An initial discussion on major controlling factors on formation of coal-formed large-medium gas fiel

An initial discussion on major controlling factors onformation of coal-formed large-medium gas fieldsDAI Jinxing, XIA Xinyu, HONG Feng, ZHAO Lin, SUN Dongmin SHI XinResearch Institute of Petroleum Exploration and Development, Beijing 100083, ChinaCorrespondenceshouldbeaddressedtoDaiJinxing(e-mail:ripedgas(@publicbta.net.cn)Abstract The so-called"large-medium gas field "means a gas field with proven reserves equal toor larger than 100X10 m. Up to 1997, 23 coal-formed great-medium fields are discovered in Chinaexcluding two in Taiwan Province. Their proven reserves take 50.9% in total natural gas reservesTherefore, the coal-formed great-medium gas field plays a decisive role in natural gas reserves andthe study of their controlling factors is very important both in theory and in practice. There are sixmajor controlling factors on the formation of coal-formed large-medium gas fields:(i)Thegenerating center of coal-formed gas and its surrounding area; (ii)low potential area;(iii) latepool-forming period; (iv) traps related to the paleo-uplifits in coal-formed gas area;(v)above oretween abnormally pressured compartments; (vi) faulted traps related to coal-bedKeyworks: coal-formed gas, large-medium gas field, controlling factor.The so-called"large-medium gas field"means a gas field with proven reserves equal to or largerthan 100X 10 m. Generally, coal-formed gas refers to the gas dervied from humic coal measures ordispersed humic organic matter during the course of thermal evolution. In fact, the gas derived fromthem during biochemistry process is also called the coal-formed biogas. Up to 1997, 23 coal-formedlarge-medium fields have been discovered in China(excluding two in Taiwan province), being locatedin ll basins. Their proven reserves take 50.9% in total natural gas res中国煤化工sxrge gas fields whose proven reserves are larger than 500X10 m, andCNMHGChangqing, Dongfang 1-1, Ya 13-1 and Shan 141 Gas Fields ) Thelmedium gas field plays a decisive role in natural gas reserves and the study of their controlling factorsis important both in theory and practice. The major controlling factors on the formation of coal-formedlarge-medium gas fields are as follows39为方数据Chinese Science Bulletin Vol 45 No 5 March 2000REVIEWS1 Coal-formed gas generating center and its surrounding areaThe coal-formed gas generating center is the area with the largest gas generating strength in coalmeasures or submeasures, it is the comprehensive reflection of the thickness of coal and dark mudstone,organic matter abundance, type and combination of maceral, and the evolution stage of coalificationThe gas generating center and its surrounding area can not only acquire continuous and high-abundancether"gas source rocks(coal measures), but also avoid the large amount of lossesdue to the short migrating distance. If larger traps exist, the formation probability of large-medium gasfields is high. According to the relationship between the generating center and beds in which large-medium gas reserves lie, gas generating center can be classified into different types, which havedifferent distribution probabilities of large-medium gas fields(i) Intra-reservoir generating centerThe source rock of gas generating center islocated in the same bed with the reservoir itgenerated. Intra-reservoir generatingDong shengcenters often develop in tectonically stablebasins or areas. It is easy to form largemedium gas fields in large traps, which arelocated in the same beds of gas generatingEtuoke Qcenter. due to stable structure. few fractureshigh strength of gas source and thedifficulty to migrate into overlying strata inlarge amounts. For example, Shan 141large gas field is generated from the intraZhenhuareservoir gas generating center formed byCarboniferous-Permian fm. taiyuanand Fm. Shanxi coal measures in OrdosBasin (fig. 1). With the development ofploration, it is expected that other coaZhidanformed large-medium gas fields will bediscoveredSanhu depression of Qaidam Basin, threelarge-medium gas fields, Tainan, Sebei NoQingyang1 and Sebei No. 2 have been discoveredhich are located in the northern margind west of QIgenerating center formed by salt lacustrine04080kmand swampy environment. These gas fieldsare located where the gas generating○3strengths are 35X m/km- and 30X10 Fig. 1. Upper Palaeozoic gas generating strength and large-mediumm/km", respectively. In the coal-formed gas fields in Ordos Basin. 1, Gas generating strength(10%m/km2);intra-reservoir gas generating center mainly 2, Lower Palaeozoic gas pool; 3, Upper Palaeozoic gas poolformed by Cenomanian in the north areasof west Siberia Basin in Russia, there are the richest large-medium gas fields and reserves in the worldwhere 9 ultra large gas fields have been discovered, whose reserve is up to 1X10crudeUrengoy gas field, the greatest gas field in the world, its reserve is 11.32X% m. From the discussionabove, the distribution probability of large-medium gas fields is very high in the intra-reservoir gasnd the surrounding area(11)Sub-reservoir gas generating center. Large amount of coal-formed gas from a gasgenerating center do not mainly accumulate in the same bed, but migrate into the overlying bed andform a large-medium gas field. Sub-reservoir gas generating center is usually located in the basins orareas which have the characteristics of flexible structure, and the gypsiferous salt bed and good or thickChinese Science BulleVol 45 No 5 March 2000395中国煤化工CNMHGREVIEWS■AndaDong 8ang a益屋互Zhaozhou口 NeijiangChang 2■ Baimiao囫四□Fig. 2. Some large-medium gas fields with sub-reservoir gas generating center and its surrounding area in China. 1, Gasfield; 2, gas generating strength(X% m/km2); 3, strata pinch-outs boundary; 4, structural boundary; 5, industry gaswell. (a) Moxi gas field in Sichuan Basin(Longtan gas generating center);(b) Wangjiatun gas field in Songliao Basin(Shahezi formation gas generating center);(c) Wenliu gas field in Bohai Bay Basin( Carboniferous-Permian gasgenerating center); (d) Suqiao gas field in Bohai Bay Basin( Carboniferous-Permian gas generating centermud acting as seals with fractures. Coal-formed gases generated by gas generating center migrate intothe traps of overlying bed through fractures, which are migration channels of large amount of naturalgas, and accumulate to gas pools. For example, in Kuqa and Taxinan depression in Tarim Basincoal-formed gases from Middle, Lower Jurassic gas generating center in coal measures migrate throughfractures and accumulate in the traps of Upper Tertiary and Upper Cretaceous, then Yaha, Yangtake,Yingmai 7, Kela 2 and Kekeya large-medium gas fields formed In Sichuan Basin, Upper Triassic gas396Chinese Science bulletin Vol 45 No 5 March 2000中国煤化工CNMHGREVIEWSgenerating center is located in Chuanxi depression. Xinchang medium gas field is located in this subreservoir generating center, where the gas generating strength is about 90X m, whose reservoir andgas source rock are Jurassic and Upper Triassic coal measures, respectively. In Bohai Bay Bformed gases of Carboniferous-Permian gas generating center migrate into Paleocene Shahejieformation and accumulate to Wenliu and Suqiao medium gas fields In Songliao Basin, coal-formedgases from gas generating center in Fm. Shahezi migrate into Quantou formation and accumulateWangjiatun medium gas field In Sichuan Basin, coal-formed gases from gas generating center in UpperPermian Fm. Longtan migrate into Triassic Fm. Leikoupo and accumulate to Moxi large gas field(fig2). Large amounts of large-medium gas fields have been discovered in lower gas generating centers inKarakumy Basin in the west area of Middle Asia coal-formed gas accumulation realm, in English Basinin the western area of middle Europe coal-formed gas accumulation realm, and in northwestern Basinof Germany. The distribution probability of coal-formed large-medium gas fields is also high in theareas with sub-reservoir generating centersto(iii) Supra-reservoir generating center. Large amounts of coal-formed gases generated by sourceock of gas generating center accumulate to form gas pools in lower beds. There are supra-reservoirgenerating centers in Ordos Basin and Bohai Bay Basin, gases from which acumulate to coal-formedgas pools in Ordovician system. In these two basins, there are no sediments for 140 million years afterOrdovician Ordovician carbonate rocks formed palaeokarst due to long term of weathering. After thatperiod, Carboniferous-Permian coal measures were deposited and gas generating center formed, gasesfrom which migrate into lower palaeokarst traps and accumulate to large-medium gas fields. Thenatural gases in Ordovician pools of Changqing gas field are mainly coal-formed gases coming fromCarboniferous-Permian coal measures. This large gas field is located in the northern part of Yan'anWushenqi supra-reservoir generating center in Carboniferous-Permian, where the gas generatingstrength is about(38-45)X10 m/km(fig. 1), which is the richest in this area. Concluding from thematerials all over the world, the distribution probability of discovering large-medium gas fields is lowin the area with supra-reservoir generating centersLow gas potential areNatural gases migrate horizontally from high gas potential areas to low gas potential areas andaccumulate to gas pools in low gas potential areas, and in vertical direction from high gas potential bedsto low gas potential beds and accumulate in low gas potenital beds. These regular patterns are evidentin large-medium gas field formation In western Sichuan depression of Sichuan Basin, Jiangyou andMianyang are present low gas potential areas of Upper Triassic Xujiahe formation coal measures,here Zhongba and Xinchang medium gas fields are discovered (fig 3). Ya 13-1 gas field has been inclosed low gas potential area since 11. 5 MPa years ago, and coal-formed gases still accumulate in Ya13-1 structure till now. Dongfang 1-1, the largest offshore gas field in China, is also located in low gaspotential areaVertical gas potential distribution proves that natural gases in Ordovician reservoir of Changqinggas field are chiefly coal-formed gases from Carboniferous-Permian coal measures. This issue has beenwidely studied:( i)In Ordos Basin, gas potential in the middle interval of Carboniferous has beengreater than that of bottom of Carboniferous after the sedimentation of Triassic Yanchang formation, socoal-formed gases can migrate from middle to the bottom of Carboniferous. During the wholegeological ages, gas potential in the bottom of Carboiferous has been greater than that of the top ofOrdovician system; therefore, coal-formed gases in the bottom of Carboniferous can migrate into thetop of Ordovician. (ii)Based on the values of gas potential, Sun Dongmin et al. sI calculated themigration ratios of Carboniferous natural gases upward and downward in the middle area of OrdosBasin during Permian period, there were about 20. 7% Carboniferous natural gases migrating upwardand 79.22% to the top of Ordovician system; during Early and Middle Jurassic periods, about 41.5%natural gases migrated upward and 58.5% downward; during Late Jurassic and Early Cretaceousperiods, 27% and 73% natural gases migrated upward and downward, respectively, during LateCretaceous period, Tertiary and Quaternary, 33% natural gases migrated upward and 67% to Ordoviciansystem. From these figures we can know that Carboniferous coal-formed gases mainlyChinese Science BulleVol 45 No 5 March 2000中国煤化工CNMHGREVIEWSSuin口 Renshou口 LeshanO WeiyuanFig. 3. Present gas potential of Upper Triassic in Sichuan Basin(unit: MJ/m)Ordovician system and the ratio upward is very small in the middle area of Ordos Basin during all the3 Large-medium traps with late pool-forming periodThe texture of each component in natural gas is simple, the molecule, density, viscosity andabsorption capacity are small, so natural gas is easy to be solved, expanded and volatilized. If the gaspool forming period is early and there was no gas source supply, especially to large-medium gas fieldsit is difficult to be preserved for lots of gas pools; therefore, all of coal-formed large-medium gas fieldsare discovered in the large-medium traps whose pool-forming periods are late. Liujiazhuang gas field,located in the western fringe of Ordos Basin, was a large gas field with 454.9x10% m reserves 50 Mayears ago. As the pool forming period was quite long ago, it is only a small gas field with only 1.9X10m reserve because of 453X 108 m diffusion losses during 50 Ma years after the gas field formed. IfLiujiazhuang gas field formed 10 Ma years ago, it would be a large-medium coal-formed gas fieldtoday. Lots of coal-formed large-medium gas fields in East China Sea Basin, Yingqiong Basin, Sanhuarea of Qaidam Basin, Bohai Bay Basin, Kuqa and Taxinan depression of Tarim Basin, and WesternSichuan depression of Sichuan Basin all formed during Tertiary and Quaternary, and the forming periodis late, so the diffusion losses are relatively small and large amounts of natural gases preserved andaccumulated to form large-medium gas fields(fig 4). The high discovery ratio of large-medium gasfields in coal-formed gas fields which have the characteristics of late pool forming period does notmean that it is impossible to form large-medium gas fields for early-formed gas pools. Large-mediumgas fields can also be discovered in the areas where there are excellent gas forming conditions, such asChangqing gas field, the largest one in China, which is a large coal-formed gas field formed very earlyChangqing gas field has been a large one till now because it is located in a very stable area on one hand;on the other hand, due to strong hydrocarbon concentration sealing of its cap rocks-Taiyuan andShanxi formation, diffusion strength is not high(average cumulative diffusion strength is 0. 17X10m/km), which is an advantage for coal-formed gases in Ordovician weathering crust to be preserved398Chinese Science bulletin Vol 45 No 5 March 2000中国煤化工CNMHGREVIEWSCGas fieldPingluobagYingmai 7eeTainanSebei No.2Fig 4. Forming period of major coal-formed large-medium gas fields in China. l, Main source rock; 2, secondarysource rock; 3, peak phase of gas generating; 4, traps forming phase; 5, gas pool forming phasefor a long time4 Traps related to the paleo-uplifts in coal-formed gas generating areaCoal measures are"all-weather"gas source rocks. Gas is constantly produced during the wholecoalification. Therefore, the paleouplifts in gas generating area are good effective traps to form largemedium gas fields because they constantly received the coal-formed gas. There are three types ofpaleostructure traps in forming the great-medium gas fields.i) Paleostructure and gas accumulation formed at the same time(Sebei type). This type meansthat the forming process of the paleostructure is at the same time, at almost same time of or slight laterthan the phase of gas accumulation, Very good examples are found in the sanhu Depression in QaidamThe Sanhu Depression was strongly subsided during 2 Ma of Quaternary. The Quaternary hugethick salty lake facies with swampy facies was rapidly deposited that the maximum thickness is over3 200 m and about 1 500 m gas source rocks developed in the lower part, intercalated with argillaceousfine sandstone and siltstone as good reservoirs. They formed the gas forming combination of source andreservoir in the same bed. The gas source rocks cover an area of about 15 000 km, and the mostChinese Science BulleVol 45 No 5 March 2000399中国煤化工CNMHGREVIEWSfavorable area is about 4 500 km. Due to the affection in the end of Himalayan Movement when thissuite of strata was strongly subsiding, a series of syngenetic anticlines were formed. Thesepaleostructures are of gentle dip angle (generally less than 2 )and small amplitude(with closure lessthan 100 m). The Tainan, Sebei No. I and Sebei No. 2 large-medium Gas Fields discovered in SanhtDepression are in the paleostructural trapsThe data of natural gas components and CHa carbon isotopes in Tainan, Sebei No. I and Sebei n2 Large-medium Gas Fields indicate that the natural gas is dry, the heavy hydrocarbon gas content israre (less than 0.35%), and the 8CI value ranges from.90% to-68.54% with typicalcharacteristics of biogas. This indicates that the natural gas in those syngenetic anticlines is not fromthe undrylying strata(high maturity), but from the Quaternary gas source rocks. It has been proved thatthe natural gas accumulation in Tainan, Sebei No. 1 and Sebei No. 2 Large-medium Gas Fieldsoccurred at the same time of forming those paleostructure(syngenetic anticlines ). Dai Jinxing namedthis kind of gas fields the Sebei type(l1)Gas accumulation after formation of paleostructure(Ya 13-1 type). This type means the gasaccumulation occurred after the paleostructure had formed ol, The large-medium gas fields of the typeare common in China and abroad, such as Zhongba, Ya 13-1, Wangjiatun and Pinghu Gas Fields. Ya13-1 Gas Field is located in the inherited Nw brachy-paleostructure on the basement uplift fault beltIts major reservoirs are sandstones of Lingshui Formation of Oligocene. There is a distinctunconformity between the Miocene Sanya, Meishan Formations and Oligocene Lingshui Formationalong the updip direction of the top of the structure obviously. The structural closure and area are largein the lower part and small in the upper part. The paleostructures in the two limbs become thick, whichmainly formed in Oligocene!. The major gas pools of Ya-13-1 Gas Field are in Lingshui FormationTheir source rocks are dominated by swampy coal-bearing beds of Yacheng Formation. Quaternary wasthe major gas generating period of Yacheng Formation!, so Ya-13-1 Gas Field was enriched in a largeamount of Quaternary coal-formed gas after the formation of the oligocene paleostructure in stead ofcoal-formed biogas during the formation of syndepositional structure(iii) Superimposed type of paleostructure and gas accumulation. This type of reservoir implies asecondary reservoir that formed near or upon a tectonically controlled old reservoir by modification ofpost-tectonic activities. The discovery rate of this type is low and its scale is relatively small. Xinchangand Pingluoba Gas Fields in West Sichuan Depression of Sichuan Basin belong to this type, that iancient gas pools controlled by paleostructure in Yanshanian Period formed the secondary gas poolJurassic through the modificationThe ancient structural trap occurred in Xinchang Gas Field in Yanshanian Period. The magnitudeof the trap was at least 25 m in the basal plate of the Upper Jurassic low Penglaizhen Formation in theend of Jurassic. Because Xujiahe Formation in West Sichuan Depression had been at the peak stage ofgas generation in Cretaceous, the paleotrap at that time formed the ancient gas pools. Xiaoquan-Xinchang area was located in regional inherited paleouplift during 150 Ma from end Jurassic. The scaleof the ancient gas pool became larger and larger. Due to the strong Himalayan Movement, fracturenetwork formed in the source rocks in Triassic Xujiahe Formation and overlying Jurassic, which madethe natural gas in the ancient gas pool of Xujiahe Formation migrate up to Jurassic system so as to formpools in Penglaizhen Formation, Shaximiao Formation and Qianfoya Formation. This shows the poolformation process of superimposed type of paleostructure and gas accumulation5 Large-medium traps above or between pressured compartments related to coal measuresOver-pressure usually exists in buried coal-bearing strata in China to form over-pressuredcompartments. For example, over-pressure appears in the coal measures in Sichuan Basin, YinggehaiBasin, Qiongdongnan Basin, southern Junggar Basin and Kuga Depression. The exploration proves thatthey are the favorable areas to search for great-medium gas fields above or between compartmentsBased on the theory, a number of great-medium gas fields are discovered, such as Dongfang 1-1 GasField, Ledong 15-1 Gas Field, Ledong 22-1 Gas Field, Ya 13-1 Gas Field, Hutubi Gas Field and so(i) Supra-compartment large-medium gas fields (Yinggehai type). There are three400Chinese Science bulletin Vol 45 No 5 March 2000中国煤化工CNMHGREVIEWScharacteristics in central subsiding zone of Yinggehai Depression in Yinggehai Basin: i)Very rapiddepositional rate of Upper Tertiary(The estimated thickness is 8 000 m, that of Meishan Formation is 3600 m) with the average of 0. 78 mm/a and the maximum of 1. 4 mm/a. ii) High geothermal gradientand high heat flow(The geothermal gradient ranges from 437C/100 m to 4.70C/100 m, the heat flowvalue from 68.78 mW/m-to 80.09 mW/m). iii)There exist many shale arching anticlinal zones andshale arching activities. There are 4 rows of Ns shale arching anticlinal zones in en echelonarrangement in the depression center. Each zone includes many shale arching anticlines. There are threeor more activity phases of shale diapirs. Due to the rapid sedimentation since Miocene, the stratacontained much water. And due to the high geothermal gradient and high heat flows, the occurrence ofaquathermal pressure boost and great increase of newly generated hydrocarbon in Miocene MeishanFormation led to the formation of abnormal pressured compartments in the Lower YinghuangFormation and Meishan Formation, and the Quaternary and the Upper Yinghuang Formation are thestrata above the compartments with normal pressure(fig. 5)Qmmmm N2y400N m 12↑-o8000ig. 5. Sketch map showing the type of supra-compartments (Yinggehai typHydrocarbon gas pool; 2, CO2 gas pool; 3, the top limit of over pressure system; 4migration direction; 6, CO, migration direction; 7, mature hydrocarbon source rock; 89, Upper YinghuangFormation; 10, Lower Yinghuang Formation: 11, Huangliu Formation; 12, Meishan Formation; 13, Sanya FormationThe three large-medium gas fields( dongfang 1-1 Gas Field, Ledong 15-1 Gas Field and Ledong22-1 Gas Field) are in shale diapiric traps. Their major gas pools are in normal pressure zones. Forexample, the measured pressure coefficient of the gas bed at the depth interval of 1 284--1 296 m inDongfang 1-1-2 Well of Dongfang 1-1 Gas Field is 1. 1 and that of 1 417--1 557 m in Ledong 15-1-1Well of Ledong 15-1 Gas Field is 1.06, belonging to the gas pools in the normal pressure zone abovecompartments. However, the deeply buried low-producing gas beds are of an ultra-pressured characterFor example, the measured pressure coefficients at the depth of 2 563. 7 m of Dongfang 1-1-1 Well and2 212.5 m of Le-15-1-1 Well are 2.12 and 2.01 respectively, belonging to abnormally pressuredcompartments. The forming model for major gas pools above compartments in Yinggehai (Dongfang 11, Ledong 15-1 and Ledong 22-1 Gas Fields)is as follows: The average organic carbon contents of themudstone in Meishan and Yinghuang Formations within the high pressure seal-bounded compartmentsare 1.33% and 0.45% respectively, which are the source rocks in Yinggehai Basin. The kerogen insource rocks is generally of type Ill, which mainly generates gas. The aquathermal pressure boostChinese science Bulletin Vol. 45 No 5 March 2000中国煤化工CNMHGREVIEWSoccurred and at the same time a large amount of natural gas was produced due to the rapidsedimentation rate, lots of water in undercompaction strata and high geotemperature and heat floThen, the abnormally pressured seal-bounded compartments were formed, in which stored a largeamount of over-pressure gas-water miscible flows. Strike-slip faults of Nw are active. N-S fracture(fault)belts in the central subsiding zone occurred and became an active belt of high pressure gas-waterflows in compartments and mud flow activity of undercompaction mudstone. The uplift of overpressure gas-water flows along the N-S fracture belts or points formed the shale arches, at the sametime, the diapir anticlinal traps were formed at the lower of Yinghuang Formation. When the overpressure gas-water flows broke through the sealing beds and moved upwards, the pressure was reducedto the normal ones. So a large amount of free gas was formed and accumulated in the diapir traps. Theactivity of shale diapirs is of many stages, such as three stages in Dongfang 1-1 Gas field. It alsoindicates that the breakthrough of high pressure gas-water-mud flows is of many stages, as well as thegas enrichment in a gas pool, that is the impulse miscible surging flow accumulation and gas poolformation(ii) Between-compartments of large-medium gas fields. The coal-formed medium gas fieldHutubi Gas Field in Junggar Basin has been preliminary explored. The gas pools are located in theZiniquan Formation(E12) of Paleogene and the gas sources are from Jurassic coal measures with thedepth of 6 000-10 000 m. The normal pressure gas pool of the Ziniquan Formation is the type ofbetween-compartments. The upper is an over-pressured compartment in Anjihai Formation(E2.3a) andthe lower is over-pressured seal-bounded compartment in Donggou Formation of Upper Cretaceous andK|9Fig. 6. Sketchapartments of Hutubi Gas Field. I, Pool; 2, over-pressured layer; 3, gas migrationdirection; 4, Dushanzi Formation; 5, Taxihe Formation; 6, Shawan Formation; 7, Anjihai Formation; 8, Ziniquan Formation9. Cretaceous: 10. Jurassic402Chinese Science bulletin Vol 45 No 5 March 2000中国煤化工CNMHGREVIEWS6 Effective fracture trap related to coal measuresDue to the frequently tectonic movements in China, there are a number of tectonic movements toin oil- and gas-bearing basins except for Ordos Basin and middle Sichuan Area inFaults are common in buried coal-bearing areas and basins of large area, such as KuqaTaxingSichuan Depression, which exert strongly influence on the formation of coal-formed gasThe below-mentioned fractures only refer to foundation fractures, common faults and fracturesthat could control the source rocks(coal measures) and oil-gas fields. The fractures exert a dual(constructive and destructive) effect on oil and gas. Here, the constructive fractures are only discussedwhich are named"effective fractures". The effective fractures affect the oil and gas on controlling theformation of source rock, acting as migration pathway, modifying dense reservoir and forming fracturetraps and so on. The effective fracture traps in buried coal-bearing basins (or areas)are favorable placesto form coal-formed gas fields. A lot of large-medium gas fields are found in these basins, e.g. the YahaYingmai 7, Yangtake, Kela 2 Gas Fields in Kuqa Depression and Luntai Arch in Tarim Basin, HutubiGas Field in Junggar Basin, Xinchang, Pingluoba Gas Fields in West Sichuan Depression, WangjiatunGas Field in Songliao Basin, Wenliu, Xuqiao Gas Fields in Bohaiwan Basin and Ya 13-1 Gas Field inQiongdongnan basinThe effective fracture traps in Kuqa Depression and Luntai Arch have great effect on the formationof coal-formed large-medium gas fields. The middle and lower Jurassic coal measures in KuqaDepression form a gas generating center with the largest gas generating strength of 120X 10 m/km2.At present, the burying depth of the source rocks is about 5 000--8 000 m, so it is too deep to explorethe reservoir in the same bed with the gas generating centers. However, the gas can migrate upward toCretaceous, Upper and Lower Tertiary strata through the effective fractures. So the four large-mediumgas fields(Yaha, Yingmai 7, Yangtake and Kela 2 Gas Fields)were discovered in Luntai Arch andKuqa Depression. These coal-formed pools are distributed like a string of beads along fractures innumerous fracture structures. For example, Yaha fracture structures are controlled by Yaha normal faultwith the length of about 80 km, the NEe strike and s dip. And six fracture trap anticlines form in thefracture belt. Multi-phrase activities happened in Yaha fracture, which stretches from the low KuqaFormation of the Upper Tertiary to basement. The upper Yaha fracture is normal but the lower is averseThe middle and lower Jurassic coal-formed gas migrated upward to the Upper Teritary Kuga-KangcunFormation, Jidike Formation, the Lower Tertiary Suweiyi Formation and the Upper Cretaceous(the lastthreedominant).9 gas condensate fields and 2 oil pools are distributed along the tendency ofthe fractures like a string of beads. The fault throw controls the structure amplitude, gas column heightand gas pool horizon in Luntai Arch. Oil and gas could exist only if there were faults in the bed. TheYaha fault throw is 100--200 m and Yingmai 7 is 30--80 m. The oil-gas column height between themis 30-60 m. The Hongqi anticlinal fault throw is 10-30 m with the oil-gas column height of 10--12Faults extended to the bottom of the Upper tein Jidike Formation. Yingmai 7 anticlinal fault only extended to the bottom of lower Tertiary, soand gas do not occur in Jidike Formation. The Jurassic gas from the depth of 6 000 m migratedto Cretaceous through fractures in Kela 2 Gas Field(fig. 7). The formation of the effective fractureneeds two factors: one is that fractures generally do not extend to the regional cover, the other is that thethick high quality covers exist. For example, there are Teritary strata covers above Middle, LowerJurassic coal measures in Kuqa Depression with the whole thickness of 800--l 400 m which aremainly composed of mudstones, gypsoliths, gypsiferous mudstones, muddy gypsoliths and salt rocksThere are 10--15 beds of gypsiferous salt rocks with a single bed thickness of 5--20 m and the totalthickness of 200-300 m. It is easy to know the effect of effective fractures on gypsiferous salt bedsThe gypsiferous salt bed is a type of high-quality cover. For example, due to the gypsiferous salt bedthe fractures become effective ones in Wenliu gas Fields in the Bohai Bay BasinTo sum up, it is mostly possible to find the large-medium gas fields if there are two or more majorgeological factors in an areaChinese Science BulleVol 45 No 5 March 2000403中国煤化工CNMHGREVIEWSKela 2 Well处235d|6KN=A12Fig. 7. Cross section of Kela 2 Well. I, Gypsiferous salt; 2, dolomite; 3, mudstone; 4, sandy conglomerate; 5, sandstone; 6.gas pool; 7, Shushanhe Formation; 8, Baxigai Formation; 9, Bashijijike Formation; 10, Paleogene; 11, Jidike Formation; 12Kangcun foReferences1. Dai Jinxing, Song Yan, Duin on coal-formed biogenOrganic Chemistry and Non-marineds. Huang difan Hu2. Dai Jinxing, Song Yan, Zhang Houfu et aL., Natural gas accumulationones in China (in Chinese), BeijingScience press. 1997.3. Guan Deshi, Zhang Weizhen, Pei Ge, Hydrocarbon sources of Ordovician payzones in central gas field in Ordos Basin, Oiland Gas Geology (in Chinese), 1993, 14(3): 1914. 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Zhou Xingxi, Liu Guangdi, Li Shaoji et al., Natural Gas Formation Condition and the Pattern of Distribution in Tarim) Beijing: Petroleum Industry Press, 1998, 1-3, 131-133May3l,1999)Chinese Science bulletin Vol 45 No 5 March 2000中国煤化工CNMHG