Petroleum geological characteristics of Kela-2 gas field

Petroleum geological char.depth is 4130 m. The bottom layer is Lower CretaceousShushanhe Formation. 34 1-m-thick gas layers are discov-acteristics of Kela-2 gas fielderied. The confirmation of Kela-2 gas field is an importantmilestone of Chinas oil and gas exploration in the area offoothill structure belts. This paper focuses on full descrip-Jia Chengzao, ZhoU Xinyuan, WaNg Zhaomingtion of petroleum geological characteristic for the Kela-2LI Qiming2, Pl Xuejun?, CAl Zhenzhong2gas fieldHu Xiaoyong1 Regional structure1. Petro China Company Limited, Beijing 100083, China;Kuqa Depression of the Tarim Basin, in which theCorrespondence should be addressed to Jia Chengzao(e-mail: jiacz@ Kela-2 gas field is located, is a foreland basin in thepetrochina. com. cn)southern Tianshan Mountains structural belt. The depression borders the southern tianshan mountains structuralAbstract The Kela-2 gas field is located in the center of belt on the north and Tabei uplift on the south with anKelasu structural belt in Kuqa Depression. This trap is oneof a series of traps in the folded belts which are distributed in east-west length of 550 km and a south-north width of 30a string of pearls in the dual structure The primary gas80 km as well as an area of 28515 km. It can be furtherbearing layers are sandstone of Lower Cretaceous K, bs while divided into four structural belts and three sags (fig. 1)the secondary layers are dolomite member and gluteninFrom the north to south the four structural belts are themember of Lower Tertiary E.xkm and sandstone of Lower northern monocline belt, Kelasu-Yiqikelike structuralCretaceous K,b. The main component of natural gas is Qiulitage structural belt and the frontal uplift belt. Frommethane whose content is higher than 97%o, It is characte r- the west to the east, the three sags are Wushi sagod and then became a reservoir in the late time. The reser- in the center of Kelasu belt. Kela-2 gas field is locatedThe Kela-2 structural trap was formed during the Xiyu periBaicheng sag and yangvoir formed late and the thick seal rock of Lower Tertiarygipsmantle are the avail reason why the giant Kela-2 gas fieldhas been well kept. The abnormal high pressure of the Kela-. km2 gas field results from the strong structural compression inthe northern part during the xiyu periodKeywords: Tarim Basin, foreland basin, thrust fold belt, Kela-2 gasfield, abnormal high pressure.可③园3The mesozoic and cenozoic basins in western ChinaFig 1. Division of structural units in Kuqa foreland basin and locationhave suitable geological conditions for oil and gas reser- of Kela-2 gas field. 1, Basin boundary, 2, the first grade tectonic line; 3voirs. The Triassic-Jurassic period was characterized by the second grade tectonic line; 4, road; 5, field; 6. reservoirs. a, Wushian extensional structure movement and the damp palaeo- sag: b, Wensu uplift; c, front uplift, d, Qiulitage tectonic belt; e,climate environment which formed a set of coal measurestrata with the lacustrine bog hydrocarbon-rich faciesangxia sag: i, north monoclinal slope; j, Tianshan Mt A, Kela-2 gaswith the strong compression in the Late Himalayan periodthick deposition of molasse caused hydrocarbon sourceThe Kelasu structural belt borders baicheng sag onrock to become mature increasingly. The thrust structural the south and the northern monocline belt on the northbelt which was developed extensively in the front of the Formation and evolution of this structural belt are mainlymountain became a favorable place for oil and gas accu- under the control of the Dawanqibei-Kelasu faulted beltmulating in abundance. 2. The exploration of Kuqa pied- The fault-related folds of various types were developed inmont thrust belt was started in the 1950s. The first explo- the structural belt( such as fault dogleg fold, eruption fold,ration target is the shallow structure, which can be seen on fault propagation fold and dual structure)5.The Kela-2the ground, The exploration entered a new period since structure is an eruption fold against the background of thethe 1990s. Drilling of Kecan-l exploration well in 1995 dual structurerevealed the high-quality assemblage of reservoir and itsThe foothill structural belt of Kuqa Depression isseal rock, which is composed of Lower Tertiary gypsifer- characterized by the progressive extension from south toous salt rock and Cretaceous sandstone Drilling of Kela-2 northwell started on March 25, 1997. dSt was conducted on Mie中国煤化工 ne belt was formed in theelasu structural belt wasLower Tertiary dolomite on January 20, 1998. with a 6.35 deterCNMHGPeriod. In the late periodmm choke, the daily gas production reached 27.71x10of Kuqa, the Qiulitage structural belt came into being, butthen the Kela-2 gas field was discoveried. Drilling of Yaken anticline in the southern part of the belt was stillKela-2 well was completed on May 24, 1998. The well under the reinforced deformation period at that timeChinese Science Bulletin Vol 47 Supp. December 20022 Stratathe perforated intervals are Neogene nij, and Eogene E,3SThe outcrop stratum of Kela-2 gas field is Upper and El- km, Lower Cretaceous K, bs and K, b Upper CreTertiary N-k. From the upper part to the lower parttaceous was omitted(fig. 2). E,km and k,bs are theShallow RiStrataLog explanationOHMMLitho Deeper o F R egamma I columnOHMM)中(%)(×10umⅢ36000I38503900TH中国煤化工CNMHFig. 2. Strata column of Kelasu structure belt. 1, Gypsiferous salt rock; 2, gypsum-mud; 3, gypsiferous mud; 4, carbonate; 5, mud; 6,pelitic siltstone: 7, siltstone: 8, finestone: 9, medium sand; 10, conglomeratic mediumChinese S序嫂弻 etin Vo.47Supp. December2002gas-bearing intervals of Kela-2 fieldtributary channel bend, sand sheet, channel mouth barThe drilled thickness of Ej.km is 500-1000 m. distal braided channel and shallow-lake mud(table 1)I8)From the upper part to the lower part, it can be dividedEogene Ej,km is mainly of the evaporation marginalinto five lithological members in detail. They are mud- marine facies including fan delta subfacies and evapora-stone, gypsiferous salt rock, dolomite, gypstion tidal flat subfaciwell as such microfacies asand glutenite. The gas-bearing layers are in the dolomitesubaqueousbraided channel, epilittoral zone, subtidaland glutenite members. Gypsiferous salt rock is a set of zone and bioclastic beach, arenose beach oolitic shoalhigh quality regional seal rock with a thickness of 400--the key factor for the oilThe glutenite member is the sedimentation of low-orderabundant in Kuqa front land thrust belt). With a thickness sys tem tract while the dolomite member is the sedimenta-of 4-10 m, dolomite member is gray dolomition of transgressive system tract(1. Clastic rock reservoir. The Cretaceous K,bslike echinoderm. With a thickness of 15-20 m, glutenite reservoir rock type of the Kela-2 gas field is mainlyis shallow gray glutenite and gravel-bearing fine brown medium and fine-grained lithic sandstone. Thesandstone in medium thickness layers. From the upper average content of quartz is 46.15%-48.9% while thatpart to the lower part, Lower Cretaceous K, bs is divided contant of feldspar (mainly alkali feldspar) is 9.34%into three lithological members. The first and second 16.02% and that of debris (mainly the debris ofmembers are sand interbedded with a small amount of metamorphic rock and sedimentary rock) content 39.53%mudstone and pelitic siltstone. The thickness of the first.48%. The components are generally of low maturity.and second members is about 90--100 and 150-160 m The structural maturity is medium and slightly higher 9)respectively. The thin mudstone interlayer in the second K, b sandstone is of relatively high maturity. The type ofmember is relatively pure. The nature gamma value of the rock is mainly lithic sandstone The average content ofmudstone interlayer is higher compared with the first quartz is 46.3%, that of feldspar content 12.8% and that oflithological member. The mud thickness is 0.5-2 m. The debris 40.9%third lithological member has a thickness of 110--150 mThere is the thick brown-red lithic gravel interbedded withThe reservoir type is porous while the reservoirfine-grained sandstone. The electric property of this space is mainly residual primary intergranular pore.Themember is characterized by high log resistance. It issecondary types are dissolution pore, intercrystal pore,another mark layer of the strata division and comparisonmicropore, structural fracture and constricted fisfor the Kela-2 gas field. Lower Cretaceous Kb is lighter-The type of reservoir rock pore throat is mainly fewbrown blocked siltstone and fine-grained sandstone with a sheet pore throat. Basal cement is quite common in thethickness of 120-150 m. The Sushanhe formation in the layer with a high content of interstitial matter. The inter-Lower Cretaceous is brown mudstone and siltpelite in megranularity is filled with interstitial matter with the pipedium-thick layerbundle throats developed. The mercury injection curve3 Characteristics of sedimentation and reservoirssandstone reservoir reflects a low pressure for expulsion,0.76 MPa on average. The pore throat sorting is relatively(D)Sedimentation. Kbs, the main target layer of desirable. The radius of the main flowing pore throatKela-2 gas field, is the sedimentation of subfacies as fan 0.4-16 um. Generally speaking, the pore structure isdelta foreland, braided delta foreland and braided alluvial characterized by medium-coarse pore throatplain, including the microfacies of subaqueous gravityflow, subaqueous distributaryrashanneaciey, agase microfacies of Eogene- Cretaceous reservoirsStrataLithologic memberSubfaciesMicrofacieszonedolomite membertranserdal flatbioclastic beach, arene beach and ooliticnarine faciessubtidal zoglutenite memberlow-order systemfan delta frontea chafirst memberhigh-orderbraided delta braidedCretaceousfaciesalluvial distal plain distal braided channeldistributary channel, channelTract中国煤化工 subfluvial distributary riverCNMHGsheetthird memberfan deltaan delta frontfluvial distributary channelfaciessubfluvial gravity flowfront fan deltashallow lake mudChinese Science Bulletin Vol 47 Supp. December 2002At the Kela-2 gas field, porosity of Eogene glutenite crystal dolomite. The fractures are mainly of structuralmember-Cretaceous Kbs is mainly distributed at 8%0fracture20%, the maximum is 22. 4%o, the4 Structural characteristics of gas fieldaverage 12.56%0. The permeability is distributed mainly at(I Faults. The Kela-2 structure is mainly under0.1--1000 md, the maximum of 1770 md and the average the control of two southern and northern anticline thrust49.4 md. The porosity and permeability have a good linear faults(fig. 3 ) The northern-border controlling fault is inrelationship, reflecting that the pores of reservoirs are the eW-ne direction. The fault throw is more than 1000distributed homogeneously with good physical properties m with the cross-member tilting towards S-SE direction. Itfor reservoirsis a large regional fault extending about 23 km. The faultphysical properties of K b are apparently poorer in the south border includes a series of thrust faults in thethan those of k bs. The porosity ranges from 2.67% to nW.ne direction The cross-section tilts towards the n13.7% with the average of.9%. The range of permeabil- NW direction. The extension length is 5-15 kmity is 0.04 6.62 md with the average of 0.78 mdThe various sedimentary microfacies and thicknessof Cretaceous K,bs are distributed steadily in the area of4 kmKela-2 gas field with the continuity of sand body beingdesirable. Mudstone is distributed sporadically and discontinually. K, bs had a number of source rocks during itssubsidence with an adequate source supply. The containing space is low or very low while the effect of fluvialsedimentation is relatively strong. The channels wereeroded and cut frequently. The sedimentary facies werecentered on the braided alluvial plain and braided delta Fig 3. Kela-2 gas field tectonic map of the dolomite top in Eogenefront!o, forming a set of plate-shape sand bodies withThe influence of south-north compression led to thegood vertical and horizontal communication, large thick- development of three groups of shear faults and tensionalness for sedimentation and extensive distribution on the positive faults along the structural axis in the Nw-SEE(Lii) Carbonate rock reservoirs. The types of Eo- tion. There are a total of 34 faults. The fault throw is 10gene E, km dolomite member are mainly bright-crystal 90 m and the extensional length is 1- 4 km. The faultlithic dolomite, micrite lithic dolomite, micrite bioclastic throw of those small faults is relatively small comparedwith the reservoir thickness and the gas column heightdolomite,silt-crystal dolomite and pelitic-dolomite gyp- The upper part is covered by thick gypsiferous salt rocksiferous rock and gypsiferous dolomiteLower Tertiary dolomite and development desa e on the state of the gas reservoirThere is little influreservoir at the Kela-2 gas field is 11. 4%o while the aver(1 )Description of regional structure. The Kela-2age permeability is 3.6 md. The type of reservoir pore in the east-west direction and the two sides basically inyspace is mainly of pores and then fracturesymmetry. The long axis is 18 km, the short axis 3-4 kmThe types of pores are mainly biological moldic andhe ratio of long and short axes 4.5:1. tipores, intergranular solution pores, intercrystal pores, stratigraphic dip of the northern side is 160-20 whileintercrystal solution pores and selective solution porethe dip of the southern side is 19-22. The closed con-The moldic pores are mainly developed in micrite dolo- and the amplitude 510 m. The spill points are located atmite and bioclastic dolomite. The intergranular solution the western end of the structure the closed line is -2468pores are mainly in lithic dolosparite. The intercrystal m gas-water interface. The gas-bearing area is 48.06 km2solution pores and intercrystal pores are developed in silt-(table 2)Table 2 Structural elements of Kela-2 gas fieldea level ofAmplitudeClosed lineClosed areaas areaheight/mTop of Lower Tertiary dolomite member1970中国煤化工49.5648.06Top of Lower Tertiary glutenite member-2000Top of the third lithologic member of Cretaceous K, bsCNMHGTop of Cretaceous K, b2330-2770Chinese S序嫂弻 etin Vo.47Supp. December20025 Characteristics of gas reservoirpressure characteristics. The natural gas differential factor(1) Temperature and pressureis slightly high under the original gas reservoir conditionsThe formation temperature in the center of Kela-2 (1.4612--1 4900). The volume factor is slightly lowgas reservoir is 100.58( and the temperature gradient (2.516x10-3-2587x10-m2/m ).The visccosity is slightly2. 1880/100 m, belonging to the normal temperature sys- high(3.403x102-3591x10-mPa-"s)atural gas belongs to high-maturity coal-bed metEogene dolomite member, glutenite member and the ane. The natural gas carbon isotope of the Kela-2 gasthree lithologic members of Cretaceous belong to the field is slightly heavy, with 8C being-27.07%same pressure system. The formation pressure in the cen27.8 %and 8C, being-17.87%0-19.4 %e belongingter of the gas reservoir is 74.41 MPa and the average pres- to typical CBM[16, 17. Based on the relation between car-sure factor 1.95--2.20, belonging to the super-high presbon isotope and maturity, the maturity of hydrocarbonsure gas reservoir. The abnormal high pressure can be source rock (R )is estimated to reach 2.2%o at maximumattributed to a number of factors. Based on the strong Natural gas came mainly from the middle and Lowerstructural compression on Kuga foreland basin during the Jurassic Kezilenuer Formation and the Yangxia Forma-Xiyu period of Himalayan Movement and combined with tionrestoration of palaeostress field in this region, the south2) Formation water. The stratum water is of CaCl2north compressional stress in this region has gradually type. The density is 1.082--1 111 g/cm, the chloriderisen from Cretaceous to the present time. The structural 7X104-10x10 mg/L and the total salinity 12x104compression caused the particles of rock to become more 165x10 mg/L, indicating good seal conditionscompact so that the pore space dwindled with the fluid(L11)Type of gas reservoir. The reservoir-caprockpressure rising. The screen effect of the thick gypsiferous assemblage of the Kela-2field is composed of thesalt layer prevents the increasingly high pressure from Eogene dolomite reservoir with a stable thickness, Eo-discharging outwardly, thus leading to the abnormal high gene-Cretaceous sandstone reservoir and Eogene gypsiferous salt rock and mudstone. a complete anticline(1) Properties of fluidabove the structure. The small faults in the upper part give(1)Natural gas. Natural gas has a high content of rise to a 20-m mudstone member separating Eogenemethane and a low content of non-hydrocarbon gas. It is dolomite member from glutenite member, causing all thehigh-quality natural gas. The molecular quantum of natugas-producing intervals to communicate with each otherI gas was the lowest in the Kuqa foreland basin(16. 4and have a uniform pressure system and a uniform gas16.7). The relative density is low, and the average is 0.569. water interface. They belong to the same gas reservoir(igThe methane content is high, 97. 265% on average. There 4)is little content of heavy hydrocarbon(Ct). The n,contentThe Kela-2 gas reservoir has a large amplitude. THis low, 1.580% on average. The acid gas content is low sea level of the height is-1970 m on the structural mapand the CO, content is 0.686% on average. The drying for the top of Eogene dolomite member. The gas-waterfactor(C/CI*)is high, belonging to dry gasinterface is-2468 m, the gas reservoir amplitude 498 mPVT properties of natural gas show super-high and the closure of the trap 510 m. The gas reservoirKela-203Kela-2011750(projection)(projection) Kela-2042000中国煤化工4 kmCNMHGFig. 4. E-w profile of Kela-2 gas reservoirChinese Science Bulletin Vol 47 Supp. December 2002thickness(Kela-2 well) is 561 m, far larger than the trap 1904.94 x 10m of gas, 4.33 m of oil and 53.55 m ofamplitude. The gas reservoir is characterized by bottom water By using of the 21.57 mm nipple for well testinwater and blocked shapethe daily gas production reaches 300.44x10m3The bottom water of Kela-2 gas reservoir is not ac-Using 3D seismic data, having six wells be contive. The third lithologic member of Cretaceous K,bs is a trolled and calculating on the basis of volume measuconglomerate layer. The physical properties are exten- ment, the proven natural gas in the place of the Kela-2 gassively poor with many mudstone interlayers. The testing field is 2840. 29 x10 m while the recoverable reserves arresults indicate that they are of dry or low-production 2130.22x10m. Based on Chinas assessment standard onyers, preventing the bottom water of the gas reservoir natural gas reserves, the Kela-2 gas field is a giant onefrom transmission of energy. However, the marginal water with deep intervals, high abundance and high productivityof the gas reservoir may be active with adequate energyReferencesGenerally speaking, it belongs to a water-drive gas reservoir of medium levelI. Jia Chengzao, He Dengfa, Lei Zhenyu et al. Oil and Gas Explo-C-iv Productivity. All the five wells drilled withinration Foreland Thrust Belt(in Chinese), Beijing: Petroleum hdustry Press, 2000, 1"-35the scope of the Kela-2 gas field(Kela-2, Kela-201, Kela- 2. Wei Guoji, Jia Chengzao, Shi Zhongshen et al, Structural char-203, Kela-204, and Kela-205 ) obtained high-pro-ductionacteristics and oil and gas of cenozoic compound regeneratedindustrial gas flow. They are characterized by high singlewell productivity and high stratum pressure. Of thos3. Lu Huafu, Jia Dong, Chen Chuming et al., Cenozoic structuralwells, Kela-201, Kela-203 and Kela-205 were put underproperties and transformation time in Kuche, Geoscience Frontier( in Chinese),1999,6(4):21corrected isochronal flow test4. Zhang guangya, Conversion form and genesis of northern strucEogene dolomite member: This member of Kela-201ture in Tarim Basin, Modern Geology, 1997, 11(4): 45was put under the corrected isochronal flow test with the 5. Jia Dong, Lu Huafu Analysis of Kuche foreland fold-thrustproduction being stable and the tested well member being3600--3607 m. With the nipples of 6.35 and 6.48 mm, the 6. Tian Zuoji, Song Jianguo, Structural characteristics and evolutionof cenozoic foreland basin in tarim. Acta petrolei sinica. 1999spectively. The effective permeability is 37. 27x10-um 7. Jia Jinhua, ( u Jiayu, Guo Qingyin et al, Study of sedimentary fa-20(4):7nd the skin factor 68.24Eogene glutenite member: Kela-2 well or Kela-201cies for Cretaceous reservoir of Kala-2 gas field in Tarim Basinwell each has one layer under the test. There is little 8. Gu Jiayu, Fang Hui, Jia Jinhua, Cretaceous braided delta sand bo-change in production. The tested member of Kela-2 welldy diagenesis and reservoir characteristics of Kuche Depression inis 3567--3572 m. With a 7.94 mm nipple, the daily gasTarim Basin, Sedimentation Magazine, 2001, 19(4): 157production is 60.57x10m. The tested member of kela- 9. Jia Jinhua, Crous kbs sedimentary stratigraphy and reser201 is 3630--3640 m With a 4.76 mm nipple put into usevoir research of Kuche Foreland Basin. Geoscience Frontier. 2000for 48 h, the daily gas production is 9.22x10*m10. Li Xiuxiang, Jin Zhijun, Zhou Xinyuan et al., Kuche DepressionCretaceous K bs: The first and second lithologicin Tarim Basin and gypsiferous salt rock-related oil and gasmembers of Cretaceous K bs are separately put under theaccumulation, Petroleum Exploration and Development, 2002corrected isochronal flow test at Kela-201. The well 11. Ma Qifu, Chen Sizhong, Zhang Qiming et al, Super-high Pressuremembers are 3665--3695 and 3770--3795 m respectively.Basin and Oil and Gas Distribution, Beijas for the 3665--3695 m well member. with a 4.76 mmblishing House, 2000, 1"-254nipple, a 5.50 MPa production pressure differential and a 12. Kinji magara, Compactness and Fluid Migration, 1978(translated56.20 MPa wellhead pressure, the daily gas production isby Chen Heli), Beijing: Petroleum Industry Press, 198121.1x10m. The effective permeability is 14.52x10-3um2 13. Zhang Qiming, Liu Funing, Yang Jihai et al, Super-high pressureand the skin factor 40. 01. As for the 3770--3795 m wellystem of Yinge Sea Basin and oil and gas distribution, ChinaOffshore Oil and Gas, 1996. 10(2): 65member, with a 11.904 mm nipple, a 3.930 MPa produ14. Gu Jiayu, Fan Tuzhi, Fang Hui et al., Fluid and oil and gaspermeability is 7, production is 105x10-m Theer prestion pressure differential and a 48.2 MPa welllservoirs in Tarim Basin, Geological Review, 2001, 47(2): 20115. Hubbert, M.K., Entrapment of petroleum under hydrodynamic10-Hm2 and the skin factor 20.01conditions, AAPG Bulletin, 1953, 37(8): 1954As for Kela-203 well, the Lower Tertiary glutenite 16. Qin Shengfei, Jia Chengzao, Tao Shizhen, Characteristics of oilmember and the first and second lithologic members ofand gas reservoirs in Kuche Depression of Tarim Basin, ChinaGeology (in Chinese), 2002, 29(1): 104Cretaceous Kbs are put under the corrected isochronal 17. Zhao Mengiun. Zhang Shuichang, The genetic groups of naturalflow test. The well member is 3698.5-3916.5 m. with a中国煤化工 servoir in Tarim Basin, China,in21.36 mm nipple and a 26.7 MPa wellhead pressure, they of Tarim Basin in China anddaily gas production is 201x10-m. The perforated thickC GIa Chengzao, Sennikov, N. v)ness at Kela-203 well is 159 m. With six different nippleseijing: Petroleum Industry Press, 2001, 54-62for back-prtesting(7.94,11.11,13.04,14.63,1634(Received August 9, 2002)and 17.96 mm), the well has cumulatively producedChinese S序嫂 letin Vo.47Supp. December2002