.Because the well reached its total depth within the source rock interval,the actual total thickness of the source rock interval likely is greater.Ulmishek (1984)T a b l e 3.V i s u a l K e r o g e n a n d V i t r i n i t e R e fle c t a n c e A s s e s s m e n t f o r C o r e S a m p l e s *K e r o g e n M a c e r a l C o m p o s i t i o nI n d i g e n o u s V i t r i n i t e D a t aS a m p l e I D T y p e L o c a l eF l A m N F l A m L i p t i n i t e V i t r i n i t e I n e r t i n i t e T A I V i t r i n i t e R e fle c t a n c e #o f R e a d i n g s S t a n d a r d D e v i a t i o nV i t r i n i t e R e c o v e r yR e m a r k s96S I 128C o r e S h i z i g o u 2080T T T 2/2םN o t r e q u e s t e d a b u n d a n t p y r i t e 96S I 134C o r e S h i z i g o u N o t r e q u e s t e d 0.68**80.07v e r y p o o rw h o l e r o c k R o*A m o r p h i n i t e (A m )i n c l u d e s n o n s t r u c t u r e d a m o r p h o u s d e b r i s םa m o r p h o u s s a p r o p e l s ;F l A m סflu o r e s c i n g a m o r p h o u s ;N F l A m סn o n flu o r e s c i n g a m o r p h o u s ;l i p t i n i t e סw a x y a n d r e s i n o u s m a t e r i a l s g e n e r a l l y h a v i n g a c h a r a c t e r i s t i c f o r m (i.e .,p l a n t c u t i c l e ,p o l l e n ,r e s i n s ,e t c .);v i t r i n i t e i n c l u d e s w o o d y ,c o a l y ,a n d v i t r i n i t i c m a t e r i a l ;i n e r t i n i t e סc h a r c o a l ,h i g h l y o x i d i z e d o r r e w o r k e d m a t e r i a l o f a n y o r i g i n ;T סt r a c e .**I n s u f fic i e n t i n d i g e n o u s v i t r i n i t e f o r a r e l i a b l e v i t r i n i t e r e fle c t a n c e d e t e r m i n a t i o n .。