Interaction of fluid dynamic factors in the migration and accumulation of natural gas

strating that the method of trend analysis of sediment grainsize is fit to the depositional area of fine-grained sedimentson large-scale continental shelf. Constant sampling intervaland suitable comparative distance(Der) are the key to leading grain size trend analysis to be reasonableInteraction of fluid dynamicAcknowledgements This work was supported by the Project of"China- factors in the migrationific Foundation of State Oceanic Administration( Grant No. 97401)and accumulation ofReferencesnatural gas1. Mclaren, P, An interpretation of trends in grain size measurements,Journal of Sedimentary Petrology, 1981, 51: 6112. Mclaren, P, Bowles, D, The effects of sediment transport on grainSONG Yan2, XIA Xinyu, WANG Zhenliang,WANG Yi%& HU Shengbiao53 SIze distribution, Journal of Sedimentary Petrology, 1985, 55: 457t sediment transport patterns inferred from 1. Research Institute of Petroleum Exploration and Development, Beijinggrain-size trends, based upon definition of"transport vectors"-reply,100083 China.Institute of Geochemistry, Chinese Academy of Sciences, Guiyan50002. China4. Gao, S, Collins, M, Analysis of grain size trends, for defining 3. Northwest University, Xi'an 710069, Chinasediment transport pathways in marine environments, Journal 4. Chinese University of Petroleum, Dongying 257062, China;Coastal Research, 1994. 10: 705. Institute of Geology, Chinese Academy of Sciences, Beijing 1000295. Gao Shu, Collins, M, Sediment grain size trend and marine sediment dynamics, Chinese Scientific Foundation(in Chinese), 1998 Correspondence should be addressed to Song Yan(e-mail: sya@ petro-12(4):2416. Cheng Peng, Gao Shu, Characteristics of grain size parameters and Abstract The migration of fluid petroleum gas, describecnet transport trend of seabed sediments in the North Yellow Sea, as fluid potential, depends only on gravity, fluid pressure con-Oceanologia et Limnologia Sinica(in Chinese), 2000, 31(6): 604trolled by depression and capillary force during tectonically7. Qin Yunshan, Zhao Yiyang, Chen Lirong et al., Geology of the stable period; but on tectonic stress during the tectonicallyYellow Sea (in Chinese). Beijing: China Ocean Press, 1989, 65- active period with severe compression. This method is appliedin the junggar Basin, showing that the migration of Jurassic8. Liu Minhou, Wu Shiying, Wang Yongi et al., Late Quaternary gas during Cretaceous and Eocene and the migration of perGeology of the Yellow Sea (in Chinese), Beijing: China Oceanmian gas from Jurassic till the presetermined byPres,1987,24-44;362-70capillary force and fluid pressure (including overpressure)whi-ch is controlled by depression; the migration of jurassic gas9. Zhao Yiyang, Li Fengye, Qin Chaoyang et al., Discussion on the from Eocene till the present and the migration of Permian gassource origins and genesis of the Central Southern Yellow Sea Mud, during Triassic are controlled by tectonic stress10.Park, Y.A., Origin and Distribution patterns of Muddy Deposits in Keywords: petroleum migration, fluid potential, tectonic stress, fluidthe Yellow Sea, Proceedings of the First International Conferencpressure, Junggar Basin.on Asian Marine Geology, Beijing: China Ocean Press, 1990, 335350.Petroleum migration is one of the most significant11. Shen Shunxi, Li Anchun, Yuan Wei, Discovery of the cold water problems in basin analysis and petroleum exploration. Thegyre and channel deposits in the southern Yellow Sea, Oceanologia research of fluid dynamic history is an efficient methodet Limnologia Sinica( in Chinese), 1993. 24(6): 563.study petroleum migration and accumulation l. .Natural12. Tang Yuxiang, Zou Emei, Lee Heungjae, Characteristics of circula- gas migration is influenced directly or indirectly by thetion in the South Yellow Sea, Acta Oceanologia Sinica (in Chinese), following physical conditions of fluid and solid in the strata2000,22(1):1the buoyancy of hydrocarbon, formation water kinetic po13. Hu, D. X. Some striking features of circulation in Huanghai Sea tential, capillary forces, fluid pressure, tectonic stress andd East China Sea, Oceanology of China Sea, 1994, 1: 27elys). Mathematical and physical simulations were applied(Received February 21, 2002) in petroleum migration and accumulation study consideringsome of the above physical parameters in some basins 4.51However中国煤化工rbon buoyancy andfluid pressisSure brougCN MH Grocarbon generationduring depression) are considered in detail while the othertwo important factors of tectonic stress and capillary forceare usually omitted. The interaction of these factors lacksChine丹与数据?l∥ tin Vol47No14l?0021207de t ailed eI n gravity and fluid pressure, while the effect of capillaryfact, during most conditions of migration, petroleum is force is not obvious. For lateral migration, gravity and fluidunwettable to the surface of rock pore, the significance of pressure change slowly, thus capillary becomes the mostcapillary force cannot be omitted; besides, in the compress important factor controlling the migration and accumulationbasins in the western part of China, tectonic stress played of natural gas. It determines the migrating direction andan important role in petroleum migration and needs more accumulation area where migration stops. Due to the restriction of data acquisition, capillary force is totally omit-In this note, the interrelationship between the factors ted in some cases of basin simulation, so the oil potential orinfluencing the migration and accumulation of natural gas gas potential may be only correspond to structural surface,is studied theoretically. The results were applied in the which is not able to reflect the direction of petroleum miJunggar Basin as an examplegrationInterrelationship between the factors influencing theFrom formula(2), in order to gain the accurate valuemigration and accumulation of natural gasof capillary force, the radii of pore and throat should beanalyzed, meanwhile the surface tension of unwetting fluidThe migration of formation fluid can be described as is also needed. This method to the calculation of capillaryfluid potential defined by Hubbert aslldespite theoretical completeness, is impractical due to the8(1) following two reasons: there are not sufficient data of surPfface tension between petroleum and mineral under differentwhere p, is fluid potential of per unit mass of fluid;g temperatures and pressures; the radii of pore and throat aregravity acceleration; H height of the concerned point todifficult to describe mathematically. Here we tried to subcertain base plain; p fluid pressure; P, fluid density; P cap- stitute the capillary force by the median pressure in theillary force during petroleum migration. Formation fluid mercury-injection method in order to calculate fluid potenalways migrates from area with higher fluid potential to tial. The base for this method is that oil accumulation apthat with lower fluid potential, meanwhile prefers to mi- pears usually when the oil saturity in the pore of the reser-grate through where the gradient of fluid potential is the voir reaches up to 50%o. Based on the capillary force cur-mostve gained in the mercury-injection method, the statisticFormula(1)indicates that gravity, fluid pressure and relation between the median pressure and porosity in rockpillary force are the three main factors controlling fluid of different areas can be acquired, which can be used topotential (flow kinetic potential can be omitted due to the calculate capillary force of different stages and buriallow velocity of ground water). Other physical parameters depths with the ancient porosity calculated in numericalinteract with the above three factors to influence fluid pres- simulationsure. The influence of thermal condition (reflected as for-(1. Tectonic stress. The mechanism of themation temperature) on gas migration is not expressed in ence of tectonic stress on petroleum migration cand density directly; and. 1, tmpeling to the elastic deformation theory,locity of the physical and chemical reactions in the strata△V(including hydrocarbon generation, mineral reaction andV Kdissolve and deposition of minerals), thus affecting the where V is volume of rock, AV volume change of rock, ofluid amount, pore volume and permeability, and affecting average stress and K volume elasticity modulusfluid pressure directly or indirectlyAccording to the principle of effective stressOf all the physical parameters involved in gasG0=00-p,(4)tion, capillary force and tectonic stress are hard to consider.( I)Capillary force. When fluid of two facies whichwhere oo is average effective stress, o o average stress, pcannot dissolve each other coexists in a pore medium, cap-fluid pressure, thusillary force will appear when the unwetting fluid passesVoo V(oo-p)through the pore whose surface is covered by the wettingKKfluid, it is controlled by the radius of capillary tube re, surThe change of nore fInid under pressure p, AV, can bece tension 8, and wetting angle eexpressed a中国煤化工2δ.cosCNMHrcolume compression coef-1208Chinese science bulletin vol 47 No 14 July 2002Compared with fluid, the grain of rock can be regard(I Hydrocarbon generation and tectoniced as uncompressible, the change of rock volume is equal There are two sets of source rocks in the Junggar basinto the change of pore volume, thus formula(5)equals(6), Jurassic coal measure humic source rock and Permian lacus-thentrine sapropelic source rock. Mature Jurassic source rock isPdistributed in the southwest part of the basin, the LowerJurassic source rock in most part of the foreland depressionKreached its hydrocarbon generation climax at the end ofCretaceous with vitrinite reflectance (R)of over 1.0%; atSince pore fluid (mainly composed of water) is much the end of Eocene, the area of mature Jurassic source rockless compressible than rock skeleton, i.e. K,>xK, formula expanded, it reached the wet-gas stage in the center of the(7)is close to 1, thensouthwest foreland depression; at present, most part of(8) Jurassic source rock in that depression reached the wet-gasFormula(8)indicates that the total tectonic stress is stage while part reached the dry-gas stage. The main hydrotion. Due to the complicated variance of geological factors, end of cretaceous till the presell c source rock is from theit is very difficult for all the total tectonic stress to changePermian source rock is distributed widely in the Jungto pore pressure. Nevertheless, formula(8)reflects the gar Basin. At the end of Early TiPermian sourcemechanics of how tectonic stress drives the petroleum mi- rock reached its hydrocarbon generation climax in thegration numerically. In fact, totally sealing is a special con- southwest depression, West Pen-1 sag and Manasi-Lake sagdition during petroleum migration; with the eventually and reached wet-gas stage in each center. By the end ofexpelling of pore fluid, pore pressure and effective stress in Jurassic, Permian source rock reached the dry-gas stagerock will change, of which the velocity depends on both the most part of the above area. The main hydrocarbon geneflow velocity and load. Generally, pore pressure will d- tion period of Permian source rock is from Late Triassic tillcrease and the effective stress on the grain will increase Jurasseventually.There are three tectonic movements in the junggarFormula( 8) indicates that during the tectonicallyBasin from permian i.e. the Indosinian mowt. thetive period(especially with severe tectonic compre-ssion), Yanshan Movement and the Himalayan Movement. Severetectonic stress is the dominant factor controlling petroleum structural compression happened during the Indosinianmigration. The reason is that during the tectonically active Movement; while the inner part of the basin, where theperiod, the vertical change of tectonic stress can be as high Permian source rock is mainly distributed, is stable duringas 25 MPa/km, far more exceed static water pressure gradi- the other two movementsent(about 10 MPa/km); from the boundary to the center ofAfter the mature of jurassic source rock. the souththe basin concerned, tectonic stress also tends to decrease west part of the Junggar Basin experienced the late Yanshanobviously(while the change of capillary force is random). Movement and the Himalayan Movement. The YanshanThe variance of tectonic stress determines the change of Movement formed the thrusting belt only in the forelandfluid potential, it determines fluid pressure and"masked" zone with little effect in the inother factors that might influence petroleum migration indicating that the Jurassic petroleum system endured the(including the overpressure by depression and capillary relatively stable tectonic condition from Cretaceous to Eoforcecene. The Himalayan Movement of Neocene was veryThe above conclusion expressed the interaction of severe in the southwest of the basin, the North-Tianshandifferent physical factors controlling petroleum migration thrusting belt compacted and extended towards the basin onand accumulation, indicating that the two cases of tectoni- a large scale, and brought about thrust and fold in the fore-cally active period and tectonically stable period should be land strata, forming the fold belt parallel to the fault of thedistinguished. The study of petroleum migration during the main nappetectonically stable period should be focused on flu(1) Result. From the hydrocarbon generation andsure(including overpressure derived from compaction tectonic history of the Junggar Basin mentioned above, itdiseliduibrium and hydrocarbon generation during depres- could be concluded that when considering the migrationsion)and capillary force, while the study of petroleum mi- and accumulation of petroleum from Permian source rock,gration during the tectonically active period should beectonic s"ement should be fomainly focused on tectonic stresscused on dr中国煤化工 pressure(especiallythe overpreCNMHGSSion)and capillaryFrom the above method, we studied natural gas mi- force should be taken into account after Jurassic; whengration and accumulation in the Junggar Basin of different considering the migration and accumulation of petroleugeological stagesfrom Jurassic source rock, fluid pressure(especially theChine丹与数据?l∥ tin Vol47No14l?0021209verpressure derived during depression) and capillary stage Gas potential is calculated from formula(1) considforce should be taken into account during Eocene, while ering all of the gravity, fluid pressure and capillary forcetectonic stress in the Himalayan Movement should be fo- Overpressure in geological time is taken into account by thecused on after neocenemethod introduced in ref. [5]. Capillary force is representedWhen analyzing the migrating direction and accumu- by median pressure in the mercury-injection analyses, thatlation place of natural gas by the method of liquid potential, calculated by the simulated relationship between porositythe concept of liquid migration-accumulation system should and middle pressure, where the porosity is gained by basinbe introduced. In the fluid potential map of certain geologi- simulationcal stage, the low-potential areas circled or half-circled byAs presented in fig. 1, there are four migration accuhigh-potential area are easy to form reservoir because mulation systems for Jurassic gas: southwest part, wellpetroleum from different high-potential places can accu- Pencan-2 area, Hutubi and the area north to Urumqi. Frommulate there. Besides, petroleum is favorable to accumula- Cretaceous to Eocene, the migration of Jurassic natural gastion where a long-period low liquid potential developed, is controlled by these four systems; especially the Hutubiwhile the lower-potential area that developed from a area, circled by multiple migration-accumulation system, ishigher-potential area or on the contrary is not favorable to the most favorable area for gas accumulation due to thepetroleum accumulation. The liquid migration- sources from multiple directionaccumulation systems are divided by the middle lines ofFig 2 shows the migrating direction of jurassic natuhigh-potential areas, each forming an independent fluid ral gas in Neocene and Quaternary in the Junggar Basindynamic system where natural gas can accumulate. Com- Since the basin is in an tectonically active period, the fluidpared with the concept"petroleum system"which stresses potential was controlled by tectonic stress, thus the direc-on the significance of source rock and regional caprock, the tion where tectonic stress decreases quickly is the naturalconcept"migration-accumulation system"stresses the dy- gas migrating direction(the direction where fluid potentialnamic procedure of natural gas migration and accumulation decreases quickly). This figure is based on paleo-tectonicfrom the evolution of the factors controlling the migration stress, which is simulated by the finite-element method; inof the fluids of oil, gas and water. The migrating direction order to establish its geological model, the paleo-structuraland accumulating place of natural gas is restricted by the map thickness isogram and depositional facies map wereboundary of natural gas migration- accumulation system, applied; the boundary of the basin is represented by thei.e. natural gas reservoir in the same migration- natural boundary of the basin; mechanic parameters ofaccumulation system generally has a source from the same different type rocks were based on the analyzed data ofdirectiorocks from this basin(1) Migration and accumulation of petroleum fromJurassic source rock. Fig. I shows the migration -accup口2四3目4圖5■623=4目56Fig. 2. Map showing Jurassic natural gas migration and accumulation inthe Junggar Basin during Neocene and Quaternary. 1, Oil/gas field; 2,Fig. I. Map showing Jurassic naturalgas migration and accumulation in petroleum migrating direction;: 3, isopleth of tectonic stress in the Hima-the junggar basin during cretaceous and eocene1, Oil/gas field; 2, layan Movement(MPa): 4, area where Jurassic source rock generatespetroleum migrating direction; 3, boundary of the migration-accumuand gas at present(R 0.8%-1.3%0;5, area where Jurassic source rocklation system at the end of Early Cretaceous; 4, boundary of the migration- generates wet gas at present (R. 1.3%-2.0%); 6 area where Jurassicsource rockn%)rock generated oil and gas by the end of Eocene(R 0.8%-1.3%);6,area中国煤化工where Jurassic source rock generated wet gas by the end of Eocene(R>F1southwest margiof1.3%)CN MH Gely high-stress areamulation system division and migrating direction of natural natural gas generated there migrated northwards. Meangas from the Jurassic source rock during Creta- ceous to while, the local low-stress area also exists in the southwestEocene. This map is based on Jurassic gas potential of this depression, presenting beneficial area for natural gas accu1210Chinese science bulletin vol 47 No 14 July 2002lation, e. g. Hutubi and dushanzimiddle and southeast part of the basin in different stages(2)Migration and accumulation of petroleum from from the end of Triassic to Tertiary in the isogram of fluidPermian source rock. The hydrocarbon generation climax potential. The beneficial areas for gas accumulation includeof Permian source rock is corresponded with the Indosinian the southwest margin, Mobei Uplift, Lunan Uplift, and theMovement. In the Indosinian tectonic stress isogram, the area east to Fukang(fig 3)Manasi-Lake Sag the West pen-1 Sag and the southwest3 Conclusionsdepression are higher in tectonic stress compared withneighboring areas and are beneficial for natural gas to miDuring the tectonically stable period, the fluid potengrate outwards. The beneficial accumulation area tial, which reflects the migration of petroleum, dependsgravity, fluid pressure controlled by depression, and caplary force. During the tectonically active period with severecompression, tectonic stress is the dominant factor controlling fluid potential and determines the migration andaccumulation of natural gasIn the Junggar Basin, the migration of Jurassic gasduring Cretaceous and Eocene and the migration of Permica·2an gas from Jurassic till the present are determined by cap-illary force and fluid pressure (including overpressurewhich is controlled by depression. The migration of Jurassic gas from Eocene till the present and the migration ofPermian gas during Triassic are controlled by tectonicstress. This thought can be applied to studying the benefp12四3圈4cial area for the accumulation of natural gasFig3. Map showing Permian natural gas migration and accumulation in Referendmigrating direction; 3, isopleth of tectonic stress in the Indosinian Move- 1. Chen Heli, An efficient approach to hydrocarbon migration re-ment(MPa); 4, area where the r of Permian source rock exceeded 1.0%arches, Oil Gas Geology (in Chinese), 1995, 16(2)by the end of Triassic.2. Helset, H. M, Lake, L. W, Three-phase secondartion ofhydrocarbon, Math. Geol., 1998, 30(6 ): 6373. Yassir, N. A,Bell, J. S, Relationship betweenstresses, and present-day geodynamics in the Scotian Shelf, Offshore Eastern Canada, AAPG Bulletin, 1994, 78(12): 1863Karan4. Lee, M.K., Williams, D D, Paleohydrology of the Delaware BasinWestern Texas: Overpressure development, hydrocarbon migration,and ore genesis, AAPG Bulletin, 2000, 84(7): 9615. Wang Zhenliang, Chen Heli, A palaeohydrodynamic analysis ofUpper Palaeozoic Group in Middle Ordos Basin, Acta Sedimen6. Hubbert, M. K, Entrapment of petroleum under hydrodynamicconditions, AAPG Bulletin, 1953, 37(8): 19547. England, w.A., Mackenzie, A S, Mann, D. M. et al., The movement and entrapment of petroleum fluids in the subsurface, J. Geo-logical Society, 1987, 144: 3278. Burrus, J, Overpressure models for clastic rocks, their relation to回!□-23E45〓hydrocarbon expulsion: a critical reevaluation(eds. Law, B. E, UIFig 4. Map showing Permian natural gas migration and accumulation inmishek, G. F, Slavin, V. I ) Abnormal Pressures in Hydrocarbonthe Junggar Basin from Jurassic to Tertiary. 1, Oil/gas field; 2, petroleumEnvironments. AAPG Memoir 70. 1998. 35-63migrating direction; 3, boundary of the migration-accumulation system at9. Wang Zhenliang, Development characteristics of fluid dynamics inthe end of Middle Jurassic; 4, boundary of the migration-reformed basins, Oil Gas Geology (in Chinese), 2000(1): 24end of Early Cretaceous: 5, boundary of the migration10. Song Yan, Wang Zhenliang Wang Yi et al., Accumulation of natu-ystem at the end of Eocene; 6, area where Permian sourcegas in the Jungar Basin (in Chinese), Beijing: Science Press,oil and gas by present (R. 0. 8%-2.0%0): 7, area where2000.62-9Permian source rock generates dry gas at present(R, >2%)for petroleum from the Manasi-LakeSag is the northymargin of the basin, while the beneficial accumulation ar-eas for the West Pen-1 Sag are Zhongguai area, MobeiUplift, and Lunan Uplift(fig. 3)During the Yanshan Movement and the Himalayan中国煤化工Movement, the inner part of the Junggar Basin is tectoniCNMHGcally stable, so the migration of Permian petroleum dependson its depth, p-V-T condition of petroleum and capillaryforce. There are three areas with low gas potential in theChine丹与数据?l∥ tin Vol47No14l?0021211