Characteristics of carbonate gas pool and multistage gas pool formation history of Hetianhe gas fiel

17. Graham S. A. Brassell S. A. Carroll. R. et al.. Characteristics ofselected petroleum source rocks, Xinjiang Uygur AutonomousRegion, Northwest China, AAPG Bulletin, 1990, 74: 493Characteristics of carbonate18. Hanson, A. D, Zhang, S. C, Moldowan, J M. et al., Molecularorganic geochemistry of the Tarim Basin. northwest China. AAPg gas pool and multistage gasBulletin,2000,84(8):1109pool formationhistory ofan Buchem, F. S. P. Razin. P, Homewood, P. w. et alStratigraphic organization of carbonate ramps and organic-rich Hetianhe gas field, Tarimntrashelf basins: Natih formation (middle Cretaceous) of North- Basin. northwest China20. Reed, J. D, Illich, H. A. Horsfield, B. Biochemical evolutionary ZHOU Xinyuan, JIA Cheengzaosignificance of Ordovician oils and their sources. Organic Geo-WANG ZhaomingWANG Qinghua& YaNG Wei?hemistry,1986,10:34721. Longman, M. W, Palmer, S.E., Organic geochemistry of mid- 1. Tarim Oil Field, PetroChina, Korla, Xinjiang 841000, China(e-mailzxy@tz.cnpc.com.cn)continent Middle and Late Ordovician oils, AAPG Bulletin, 1987, 2. PetroChina, Beijing 100724, ChinaAbstract Hetianhe is a big carbonate gas field which is22. Prouty, C E, Trenton exploration and wrenching tectonics- Mi- found and demonstrated in the period of"Chinese Nationalchigan basin and environs, The Trenton Group(Upper Ordovician Ninth 5-Year Plan?. The proved reserve of Hetianhe gas fieldSeries)of Eastern North America(ed. Keith, B, D ) AAPG Studi-is over 600. X08 m. Its main producing layers are Carbonif-erous bioclastic limestone and ordovician carbonate com-es in Geology 29, 1989, 207-236posed of buried hill. The former is stratified gas pool with23. Obermajer, M, Fowler, M. G, Snowdon, L. R, Depositional water around its side and the latter is massive gas pool withenoironmetn and oil generation in Ordovician source rocks from water in its bottom. The gases in the gas pools belong to drySouthwestern Ontario, Canada: Organic geochemical and gases with normal temperature and pressure systemsBasedof gas and source rock, the gases areological approach, AAPG Bulletin, 1999, 83(9): 1426.mainly generated from Cambrian source rocks. According toWei, Ding Yong, Zhou Xiaofen et al., Theforming the researches on source rock and structure evolution, andondition analysis on Bachu-Maigaiti Slope, The Exploration and the observations on the thin section to reservoir bitumen andDevelopment Symposia for Tarim Basin s Northern Oil and Gashe studies on homogenization temperature of fluid inclusions, the gas pool has been identified and divided into threeFields(in Chinese), Beijing: Geological Publishing House, 2000, formation periods. The first is Late Caledonian when the oilgenerated from the Cambrian source rocks and migratedZhou Xingxi, Accumulation Conditions of the cracking gas in along faults, as a form of liquid facies into Ordovician car-platformal basin region of Talimu Basin, Natural Gas Industry (inuplifted, the oil reservoir had been destroyed. The second ishinese),1999,19(2):24.Late Hercynian when condensate gases generated from the26. Drozd, R.J., Cole, G. A, Point Pleasant-Brassfield( petroleum Cambrian source rocks and migrated into Ordovician resersystem, Appalachian basin, U.S.A The Petroleum System-From voir, as a form of liquid facies. Since the fractures hadSource to Trap(eds. Magoon, L. B. Dow, w. G.). AAPG Memoir reached P strata, so the trap might have a real poor preser.60,1994,387-398not happened. The third gas reservoir formation period27. Jiang Huashan, Ye Deshang, Wang Shaoli et al., The Ordovician occurred in Himalaya. The fractures on both sides ofhydrocarbon reservoir characteristics of Tahe oil field, The Explo- Hetianhe gas field developed violently under the forces ofration and Development Symposia for Tarim Basini's Northern Oil compression, and thus the present fault horst formed. Thedry gases generated from Cambrian source rocks and miand Gas Fields(in Chinese), Beijing: Geological Publishing grated upwards as the form of gas facies into Ordovician andHouse,2000,56-67Carboniferous reservoirs, and the large gas pool as discov28. Zhang Guangya, Song Jianguo, Control of basin reworking on hy- ered at present was formed finallhrocarbon accumulationand preservation in the Tarim Basin, Keywords: multistage pool formation, characteristics of gas poolNorthwest China, Geological Review(in Chinese ), 1998, 44(5): Hetianhe, Tarim Basin, carbonate rock中国煤化工ate rocks are main pro-29. Zhao Jingzhou, Timing marine accumulation in the Tarim BasinCNMHGWest china such as siby oil/gas-water contact retrospecting, Petroleum Exploration andchuan basin Ordos basin and Tarim basin( HoweverDevelopment (in Chinese), 2001, 28(4): 5the migration and accumulation of natural gases in Pa-(Received August 9, 2002) lacozoic carbonate rocks in China are more complicatedChinese Science Bulletin Vol 47 Supp. December 2002than those overseas[5,6. Since Palaeozoic era. the structure history of reservoir formation will certain enrich the oilmovementand pool formation theory for carbonate rocks in this area,occurred frequently in Chinal. 8. This had strongly deand help to reveal the rules of oil and gas pool formationstroyed the former oil and gas pool. As the structure and gulde Turther oil and gas exploration in this areamovement occurred, the gas pools had experienced the I Gas pool characteristicscycling of formation-destroyed and reformed-redestroyedsTratum and sedimentary facies. The drilledmovements, and thus they have different reservoir or ure strata from top to bottom in the Hetianhe gas field are asin many times. Different areas have different strucfollows: Quaterary, Neogene, Lower Permian, Carbonif-tion characteristics 249. So, it is essential to make a syserous and Ordovician. Mesozoic, Upper Permian, Devonian and silurian are absent. The Carboniferous stratum cantematic study on the property and history of gas reservoir be divided into eight lithologic intervals as a sequenceformation, especially on the key gas poolfrom top to bottom: Xiaohaizi limestone interval, sandsto-Hetianhe, a big carbonate gas field, with more than ne and mudstone interval, upper mudstone, standard lime600.>10%m reserves, is located in the Mazatage fault stone. middle mudstone bioclastic limestone. lower mud-ructure belt on the southern side of Bachu bulge, central stone and sandy gravel interval o(fig. 2). Commercial oiluplift, Tarim Basin(fig. 1). The exploration of oil and gas and gas flow have been discovered in Carboniferous stanon Bachu bulge, Tarim Basin has been being carried on dard limestone, bioclastic limestone, two sandstone layerssince the late 1950s. drilling work began from 1966in sandstone and mudstone interval, thin sandstone layerter the effort for near half a century, in 1995, the Niaoshan of lower mudstone, sandy gravel and Ordovician buriedstructure gas pool was found on the western side of the hill carbonate rocks. Carboniferous in this area is shallowMazatage structure belt. The reservoir is carbonate rocksepicontinental sea sediment and Ordovician is the carbonin age of Ordovician. In 1997, in drilling well Ma-4 a gas ate platform sediment!llpool was discovered in Carboniferous bioclastic limestone1: Type of gas pool. The Hetianhe gas fieldin the Mazatage structure belt, and thus the Hetianhe gas mainly develops 5 carbonatefield was foundbioclastic limestone pools in structures of Ma2, Ma4 andHetianhe is the largest carbonate gas field found in Ma8, and Ordovician buried hill pools in structures Ma8the Tarim Basin nowadays. The reservoir characteristics and Ma4(fig. 3). Carboniferous bioclastic limestone isof the Hatianhe gas field are analyzed here. Based on the individual stratified anticline gas pools with water aroundobservation on thin section of reservoir bitumen and the with its Upper Carboniferous sandy gravel and form atudies on homogenization temperature offluidInclusionsuniform reservoir, belonging to massive gas pools withit could be inferred that the gas pool has experienced three water in its bottomeriods of gas pool formation The last one should bemost important to the present gas pool. The study on theMazatage fault structure beltMa8●Ma3·Ma2MaoMa 401 0Hetianhe gas fieldMa 4Tarim Basin☆ Gas field location中国煤化工CNMHFig. 1. Location of Hetianhe gas field and its wellChinese S序嫂弻 etin Vo.47Supp. December2002147Memberprotilediplay rockXiaohaizI161-201Maand-mud241, 5-346"Fstoneunmud 32-58Mas96-177limestonemId- mud128-164Madstone 76-104MadMas回园园国回回stone:3, gypse: 4, dolomite; 5, sand-gravel stone; 6, mud stone; 7, bio-limestone: 8, sand; 9, gas bed; 10, reservoir; 1 l, cap rockMa8 structure Ma2 structureMa4 structureand the content of CO, is greatly changed, ranging be0.16%-14.3%.Ma8Ma3 Ma2 Ma7 Ma5 Ma401 Ma4 Ma402The oil is typical condensate with low density, lowKZ_solidification point, low sulfur content, low wax contentlow resin and low asphalt content. Its density is 0.750.82 g/cm generally 0.8 g/cm:. The solidification point islower than-30., and sulfur content commonly less than0.2%.The formation water is only generated at the bottomof Ordovician gas pool, for which the density ranges fromGas deposit Bed boundary [n Well1.0738 to 1.090 g/cm, total mineralization degree(11.213. 1)->10 ppm, and pH 6-7. Therefore, the water be-Fig. 3. Geological profile of gas pool section of Hetianhe gas fieldlongs to CaCl, typeC1V Temperature and pressure systems. Two of the(L11)Fluid characteristics. Both commercial gas gas pools in Ma8 structure of Hetianhe gas field haveflow, and a small quantity of oil have been gotten in the loweradient recnectively 1 27 0100 m andHetianhe gas field. The gases have the character of high 1.833中国煤化工 betw2.28and2.4CHA and N2 content, in which CHa content is 75%-85%. 100C Gal temperature system/I2)The gases belong to a high maturity dry type with drThe pressure coefficients of the gas pools range betweencoefficient of more than 0.95 2. The non-hydrocarbon 1.07 and 1.17, belonging to a normal pressure systemgases are N, and CO The content of N, is 10%-18%, tool2Chinese Science Bulletin Vol 47 Supp. December 20022 Source rocks and their evolutionthreshold around the end of Ordovician. Only around theGenerally, three sets source rocks developed onend of Silurian, had the area north to Hetianhe gas fieldBachu bulge, which are in age of Cambrain, Ordovician matured, but the most as southern areas of the field hadand Carboniferous. Cambrian hydrocarbon source rock not matured yet. At the end of Devonian, part of thewas composed of a set of lagoon facies sediments 13, 141 northern area of Hetianhe gas field had reached a highmade of deep gray, gray dark pelitic micrite dolomitic mature stage; however, the southern area had completelylimestone with salt and gypsum Content of Total organic entered the generation threshold. Therefore, the Latecarbon(TOC) is 0. 21%6-243%6.0.81% on average. The Cered in bhe erey nide eh end bf che oaiferou genheatirenorganic matter is sapropel typell2. The measured bitumen north to Hetianhe gas field had completely reached a highreflectance is 1.65%--2.32%(as Ro value), belonging to a mature stage, and then the southern area matured furtherhigh-to post-matured stagell-4. 51. The Ordovician hydro- About the end of Permian, the northern part of the Hetiancarbon source rock was composed of a set of gray-deep he gas field, the Cambrian source rock had reached a con-gray marly limestone, in which TOC ranges from 0. 21%0to 1.99%0, on average 0.93%0, and the organic matterdensate stage(table 1). Nowadays, apart from part of thesapropel and mixture types 2. The organic matter is anorthern area which has entered the post-mature stage, theCambrian in most part of this area is still in the late stageready in the stage of condensate with the Ro values rang- of a high mature, dry gas stageing between 1.10%--1.40%0: however, the source rock hasa limited distribution area. The Carboniferous hydrocarTable 1 Thermal evolution data of Cambrian source rock in hetianhebon source rocks were composed of both mudstone andgas field(%)rbonate. The carbonate source rock is a set of gray-deepsouth partgray pelitic micrite limestone, its TOC is 0. 1%0-1.27%, End of Ordovician0.450.50.6-07averagely 0.62%. The mudstone hydrocarbon source rock End of Silurian0.50.69-1.1is humic and mixture types, and its residue toC is 0.2% End of DevonianEnd of Carboniferous0.65-0.817-1.87.09%, averagely 0.26%1.97%. The Ro value is End of Permian10-1.20. 65%0-0.87. The organic matter is at low mature andmature stages and the Ordovician and Carboniferous hy- 3 Structure evolution and trap formationdrocarbon source rocks cannot be effective for gasdrocarbon source rock reached a high maturity stage, so it Ddchu bu, Mazateatt 16 7I and sandwiched by two reversegeneration because of low maturity. Only Cambrian hyfault structure belt is located onfaults on its south and north sides. The trend of the belt'scould be an effective source rock in the studied area(fig. axial is commonly consistent with that of the fault.Thebelt from south to north has a width of 5 km. and a lengthof 90 km. The west part of the belt is wider than the eastpart. The structural belt from west to east has successivelydeveloped the three local structures(fig 3)as Ma8, Ma2and Ma4. The west part of the belt is higher than the easton the whole. Under the regional structural stress, thestructural belt has suffered four times tectonic move2000cIn Sinian-Ordovician, affected by the extension, theHetianhe gas field developedte normal圣300faults along the northern edge of Hetian ancient uplift, andthen an extensive fault horst belt was formed. In LateCaledonian-Hercynian, the south part of the Tarim Basinrose on the whole since ancient Tethys underthrust andQiangtang ancient land migrated northwards. The bachubulge and Maigaiti slope formed a unique northward clinoform. In the Indo-Chinese and Nevadan epoch, theBuv凵中国煤化工Fig. 4CNMHGalso the top of PermianAccording to the analysis of thermal evolution histo- was strongly erased. The structure acted violently and wasry in this area, most of the source rock at the bottom of finalized in Hymalaya. In Early-Middle Hymalaya,theCambrian had not entered a hydrocarbon generation former northward clinoform was raised due to the rise ofChinese S序嫂 letin Vo.47Supp. December2002Kunlun Mountains and the formation of Southwest De- easily combine with colloidal state asphaltenes and formpression In the Late Hymalaya epoch, compression force solvent film which would be favorable for stabilizingmade Tertiary stratum slide along the gypsum salt layer at colloid. When the light hydrocarbon appears as gas faciesthe bottom of lower Tertiary, and formed the present Maz- asphaltenes would be deposited. Although the depositionstructure body below. So the fault anticline structures Ma8, tion is the result of oil pool intrused by gase dant deposiatage Mountain. The force had a little effect on the main begins from the migrating of oil facies, abuMa2 and ma4 all remained. The structures matched with(1 )Hydrocarbon fluid inclusion. Many studiesCarboniferous and Ordovician beneficial reservoir and have been made on inclusions within the Ordoclan andcap rocks(fig. 2), and formed favorable traps on the Maz- Carboniferous carbonate reservoirs. The inclusions in-atage structure beltclude the kids of single-phase salt-water inclusion, single4 Times of gas pool formationphase gaseous hydrocarbon, single-phase liquid hydrocarbon and double-phase salt water. The homogenizationDuring the reservoir diagenesis, some markstemperatures of hydrocarbon inclusions have been estiflecting the pool formation can usually be generated to- mated for this studysuch as fluid inclusions and reservoir bitumen TheAccording to the analysis results of homogenizationgenerated times of reservoir bitumen/i] and the homo- temperatures, two generating times of inclusions in thegenization temperature of fluid inclusions should be used Ordovician reservoir can be identified. The peaks of o-to indentify the generated times and difference episodes ofoil and gas pool 9-22mogenization temperatures are 60--80( and 90-120.Csome of them reach 130-140. ((fig. 5). However, lowD)The thin section research on reservoir bitumen. temperature and very high inclusions were seldom develBased on the observation on reservoir bitumen thinoped within the carboniferous reservoirs. Only one gen-section of Hetianhe gas field, two phases of bitumen could erating episode of inclusions, with its homogenizationbe easily and obviously discovered. The first phaseblack lumpy-like and the second is scattered or agglomertemperatures being 75--90C individually up to 115.Ccan be fouate bitumen sedimentsThe first phase bitumen of black-lumpy tpye is filledin the former intergranular pores or intergranular enlarged 220.00173919571848this phase's bitumen also is filled in intercrystalline solu-25003s 10 ',pores which is often incised by structure fractures. Someof the fractures could be either filled, or not. Sometimes, 10.00tion pores. This kind of black bitumen all belongs to atype of degradation, for which a chromatography of saturated hydrocarbon shows characteristics of destroyedancient oil pool. It is biodegraded oil because of its obvi- carbon inclusions within Ordovician reservoirs, Hetianhe gas fieldously high hopane content), for which a normal oil poolseldom shows. The idea has been proved by biomarkers(ii)Multistage pool formation history. Observaextracted from the reservoir bitumen. The biomarkers also tions on thin sections of reservoir bitumen and analysis ofshow that the oil has been generated from Cambrian sour- homogenization temperature of fluid inclusions and thece rock. This type of bitumen has been frequently found in evolutional characteristics of source rocks and structurethe reservoirs of drilled wells Gudong 1, Gudong 2, Shan indicate that the gas pool has suffered at least three periI and Ma 3 on the west of Hetianhe gas field, and spora- ods of gas pool formation: Late Caledonian, Late Herdically distributed in the wells of the east parted cynian and Himalaya Period respectively. The Himalaya isThe second phase of bitumen is irregular, scattered the key period of hydrocarbon migration and accumulaand or agglomerate bitumen sediments coexisting with tion.light oil. Some of small oil spots have little asphaltenes(1) Late Caledonian. Around the end of Palaeozoicand some of oil spots exist at the marginal part of the first the structural pattern that the south part was higher thanphase bitumen filled in intergranular pores. The black- the north had been formed. At that time, the former extenagglomerate bitumen sediments can be divided into two sion fractures had been translated into compression typekinds. One is distributed in early stylolite caused by the due to the transition of stress field o. The oil generatedneutralization of oil and minerals in stylolite. The other from中国煤化 Trated upwards along faultsexists in intergranular solution pores formed by the gas intoservoir and accumulatedntrusion when gases migrated into early accumulated oil. theCNMHGstration includes: 1)TheWhen gas intrusion occurred, a great deal of hydrocarbon black bitumen formed in the first period is filled in earlierwith light molecular weight exists as gas facies. However, intragranular porosity and the intragranular dissolvedthe polar compounds such as benzene and toluene can porosity that was generated by diagenesis is likely to beChinese Science Bulletin Vol 47 Supp. December 2002the trace of oil pool formation. 2) The hydrocarbon inclu- tiny or fine fractures. The majority of the fractures andons with the homogenization temperatures ranging be- pores had been cemented and the bitumen sediments cantween 60-80( is possibly formed in this period of pool be easily found within the early generated suture. 2)Theformation. At that time the buried depth of Ordovician volcanic activities made the geothermal gradient becomereservoir was down to the interval of 1000-2000 m. with higher. When the buried depth of reservoir was down toa high geothermal gradient(fig. 6)20003000 m(fig. 6), the temperature of reservoir canreach over 120'C and the hydrocarbon fluid inclusionwith homogenization temperatures of more than 120(can be caught in the Carboniferous bioclastic limestoneOnly a few of hydrocarbon fluid inclusion with very highhomogenization temperature could be generatedthe studied area was continuously in a state of upliftingduring the Indo-Sinian epoch In the Himalaya Period, thefractures on both sides of Hetianhe gas field developedviolently under the forcing of compression, and thus thepresent fault horst formed. And beneficial traps were alsoLate-HercynIanformed in Carboniferous and Ordovician strata. andprovided beneficial places for accumulation of oil and gasThe Cambrian source rock had entered the dry gas stagethat movement. The dry gas generated from Cambriansource rock migrated upwards, and then the gas pool ofLate-CaledonianHetianhe gas field was generated. The proof of gas poolformation in this period includes: 1) the second periodagglomerate black bitumen which is distributed in earlyintergranular solution pores is very likely the result of oilBefore mid-Late Ordovicianmigration in this period. The dry gas generated fromCambrian source rock entered the remaining oil poolformed during the former two pool formation eventsI0 kmThen the erosion happened, and the bitumen sedimentsformed. 2) The hydrocarbon inclusions in the OrdovicianFig. 6. Pool formation model of Hetianhe gas fieldreservoir with the homogenization temperatures of 90(2)Late Hercynian Period. In the studied area oc- 120. and in Carboniferous bioclastic limestone is likelycurred a large transgression, with 800--1000 m sedimentsformed in this pool formation period. Since the reservoirin Carboniferous, around Late Hercynian and the end of had the depth of 2000-3000 m at that time, geothermaearly Permian, volcanic activities happened in a huge area gradient was higher than that of the present (ig. 6)in the Tarim Basin. The geothermal gradient obviously 5 Conclusionsbecame higher, leading to the maturity of Cambian sourcerock(fig 4). The Lower Cambrian source rock developedThe main producing layers are Carboniferoron the north part of Hetianhe gas field had entered the clastic limestone and Ordovician carbonate buriedcondensate stage. Meanwhile, the volcanic activities acti- carbonate, the former is a stratified gas pool with watervated the faults on both sides of the gas field. The faults around its side, and the latter is a massive gas pool withcut through the Permian stratum and forced the gas to water in its bottom. The gases belong to dry type withmigrate vertically. Due to the tectonic movement, the normal temperature and pressure systemstraping condition was unstable. The oil and gas had notCambrian is the main hydrocarbon source rock ofgathered on a large scale. The reservoir retains some of this gas field. Himalaya is the main period of the naturalthe evidence of oil migration: 1)The agglomerate black gas accumulation In the pool occurred three events of oilbitumen formed in the second period which was distribut中国煤化工ed in early stylolite is probably the result of oil migrationthe oil generated from thein this period. In the tiny or enlarged solution fractures CarCN MH Ged along faults into thecaused by the tectonic movement, due to anti-co-agulaOrdovician carbonate reservoir. After that, the crust uption and absorption on the mineral surface, the generated lifted, the formed oil reservoir had been destroyed. In Latelight oil is mainly left in the matrix pores of Hercynian, the condensate gases generated from the CamChinese S序嫂 letin Vo.47Supp. December2002brian source rocks migrated into Ordovician and Car- 11. Yangwei, Wang Qinghua, Liu Xiaozeng. The genesis of lower Orboniferous reservoirs, but no large-scale gas pool develped. In Himalaya, the fractures on both sides of Hetianhedovician dolomite in Hetianhe gas field of Tarim Basin, Actagas field developed violently under the forcing of com-Sedimentologica Sinica(in Chinese), 2000, 18(4): 544.pression, and thus the present fault horst formed. The dry 12. Dai Jinxing, Pei Xigu, Qi Houfa, China Natural Gas Geology(Volgas generated from Cambrian source rocks migrated1)(in Chinese), Beijing: Petroleum Geology Press, 1992, 5-146wards to form the present huge gas field13. Feng Guoxiu, Cheng Shengji, Relationship between bitumen andReferencesvitrinite in sediment rock, Natural Gas Industry (in Chinese), 19881. 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