The Genetic Mechanism and Model of Deep-Basin Gas Accumulation and Methods for Predicting the Favora

Vol. 77 No.4ACTA GEOLOGICA SINICADec. 2003547The Genetic Mechanism and Model of Deep-Basin Gas Accumulation andMethods for Predicting the Favorable AreasWANG Tao'2, PANG Xiongi-3, MA Xinhua24, JIN Zhjun2.5 and JIANG Zhenxue231 China National Petroleum Corporation (CNPC), Liupukang, Beijing 1007242 Key Laboratory for Petroleum Accumulation at University of Petroleum, Ministry of Education,Changping, Bejing 102249; E-mail: pangxq@ bjpeu.edu.cn3 Basin and Reservoir Research Center at University of Petroleum, Changping, Beijing 1022494 Institute of Geology and Geophysics, Chinese Academy of Sciences, Qijiahuozi, Bejing 1000295 Exploration and Production Research Institute, SINOPEC, Haidian, Bejing 100083Abstract As a kind of abnormal natural gas formed with special mechanism, the deep-basin gas, accumulated in thelower parts of a basin or syncline and trapped by a tight reservoir, has such characteristics as gas-water inversion, abnormalpressure, continuous distribution and tremendous reserves. Being a geological product of the evolution of petroliferousbasins by the end of the midde-late stages, the formation of a deep-basin gas accumulation must meet four conditions, i.c.,contimuous and sufficient gas supply, tight reservoirs in continuous distrbution, good sealing caps and stable structures.The areas, where the expansion force of natural gas is smaller than the sum of the capillary force and the hydrostaticpressure within tight reservoirs, are favorable for forming deep-basin gas pools. The range delincated by the above twoforces corresponds l0 that of the deep-basin gas trap. Within the scope of the deep-basin gas urap, the balance relationshipbetween the anounts of ingoing and overflowing gases determines the gas-bearing area of the deep-basin gas pool. The gasvolume in regions with high porosity and high permeability is worth exploring under current technical conditions and it isequivalent to the practical resources (about 10% -20% of the deep-basin gas). Based on studies of deep basin gasformation conditions, the theory of force balance and the equation of material balance, the favorable areas and gas-containing ranges, as well as possible gas-ich regions are preliminarily predicted in the deep-basin gas pools in the UpperPaleozoic He-8 segment of the Ordos basin.Key words: deep-basin gas, geology of natural gas, genetic mechanism, Ordos basin, China1 Introductionand study the prediction methods with a case of theirapplication.The discovery of the deep basin gas pool with 100x1012 m2of geologic reserves in the Alberta basin (Fig. 1) byJ. A.2 Formation Mechanism and ModelMaster from Canadian in 1979 surprised and inspired thepetroleum community all over the world McMaster, 1981;1.1 Natural gas is confined by tight reservoirs andCant, 1983; Dai, 1983; Law, 1986; Yuan et al., 1996; Li etaccumulated in the lower parts of the basinal, 1998; Min, 2000; Fu, 2001). To our surprise, under theTight Reservoirs are the essential factor for deep-basin gasconditions of tight reservoirs, sufficient gas supplied byto be accumulated in deep depressions. After gas enters thesource rocks etc., matched with a novel mechanism, it isdepression from the bottom of a tight reservoir, it does notpossible to form a new type of natural gas pools withmigrate upward by buoyancy due to the restriction ofcompletely different characteristics in the lower parts of acapillary force, and accumulates in the lower parts of thebasin where oil and gas explorations used to be thoughtbasin. With the accumulation of more and more gas, theimpossible. It is inspiring that this new type of gasstrong force of gas expansion will drain the porous water inaccumulation with huge reserves is on the verge ofthe tight reservoit from bottom to top and from center to theeconomically recoverable reserves under current technicalshallower border areas, so that the gas-bearing arcaconditions, of which about 10% to 20% of gas can bebecomes larger and larger until the source rocks stoptransformed into economic resources. This paper, based onsupplying gas or the pore throat radius of the upper parts ofpractical explorations of deep-basin gas pools in China inthe reservoir is too large to confine gas. This mechanism isrecent years, aims to summarize the identification criteriacompletely different from that of normal gas accumulationand formation conditions of deep-basin gas accumulations,中国煤化工expound their genetic mechanisms and geological model,YHCNMHG548Deep-basin Gas Accumulation and Methods for Predicting Favorable AreasWang et al.at the bottom of a single glass tube full of water when the2tube has a diameter more than 0.3 cm; or natural gas cannotbe accumulated either at the bottom of a glass tube full ofsand grains and sealed by water when the grains exceed 0.1BritishAlbcrta\Columbiacm in diameter. But to a funnel-shaped glass tube, the gassealed threshold varies with the rate of gas injection and theangle of the funnel bottleck, which is between 0.102 cmDand 0.359 cm. Theoretical researches indicate that thethreshold for forming a deep-basin gas pool is contolled bythe force acting on the boundary between gas and water.Just on the point of the threshold, the dynamic force silingupwards reaches a balance with resistance, so that naturalFreshwatergas cannot be accumulated; above this point the dynamicforce is stronger than resistance, gas cannot be accumulatedeither; but below this point resistance is stronger than theEdmontondynamic force, and so natural gas can be accumulated. Therelationship between them is represented as follows:where Pp represents the dynamic force of deep-basin gaspools，which is produced upwards by gas molecule①Range of dcep bosin gasexpansion after deep-basin gas has been accumulated and isCalgarycorrelated with temperature, pressure, volume of gas。Tar reroir in the Lower Ceucoucontained and molecule numbers; Py denotes a kind of forcewhich prevents deep-basin gas pool forming and waterUnin of tickoestdrained to the surroundings and consists mainly of theoverlying water static pressure (Pw), the reservoir capillaryforce (P) and the probable overpressure occurring in theTrap for deep-basin gas(6)overlying fluids (OPw ).According to the balance of these forces, the theoreticallimits of deep-basin gas traps can be calculated. Figure 3shows the conceptual model.500-Studics reveal that the ranges of traps favorable for theformation and development of deep-basin gas poolsbecomne smaller with increases in the dips of the reservoirs,but tur larger with increases in burial depths and decreases者i Pofle iin diameters of sand grains (Pang et al, 2001).1000-Marure source rockVcalean1.3 The material balance theory determines the gas-正Dep-basin gas pol三二waterbearing ranges of deep-basin gas poolsWhether natural gas can be accumulated in the deep-basinFig.1. Geological charactenistics of the deep basin gas accu-gas traps lies in the fact whetber gas enters the trap and howmulation in the Alberta basin.much gas enters. The deep-basin gas pool is a dynamically(a) Plan of the Alberta deep- basin gas pool distribution; (b) profile of hebalanced gas pool scaled by water. If the amount of gasenters into the pool is less than that spilled out from theboundary between gas and water, the gas-bearing range1.2 The force balance theory constrains the deep-basinvill decrease or even disappear completely. On thegas distributioncontrary, the range will increase, but no larger than theStatistics and research results show that the porosity oflimits of the traps. Figure 4 gives an example showing thereservoirs of deep- basin gas accumulations is generally lessconceptual model. The case study shows that the gas-than 12% and the permeability below 1x10-3 μm?. Thebearing area of a deep-basin gas pool increases with theresults from physical simulation experiments havethicknesea richnesc and evnlution degree of sourcedemonstrated that there lies a threshold for the formation ofoC中国煤化工oluion time.a deep-basin gas pool. For example, natural gas cannot beTYHCNMHGVol. 77 No.4ACTA GEOLOGICA SINICADec. 2003549φ<12%k>Ix 10'um3k<1X10'μm2Regional capP.-Water volume presure←R→|P-Capillary foreP,-Gas expanded forcemmumm个p。ReservoirBottom sealed tightly(a) Natunl gas in the reservoir withhigh porosity and permeabilityReservoir9 9(b) Natural gas restricted by tightmigrating to form a normal gas poolsingaSpooreservoir to form a deep-basin gas poolFig. 2. Comparison of pol-foming mechanisms between deep-basin gas and normal natural gas.48000 tsurfaceb)42000 t36000H= 100 m,- Trap range ➢(a30000 .D=0.0039 mmx=5°, 24000E 18000a=10°_| Point of forces balance12000a=30°600002000 3000 4000 500060000 70000Maximum depth (m)78000 pH=100m,9yP,- Power for gas72000z_=6000 m2-_RpIgration66000- Resistance to gas合6600 tE 60000thicknessa- dip angle of reservoir品54000司48000公|公A/g 42000a=5°日36000Sourceroc30000a=7°24000123售4售5678910Gnain size(φ)H- rservoir ticknes (m); L- dep basin gas pool range (m); φ -log: D, D- grain size (mm),Fig. 3. Rlatioship between boundary conditions of force balance and dominant factors of depbasin gas pools. (由) Boundaryof force balance; (b) charts showing the rlationships among deep basin range, grain size and reevoir dip.evolution, compaction and diagensis besides the sand grain1.4 The middle-late stage of basin's evolution isize. Generally, only after the reservoir burial depth reachesfavorable for the formation of deep-basin gas poolsa critical value, will the reservoir become tight on a largeTwo conditions are fundamental for the formation of deep-scale (Wang et al, 2002), so will hydrocarbons generatedbasin gas pools. One is tight reservoir, and the other is that中国煤化工TrockBasedonthethe source rock has begun to generate and expel gas in largerela I:ory of basin evolutionarmounts. The compactness of reservoirs is correlated to itsnd:YHC N M H Gan be divided into three550Deep-basin Gas Accumulation and Methods for Predicting Favorable AreasWang et al.Surface10500↑9000(8)Zm=4000m-Trap range ()一 +宜7001= 10 Ma6000t Poolrange ()4500宜30001500富Diffusing gas500 1000 1500 2000 2500 3000Ln (m)13000C%- Organic carbon content11500z-------KTI- Kerogen type index10000Ln=1000 mthickness (Hs)18 bRo- Transformation degreeZm=5000 m.1- timing of pool formed8500-Ln- - source rock area7000Source rock] Clr Kthicknes (Hn)-g 5500CGas supplied and expelled4000by the souree rock25006080100120140160180 200t(Ma)Fig. 4. Relationship between the material balance conditions and the dominant factors of deep-basin gas pools. (a) Condition formatter balance of deep-basin gas pools; (b) relationships among the ditribution of deep basin gas pools, characteristics of source rocksand timing of the pool formed.a - Reservior dip (5*C); H, - reservior thickness (50 m); D - grain size (0.0156 mm); KTI - kerogen type index (25); TOC - organic cartbon content (4%).Strata pressureArea for deep-basin gas pool.-Normal gas pool-Normal gas pool withTrap spill pointabnormal high pressureGentle and stable structureTight reservoir restricts食gas to be accumulated9in the lower ares.Deep-basin gaspool with abnormalGas supplied langelylow pressureby the source rockBottom sealed tighly10 form a poolis favorable for 825preservation.souerok esrorC吉 Normal gpoll C至]oep bso gspoo ~~ ]waerFig. 5. Conditions and genetic model for deep- basin gas accumulation.stages.Stage 1: Deep-basin gas pools were not formed. At this中国煤化工iexpelled because of thestage, the high reservoir porosity and perrmeability cannotTYHCNMH Ction degree of the sourceVol. 77 No.4ACTA GEOLOGICA SINICADec. 2003551rock.beds is less than the hydrostatic pressure in a normalStage 2: Deep-basin gas pools were formed on a largecompacted basin, with a pressure coefficient less than I. Toscale. At this stage, the low reservoir porosity anda single gas-bearing sand bed, the pressures at differentpermeability (中<12%, k<1 mD) were favorable for deep-burial depths are distributed in a straight line on the profilebasin gas pools to form, meanwhile the source rock began(Fig. 6b).to expel natural gas in large amount.(4) The gas-bearing beds are distributed in gentle andStage 3: The tight reservoir was full of natural gas; thestable structures, such as synclines and slopes with dipgas that entered the trap Later would spill out from theangles less than 159, and the boundary between gas andboundary between gas and water (trap spill point), or thewater is often coincident with the transform belt. Steepergas generated and expelled from the source rock wasreservoirs, in which gas accumulated may have strongerexhausted due to the too long time diffusion or interruptedbuoyancy, are unfavorable for gas preservation.by the erosion resulting from later uplift of the overlying(5) The bottom is tightly sealed. Normal gas pools liestrata. .under a regional cap, whereas the deep-basin gas pools areAfter these three stages, the formed deep-basin gas willconcentrated over a tightly sealed floor. When the cap isbe preserved and adjusted. The pools formed in the areas destroyed, the deep-basin gas will spill upwards underwith gentle and stable stuctures are easy to be preserved,buoyancy.and gases accumulated in the overlapped reservoirs inBesides the above-mentioned characteristics, deep-basincontinuous distribution which are interbedded in the thickgas pools are not easily discovered during the process ofor tight source rocks, especially when thick and tightdrilling, show low gas production or as dry holes duringsealing caps developed on the bottom of the reservoirs, willproduction tests, bave gas shows on a large scale, and havenot easily spoiled and diffused. On the contrary, the areasthe source rock and reservoir combined in one.with frequent movements and active hydrodynamic forceare not favorable for deep-basin gas pool formation, and2.2 Prediction methods of the deep-basin gas poolspreservation even if they have been formed. Figure 5 showsThere are three levels to predict the deep-basin gas pools,the fundamental conditions for deep-basin gas poolwhich are the traps’ limits prediction, the gas-bearingformation and its genetic model.ranges prediction and the "sweet spots" rich in gasprediction. Their relationship is shown in Fig.7.2 Identification Criteria and Prediction(1) Prediction of deep-basin gas traps: Two methods areMethods of Deep-Basin Gas Poolsused to predict the traps; one is the method of overlappinggeological conditions- first， delinecate the range of the2.1 Identification criteriapotential source rocks, the area of reservoir developmentAlthough the mechanism and model of deep-basin gasand the scope of stable structures that are favorable for thepools are simple, consensus has not reached on how toformation of deep-basin gas pools, mark them as OI, O2 andidentify deep-basin gas pools according to theO3 respectively, and put them logether on the samecharacteristics of their occurrence (Song et al, 2001;coordinate system, then the parts overlapped are thZhang, 2001). The following criteria are summarized fromfavorable areas for the deep-basin gas trap developmentthe features of deep-basin pools discovered all over the(Fig. 8). It can be seen from the figure that with this methodworld (Fig. 6).two favorable areas are predicted with areas over 600 km2(1) Gas-water inversion, i.e., gas is accumulated at thein the Turpan-Hami basin. The other is the method tobottom of a reservoir and water overlies to seal the gas; thenumerically simulate the force balance (Fig. 3) (Pang et al,boundary between water and gas is not controlled by the2001).structural contour. This feature is just opposite to the(2) Prediction of the gas -bearing limits the pools: Fouroccurrence of normal pools (Fig. 6a).approaches can be used: the first is to calculate the limits(2) The reservoirs are tight and in continuousbased on the material balance equation, the second is toditribution, generally φ<12% and kAThreshold of pool formationrich in gasPoint of material balance- Source rock and cap sealing on the bttomArea with high porosityand high permeabilityB. Prediction of favorible gas-bearing areas based on the materialC. Prediction of gas-rich areas with high porosity and high permeability.Fig. 7. Three prediction levels of the prospective area for deep basin gas accumulation and the principle.(3) Prediction of “sweet spots'":Sweet pots are delineated with the“overlapping”method, on the basis ofanalyzing the sedimentary facies ofthe target reservoir within the gas-bearing scope， and studying itsdiagensis and predicting its thickness,porosity and permeability, combinedwith considerations of the results ofgas-bearing volume in the targetformation from driling, the resultsBasin boundaryfrom geophysical data and surface" ) 0, Tectonically stable areageochemical exploration data iraddition to sand body thickness,O2 Center of gas source and range of gas suppliedporosity and permeability.! O, Region of tight and continuous reservoir developmentO。Area favorable for sealing and storage of gas3Prediction of Areas! O Area favorable for deep-basin gas pool developmentFavorable for Deep-BasinFig. 8. Prediction methods and principles of the favorable areas of deep basin gas pools.Gas Exploration in the UpperPaleozoic He-8 Segment ofthe Ordos Basinthe Xiaocaohu depression of the Turpan-Harmi basin, isenriched with large amount of deep-basin gas and theUsing the above methods, the areas favorable for deep-western slope area close to the Hongtai gas pool is the mostbasin gas exploration in the Upper Paleozoic He-8 segmentfavorable area for drilling of "sweet spots", where Wellsof the Ordos basin were predicted by three levels, withHongtai-6 and Hongtai-8 that are presently drilled havereference to Wang et al. (2002). The results are shown inobtained test gas recovery of 25x10 m' and 5.3x10* m' perday, respectively, which indicates that a new deep-basin .中国煤化工an area of I510km)gas pool will be found.TYHC N M H GPaleozoic He-8 segment554Deep-basin Gias Accumulation and Methods for Predicting Favorable AreasWang et al.deep-basin gas pools is the gas-water_▲▲_▲-A▲inversion.Waler,2) Reservoirs withporosityandpermeability greater than 12% and 1x10-3langjingDongshengum2' respectively are unfavorable for deep-++ tegionbasin gas accumulation and the boundary ofand /watey品force balance agreeable with the criticalconditions of φ= 12% andk= 1x10~ um21corresponds to the spill point of the trap fordeep-basin gas, which restricted the limits10representing the maximum gas -bearingrange of a deep-basin gas pool in theory. Thetight reservoir is the most essential卤冒geological feature for deep-basin gas pools.Jiaxian(3) The gas volume flling a deep basin gastrap is determined by the balance betweenthe gases going into and flowing out of the手Dingiam0trap. If the amount of gas ingoing is largerthan that of overflowing, the gas bearingarea will increase and it cannot exceed theLegendtrap area; on the contrary, the gas-bearingf-25soline of sand ticknes (m)area will decrease and even disappearsProable adHuanxiancompletely. A dynamic balance is theMot Dworobe gasbaringprerequisite for the preservation of a deep-Fearstoerbasin gas pool.Qingyang(4) The deep-basin gas pools were formedand developed in the midde-late stage of thebasin evolution. At the early stage, theFig. 9. Prediction results of favorable areas for deep-basin gas accumulation inthe He-8 segment of the Ordos basin.source rocks could not supply sufficient gasand the reservoirs unable to restrict themigration of natural gas, so that natural gasof the Ordos basin nearly covers all the northern part, wherecould not be accumulated in the lower parts of the basin,developed conditions for deep-basin gas accumulationbut in the middle-late stage, the two conditions are inexcept those areas with small burial depths and strongproper match, resulting in large accumulation of deep-basintectonic movements. From the gas-bearing range of thegas. After the middle-late stage, the formed pools weredeep-basin gas pool defined by current dilling, the areaunder adjustment and preservation as the source rockswith sand thickness over 20 m is more than 9.4x10* km2,stopped supplying gas or the traps were full.and the future resources are estimated to reach 14.83x1012(5) The favorable areas for deep-basin gas explorationm'. There are 7 gas-bearing areas with high porosity andcan be predicted in three levels. Firstly, predict the areashigh permeability in the He-8 segment, the first four offavorable for development of traps, usually by the methodwhich have an area of 2.3x104 km? and an amount ofof overlapping geological conditions and theory of forcepractical resources of 3.5x1012 km'.balance methods; secondly, predict the gas-bearing scopeof the decp-basin gas pools, which can be calculated by .4 Conclusionsmaterial balance simulation or delineated according to theresults from drilling, geophysical survey and geochemical(1) The fundamental cause for the deep-basin gasexploration; finally, predict the high-porosity， high-accumulated in tight reservoirs in the lower part of a basinpermeability and gas-rich“sweet spots" in the pools, whichlies in that the capillary force prevents the natural gasescan be determined based on the comprehensive results fromfrom migrating upwards by buoyancy. The deep-basin gasseismic facies, sedimentary facies and diagensis as well asaccumulates to form pools and drains porous water to the中国煤化工potential tests of theshallower strata by the gas expansion force to continuouslyl survey, geochemicalenlarge its area. The most obvious geological feature of theTYHCNMHGVol. 77 No. 4ACTA GEOLOGICA SINICADec.2003555Water-”bearingI HangjinqiGasan/DongshengionwateronforWusheng■YinchuanIV.)AreLilNMeng-a国Jiaxian8as020406080kmDingbian/;n Anbian JingbianCVEroded sndsone/30 Isoline of sand thickness(m)Direction of material sourceboundary|/’”、Boundary between gas andbearing region and gas &一water transition zone and areaRelatively favorable gas-rich areawater fransition zonefor-deep basin gasI I ShenmuLocationMost favorable gas rich arca⑥] Popectve urea. Yi-s5| Well position and nameFavorable gas-rich areaFig. 10. Prediction results of areas rich in deep-basin gas in the Upper Palcozoic He-8 segment in the Ordos basin.overpressure occurs in the overlying hydrostatic pressure5 Discussionsystem sealing the deep-basin gas, the pressure inside thepool may be greater than the hydrostatic pressure. TheseAbnormal negative pressure is an indicative feature ofhave been discussed in some literature (Ma et al, 2002;deep-basin gas pools, but abnormal overpressure cannot beWang et al, 2002).excluded completely from those pools. Studies haverevealed that abnormal overpressure occurs in the deep-Acknowledgementsbasin pools when the gas supply ratec from the source rocksis greater than that of water expelled upwards by gasThis study is part of the National Key Basic Researchexcess gas accumulated in the pool will destroy theProject (973) of the "Formation and Distribution of Oil andbalanced pressure system with negative value during theGas in Typical Superimposed Basins in Chinadeep-basin gas pool formation. By the way, in the early(G19990433)", and is supported by the Ministry of Sciencestage of the deep gas pool formation, the pool would beand Technology of China.able to enlarge its gas-bearing area easily only when thepressure inside the pool was greater than the sum of the中国煤化工l received Dec. 25, 2002hydrostatic pressure at that time and the capillary forceaccepted April 16, 2003during the driving of hydrostatic pressure. In addition, ifMHCNMHGoZiguoandZhuXiling556Deep-basin Gas Accumulation and Methods for Predicting Favorable AreasWang et al.Masters, J. A., 1979. Deep basin gas trap, westemn Canada. AAPGReferencesBulletin, 63(2): 152- 181.Cant, D. J，1983. Spirit River formnation: a stratigraphic-McMaster, G. E., 1981. Gas reservoirs, deep basin, westemCanada. The Jourmal of Canadian Petroleum Technology, 20diagenetic gas trap in the deep basin of Alberta. AAPG Bullerin,67:577-587.(3): 62-66.Chen Yuanrong and Jia Guoxiang. 2000. Research and applicationMin Qi, 1998. 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