An investigation of Water-gas interface migration of the upper Paleozoic gas pool of the Ordos Basin

ARTICLESChinese Science Bulletin 2004 Vol 49 No. 7 735-739On the basis of these characteristics some scholarshave believed that the Upper Paleozoic gas pool of theAn investigation of water-gas Ordos Basin is a huge deep basin gas trap after comparinginterface migration of the up- ada i-s). Although there is til a question of whether theper paleozoic gas pool of thegas pool is a deep basin gas trap, the characteristics for theformation of the gas pool are quite clear. For exampleOrdos basin using reservoircontrolled by hand shaped reservoir sandstone bodies influid inclusion informationthe south-north direction, this gas pool consists ofmulti-small isolated gas pools, and these small gas poolsall began to develop from the south area with a regional山x:XIAO Xianming, LIU Dehanmigration of their water-gas interface from the south to theshen jiaguinorth areaOn the basis of previous work, the authors would trof Geochemistry. Chinese Academy of Sciences. Guangzhou 510640, to reconstruct the migration history of the water-gas interChina2. School of Civil Engineering, Hunan University of Science Tech- only provide important information to address the for-nology Xiangtan 411201, ChinaCorrespondence should be addressed to Mi Jingkui (e-mail: mijkmation and evolvement of the gas pool, but also documengig. ac. cn)approach for migration processAbstract There is a particular characteristic in the for-I Principle and methodmation of the Upper Paleozoic gas pool in the Ordos BasinThere are lots of important information in fluid in-that is its water-gas interface migrated regional during geo- clusions about the formation of oil and gas reservoir. Aslogical history. However, there has been lack of detailed re- well known, a necessary condition for the formation ofsearch on this problem and also no similar report on other fluid inclusions is that there should be some water in rocksbasins. In this paper, the formation time of the nuid inclu- Three different zones have existed during the formation ofsions formed in the water-gas transition zone of the gas poolwas deduced using their trapping temperatures and combinthe Upper Paleozoic gas pool of the Ordos Basin"2, theying of the burial with geothermal history of the basin On the are gas zone, water-gas transition zone and water zone.basis of this, the isochrone of water-gas interface migration sInce natural gases have a very low solubility in water andfor the gas pool was mapped. The result shows that the gas the gas migrate by diffusion", only aqueous inclusionspool began to form around the Yanan area at about 165Ma, and gas-bearing inclusions can form in the water zoneand then developed and enlarged toward the north direction. However, the situation in the water-gas transition zone isThe gas pool finally formed at about 129 Ma. Since the basin different from that in the water zone, all kinds of unmixeduplifted from the late Cretaceous and gas supply decreased, water and gas fluids occur widely, and different types ofthe water-gas interface of the gas pool migrated back to thefluid inclusions with a variable vapor/liquid ratio can grow,tionincluding a type of boiling fluid inclusions. After a reserKeywords: Ordos Basin, nuid inclusion, trapping temperature, wa. voir rock was fully filled with gases, there was no water inthe rock pores and not any types of inclusions can formDUI:10.1360/03wdU13lThus the formation time of the fluid inclusions from thewater-gas transition zone would record the time whenLots of geological and geochemical data obtained gases injected into the reservoir rock and the isochronerecently from the Ordos Basin have shown the following the inclusion formation would represent the evolution ofcharacteristics of the Upper Paleozoic gas pool 9:(1) the water-gas interface of the gas pool during geologicalThe source rock of the gas pool is the Carbonifer- history. Therefore, the migration of the water-gas interfaceous-Permian coal-bearing stratum and the reservoiis in a gas pool could be deduced using fluid inclusionthe Carboniferous-Permian Light sandstone with low po-information in reservoir rocksosity and low permeability; (2)the tectonic framework ofa key problem to reconstruct the migration of thethe basin is a large asymmetric syncline, dipping steeply water-gas interface of a gas pool is to identify the fluidnorth:(3)the gas-bearing zone is distributed in the tec- distinct characteristic of inclusions formed in water-gastonic slope area of the basin Toward up-dipping direction traninclusions coexisted within the North and East areas, there is a water-gas transition gas-b中国煤化工 queous inclusion,thezone, and then a water zone; and ( 4) the regional trap of aqueolCNMHG homogenization tem-the gas pool is a hydrodynamic regime, with water on the peratutop of gas, showing a reverse water and gas relationshipwith gas inclusions were selected to measure Th(Fig. 1).Chinese Science Bulletin Vol 49 No. 7 April 2004735ARTICLESaverage Th of 1125C and 93"C, respectively. The paleogeothermal temperature evolution curve of this samplecan be drawn based on its burial history and paleogeothermal gradient(Ren, Z. L, 1994). The cross point be8Aqueous inclusiontween the horizontal line passing the Tt and the paleogeo-thermal temperature curve represents the trapped point ofGas-bearingthe inclusions, and the correspondent time in the X-axis isnclusionthe formation time of the inclusionsThe real line and dashed linthe two象 Gas inclusiongroups of inclusions formed before and after the basinuplifting, respectivelyFig. 1. Different types of fluid inclusions formed in the water and gas( Permian sandstone from well ZT-1, I depth of 2994 m)The procedures to investigate the migration of the wa-ter-gas interface transition zone in a gas pool are in thefollowingl125℃(1) Measurement of fluid inclusion homogenizationtemperature. The THMSG-600 micro-stage made by93℃Linkcam Company was used. The increasing rate of temsions should be distinguished from hydrocarbon-bearingnclusions before measuring their Th, since only theaqueous inclusion can represent its hosted rock tempera35ture. With a cooling-heating stage, it is easy to distinguishinclusion from a gas-bearing liquid inclusionA gas-bearing liquid inclusion has a higher Th, usuallyabove 170C, and its melting temperature is below-100T300150but the Th of an aqueous inclusion is lower, in the range ofFig. 2. Plot showing the determination of formation time of inclusions80-140C, and its melting temperature is above-10C(2)Calculation of fluid inclusion trapping tempera- 2 Geological distribution of samplesture. Generally, only the trapping temperature(Tt)ofThe studied samples are all tight sandstone, takenaqueous inclusion can represent its hosted rock tempera- from the Shanxi Formation and Lower Shihezi Formationture when the inclusion grown, and the Th of an aqueous from the Ordos Basin. Table I presents the geological disinclusion is lower than its Tt. Only when water is satu- tribution of the samples, and Fig3 shows the well locarated by gas in an inclusion, will the Th be equal to the tions from which the samples were takent">. Under geological conditions, the water in aqueous 3 Results and discussioninclusion is seldom saturated by gas. Thus, the Tt of inclu-sions should be obtained to present fluid temperatures(i)Characteristics of fluid inclusionswhen the inclusions formed In this study, the Tt was calThere are abundant secondary fluid inclusions occur-culated by combining isochores equations of a petroleum ring in micro-fissures of the reservoir sandstone frominclusion and its coeval aqueous inclusion using the the Ordos Basin. The inclusions are small size, and mostLeica-Qwin and PVTsim software[I6b-iNof them are non-fluorescent. Fluorescence inclusions were(3)Determination of formation depth of inclusions. only found in the samples from the wells located in theThe method was suggested by Xiao et al. (2002). In northeast area of the basin. The fluid inclusions can bethis study, the Tt of an inclusion was used instead of its subdivided into three types1)Petroleum inclusion. This type of inclusions(4)Formation time of inclusions. The formation can中国煤化 Tee kinds:i) Liquidtime of an aqueous inclusion can be determined directlyhyeacterized by a small size,presented in Fig. 2. There are two groups of aqueous in- withCN M H Gor/iquid ratios less thanclusions in the sample from the well M5(1890 m), with 20%. ii)Gas inclusion: This kind of inclusion has aChinese Science Bulletin Vol 49 No. 7 April 2004ARTICLES1 TI and Th data of fluid inclusions and their formation timeLocationDepth/'mTh℃TtCFormationFormationdepth/mtime/Ma2555-2560.5103(93-186l125107(98-120)2714.3T-Z2543.8497(92-13)114.54292840.6113.8(108.117)11732991.5-2994.7P11494123)28602866.5P119110-123)1526113(112,114)91-310134562-34601185(121,116147.3G-Z3655.1123(1213-1257)26.53185.7162,73346-3370PPPPPPpPPPl17(108126)3014.33998-4072121.1(1087-128)313l4Sh172983.5125(1001363242.9Y4492655-2660127(124-132)130.53300l61.5G8-3G-Z3565-3600124(110-127)32143G4-53348-3398l18(115-127)121.53042928892938127(120-133)130.5163.1ll4(108-123)29286146.8G-Z1155(l10-122)11929714148.6G-Z1276(118136)131.11646Z3189PPPll73(116,I8.63022.91523127.5(120-136)Sh131G-Z3268.5125.2(I16138)12872486167.3ZcSG-Z1144(90-1248)1792925.7148.6G-Z24015126.91593r-Z, Water and gas transition zone: G-Z, gas zone. tt is calibrated except for the wells M4 and M5 using the difference between the Th with Tt ofinclusions from wells M4 and M5. The calibration formula: t-Th+AT.5Clarger size, with Th of 80--115C, and vapor/liquid ratios tioned above, the Th of an aqueous inclusion can representof 63%--75%. iii)Three phases inclusions: This kind of its Tt only when the water in an inclusion was saturated byinclusion consists of water, petroleum and gas phases. gas. It is obvious that the aqueous inclusions coexistedUnder the transmitted light model, the boundary between with pure-liquid inclusions and gas inclusions were notthe water phase and the petroleum phase is not clear, but saturated by gas. Thus, the isochore of the aqueous incluunder fluorescent light, the petroleum phase has blue sion is not a vertical line, but an incline. An accuratefluorescence, and the water phase have no fluorescence. method to calculate the trapping pressure and temperatureThe Th for petroleum and gas phases is 85-120C, and is to combine the isochore equations of both petroleumthe Th of three phases is higher than 200Cinclusions and its coeval aqueous inclusions. However(2)Aqueous inclusion. This type of inclusion haspetroleum inclusions were only found in the samples fromtwo kinds: i )Pure liquid phase inclusion: It has ellipse wells M4 and M5. The Tt of the inclusions in samplesor irregular shape and variable size with only one phase, it where no petroleum inclusions were found was calibratis colorless and transparent under the transmitted lighting the difference between the tt and the Th of the inmodel; ii ) aqueous inclusion: It is two-phase inclusion clusion in the wells M4 and M5 samples. The results showwith vapor/liquid ratios of less than 20% and Th of 80- that the Th of an inclusion is obviously lower than its Tt140C. Most of the secondary inclusions belong to this and the difference is 3-4C, with an average of 3. 5CkindThe Th of inclusion in Table 1 is the calibrated tempera(3)Gas inclusion. It includes two kinds: i )Pure turesgas inclusion: It is one phase and black color under the中国煤化工for the formation oftransmitted light model; ii)gas inclusion: It has two the ingsamples. The Upperphases with vapor/liquid ratios of more than 50% and the PaleozoCN MH Ghe Yanan Area beforesize in the range of 10--20 um.165 Ma (Late JurassIc and tnen extended toward the(ii) Trapping temperature of inclusions. As men- north direction, with a regional migration of water-gasChinese Science Bulletin Vol 49 No. 7 April 2004ARTICLESBaotou口ER3·ERLWushenzhao口130z·,140●S231o YulinSulG8-302·卫9S101S219FaultD Qingyang口 FuxianDivision ofg. 3. A map showing the isochrone of water-gas interface migrationinterface from south to north. The gas pool finally formed rocks were matured into gas generation peak stage andbefore 129 Magenerated a great deal of gases. The gas zone expendedIt should be noted that there were two groups of and the water-gas interface migrated from the south to theaqueous inclusions in the water-gas transition zone(sam- north direction. The water-gas interface passed throughples from wells M4, M5 and D6). For example, the two this area to form the inclusions with higher Th, and thengroups of inclusions in the samples from well M5 have the this area became a gas zone. In the late Cretaceous, theTh of 82-93'C and 95--118C, and the vapor/iquid basin began to uplift and about 1000--1600-m-thick strataratios of 6. 12%--7.55% and 9.25%12.55%, the mini- were eroded. The gas supply from the source area demum trapping pressures of 16.6 MPa and 23. 2 MPa,recrease中国煤化工8 as resource were notspectively. The two groups of inclusions could be ex- enough-I of the hydrodynamiction ofCNMHGback to present posigeological history. During the early Cretaceous, the source tion to form the inclusions with lower Th and this area738Chinese Science Bulletin vol 49 No. 7 April 2004ARTICLESbecame into a water-gas zone again. The trapping tem- 7. Ma, Z. E, Cheng, Q.R, Zhou, S.x. Re-recognition on gas andperature and pressure of this group of inclusions are con-water seams in Upper- Paleozoic lower permeable strata of the Orsistent with the present strata temperature and pressuredos Basin, Lower Permeable Oil and Gas Field (in Chinese), 1998A conclusion can be made from the above discussion3(2):2931.that the water-gas interface moved forward and back dur-ing geological history is the reason for the two groups of8. Dai,J.X, wang, T.B., Song, Y, et al, The Distribution and For-aqueous inclusions formation in the present water-gasmation Conditions of Large Gas Fields in China, Beijing: Geologzone. Some scholars have also discussed this phenomenoncal Publishing House, 1997, 93-98of the water-gas zone migration of the gas pool from the9. Yang, J. J, Xigu-Pci, X. G, Gcology of Natural Gases in Chinageologic background and had a similar view with thisrdos Basin (in Chinese). Beijing: Petroleum IndustryPress43-55,4 Conclusion10. Min, Q Fu, J H, Xi, S L et al., Accumulation and migration ofUpper-Paleozoic natural gas pool in the Ordos Basin, PetroleumThe migration of the water-gas interface of a gasExploration and Development(in Chinese), 2002, 47 (4): 26--29pool can be reconstructed using fluid inclusion informa- 11. Fu, S.Y., Peng, P A. Zhang. W.Z. et al. Study on the migration oftion, with the combination of the geologic background ofthe basin. The results show that the Upper Paleozoic gaswater and gas interface using VRo and inclusion data, Petroleumpool of the Ordos Basin began to form near the Yanan areaJournal (in Chinese), 2003, 24(3): 46-51at about 165 Ma, and then enlarged toward the northem 12. Xiao, X M. Liu, D H. Song. ZG et al. Dating gas pool forma-area of the basin. The gas pool finally formed at about 129tion by means of nuid inclusion information in the reservoirs-aMa. The water-gas interface of the gas pool migrated backcase study of the Ordos Basin, Chinese Science Bulletin, 2002,to the present position since the source rocks could not18):1567-1572supply enough gas after the late Cretaceous.13. Huang, Z L, Hao, S.S., Study on diffusion and sealing of naturalAcknowledgements This work was supported by the State 973'gases, Petroleum Journal (in Chinese), 1996, 17(4): 36-40Project (Grant No. 2003CB214607) and the Project of the Chinese 14. Welte, D. H, Stoessinger, w, Schaefer, R G et al., The NaturalAcademy of Sciences( Grant No. KZCX2-1l0Gas Generation and Migration in Western Basin, Canada a Caseeferencestudy of Deep Basin Gas Trap(translated by Hu, Z. Y), Beijing.1. Zhang. W. Z, Li, J F, Zan, C. L, An investigation on gas sourcePetroleum Industry Press, 1990, 39-rocks of the Upper-Paleozoic deep basin gas trap in the Ordos Ba-15. Tilley, B. Ebrucee, B Longstaffe, F, J., Thermal history of Al-in, Lower Permeable Oil and Gas Field(in Chinese), 1998, 13(2)berta Deep Basin: comparative study of nuid Inclusion and vitrinite13-23.reflectance data, AAPG 1989, 73(10): 1206-1222Li, Z D, Hu,Y, J, Wang, X, Study on the Upper-Paleozoic deep16. Mi, J. K, Xiao, X. M,, Liu, D. H. et aL., Calculation of pa-asin Gas trap in the Ordos Basin. Natural gas Industry (in Chileo-pressure of gas reservoir using the PVT characteristics of inclu-ese),1988,18(3):1016ions, Science in China, Ser. D(in Chinese), 2003, 35(7): 6793. Min, Q Deep basin gas trap and Ordos basin, Lower PermeableOil and Gas Field (in Chinese), 1998, 30(2): 1-617. 17 Liu, D H, Xiao, X.M., Mi, J K. et al., Determination of trap-4. Zhang, J. L, Chang, X. C,, Zhang, J. G, Study on the Upping pressure and temperature of petroleum inclusions using PVTper-Paleozoic deep basin gas trap of in the Ordos Basin, Petroleumsimulation software a case study of Lower Ordovician carbon-Exploration and Development(in Chinese), 2000, 27(4): 30-36.ates from the Lunnan Low plift, Tarim Basin, Marine and Petro-5. Min, Q, Liu, X H, wang, x, et al., Study on the geologic charac-Icum Gcology, 2003, 20(6): 29-43leristics of Upper-Paleozoic deep basin gas trap in the Ordos Basin, 18. Aplin, A C Larter, SR, Bigge, M.A. et al. PVTX history of theLower Permeable Oil and Gas Field(in Chinesc), 1998, 3(2):7-North Sea's Judy oilfield, Journal of Geochemical Exploration,2000,60-70:641-6446. Huang. Y.M. Huang J. S, Liu, S. B. et al. Study on the reservoir 19. Ren, Z.L. Zhao, Z.Y, Study on the paleogeothermal temperatureat sandstone of Upper- Paleozoic deep basin gas trap in the Ordosin the Ordos Basin, Sediment Journal (in Chinese), 1994, 12(1): 17Basin. Lower Permeable Oil and Gas Field(in Chinese), 1998, 3(2):(Received November 4, 2003; accepted January 7, 2004)中国煤化工CNMHGChinese Science Bulletin Vol 49 No. 7 April 2004739