Geological constraints of giant and medium-sized gas fields in Kuqa Depression

Geological constraints of giant bna d ade so h mean e, mpor ant da van es aveand medium-sized gas fieldsbearing belts of the Kuqa Depression. The Triassic andJurassic source rocks have been determined as the mainin Kuga depressiongas source rocks". The combination of Tertiary gypsumrocks with underlying Cretaceous and Eogene sandstonesJIA Chengzao1, GU Jiayu?& ZHANG Guangyaprovide the most favorable reservoir-seal assemblage. Theforeland basin consists of the Kuga Depression and TabeiUplift, and is different from the typical one owing to the2. Research Institute of Petroleum Exploration and Development, Petro-China, Beijing 100083, Chinaabsence of the passive continental margin and marinesequence. Many geologists believe that the hydrocarbonAbstract There is a gas-rich and well-charged petroleum pools in the Kuqa Depression are late-accumulated andsystem in the Kuqa Depression where Triassic and jurassic the conditions are favorable. Although each of the factorssource rocks play important roles. Distributed in an area of that constrain the formation of oil and gas pools in themore than 10000 km and with a thickness of up to 1000 m, Kuga Depression has been deeply probed, few studiesthey are composed of dark mudstones, carbonaceous mud- have concerned about the space-time match of variousstones and coal seams containing 6%0, 40%0 and 90% of ToC,respectively, and are mainly the humic organic matter. As geological controls and the formation and distributionhigh-quality regional cap rocks, the Neogene and Eogeneregularity of giant and medium-sized gas fields have notgypsum rocks and gypseous mudstones matched well with been understood well. This paper mainly summarizes thethe underlying Neogene and Cretaceous-Eogene sandstones. geological constraints of giant and medium-sized gasThey have formed the most favorable reservoir-seal assem- fields and the gas accumulation theory in the Kuqa deblages in the Kuqa Depression. Also the Jurassic sandstones pression. It is believed that the achievements in this paperand mudstones formed another favorable reservoir-seal will clarify much more the prospecting direction in theassemblage. The traps are shaped late in the fold-thrust belt, research area and act as a guide to the exploration of othermainly fixed in the Tertiary-Quaternary, where ten structure foreland basins in central and western Chinstyles have been distinguished. These traps spread as a zonein N-S are scattered like a segmental line in W-E and show 1 Stratigraphy and reservoir-seal assemblagestier-styled vertically. The best traps are gypsum-salt coveredfault-bend anticlines related to the passive roof duplex. This(1)Stratigraphy and reservoir-sealhe stratigraphy in the Kuqa Depression consistthe early Himalayan Movement, mainly gas condensate and mainly of the Meso-Cenozoic successions and their totaloil accumulated and were distributed in the outer circular thickness is up to 10000 m. The Mesozoic successions,region of the kitchen; whereas in the middle and late Hima- having a thickness of 2000--3000 m(4000 m to thelayan the gas accumulations mainly formed and were dis- maximum), are lacustrine-swamp-fluvial deposits of graytributed in the inner circular region near the kitchen. The colored alternations of sandstones and mudstooverpressure of gas pools is common and is formed by seal bedded with some coal seams. carbonaceous mudstonescapacity of thick gypsum layers, extensive tectonic compres- and oilshales in the lower and middle parts. With a thick-sion and large uplift. The well-preserved anticline trapsunderlying the high-quality regional cap rocks of the Terti. ness of 3000-5000 m(8000 m to the maximum), theary gypsum rocks and gypseous mudstones are the main Cenozoic successions are lacustrine- fluvial deposits com-targets for the discovery of giant and medium-sized gas fields. posed mainly of brown alternations of sandstones andAbove conclusions are important for the petroleum geology mudstones; two sets of gypseous mudstone and gypsumtheory and the exploration of the fold-thrust belt in foreland rocks are developed in the lower-middle parts, and thinbasins in central and western Chinadolostones are interbedded near to the bottomKeywords: coal-bearing source rock, fault-related fold, oil and gasThree main reservoir-seal assemblages are developedpool, giant and medium-sized gas field, Kuqa Depressionin the Kuga Depression. (1) The Jurassic assemblage ofcoal-bearing mudstones of the lower Kezilenuer formaThe Kuqa Depression lies in the northern Tarim tion to the upper Y angxia Formation combining with fine-Basin with an area of 28500 km, and can be subdivided grained sandstones and gravel-bearing sandstones of theinto Yangxia, Baicheng and Wushi sags from east to west. middle Yangxia Formation, where the Yinan-2 gas poolThe significant breakthrough has been made in the fold- has been found, occurred in the northeastern part of thethrust belt in the Kuqa Depression in recent years. A large deprgas accumulation area has been found, which comprises a rock中国煤化 Te or the Eogene gypsumcombining with the eo-group of gas fields and gas-bearing structures such as genCN MH Geous Bashijiqike FormaKela-2, Kela-3, Dabei-1, Yinan-2, Tuziluoke, Dina-1, tion sandstones, in which Kela-2, Kela-3, Dabei-1Zhang Guangya, Li Honghui, Jia Jinhua et aL., Evolution and hydrocarbon-resource assessment of Kuqa foreland basin, Tarim Oilfield Corporation andResearch Institute of Petroleum Exploration and Development of PetroChina Company Ltd. 1999. restricted reportChinese S序嫂弻 etin Vo.47Supp. December2002Dina-1, Dina-2 gas fields(pools) and Quele-1 oil pool Kela-2 gas fieldhave been found, spreads over the central and westernThe Tertiary is characterized by the deposits of fluparts of the Depression and is the best assemblage in the vial, littoral and saline lake in the dry climate, such as theKuqa foreland basin. (3) The Neogene assemblage of the more or less gypseous conglomerate, sandstone, mudstone,Jiedike Formation gypseous mudstones combining with micritic limestone and anhydrite. Commonly, the Eogenethe siltstones and sandy conglomerates, where Tuziluoke, reservoir sandstones are thick and coarse in the easternDina-I and Dina-2 gas pools have been found, is distrib- part and thin and fine in the western part. With secondaryuted in the eastern and southern parts of the depressionpores as main reservoir spaces, the basal sandstone isi) Reservoirs. The main targets of the explora. important reservoir for the Kela-2 gas field with the potion in the Kuqa Depression are the sandstones of the rosity of 3. 97%0-18.14%(12.3% as the mean) andJurassic, Cretaceous Bashijiqike Formation, the Eogene, meability of 0.03.X10-3-202x10-Hm2(52.6.x10-3and the Neogene Jidike Formation. Sandstones of the as the mean)Cretaceous Bashijiqike Formation and the eogene are themost important reservoirs among themThe jurassic Ahe Formation Yangxia Formation andKezilenuer Formation are main reservoir intervals. TIare feldspathic litharenite and become thicker northwardand thinner southward with the thickness of 407--1336 melI KLThe sand bodies of channels and subaqueous channels ofbraided delta are the main reservoirs. With huge thicknessand rare muddy interbeds, these sand bodies are regionallyand constantly distributed. Their reservoir properties be-come worse from east to west. Reservoirs along tugeer⑧!2回3国:4國和5ming Anticline-Tuziluokegou in the east have the porosityof 10.7%-17.25% and the permeability of 0.71.x10Fig. 1. Braided-delta depositional model of the first and secor418.>10-?with mainly residual primary intergranular alluvial plain: 3, braided delta front: 4. braided prodelta;5, wellpores, frame-grain dissolution pores and intergranularenlarged pores. Reservoirs in the western part are lowThe Neogene Jidike Formation reservoirs are alluvialpermeability reservoirs or fractured compact reservoirs fan-lacustrine coarse-grained sandy conglomerates withwith the porosity of 6%--8% and the permeability of <1 the total thickness of about 120 m. It can be subdivided>10-um?. The frame-grain dissolution pores are the into five lithological intervals with the stable area distri-main reservoir spaces and primary intergranular pores bution, of which the sandstone and mudstone intervals andhave mostly vanishedbasal conglomerate interval are the main reservoirs OutThe sandstones of main cretaceous reservoirs are crop observation in the eastern Kuqa Depression indicatesfeldspathic litharenite and litharenite deposited in the that the littoral mudstones and gypsum permeability barribraided delta and lacustrine littoral environments. They ers are well developed in the sandstone and mudsstonhave low compositional maturity and moderate textural intervals. The single bed of sandstones is thin and has amaturity. Their thickness ranges from 100 to 300 m. of bad vertical connection, so that the sandbody architecturewhich the Bashijiqike Formation takes the most part.is of the "multilayer sandwiched style. The sandstonesFrom top to bottom, three lithological intervals can be have a porosity range of 8%-16% and permeabilitysubdivided. The first and second ones have good reservoir range of 0.1.x103--10x10um(209->10um as theproperty and are characterized by the medium porosity maximum)and medium permeability or the medium-low porosity andi1)Cap rocks. There are three main regional caplow permeability. With residual primary intergranular rocks in the Kuga Depression: the gypsum rocks and gyppores as the main void spaces, their mean porosity is seous mudstones of Neogene and Eogene and the Jurassic13%-13.74%(22.4% as the maximum) and the mean coal-bearing suites. The Neogene and Eogene cap rockspermeability is 14.76.>10-3-6.96x10-3 Hm2(2340.> are the most important regional cap rocks and most oil10-um as the maximum). When they were deposited, the and gas accumulated beneath them. The Eogene gypsumtopographic surface was relatively flat. Many streams rocksunloaded sediments in front of the mount foot and shaped theH中国煤化工 are distributed mainly ina thickness of more thanbraided the alluvial plain and a wide distribution of sand- 300CN MH Giffusion coefficient of gasbodies(fig. 1). The reservoir model is described as a su- through gypseous mudstones is as low as 9.98.x10-13m2/sperimposed type with good connection and rare perme- but the one through mudstones, 1.89..>10-3 m?/s.Theability barriers as the main production interval of the extremely strong seal capability of gypseous mudstones isChinese Science Bulletin Vol 47 Supp. December 2002the fundamental factor for the formation of the Kela-2(1. Structural framework. The strong compressiongiant gas field. The gypsum rocks and gypseous mud- of South Tianshan Mountains resulted in the formation ofstones of the Neogene Jidike Formation are developed in fold-thrust belts in the Kuga foreland basin, which is disthe eastern Depression and connected spatially with the tributed in lines and zones(fig. 2)8-11. In addition of theEogene high-quality cap rocks and formed a united cap zonation in the south-north direction. the structural stylerock for the whole Kuqa Depression. With a thickness of distribution also shows segmentation in the west-eastabout 330 m, the Jurassic coal-bearing cap rocks of the direction and tiers in subsurface. The segmentation in thelower Kezilenuer Formation to the upper Yangxia Formation are mainly composed of thick-bedded carbonaceous west-east direction is obvious and the basin is subdividedmudstones. mudstones and coal seamsroughly into three segments by the Kuqa River and the2 Structural features and trapsKalayuergun Fault. This segmentation is mainly deter-mined by the changes of the magnitude and direction of(1)Structural evolution. The Kuqa foreland basin the structural compressing stress along the structuralstrikelacks the passive marginal sediments of the pre-foreland The tiers in depth is mainly associated with detachmentsstage and the marine sediments in the early and middle developed at the surfaces ofperiods of foreland basins, which are the characteristics of ers of the Tertiary and marls, cypsum rocks and shale laytones, shale and coal layers oftypical foreland basins. 2. The basin formed in the Meso- the Jurassic. The spatial difference of the structural stylezoic and Cenozoic and is dominated by the continental determines the distribution of giant and medium-sized gasdeposition. It has three evolutional stages: the foreland fieldbasin in the Late Permian -Triassic. the inner- contiClose to the South Tianshan orogenic belt, the north-nental depression in the Jurassic and the reborn foreland ern monocline belt has the basement-involved thrust systhe kuga Basin is a kind of the foreland-alike basin a> Ore, tem and structural wedge controlled by back thrusts inbasin from the Cretaceous to Quaternarydepth. Toward the foreland are the Kelasu and YiqikelikThe Triassic foreland basin showed the features of Tugermin structural belts. The Kelasu belt consists of tworapid subsidence and deposition, which decreased from lines of anticlines on the surface and in the shallow depth,north to south. The basal conglomerates of the Lower which are dominated by fault-progradation folds aboveTriassic Ehobulak Formation consist of alluvial fan and salt and overthrusts along the Eogene detachment. Saltfan-deltaic molasse deposits, indicating a rapid flexural layers may form salt dome structures by themselves. Be-subsidence in the early basin development. During the neath salt, imbricate thrust belts and duplex structuresdeposition of Middle-Upper Triassic Karamay and prevail, forming fault-bend folds and back-thrust blocksHuangshanjie formations, the subsidence rate was higher and showing no accordance of the upper and lower structhan the depositional rate and fine sediments occurred in tures. The Yiqikelik-Tugermin structural belt is narrow,the deeper water. During the deposition of the Upper developing imbricate thrust systems and basement-inTriassic Taliqike Formation, structural subsidence rate volved structures. Further toward the foreland is the Qiudecreased gradually and balanced with the deposition, litage-Dina structural belt, which is divided into theresulting in the shallow water and coarse sediments of the eastern sector, Dina-easterm Qiulitage structural belt, andmeandering river and flood plain. The basin subsided the western sector, western Qiulitage-Quele structural beltrapidly both in Early and Late Jurassic, and was stable in The eastern sector is dominated by fault-propagation foldMiddle jurasabove the Tertiary salt layer and by the passive roof theFrom Cretaceous to early Tertiary was the early sta- duplex structure beneath the salt. In the western sector,ge of the reborn foreland basin, which was characterized fault-propagation folds controlled by counter-thrusts occurby weak tension. During Early Cretaceous, the subsidence above the salts and basement-involved fault-blocks bewas slow, and oxidized sediments deposited in a wide and neath the salts. Salt pillows and anticlines formed in theshallow lake. In Late Cretaceous, the whole basin uplifted salt layers. A series of faulted blocks, controlled by norand underwent different erosion In Eogene, marine trans- mal faults, developed on the forebulgegression took place and the sediments of semi-saline la(ii Structural style of fold-thrust belts. Applyingcustrine and fluvial facies deposited in the Kuqa Depres- the theory of fault-associated folds to the Kuqa Depression In Neogene, the reactivation of the southern Tian-sion中国煤化I-have been established(figshan Mountains resulted in the strong flexural subsidence, 3),cp geometry description ofwhich formed the reborn foreland basin. From the late comCNMH SUrface!). The duplexNeogene to Quaternary, folding took place and the faulted structure is one of the most important structural combina-blocks dissected by normal faults developed in the tions, which consists of a number of similar step thrustsfrontal slopeand fault-bend fold occurring in each thrust slice. WhenChinese S序嫂 letin Vo.47Supp. December2002the back-thrust develops in front of the movement, duplex sion demonstrates that the fault-associated fold model andstructures may develop into passive roof dupltures, the interpretation of key structures are reasonable. Thecalled the triangle zone sometimes. Kela-2 anticline is shortage rate is 20%0-35%0, locally up to 50%odeveloped in passive roof duplex structures, with favorC) Trap types and distribution. Traps of anticlines,able hydrocarbon charge, accumulation and preservation faulted anticlines and faulted noses exist in fold-thrustconditionsbelts 8, among which anticlines and faulted anticlines areThe fold-thrust belts in the Kuqa Depression mainly favorable for the formation of oil and gas pools, whereasformed from the Neogene to Quaternary bl. The deforma- faulted noses are unfavorable for the hydrocarbon presertion migrated from the north to the south 7, forming pro- vation. Faulted anticlines associated with normal faults asgradating thrusts and a variety of fault-associated folds. well as stratigraphic traps onlapped on the unconformityBy use of the balance section technique, the restoration in the frontal slopeand analysis of the fold-thrust belts in the Kuqa Depres010203040kmKela-l=KelaloracbAicheng口 XinhePetroleum pools and Oil seepageFracturePinch out line Linit of levelFig. 2. Major structure unit map of Kuga Depression点NFault-progradation foldFault-bend and decollement mixed foldFault-bend foldFault-progradation andDuplex structureThrust structuredecollement mixed fold中国煤化工Model of mixed-Model of stDetach foldCNMHGand fault-mixed foldFig 3. Major structural types in Kuqa DpressionChinese Science Bulletin Vol 47 Supp. December 2002Well Kela-2NI-kb40006000Fig. 4. Pool-forming model in Kelasu structural beltIn fold-thrust belts, the traps above and beneath saltsThe ToC content is 04%0--5.58% in the dark mudare quite different. Above salts, unsymmetrical or over- stone of the Huangshanjie Formation, 0.80%0-5.98% inturned fault-progradation anticlines, salt dome anticlines the lacustrine source rock of the Yangxia Formation. Theand fault noses were formed by the large-scaled detach- hydrocarbon source rock of the Kezilenuer Formation isment,which are poor for the hydrocarbon accumulation. mainly dark mudstone with ToC 0.75%-5.96%.TheA series of fault bend, fault bend-progradation and fault TOC content of marlstone and coal are 6. 1%--39progradation anticlines, related with passive roof duplex respectively and 41.5%-93.2%. In those hydrocarbonstructures and imbricate thrusts, are mainly developed source rocks, the content of exinite+sapropel is 20%within Mesozoic formations, among which the fault bend 40%, the content of vitrinite+inertinite is above 60% andanticlines related with passive roof duplex structures are the ratio of H/C is below 0.8. The carbon isotope value ofthe most favorable, such as Kela-2 structurekerogen is above -25%. All of those indicate that the3 Hydrocarbon generationorganic matter is mainly type L I(fig. 5). Through theexperiments of the thermal pressure simulation of theThe distribution of Triassic-Jurassic source rocks in mudstones. marlstones and coals from Triassic-Jurassicthe Kuqa Depression is wide, with great thickness and source rocks. it is concluded that those source rockshigh organic matter content. The type of source rock is mainly generate gaseous hydrocarbon. However, the Imainly humic. Most source rocks reach high or over- custine mudstones of the Huangshanjie Formation genermature and mainly generate gas. Thethe funda- ate relatively high oilmental materials for the formation of the giant-sized natu(1, Maturity and history of hydrocarbon generationral gas zone in Kuqa9, 2, as well as the abundant liquid and expulsion of source rocks. The maturity of sourcehydrocarbrocks in the west is higher than those in the east. In the( I) Characteristics of Triassic-Jurassic source rocks. west, the Triassic source rocks reached the hydrocarbonFrom bottom to top, the Triassic source rocks occur generation peak in early Neogene, Jurassic source rocksin the Huangshanjie Formation and Taliqike Formation, reached the oil generation peak in middle Miocene andand the Jurassic source rocks lie in the Yangxia Formation, gas generation peak at the end of Neogene. At presentKezilenuer Formation and Qiakemake Formation. The days, they are generating dry gas. In the east, the Triassicmost important formations are Huangshanjie, Y angxia and source rocks generated oil in late Eogene, generated gas atKezilenuer. The source rocks of the Huangshanjie Forma- the end of Miocene and entered the gas generation peaktion are mainly lacustrine mudstone. The source rocks of since Pliocene; Jurassic source rocks reached the oil genthe Yangxia Formation and Kezilenuer Formation are eraticmainly mudstones, coal and marlstones in coal-bearingstrata. The hydrocarbon source rocks are 12000--14000 coneH中国煤化工ne, the gas generationthey are in the stage ofCNMHhe early period,km- in area distribution. In the Kuqa River and Kapusha- Triassic-Jurassic source rocks mainly generated oil. Afterliang River outcrop section, the total thickness reaches Pliocene, they reached the gas generation peak, which1000 m to the maximumindicates that natural gas is late-accumulated in trapsChinese S序嫂 letin Vo.47Supp. December2002o Jurassic mudstoneNatural gas mainly accumulates in the frontal slope beltslight lighto Carbonaceous mudstone2)Overpressure and accumulation. The Eogene▲ Jurassic coalconglomeratic sandstones and Cretaceous sandstones in■ Triassic coalthe Kela-2 gas field are in a single pressure system. In themiddle of gas pools, the average formation pressure is74.41 MPa and the average pressure gradient is 1.92.20 MPa/100 m, which indicates that the gas pools areghly overpressured. Furthermore, above the regional capocks is the normal hydrostatic pressure system. It is genat the excellent seal capacity from thehuge and thick gypseous cap rocks, strong tectonic compression and uplift are the main causes for the overpressure in the Kelasu structure belt. In the Kela-2 gas field,gypseod gypsum capare 400 m in thickness and exhibit excellent seal capacityVitriniteSapropel+exiniteGenerally, when the formation pressure coefficient ofmudstone cap rocks reaches 1.96, the beds will beFig. 5. Triangular diagram of kerogen maceral of Mesozoic hydrocar- naturally fractured. So this coefficient value is calledbon source rock in Kuqa Depressiondead line"of the hydrocarbon accumulation for mud11)Petroleum system analyses and resource asgypseous mudstones have thesessment. The Triassic-Jurassic source rocks in the Kuqastrong plastic flowability and high breakthrough pressureDshare a unitary kitchen Oil and gas generated under tectonic compression, oil and gas pools can becapped at the pressure coefficient over the"dead line"offrom those source rocks, not only supplied for the Kuga 1.96sag but migrated laterally to the frontal slope belt andaccumulated, and formed the Kuqa petroleum system21The Kela-2 structure developed continuously fromMiocene to Pleistocene while it was charged continuouslThe petroleum system is gas-prone. The total intensity ofthe gas generation reaches 280>108 m/km?. The total by natural gas. The ultimate time of the gas pool forma-areas with the intensity of gas generation over 100.X108 tion is late Himalayan[221. After then the Himalayanmovement caused strong compression, resulting in thecharging system. By the latest resource evaluation, the oil overpressure in the reservoirs sealed or half-sealed byreserve of the Kuqa petroleum system is 4.1>10%t and overlying Tertiary gypsum rocks and gypseous mudstonesthe natural gas reserve is 2.23.X10 mDuring the period of the tectonic compression, accompanied with the relatively great strata uplift and denudation,4 Accumulation characteristics and formation condi-(for example, the denudation thickness of Well Kela-2 istions of giant gas fields1500--2750 m) the original high pressure underneathi Characteristics of petroleum accumulationregional cap rocks is preserved, which makes the strata(1) Late accumulation. By applying multi-methods, overpressure more obviousit is concluded that the petroleum pools are late-accumr(,) Main geological constraints for giant and me-lated in the Kuqa Depression. The three periods of petro- dium-sized gas fieldsleum accumulation are Eogene, Jidike-Kangcun and(1)Regional geological background. The northernKuqa-Xiyu, which correspond to the early Himalayan, Tethys basin groups including basins of Tarim, Kara-Kummiddle Himalayan and late Himalayan respectivelyand Afghanistan-Tajik in central Asia are located in theIn early Himalayan, Triassic source rocks were in the northern margin of the old Tethys Sea. They are characstage of oil generation. There were some fold-thrust terized by the similar basin type, tectonic evolution andstructures formed in the north of the Kuqa Depression. major source rocks 23-25. Those basins mutually constituteThe frontal slope and stratigraphic traps also started to theform. Petroleum accumulated in those traps was destroyed中国煤化工th Tethys, the major hy-by late tectonic activities though partly it formed some docsCNMH Giddle-lower Jurassic tersecondary oil pools such as the yiqikelike oil field. In the restrial and littoral coal-bearing strata. With the high or-middle Himalayan and late Himalayan, the Triassic and ganic matter content, hydrocarbon source rocks have theJurassic source rocks all reached the gas generation peak. high hydrocarbon generation potential. The middle-lowerChinese Science Bulletin Vol 47 Supp. December 2002Jurassic source rocks generally reach high mature or the end of Eogene to early Miocene. thrusts and anticlinesovermature. The high-quality regional cap rocks consist of formed in the north of the depression/27. Concurrently,gypsum rocks and mudstones while the Jurassic coal- faulted anticlines and faulted nose structures developed inbearing successes become the source rocks for the forma- the frontal slope. From Miocene to Pliocene, the maintion of giant and medium-sized gas fields. Jurassic and fold-thrust belt had formed. Triassic-Jurassic source rocksCretaceous sandstones combining with Eogene gypseous mainly generate the liquid hydrocarbon in early Himala-rocks constitute the high-quality reservoir-seal assemblage. yan, and gas in middle Himalayan and late HimalayanThe anticline structures in the fold-thrust belt spread in The maturity is high in the kitchen whereas it is low in thelines and zones, and their formation time matched well outer region. Accordingly, the liquid hydrocarbon gener-with that of the major hydrocarbon-genera-tion peak. All ated in the early period accumulates in the frontal slopethose petroleum geology conditions make the Kuqa De- and north of the depression. The gas generated in middlepression favorable for the formation of giant gas fields Himalayan and late Himalayan mainly accumulates in theand becomes the important part of the gas-rich zone in fold-thrust belt. Those conditions determined that in thenorthern tethouter circular region formed condensate oil-gas pools and(2) Abundance of hydrocarbon underneath Tertiary oil reservoirs such as Yiqikelike, Tugeerming, Yaha,gypseous cap rocks. The two sets of huge thick cap Hongqi, Yingmai-7, Yudong-2, Yangtake, Quele-l, etcrocks of gypsum rocks and gypseous mudstone in the whereas in the inner circular region formed gas pools ofJidike Formation of Neogene and Eogene are one of the Dabei-l, Kela-2, Kela-3, Yinan-2, Tuziluoke Dina-l andfavorable and crucial factors for the formation of giant Dina-2and medium-sized gas fields and high overpressurel26l(ll) Exploration direction for giant gas fieldTheir combination with the thin sandstone layers in theIn the Kuqa Depression, the fold-thrust belts are theJidike Formation of Neogene, the dolostones in the Ku- focal areas for the exploration. There are some favorablemugeliemu Formation of Eogene, the basal conglomerates characteristics as follows: (1) Large area of traps, highof Eogene and the sandstones in the Bashijiqike Forma- closure, favorable traps related to passive roof duplexestion of Cretaceous, has formed the high-quality reservoir- and imbricate thrust faults, and the time match of trapsseal assemblages. Most of oil and gas fields discovered with major hydrocarbon-generation and expulsion peakbelong to those assemblages.(2) the belts located in the kitchen or near to the kitchen of(3) Most favorable traps. The suites of faults Triassic-Jurassic source rocks with high hydrocarbonthrusting from north to south in the e-w strike develop in generating intensity and abundant hydrocarbon chargingthe fold-thrust belt of the Kuga Depression. Among di-(3)possessing high-quality or good reservoir-seal assemplexes and passive roof duplexes controlled by those blage: the cap rocks of Tertiary gypsum rocks and gyp-faults, wide and gentle fault-bend anticlines covered by seous mudstones and Jurassic mudstones combining withthe Tertiary gypseous rocks are the most favorable traps. reservoir sandstones of Tertiary, the Bashijiqike Forma-In those traps in the duplexes, the acting faults connect the tion of Cretaceous and Jurassic strata, such as the middleTriassic-Jurassic source rocks and Cretaceous-Tertiary to Yangxia Formation, middle-upper Kezilenuer Formationhigh-quality reservoirs, and end under the gypseous cap and Ahe Formation; and (4)plays containing abundant oilrocks. So the hydrocarbon charging and preservation are and gas resourcesexcellent, such as the Kela-2 anticline(fig. 4) The thrusts5 Conclusionabove the Tertiary gypseous rocks reach the shallow layeror ground surface and structures are mainly fault-prograThe Triassic-Jurassic source rocks in the gas-richdation anticlines. In the thick gypsum salt zone owinsystem of Kuqa are characterized by the wide spreading,plastic flow, anticlines and faulted nose structures can great thickness, and high content and maturity of organicbe formed by uplifting overlying layers, for which hydro- matterscarbon accumulation and preservation conditions are notThe Tertiary high-quality cap rocks of gypsum rocksgood and some residual oil reservoirs can form. Therefore, and gypseous mudstones are the crucial factors for thethe passive roof duplexes and fault-bend anticlines in formation of giant and medium-sized gas fields. The comDabei-Kelasu-Yinan and Qiulitage -Dina structure belts binat中国煤化工:+h sandstones of cretaceousare the favorable zones for the hydrocarbon accumulation. andreservoir -seal assemblageIn the north of those structure belts. the imbricate thrust wheCN MH GoSe good cap rocksfaults develop and in the south are shallow detached folds vided the excellent seal capacity, accumulated overpreswhere the accumulation condition is not goodsure in gas pools and preserved the charged hydrocarbon(4)Circular distribute of oil and gas pols. FromThe fold-thrust belts in the Kuqa Depression spreadChinese S序嫂 letin Vo.47Supp. December2002in zones in the n-s direction . in a segmental line in thenese),2002,23(1):6W-e direction and in a tiered style in the subsurface. Tho12. Boyer, S E, Elliot, D, Thrust Systems, AAPG Bulletin, 1982, 661196se structures formed earlier in the north region than in the13. Suppe, J, Geometry and kinematics of fault-bend folding, Amerisouth region. 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