Formation Laws of Inorganic Gas Pools in the Northern Jiangsu Basin

674Vol. 74 No.3 AACTA GEOLOGICA SINICASept. 2000Formation Laws of Inorganic Gas poolsin the northern Jiangsu BasinZHOU Liqing, YANG Shengliang and LEI YixinExploration and Development Department, East China Petroleum Bureau, CNSPC,37 South rehe Rd, Xiaguan District, Nanjing 210011, JiangSustract In the Northern Jiangsu basin there are high pure CO, gas pools, low condensed oil-containing CO2gas pools, high condensed oil-containing CO, gas pools and He-containing natural gas pools, with the 8 Cco,(PDB)values ranging from -2.87 %cto-650%0 He/'He 3.71x10-to 642x10-6, R/Ra 2.64 to 4.5, 40Ar/Ar 705 to 734belonging to typical mantle source inorganic gas pools which are related to young magmatic activity. The gas layersoccur in two major reservoir-caprock systems, the terrestrial Meso-Cenozoic clastic rock system and the marineMeso-Palaeozoic carbonate rock-clastic rock system. Controlled by the difference in the scale of traps in the two reservoir-caprock systems, large and medium-scale inorganic gas pools are formed in the marine Meso-PalacozoicGroup and only small ones are formed in the terrestrial Meso-Cenozoic strata. Inorganic gas pools in this basin aredistributed along the two deep lithospheric faults on the west and south boundaries of the basin. Gas pools are devel-oped at the intersected part of the ENe-trending faults that control the half graben and the e-w tenso-shear faults,inly distributed near the Es, Ny' and NyQ basalt eruption centresKey words: Northern Jiangsu basin, inorganic natural gas pool, formation law1 Introductionthe basin, with geological reserves of CO2 about200x108m. the reserves of condensed oil are 40x10+tThe Northern Jiangsu basin refers to the land part of and the gelogical reserves of natural gas in other CONorth Jiangsu-the South Yellow Sea basin, which lies gas fields and He-containing gas fields are H10x10north of the Yangtze River in Jiangsu Province with an m. The helium content in the Xiqiao gas pool reachesarea of 35,000 km. The basin is bounded by the Tanlu 1. 206-1. 241%, much higher than that of the Weiyuanfault in the west. connected with the suture zone of thefield in Sichuan (with the content of helium inYangtze plate and North China plate in the north, and natural gas being about 0. 2%0), which is the only hellinked with the Sunan(southern Jiangsu) uplift zone um base in Chinaby the fault along the Yangtze River. It is the largestEogene extensive fault depression hydrocarbon basin 2 The Reservoir-Caprock Association andin the land part of southeast chinaTrap Type of Inorganic Gas Pools in theIn the oil and gas exploration of the past forty and basinmore years, six inorganic gas fields have been succesively found in the basin, i. e the Huangqiao CO2 gas The Northern Jiangsu Meso-Cenozoic terrestrial basinfield, Xiqiao He-containing shallow layer gas field, evolved from a very complicated basement. TheHongzhuang CO, gas fieldng Well An-1 He- basement consists of shallow-marine platform carboncontaining gas field, Xiaoji CO2, gas field and Well ate rock, deep-water aulacogen radiolaria-bearingTianshen-4 CO2 gas field(Fig. 1). In addition, good siliceous mudrock, neritic coalseam-containing midinorganic gas shows are found in 9 wells of 7 struc- lower Triassic-lower Sinian clastic rock in the lowertures. The largest high pure CO, gas pool in China, i. e. Yangtze area, with a total thickness of 5000-6000 mthe huangqiao CO2 gas pool, lies on the south slope of During the Indosinian-Yanshanian, violent crust中国煤化工CNMHGFormation l:of Inorganic gas pools in the northern jiangsu basilZhou et a675l19°00′121°00′2040kmFuning③oYanchengao⑤Xiaoj33°20′TianchanNanxinjle a 100South Jiangsu upliftl19°002°00Lithospheric ENE-trending E-w-trending Overlapping Inorganic Inorganicfault basement fault basement fault boundary gas fieldof basinFig. 1. Relationship between deep faults and distribution of inorganic natural gas pools in the Northern Jiangsu basinHuangqiao gas field; Xiqiao He-containing gas filed; III. Anfeng Well An-1 He-containing gas filed; N. Hongzhuang CO, gas fieldXiaoji CO, gas field; V. Tianchang Well Tianshen-1 CO, gas field. Name of faults: Qintong: O Tanlu; 2 Yanjiang fault: Hongze-Funing:Tianchang-west Liubao; Gaolizhuang-Xiaoji; Qintong: Liangduo fault block; 8 Fu'an; Zhangjiaduo-Beiling: Nanxinjie: OTazili; 2 Lifa-Xichang: Yangzhou; Huaian-Yancheng: G Guanzhen-Baiju; Tianchang-Xiaoji-Hongzhuansqueezing movement spread the entire lower Yangtze a 5000-7000 m thick formation of alluvial-fluvialarea, which resulted in strong deformation of marine deep lacustrine sandrock and mudrock intercalatedsediments, forming a thrust nappe system. A regional with oil shale (Yan et al., 1979; Wang, 1983; Li,uplift resulted in erosion of the various areas to differ- 1984)ent degrees. The lower Sinian cropped out in some ofThe above geological evolution resulted in the twothe areas, and Early Mesozoic molasse formation is reservoir-caprock systems in the basin, the terrestialdeveloped only in local areas, with intact marine strata Meso-Cenozoic clastic rock and marine Meso-PIpalaeozoic carbonate rock-clastic rock. The mainIn the mid-late Yanshanian, under the action of caprocks in the former system are the thick mudrockregional extensive stress, a suite of gypsum and salt and clay layer in the middle-upper part of each formacontaining red sand and mudrock drape formation as tion and the main reservoir layers are the fine sandstothick as 1500 m was widely deposited in the late Cre- ne and gravel-containing fine sandstone in the middleaceous; in the Himalayan, under the action of exten- lower part of each formation. The main reservoirional block faulting, the Northern Jiangsu basin was spaces are the primary and secondary pores in sandseparated from the entire lower Yangtze area and be- stone in the Cenozoic Group and the fissures and o-came a fault block subsidence centre, which accepted lution fissuresH中国煤化工atter systemCNMHG676Vol. 74ACTA GEOLOGICA SINICASept. 2000the overlain Meso-Cenozoic mudrock and shale serve gas pools in the basin, two major types of gas poolsas the caprocks and the main reservoir spaces are the are divided, i.e. the co2 gas pools and the He-structure fissures, solution fissures and solution caves containing gas pools. And, according to the content ofin carbonate rock and compact sandstone. The large each composition in the inorganic gas pools, they canamount of solution fissures and caves in the carbonate be further divided into high pure CO2 gas pools, hyrock formed near the Indosinian-Yanshanian uncon- drocarbon-containing CO, gas pools, hydrocarbon andformity interface are the important reservoir spacesnitrogen-rich CO2 gas pools, He-containing naturalFault blocks, fault noses and drape anticlines are gas pools, He-rich nitrogen gas pools(table 1)extensively formed in the terrestrial meso-Cenozoiebasin. The structures are large in number, but small in 3.2 Geochemical characters of inorganic gascale, the area of a single structure is 0.5-5 km, with poolsa closure height of 0. 2-1 km. Besides, the whole lat- 3.2.1 Features of stable isotope composition oferal closure of the structures is bad, so that only small cO2and very small inorganic gas pools are formed. A lar- The aCco, values of CO, in inorganic gas pools ofge number of large and medium-scale buried hills, this area range from -2.87 %o-10.62 %o those of thedrape structures and inner structures are widely devel- associated COil fields betweenoped in the marine Meso-Palaeozoic basin. The area -34.72 %ocand-35.60 %tAble 2)of a single trap is 10-100 km, and the closure height Schidlowsdi (1988)suggested that the 8Cco, val-0.5-2 km. They are closed entirely by the Meso- ues of carbon in mantle-source diamond and the midCenozoic Group forming numerous suites of gas lay- ocean ridge basalts are -3 %oto -8 %o so that theers with a large accumulative thickness, so that large 8Cco, value of mantle origin is -5%0 Stahl et aland medium-scale inorganic gas pools are often (1971)considered that the 8Cco2 value of CO2 relatformed as long as there are plentiful gas sourcesed to young basic magmatic activity is -7 %p that oforganic originated CO2 is about-10 %do -20 %o that3 Composition Types and Geochemicalof CO, generated from decomposition of marineCharacters of Inorganic gas poolslimestone is -2 %do +2 %o It has been determined thatthe acco, value of co, that originates from high3.1 Composition types of inorganic gas poolstemperature pyrolysis of marine carbonate rock of thisccording to the revealed composition of inorganarea is 1.8%oto 3.86%o(Fei, 1987). It is clearly seenTable 1 Composition types of inorganic gas pools in the basinComposition type ofGas compositionTypical casesInorganIc gasOO2(%)C-C4(%)He(%)High-purity CO, gasKP, Ton, Pig, Cac and D w of gas pools0.9-1in Huangqiao gas field: Ed and EdinHongzhuang gas fieldGas pool S, m in Huangqiao gas field; Es'containing92.694422,90-5.802,10-2,59in Hongzhuang gas field; Fuminzhuang Esgas pool, Well Tiansheng 4 CO, gas poolHydrocarbons andGas pool Sf in Huangqiao gas field; gas-nitrogen enriched6495-89410.34-12.01.45-2290.060.21earing trap K P, gas-bearing layer O, h184-4.51Gas pool Er' in Anfeng Well An-IHe- and N-enrichedGas pool Nyin Xiqiao: gas-bearing layers6.198.857.8765.0297-32.98Ny' and k, p in Well Huai-1 in the Huaian中国煤化工CNMHGFormation laws of inorganic gas pools in the northern Jiangsu basinZhou et677Table 2 Isotope analysis data of main inorganic gas pools in the basin'He/He50Type of gas poolName of gas pool(PDB od(PDB %d(%sMOW-4.73--10.62192.1--254.69-2.87--3,8723Pool of inor-5.54-2789--31.33114.2--125.39.5-4055.87734Fuminzhuang es'6.506.42Well An-I Er'207.2Pool of organic469-45.94originXiaoliuzhuang Ed-34.72407-185that the CO2 in the basin is of a typical inorganic characteristic value of 4 Ar/Ar of the original gasesmantle origin that is related to young basic magmatic from deep mantle is 400, higher than that of the airactivity, and its 8Cco, value is higher than that of the The Chinese researchers ei et al., 1987)generall2.2 Features of helium isotope composition.organic gas in the known oil fields of the Gaoyou sabelieve that when the 40Ar/36Ar value in a gas sampleLupton(1983)considered that the He/ He ratios of ered as of a mantle source r, the gas may be consid-gases from the atmosphere, the crust and the mantle The Ar/Ar values of inorganic gas pools in there 1.4x10-6, 1.4x10-and 14x10 respectively. Jef- basin are 705-734, which are higher than those of thefrey et al.(1988)suggested that the r/ra ratio of the Cenozoic oil and gas pools(407-469). It is apparentmid-ocean ridge basalts is 3.5-6, which represents that the argon in the Cenozoic oil and gas poolsthat of the upper mantle. The R/Ra ratio for land is mainly comes from the atmosphere and crust whereas0.06-0.35. and that for the crust is.01-0.025the argon in the inorganic gas pools is mainly fromThe helium content in inorganic gas pools of this deep mantle.area is generally 0.04%-1.206%, He/He generally 3.2.4 Features of carbon and hydrogen isotopes3.71-6.42x10-6, and R/Ra 2.65-4.5, indicating an from methaneorigin between the crust and the mantle, and the val- In the inorganic gas pools of the basin there is generues are apparently higher than the 3He/4He value ally more than 1% of methane, and the highest content(7.42-10.5x10)and R/Ra value(0.5-0.7) of the may reach 46.88. Their 8CI values are generallyorganic gas in the Gaoyou sag. The above data indi-27.52%to-57.58%, and SD.-11.42%oto207.2 %o while these values for inorganic gas poolsantle that is related to the activity of basic basalts, associated with Cenozoic oil and gas pools arebut it also contains some crust-source radioactive ele- 57.58 %0 and-207. 2 %o respectively. Therefore, thements. The content of helium in oil and gas poolsmethane belongs to the typical mature and highrelatively low, which mainly comes from the atmos- mature typephere and crust.4 Distribution Laws of Inorganic Gas Pools3.2.3 Isotope features of argon40Ar/Ar=29.55 is generally taken as the representa- Studies have revealed the following distribution lawstive value of argon in the air. Ozima(1975)considered of the inorganic gas poolsthat the 40Ar/6Ar value of gases from the mantle are(1) They are distributed near the lithospheric faultslarger than 2000. Hart et al. (1979)suggested that the On the basis中国煤化工CNMHG678Vol. 74 No. 3ACTA GEOLOGICA SINICAsults such as whole crust seismic exploration and tel- km. Their intersections with the NEe-trending faultsluric magnetotelluric depth-probing as well as other are the concentrated points of stress, and hence thepetroleum geophysical exploration results, in comb]- weakest and most open parts. Therefore, they are thenation with a surface geological investigation, Chen et main paths for the mantle substances to swell up, andal.(1988, 1999)made a systematic interpretation of most of the Cenozoic basalt eruption centres are locatthe deep geological sections of HQ-13 and HQ-8 in ed at this kind of positionsthe lower Yangtze area, and considered that there existAlmost all of the inorganic gas pools occur at thethe Tanlu fault and the Yanjiang fault(the fault along intersection parts of the E-W-trending Tiancharthe Yangtze River) respectively in the west and south Xiaoji-Hongzhuang fault, the Yanjiang fault and theparts of the basin(Fig. 1). The main markers of the ENE-trending sag-controlling basement faults. Thetwo lithospheric faults are: 1) There exists a vertical vertical gas transportation at the intersection of thefracture plane which deeply cuts into the Moho in the ENE and Ew-trending faults has been proved by ahole crust reflective section. 2) The lithopheric number of data. The DiPoG processed data of thethickness is apparently different among the blocks. 3) seismic line EF75 5 crossing the intersection of theThere are obvious differences in the resistivity and ENE-trending Tianchang-West Liubao fault and the egeothermal field of the lithospheric blocks. 4) There W-trending Guanzhen-Baiju fault indicate that thereare obvious differences in the thickness and stability apparently exists a low-velocity anomalous zone disof the low-velocity high conductive layers in the crust tributed in the shape of a vertical stripe penetratingamong the blocks. 5)Apparent magnetic anomalies from the Cenozoic Group to the metamorphic bedrockexist among the blocks. 6) There are apparent differ- below the depth of 12000 m, and this is an importanceences in the tectonic evolution of the blocks. 7) There proof of the upwelling of deep source inorganic gasesare observable fracture zones on the ground surface Abundant gas shows have been found in the wellsand in the sedimentary cover. 8) There exist intensive D52 and d58 near the low-velocity anomalous zonevolcanic activity. 9) The mineralization process in the and a Co 2 gas pool is found in well Tianshen- 4 whichblocks is controlled by the faultsis located in the west segment of the Tianchang-WestAll the inorganic gas pools in the basin are distiLiubao fault belt. drilling data have revealed that theuted near the lithospheric faults, and the closer the Et and Ef, basalt eruptive rocks are formed in thisinorganic pools to the faults, the larger the scale of the region, and the belt is a path for deep mantle sub-pools. Obviously, the lithospheric faults, especially the stances to swell upYanjiang deep fault which was violently active during(3)They are the associated products of Cenozoicthe Himalayan period, are the main paths for the man- basic magmatism. There are four major stages of vol-tle substances to swell upity in the cenozoic basin,1.e(2)The intersections of ENE- and E-w-trending Es'and Ef-Et', during which basaltic magmatismbasement faults are the main areas for their distribu- dominated. All the inorganic gas pools in the basin aretion. In the basin, the NEE-trending basement faults distributed in or near the eruption centres of basichave throws of 1000-7000 m, and control the forma- magmas during the Cenozoic, especially the threetion and evolution of the Cenozoic half graben. The basic magmatic eruption belts of Ny2-Q, Ny and Esfaults deeply cut into the low-velocity high conductive For example, the huangqiao CO2 gas pool and Xiqiaolayer in the crust( Chen et al., 1988), bringing the He-containing gas field lie near the N and Es'volmantle substances that enter the crust along the lithos- canic eruption centres in Qutang, Beiling, Tazili andpheric faults out of the crust, or to the near-surface Yazhou fault belts; the Hongzhuang gas pool near thezoneNy' and Es' volcanic eruption centres in Qintong faultThe E-w-trending faults are another series of very belt; the Xiaoji CO, gas pool near the n and esimportant extensive wrench faults in the basin, with eruption centres in the Xiaoji-Gaolizhuang fault belt;the throws generally being 200-1000 m. The fault the Well Tiaoshen 4 CO, gas pool near the E-Et'andplanes are steep, and the cutting depths generally 5-10 NyQ volcanic*he Tianchang-中国煤化工CNMHGFormation l:of Inorganic gas pools in the northern jiangsu basilZhou et a679West Liubao fault belt; and the Anfeng Well An-1 Et'Manuscript received Nov 1999helium and nitrogen-containing gas pool is distributedaccepted Apr. 2000near the es volcanic eruption centre in the Fuan faultedited by Zhu Xilingbelt. It is apparent that basaltic magmas and inorganicgases are all products of the upwelling of mantle sub- Referencesstances to the surface. Therefore, the gases in inorChen Husheng, 1988. Synthetic geophysical investigation ofganic gas pools mainly originated from the mantle,line hq-13 and its geological significance of lower Yangtzearea, Oil and Gas Geology, 3: 24-35 (in Chinesemixed with some crust-originated substancesChen Husheng, Zhang Yonghong. Xu Siwen et al., 1999Most of the basic magmatic eruptions in the basinLithospheric Texture and Structure Features and Oil andwere associated with plentiful inorganic gases, someGas Resource Evaluation of Lower Yangtze Area and Itsof which accumulated into pools under favourableNear Area. Beijing: Geological Publishing House, 150-430(in Chinese)source-reservoir-caprock associations and good trap- Fei Fu'an, Luo Jikun, Li Lina et al, 1987. Natural gas gecping conditions. As strong extensional block faultingchemical features and its origin discussion of Huangqiao re-in the basin continued until the end of Ny. most of thegion of Jiangsu Province. In: Annual Report of LanzhouGeological Institute of Chinese Academy of sciences, 34-45inorganic gas pools formed at the early stage of the(in Chinese)extension activity were destroyed, and only a small Jeffrey, A.W.A., and Kaplan, I.R., 1988. Hydrocarbons andpart was preserved. Most of the present inorganic gasinorganic gases in the Gravberg-l well, Siljan Ring, Swedenpools were formed in the late stage of extensionalChemical Geology, 71: 237-255Li Daoqi, 1984. a discussion on the Environment of the largetectonic activity when the mantle substances swelledfacies of the paleocene taizhou and funing formations inup and eruptedthe Northern Jiangsu basin. Acta Geologica Sinica, 58(1)80-85(in Chinese with English abstract)5 ConclusionsOxburgh, E.R., Johnson, H E, and Axford,1986.Heli-um isotopes in sedimentary basins. Nature, 3224: 632-63Schinlowsdy, 1988 Occurrence and significance of magmaticThe following conclusions may be drawn for the for-clusions and silicate liquid immiscibility. Acta. Geol. Pol.mation of inorganic gas pools in the basinSechell, M., 1988. Genetic characterization of natural gas1) The inorganic gas pools are mainly distributedAAPG,67(12):2225-2238.around the peripheral areas of the Tanlu and Yanjiang Stahl, W.J., Gold, T, and Soter, S, 1971. The deep earthlithospheric fault belts and at the intersection parts ofientific American, 242: 154-161large ENE- and E-W-trending faults with intensiveYan Qinshang, Zhang Guodong, Xiang Lisong et al., 1979basic basaltic magmatic activity in the Nya-Q, Ny' and Funing Group (lower Paleogene ) in Jinhu depression, NorthEs' periods. The large Meso-Palaeozoic buried hillJiangsu plain. Acta Geologica Sinica, 53(1): 74-85(in Chi-structures underlying the meso-Cenozoic bodies in thenese with English abstract).southwest part of the basin are the most favourableabout the first authorareas for forming large and medium-scale high pure Zhou Liqing Born in 1964; graduated from theCO2 gas fields(2) The He-containing natural gas in the basiDepartment of Mineral Resources, Wuhan College ofGeology, majoring in oil and natural gas explorationcharacterized by high He content, shallow burial andassociation with inorganic nitrogen gas. It is originatand got a masters degree in 1990 at Beijing Petroed mainly from the mantle, and secondarily from theleum Exploration and Development Institute. Now hegas pools. Larsistant for the chief engineer of east Chinapetroleum Bureau of CNSPC(China National Stain sallow layers of the zones in which inorganic gas Is Petroleum Corporation), deputy manager and senior'ell developed are the main distribution areas forlarge He-containing gas poolspartment, engarrangement. e-mail:ecbcnspc@publicaptt.js.cn;Fax:025-8822000中国煤化工CNMHG