ol. 25 No. 1CHINESE JOURNAL OF GEOCHEMISTRY2006Generation and accumulation of Quaternary shallowgas in eastern Qaidam Basin, NW China(Agm(蘇愛國y2…, HAO Wenzhi(趙文智y, DANG Y(黨玉琪ING Shuichang(張水昌), LI Maowen(黎茂穩(wěn)), PANG Xiongqi(龐雄奇ZHOU Ruilian(周瑞年), GUAN Ziqiang(管自強),Ⅻ U Ziyuan(徐子遠)ZHANG Daowei(張道偉), Tan Yanhu(譚彥虎), chen Jianfa(陳踐發(fā)),LI Sumei(李素梅I China Uniiversity of Mining Technology( Beiing ) Beijing 1000832 Petro China Research Institute of Petroleum Exploration and Development, Beijing 100083,ChinaPetro China Qinghai Oilfield Company, Denghuang 736202, ChinaGeological Survey of Canada, Calgary T2L 2A7, CanadaChina University of Petroleum( Beiing ) Beijing 102249, ChinaAbstract This study presents an overview on the geological setting and geochemical characteristics ofPleistocene shallow gas accumulations in eastern Qaidam Basin, NW China. Five largest gas accumulations discovered in this region have a combined enclosure area of about 87 km and 7. 9 trillion cubic feettcf )of proven plus controlled gas reserves. The dofinance8D values of methane(-68 51%0 to-65 00%0 and -227 55%0 to.94%0, respectively )suggestthat these gases are biogenic derived from the degradation of sedimentary organic matter by methanogensunder relatively low temperatures( 75C). A sufficient supply and adequate preservation of organicmatter in the Pleistocene sediments is made possible by the lake basins high altitude( 2600-3000 mgh water salinity(>15%)and strong stratification. The deposition and extensive lateral occurrence ofshore and shallow lake sands/silts in beach sand sheets and small sand bars provided excellent reservoirsfor the biogenic gas generated from adjacent rocks. Effective but dynamic gas seals were provided by suchfactors as intermittent vertical variations in the sediment lithologies, hydraulic trapping due to mudstonewater saturation the hydrocarbon gradient created as a result of gas generation from potential caprocksand the presence of a regional caprock consisting of 400-800-m-thick mudstones and evaporites. It appears that the most favorable traps for large gas accumulations occur on structural slopes near the major gaslarge gentle anticlines with little faKey words biogenic gas generation accumulation; Quaternary Qaidam Basinnumber of significant shallow gas accumulations have1 Introductionbeen discovered in China such as the Qaidam Songli-ao, Bohai Bay Erlian Jianghan Subei and Baise baShallow( mostly biogenic)gas accounts for over sins( Qi Houfa et al. 1997), and the southeastern20% of the world s discovered gas reserves( Rice and offshore areas( Xu Wang, 1997)including the hangClaypool 1981: Claypool and Kaplan, 1974), and zhou Bay( Lin Chunming et al. 2004represents one of the unconventional energy sourcesThe only giant shallow gas accumulations in Chinathat increasingly attract the attention of petroleum geol- are located in the Sanhu Region of eastern Qaidam Ba-ogists. Numerous Cretaceous and Tertiary shallow gas sin, NW China. Five largest gas accumulations discovered in this region have a combined enclosure area ofworld, and some accumulations such as those in Siberi- about 87 km and 7. 9 trillion cubic feet( tcf )of prova, USA and Canada are of very important economic en plus controlled gas reserves. The largest gas accuvalue( Martini et al. 1998 Littke et al. 1999: mul中國煤化工seil, Sebei2andShurr and Ridgley 2002 ) Over the past decade,aHCNMHThe gaszones rangewith a single well dailyISSN1000-9426s This study was funded by the China 973 Key Foundation Research Deproduction rate of 3. 5-14 million cubic feet( mcfopment Project( Grant No. 2001 CB209100), National Key ScientificgenerateProgram( Grant No. 96-111-01-03) and the ESS Global Opportuniticene sediments( Zhou Zhuhong et al., 1994; Gu ShuCcmc高教,Em:gmsong, 1996 Qi Houfa et al. 1997; Guan Zhiqiang etyahoo. comSU Aiguo et al2001 Dai Jingxing et al. 2003)aoilfieldpress++AnticlineStudy area03060kmSbITEs Slopentral5oQ contour(m)o0o Gasfield4+ AnticlineSouth Slopeol Gas kitchen■ golmudFig. Ia. Map showing the oil and gas field locations and surface recognizable anticlinal structures in theQaidam Basin, NW China lb. enlarged map of the study area showing the Quaternary sediment con油小m,sms2,The origin of shallow biogenic gas is a topic in- et al., 1998 Okyar and Ediger, 1999; Lin Chunmingvolving many discussions( Wang Wanchun et al., et al., 2004). Such gases are different from routine2005). It is generally believed that biogenic gas is the ones( Liu Luofu et al. 2005), and typically rich inproduct of anaerobic degradation of sedimentary organic methane but not associated with oil and their originsmatter at low temperatures and it is typically trapped can be constrained by carbon and hydrogen isotope daI immature sediments( rice1中國煤化工,197:Jmes,1983and Claypool, 1981 ). The suitable biogenic gas habi- SheCNMH Gennett, 1983; Fabertats include swamps paddy fields, anoxic freshwater 1987, Berner and Faber, 1988; Galimov, 1988bays, glacial drifts andWhiticar, 1990, Floodgate and Judd, 1992 Jenden etrine sediments beneath the anaerobic sulfate-reducing al., 1993 Baylis et al., 1997aplanzones( vilks et al. 1974: Rashid and Vilks, 1977Schoell, 1986*1883, 1988: Whiticar, 1990: AlbertThis study on the Pleistocene gas accumulationsQuaternary shallow gas in eastern Qaidam Basin, NW China45in eastern Qaidam Basin aims to describe the gerCenozoic sediments on the top of the pre-Mesozoic base-geological settings of shallow gas pools with anment consisting of metamorphic and igneous rocksphasis on the salt lake sediments discuss the condi- Structurally the badivided into a meions required for the formation of gas pools, and pro- block zone in the north and two Cenozoic depressions invide a synthesis on the geological controls of shallow the west and south( figenlergas distributionbasin during the Jurassic were located within the northern Fault Block Zone( NFBZ), with several Jurassic2 Geological settingfreshwater lacustrine and deltaic coal-bearing petroleumource rocks being developed. During the Tertiary theThe Qaidam Basin is situated on northern Qinhai- depocentres shifted to the Western Depression(WD)Xizang( Tibet) Plateau(90°-98°20′E,35°55′with the sediments being characterized by saline lacus-39°10′N), with an area of121000km2and2600trine source rocks. The collision with the northeastward3600 m altitude above sea level. This intermontane ba- moving Indian plate during the Late Himalayan orogenysin is surrounded by Kunlun in the south, Qilian in the at the end of Tertiary resulted in differential uplift in thenortheast, and Altun mountains in the northwest( with Qaidam Basin and the depocenters shifted further to thethe altitudes ranging from 4000-4500 to over 5000 m, Eastern Depression(eD) during the deposition of theFig. 1). It is filled with over 16000-m-thick Mesozoic- Pleistocene strata( Fig. 2ADepth A I Western DepressionSanhu regionZ. Depocen10000AAltitude (m)South SlopeCentral sNorth SlopePluvial faciesFluvial delta faciesShore take faciesShallow lake faciesSanhu region87w0Fig. 2. Cross sections showing the shift in depocentres in the Qaidam Basin and the variation in Quaternary sedThe Eastern Depression with an area of approxi- contents(0. 5%-2.5%), which increase upwardnately 37000 km, contains an inland sedimentary se- within each sedimentary cyclence dominated by lacustrine clastic rocks( FigThe Pleistocene sediments in the Eastern Depres2B ). Its depocentres during the Quaternary were located immediately south of the current biogenic accumu-如Hry工中國煤化工mmlation zones. The Pleistocene sediments were deposited rositCNMHGontent(>30%in brackish water-saline lakes that peaked at approxi- little lithification( Kang Zhuling et al. 2000). Basedmately 1.5 Ma( equivalent to the K, esistivity marker on its lithology and wireline log characteristics, thein Fig. 2B ) The sediments generally have high CIPleistocene stratigraphic column can be divided fromSU Aiguo et alm)60.60Gas pay payzone porosity(%)sampling Permeability(mD)0405目5o中國煤化工CNMHFig. 3, Stratigraphic column of the Pleistocene Sebei Formation in the Tainan Gasfield area showing the po-itions of stratigraphic correlation markers gas pays and payzone divisionsQuaternary shallow gas in eastern Qaidam Basin, NW Chinathe bottom to the top, four formations: Sebei( Qi), tography and gas chromatography-mass spectrometryChaerhan( Q2), Dabusen( Q3)and Yanqiao( Q4)All of the rock samples were also characterizedThe Sebei Formation is the main gas-bearing one in the routinely for their seal and reservoir properties( minerstudy area, and it can be divided into three units alogical composition pore texture, porositFig. 3). The lower unit corresponds to the Ko -K3 ability ) Detailed bacteria counting was made on aarker in Fig. 2B, with a thickness of about 310 m. number of recent mud samples from the bottom of theIts lithologies include shallow gray-brownish gray mud, Qinghai Lake and mudstone samples from 15 explora-sandy mud, silt, muddy silt interbeds with dark gray tion wells(0-2000 m in depth )using the Maximumcarbonaceous mudstone layers above theProbable Number approach Siebert and HattinghThis unit is coarsest among the pleistocene sediment1967). Several culture sampleswith sands and silts accounting for over 100-m cumula- quent laboratory incubation experiments to estimate the) gaIn order toThe middle-upper units are equivalent to the K-K1o evaluate the key geological controls on the occurrencemarker( Fig. 2B ) with a cumulative thickness of of biogenc gas accur1450 m. Their lithologies are dominated by dark gray- mudstone isopach maps were made based on seigray muds, with some shallow gray silts and muddreflection data, calibrated using mudg and wirelinesilts, in over 10 upward coarse to finer gravel gradinglog data from approximately 110 wellscycles. The middle-upper units are generally rich inmuds(70%-80%), and are considered the main4 Results and discussionsource and reservoir units for biogenic gas in this basin. The regional seals for the gas accumulation are the4. 1 Gas geochemistry and originoverliEng Chaerhan, Dabusun and Yanqiao formationsThese formations have a total thickness of about 300Fifty-five shallow gas sampless romm, consisting of several evaporite sequences. The Yan- and Sebei-2 gas fields were analyzed for both chemicalqiao Formation is dominated by evaporites whereas the composition and stable carbon isotope ratios of theChaerhan and Dabusun formations contain mainly gray methane. Hydrogen isotope data of the methane wereto shallow brownish mudsdozen of gas samples. Themarized results Table 1) showed that the samples3 Samples and methodswere dominated by methane(>99. 9%), with minorC2+ hydrocarbons( <0. 07 %), N2 and CO2. GasUsed in this study are 55 gas samples, 320 mud- samples from the Yanhu gas field containone samples and 240 sand/siltstone samples collected CO2, as well as trace amounts of H, S( Table 1 ) Thefrom the Pleistocene strata in the Sanhu Region of east- 8C and SD values of methane in these gases rangeern Qaidam BasinThe 22 exploration and appraisal from -68.51%0 to -65 00%0 and -227.55%0towells from which the samples were collected cover all221. 94%0, respectively. The 8D values of methaneof the discovered shallow gas accumulations in the are within the range of worldwide commercially prostudy area. The gas samples collected at their well- duced gases from sedimentary basins ( -310%0toheads or separators, were analyzed routinely for chemi130%0, Schoell, 1980 Jenden et al. 1993).Thecal composition and for stable carbon hydrogen and SD values of the Sebei gases are close to those of thehelium isotope composition of methane. Oil-associated oil-associated gases from the western part of the Qaidgas samples from several oilfields in the western and am Basin(-210 8%057. 8%0), suggesting thenorthern parts of the Qaidam Basin were also collected common origin of methane from sedimentary organicand analyzed for geochemical comparison. In addition matter( Fig. 4). This is supported by the lack of a reto routaleomagnetic and paleotectonic analysis, markable depletion of deuterium in methane that isthe mudstone samples were analyzed using a Rock- commonly associated with abiotic methanogenesis(eEval/TOC analyzer for their total organic carbon con- gtent( TOC ) A portion of the mudstone samples were al中國煤化工n) These results indicateCNMHGallow gas in contrast toCI and total salt contents were determined by ion those thermogenic gases derived from other parts of thechromatography. The selected mudstone samples were Qaidam Basin. As reported in Su Aigauo et alextracted using dichloromethane for their soluble organ- ( 2003), gases derived from the Jurassic coal-bearinic content, and the extracts were subjected to routine strata in the Northern Fault Block Zone( e. g. the Nanpetroleum點靴據(jù) hical characterization by gas chroma-baxian, Lenghu and Mahai fields) contain 81%0SU Aiguo et al94% methane and 1%-10% N,+C0,, with8C1( Zhang Xiaobao et al. 2002 )contain 78%-95%values in the range of -24%0 to -37%00. On the othemeN, +Co,, with 8Chand oil-associated gases in the Western Depression in the range of -42%0 to-29%0Table 1. Chemical and isotopic compositions of natural gases in the Qaidam BasinComposition( %)61C(%o)Field Well No. Depth( m)CH4 C2H C3H8 8CBHe/HeTainan Tn41717.8-1721.299.930.060.01-68.54-46.52-32.58T6159.0-160201050.5998.240.050.03-6825-36.30-23.95-1327(4.27±0.13)×10-70.30131053,4-1056.80.6899.240.060.02-68.518-1456.40.0070.4098.230.970.22-66.32-4466.21-43S241081.2-1084.80.0399.9766.65-43.76-31.76S791026.2-1031.20.00599.2043.63-322,050.1197.84s211479.2-14820.4499.210.250.06-64.90-37.66-23.57-20.04(1.21±0.11)×10-70.09Sx1132.2-1135.62.170.6896.950.070,03-67.03-37.5721.51(4.19±0.65)×10-80.03dark mud beds are presently buried at up to 1500lepth, with a cumulative thickness of up to 1400 mThese beds account for more than 70 of the sedimentary column, distributed over an area of up to 30000cO reductionkm( Fig. 1 ). According to sporopollen assemblagesit of the sebeFormation was formed in a shallow lake under relatively-300 Sebei gasfields / Fermentationwet climatic conditions ( 1. 5-1. 7 Ma), whereas the▲ Western Depressionmiddle-upper units contain more evaporates indicatinga cooler and drier climate after 1.5 Ma because of the7535high altitudeC1(‰)The hydrocarbon source beds in the study areaclude organic-lean dark mudstones with 0. 15%00. 46% TOC and 100-200 mg extract per kg rockDang Yuqi et al. 2003)and carbonaceous muds con-taining up to 19% Toc with an average of 9. 06%)U -40F Thermogenic gaand 6000 mg extract per kg rock. Data obtained fromples and eleemente demineB( Table 2 )indicate dominantly type-III organic matterand the relatively shallow burial and low vitrinite reflectance values suggest thermally immature sedimentsDue to the large difference in paleoclimatic conditionsFig. 4. Gas isotopic compositions as indicators for the bias suggested by the sporopollen data( Gu Shusongogenic origin of gases in the study area. Fields for ther-1993 Dang Yuqi et al. 2003), the8C values of i-oogenesis, fermentation and CO, reduction were takesolated protokerogen samples from the upper-middle u-from Kaplan et al.( 1997)中國煤化工 are considerably higher4. 2 Hydrocarbon source and gas generationCNMHGfrom the lower unitThe dark mud beds of the shallow toA sufficient supply and adequate preservation oflake facies in the Sebei Formation are possible sourcesorganic matter is essential to the formation of bicfor the shallow gas( Zhou Zhuhong et al. 1994; Daigas accumulations( Oremland and Taylor, 1978). belingxing ta =2003, Ding Anna et al., 2003). The cause of the lake basin's high altitude(2600-3000Quaternary shallow gas in eastern Qaidam Basin, Nw Chinam), high water salinity(>15%)and stror00-1750 m. Mud logging resultscation, the oxidized zone of the lake sediments is nar- ration wells indicated that the maximum methane genrow leading to excellent preservation of organic matter eration in the study area occurred at the depth of 500in the Quaternary sediments under the Sanhu Re1500 m, corresponding to a formation temperature ofIn the study area, sulfate-reducing bacteria occurred 27-55C. Laboratory incubation experiments usingfrom a few centimeters below the sediment-water inter- the Quaternary sediment samples collected from theface to a maximum depth of 200 m. Anaerobic bacteria study area indicated that methanogens thrived mostlywere commonly observed at the bottom of the water col- under 62C and virtually no methane was generatedumn and within the sulfate reduction zoneowenduring experiments conducted above 75C( Deng Yu etthe most favorable conditions for methanogens to thrive al. 1996)appeared to be in the sediments with burial depth ofTable 2. Organic enrichment and kerogen composition of Quaternary sediments in thSanhu Region of the Qaidam BasinWell No. Depth( m) LithologyH/C O/C S,S3 HI OI 8C(%e)Kerogen type Ro%) TOC(%River mudDark mud1.060.3510.99395360.42Lake mudⅡTnl114. 5 Dark mud0.490.200.548ⅢDark mud0.610.190.24187623.1Ⅲ300.0 Grey mudstone5.222.290.242290Ⅲ350.0 Grey mudstone0.590.250.3022752574266520.0 Grey mudstone0.950.160.3146149-24.5Ⅲ20.780.300.9255600.21Sz6 973.0 Grey mudstone with carbonaceous band 1.06 0.25 2.22 151 68 -25.8 2 0.36 1.47Grey mudstoneband1.230.194.6926056279.481206 Grey mudstone with carbonaceous band2.51240 Carbonaceous mudstone with marlstone 0.75 0.19 1. 22 67 5522 Grey mudstone with carbonaceous band 0.98 0.24 2.14 129 60Ⅲ0.2732methane formation usually occurs via sediment column is needed for significant biogenic gasone of the two routes, reduction of carbon dioxide and generation whereas 0. 35% TOC and 50% dark mudacetate fermentation, even though the latter has both stone in the sedimentary column are necessary for faeffects: acetate can be converted to methane and car- vorable biogenic gas kitchens( Gu Shusong, 1993;bon dioxide by methanogens and the resultant fermen- Zhou Zhuhong et al. 1994). Although the specifictative carbon dioxide in turn provides carbon source for merit of these criteria is debatable this approach doesthe methanogens to add hydrogen to form methanenot give us a practical tool for preliminary biogenic gasBalch et al., 1979; Mah, 1981). As discussed ear- potential evaluation. Based on these criteria, the likelylier, methanogens are common in the Quaternary sedi- and favorable biogenic gas kitchen areas are estimatedments of the study area. The stable carbon and hydro- to be around 15000 and 4500 km2, respectively( fisgen isotopes of the methane in the Sebei Gasfield are 5). Perhaps not surprisingly all of the known biogenicsignificantly different from those of thermogenic gases gas fields occur within the most favorable gas kitchensin the Western Depression of the Qaidam Basin and with the gas generation intensity greater than 3.5 bilthose from acetate fermentation( Fig. 4). The extremely low acetate concentrations in the potential gasovef RCM ner km中國煤化工source rocks(6-50 ug/g)are clearly favorable to aCNMHGderivation from carbon dioxide reduction( Ding AnnaThe quaternary biogenic gas reservoirs in the San-etal.,2003)hu Region were deposited mainly near a shoreline-shalPrevious internal studies by PetroChina Qinghai low lake setting with the reservoir rocks being primariOilfield Company suggested that a minimum of 0. 18%ly beach sand sheets and small sand bars. The litholoTOC and 3o 9 ark mudstone within the Quaternary gy is dominated by silty detritus with siltstones and ar-SU Aiguo et alsiltstones accounting for over 90% of thereservoir volumes. Fine-grained sandstones and oolitic4. 4. 1 Differential porosity texturesandstones are seen occasionally. The proportions ofAs mudstones are dominated by fine-grained clayquartz, feldspar and other detritus are usually in theminer porerangesof38%-60%,17%-39%and5%-28%much higher breakthrough pressure than sandstonesrespectively. The sheet-like beach sands extend exten- Results obtained from the Qinghai Oilfield Companysively laterally with each gas field and some show ex- database indicated that the microporosities in the mudcellent correlation across several gas fields. The thick- stones are mainly 16-160 nm in diameter 2-3 orness of individual sand layers ranges from 1-3 mders of magnitude lower than those of sandstones. Thewith some sand bars up to 6 m. These sandy reservoirs difference in porosity texture between the different ligenerally show strong vertical heterogeneity but good thologies led toan increase Inpermeability from mud-lateral homogeneity accounting for 16%-28% of the stone, through silty mudstone and argillaceous silttotal Quaternary sedimentary columnstone to siltstone( Fig. 6). a difference of over threeand siltstone would effectively stop or reduce the esthe underlying strata, forming thebackbone of the Quaternary gas sealsThe water saturation levels of the Quaternary mud-30 kmWhen its pores are filled withith high saliniGolmudthe gas permeability of a mudstone will be drastically回c國hk②mh區(qū)uhreduced for three different reasons. Firstly the in-creased elasticity reduces the probability of microfracture formation. Secondly, the swelling of hydrophilicig. 5. Variations in gas productivity in the Sanhu Region ofclay mthe Qaidam Basin showing the relative locations of gas fieldand gas source kitchensdimension of the rock. Thirdly the presence of waterincreases the rock capillary pressure and thus the sealBecause all the Quaternary reservoirs in the study breakthrough pressure. This is in contrast to the siltarea are currently at the early stage of diage20%Booey stones and sandstones that usually show much largermostly 22%-36% ). The permeability ranges from 10 4.4. 3 Hydrocarbon gradient sealto 2694 mD, with an average of 595 mD. The porositygenerally decreases with increasing burial depth. TheDark mudstones serve as both source rocks andporosity and permeability vary with lithology( Fig. 6), caprocks for the biogenic gas. As the methanogenesiswhich is more noticeable in deeper strata. As shown inin a dark mudstone proceeds a gradient in methaneFig. 7, over 120 porous sand layers have been identi- concentration will be created, leading to diffusion offied in the sebei formation within the sebei-1 Gasfieldmethane from high potential to low potential field. TEPliocene stratadiffusto sometent stop or slow down the vertical flow of methane from4. 4 Caprocks and gas seal capacitiesthe underlying rocks, thus forming a gas seal virtuallystones in the study area areby the hydrocarbon concentration gradientcharacterized by poor consolidation and high po中國煤化工It is highly remarkable that such rocks could form ef-fective seals for such giant gas accumulations as obCNMHGthree factors is vital forserved in the Tainan Sebei 1 and 2 gas fields. Thethe formation of immediate gas seals with the biogenicfollowing four factors are considered important in thissource/reservoir beds. The presence of 400-800-mthick mudstone, anhydrite and halite above the verti-cally stacked multiple gas pays also provides excellentregional caprocks for the biogenic gas paysQuaternary shallow gas in eastern Qaidam Basin, Nw China500一一一,一一+一E≥8目Argillaceous siltstone含Mudstone0.052035Porosity (%)Fig. 6. Relationships between porosity and permeability as a function of reservoir lithology in the study areaSebei-1 GasfieldTainan GasfieldSebei-2 Gasfield0 2km22722272:蛋xyKKKFig. 7. Structural maps and cross sections of the three largest biogenic gas fields in the Qaidam Basi4.5 Biogenic gas trapsamplitude( 450 and 370 m), and steep dipping( 35and 20, respectively ) The five anticlines in the mid-Figure 7 shows the structural enclosures and cross dle row show large variations in enclosure area( 596sections of the three major gas fields in the study area. 96 km)and amplitude( 200-18 m), with relativelyA total of twelve structural traps have been identified so gentle dipping(<5). The Sebei-1 and -2 structuresfar in the Sanhu Region of the Qaidam Basin( Fithird row of the structural belt with1), 11 of which are surface structures. All of them are extr中國煤化工30′) Their enclosureQuaternary anticlines occurring along the NorthernareaCNMHGI amplitudes 60 and 17Slope of the Eastern Depression. They are distributed m, respectivelin three rows trending in an nW-se direction TheSurface erosion and fault intensity in the quateramplitude of the anticlines decreases from northnary sediments decreased from north to south. For thesouth: the two anticlines in the northernmost position three rows of structures, drilling results indicated thathave largee18sre areas(600 and 370 km2), big the residual Quaternary sedimentary columns increasedSU Aiguo et alfrom 385-470 m in the north, 848-1156 m in themiddle, and over 1600 m in the south. A number ofhighly angular normal faults perpendicular to the anti-(A)Presentcline axis were observed in the structures of the northand middle rows. However only 2 small, near-sur-face, normal faults have been observed in the sebej-2anticline of the south rowThe above structural variations generally favor the中1750better preservation of anticlinal structures thus impro-ving prospect for gas accumulation in the south rowExploration results showed that little gas has been dis-covered from the structures in the north row. therehave been found two small gas fields( tuofengshan andYanhu )and a gas-bearing anticline( Taijinaier )in themiddle row and two giant gas fields( Sebei-1 and -2)(B)After deposition of Ks markerIn addition a giant gas field has also been discovered recently in the Tainan anticlinal structure. Thisstructure is located within the Central Sag area of theEastern Depression, adjacent to the Northern SlopeFig. 1 ). It is a near E-W trending syndepositionalshort-dimension subtle anticlinal structure, with no(C) After deposition of K9 markertraceable faults or surface erosion The structure has Eextremely gentle dippin,1°15′-1°35′ in the southand1°35′-1°45′ in the north flank. Its enclosurearea is 104 km, and amplitude 110Therefore the most favorable traps for gas accu-mulation in the study area are located on the NorthFig.8. Structural evolution of the tainan anticline.Slope, near the depocenters and effective gas kitchens al amplitude reaching approximately 30 m by the timethe Central Sag area. These localities have experi-enced only mild tectonic activities, with relatively littleof the K, deposition. During the Middle-Late Pleistocene to Holocene, the Qinghai-Xizang( Tibet ) Plateauwas strongly compressed by the subducting Indian Plat-4. 6 Biogenic gas accumulation modelsform. The resultant Tainan anticlinal structure has anAll of the biogenic gas accumulations in the studyamplitude of 110 m for the Kg marker. The structuralarea occurred in weriealy stacked payzones each with evolution is similar at the Sebei-1 and Sebei-2 gas fieldseparate gas-water contact. As the Quaternary sedilocations except that the structures were formed slight-ments were deposited in rhythmic bands most of thend shale layers interbedded withsandy/silty beds can only act as dynamic barriers. On-Figure 9 illustrates the expected sequence of ely those thick mudstones located near the top of eachvents as biogenic gas migrated into a previously waterthmic cycle could be considered effective sealswet structure. The migrating oil initially entered theprevent upward gas migrationDetailed examination of the Tainan Gasfield bymudstone caprocks and only a relatively small part ofDai Jingxing et al.(2003)indicated that the Tainanthe accumulation was initially gas saturated. As moreanticline was initiated at the Early Pleistocene( Late gas migrated into the structure the increased buoyantHimalayan orogeny because of the compressing force中國煤化工 lace water and drove thefrom north and south directions( Fig. 8). During the gas-CNMHG the gas-water contactdeposition of the Ku marker, the amplitude of thewp UI une underlying porous sand/siltTainan anticline had reached a few meters. This waslayer, the gas began to accumulate in thifollowed by a relatively calm period of structural devel voir zone. Freshly generated gas, generated from oneopment. After the deposition of the K, marker the an-or both sides of the trap, was forced to advance into theticlinal d高熱輯 It was accelerated, th the structstructure and drove the gas-water contact of this newQuaternary shallow gas in eastern Qaidam Basin, Nw China(A)Stage I二二二二二=二二(B)Stage 2(C)Stage 3===-Fig. 9. Generalized biogenic gas accumulation model for the Sanhu Region of the Qaidam Basinreservoir zone further down. Eventually multiple pay- flanks of the anticlinal structure i (3) the presence ofzones with close vertical approximation would be many mudstone interbeds within the gas column, lead-formed for a single trap. These payzones can be con- ing to multiple gas-water systems (4) high free gasidered as a single gas reservoir unit, ultimately con- flow during testingtrolled by the same thick mudstone caprocks. As moreIncreasIng trap enclosure area and gas reservesgas was accumulated in the trap the gas column grew. with increasing reservoir depth for all of the major gasWhen the buoyant pressure became greater than the fieldscritical breakthrough point of the mudstone seal,someof the pores in the seal open and gas leakage occurred. 5 ConclusionsWhen gas leakage reduced the net gas buoyant pressureto or below the mudstone breakthrough point theThis study provided an overview on the geologicalstone became a gas seal again and gas accumulation setting and geochemical characteristics of the Pleistocontinued. Therefore, the formation of biogenic gas ac- cene shallow gas accumulations in eastern Qaidam Ba-cumulations in the study area is the result of highly dy- sin, Nw China. The dominance of methanenamIc processes99.9%)he 8C and 8D values of methaneTesting results showed that almost 99%o of the bio-中國煤化工and-227.55%togenic gas reserves occurred in the Lower-Middle PleisCNMHGthese gases suggested atocene strata( between K, to Ko marker ) Key charac- biogenic origin associated with methanogenesis of sediteristics of the biogenic gas accumulations in the study mentary organic matter under relatively low temperaarea include( 1) shallow burial depth( 500-1500 tures( <75C ) The formation of a large quantity ofm); (2) strong structural control on gas yields withbiogenic gas in the Pleistocene sediments of the studythe gas yields &teasing drastically from the top to the area is possible as the sufficient supploly and adequateSU Aiguo et alpreservation of organic matter is ensured by the lakearacteristics[J]. Natural Gas Geosciences. 14, 402-407( inbasins hiigh altitude( 2600-3000 m ), high water Faber E. 1987) Zur Isotopengeochemie gasformiger KohlenwasserstoffeChinese with English abstractsalinity(>15%)and strong stratification. 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