Source-Contacting Gas:Accumulation Mechanism and Distribution in China

Journal of China University of Geosciences, Vol. 15, No. 1, p. 99-109, March 2004ISSN1002-0705rinted in ChinaSource-Contacting Gas: Accumulation Mechanism andDistribution in chinaZhang jinchuanEnergy Resource Department, China University of Geosciences, Beijing 100083, ChinaJin ZhijunExploration and Production Research Institute, SINOPEC, Beijing 100083, ChinaLiu lifangEnergy resource Department, China University of Geosciences, Beijing 100083, ChinaWang Zhixin Xue Hui PujunBasin and Reservoir Research Center, University of Petroleum, Beijing 102249, ChinaABSTRACT: Source-contacting gas, which is also called basin-center gas, deep basin gas, is the tight-sand gas accumulation contacting closely to its source rocks. Having different accumulation mechanismsfrom conventional gas reservoirs that are formed by replacement way, the typical source-contacting gasreservoirs are formed by piston-typed migration forward way. Source-contacting gas accumulations ehibit a series of distinctly mechanic characteristics. According to the valid combination of these charac-teristics, the estimation for the type of discovered gas reservoirs or distributions of source-contacting gasreservoirs can be forecasted. The source-contacting gas is special for having no edge water or bottom water for gas and complicated gas-water relationships, which emphasizes the intimate association of reser-voir rocks with source rocks, which is called the root of the gas reservoir. There are many basins havingthe mechanic conditions for source-contacting gas accumulations in China, which can be divided intothree regions. Most of the basins with favorable accumulation conditions are located mainly in the centraland western China. According to the present data, basins having source-contacting gas accumulations inChina can be divided into three types, accumulation conditions and configuration relationships are thebest in type a basins and they are the larger basins in central China. Type b basins with plain accumulation conditions exist primarily in eastern China and also the basins in western China, Accumulation con-ditions and exploration futures are worse in type C basins, which refer mainly to the small basins insouthern China and China Sea basins. Main source-contacting gas basins in China are thoroughly dis-cussed in this paper and the distribution patterns of source-contacting gas in five huge basins are discussedKEY WORDS: source-contacting gas, accumulation mechanism, basins in China, distribution patterINTRODUCTIONalways used together with unconventional gas accuSource-contacting gas is also called tight-sand mulations in actual research. Generally, the currentgas(Spencer and Mast, 1986), low permeability gas understandings on source-contacting gas include: nat(Spencer, 1989: Law and Dickinson, 1985), deep ural gas is accumulated without close relationshipbasin gas ( Masters, 1984, 1979), basin-center gas with conventional traps, without edge water and bot(Law, 2002; Rose et al., 1986), continuous gas tom wa中国煤化工 tight sand reservoirs( Schmoker et al., 1996)etc. These appellations are with alCNMHGSource-contacting gasis a kind ot gas accumulations with universal existThis paper is supported by the National Natural Science Foundation ences. Spencer(1989) pointed out that this type ofof China ( Nos, 40272062 and 40172052)gas reservoirs exist in almost all petroliferous re-Manuscript received November 10,2003gions. Surdam thought that 90 of gas accManuscript accepted December 25, 2003.tions to be found in the future possibly belong to this100Zhang Jinchuan, Jin Zhijun, Liu Lifang, Wang Zhixin, Xue Hui and Pu Juntype( Shirley, 1999). a great deal of source- to is that the tight-sand gas reservoir is contactedcontacting gas has been found in the areas with high tightly and extensively to its source rocks in large arexploration degrees. According to the statistics made eas. It performs as universal gas bearing, complicat-by Law(2002), there are 70 basins in the world in ed relationships of gas and water distribution in thewhich source-contacting gas reservoirs have been dis- gas reservoir, and water over gas generally. That iscovered or speculated. Actually, current estimations to say, there is no edge water and bottom water toon source-contacting gas accumulations are still in- create buoyancy for gas. Therefore, a sourcecreasing gradually. In China, more and more correla- contacting gas reservoir can be roughly defined as ative evidences have been found to show that lots of tight-sand reservoir with complicated fluid distribu-basins especially in the middle and western areas have tion relationship and contacting closely to its sourcepotentials for developing of source-contacting gas rocks. This definition emphasizes the contacting sta(Zhang, 1999)us between reservoirs and their sources ZhangThe significance for source-contacting gas stud- 2003)( Fig. 1). Compared with conventional onesying is that it breaks the constriction of conventional- that were formed by replacement driving way, typically mechanic rules on accumulations and distributions. source-contacting gas reservoirs were formed by theSource-contacting gas reservoirs always have large piston- typed driving and migration. Based on thisamounts of reserves and accrue in the centers of syn- definition, further analysis can be done and a series ofclines or deep depressions, and the lower part of basic geologic features can be draw out, such as thatstructural slopes. Their distribution can complement the reservoirs are tight, there is no buoyancy drivingwith conventional gas reservoirs accordingly on struc- gas-bearing areas and volumes are large and reservetural belts, which always lead to the creating of newareideas on looking for gas reservoirs in deep sags andsyncline centers.DEFINITION ON SOURCE-CONTACTING GASSource-contacting gas reservoirs usually refer tothe gas accumulations with complicated fluid distribu-tIolationship under unconventional barrier condirmal trapped gasaccumulationtions. According to the instance research, this type rRaof gas reservoirs is usually formed in tight sands with 92a2sacoal bedslarge areas,occurred in the central part of synclines, arrintfinweet spotsB93383 crueldeeper part of depressions, or lower part of structural mrinclinesd water distributions ofsource-contacting gas reservoirs are obviously differ- Figure 1. Sketch map on source-contacting gas and con-ent from conventional trapped ones, their accumula- ventional gas accumulations.tion mechanisms are also obviously different fromeach other. Masters (1979) pointed out the reverseMECHANIC ACCUMULATION AND CHARACTERwater to gas relationships of source-contacting gasISTICS FOR TYPICAL SOURCE-CONTACTING GASwhich is described as deep basin gas. Law(2002) RESERVOIRSemphasized the feature of universal gas bearing of the The primary source of natural gas in a source-basin-center gas reservoirs. There is still no uniform contacting reservoir is converted from complex chemand prevalent accepted definition in source-contacting ical reaction of kerogen during the process of oil andgas reservoir research currently (Law, 20in and gas generation, which means that the gas generationZhang,199. Closing to their source rocks that can process凵中国煤化工clergy for dischargingtaken as the roots of gas accumulations is the esCNMHGsential condition for the accumulation of this kind of cause tCu vI aN Bas accumulation itselfgas reservoirs. So the words of source-contacting gas is approximately equal to the surrounding pressureand source-contacting gas accumulation are used in that is needed to compress the existing gas under thethis paperburied conditions, energy coming from source rocksTypically, what the source-contacting gas refers is the primitive power source that resulted in the acurce-Contacting Gas: Accumulation Mechanism and Distribution in China101cumulation of a source-contacting gas reservoir andbeing of compressed inflation energy of the gas.water saturatedzoneAccumulation Principle for Typical Source-ContactingWhen natural gas enters into tight reservoir fromIts source rocks, energy of itself may be lost or com-ed at any time. Migration and accumulationtransitionalprocesses of natural gas happen continuously and theenergy in natural gas accumulating body cannot be re-leased or released limitedly for the weaker seal of capillary pressures of tight sands. Source rocks comple-ment energy to gas accumulation body constantly andforce the pore water to move upward gradually whengas saturatedthe natural gas accumulation is contacted directlyzonewith its source rockOn the contrary to usual reservoirs, tight sandsare of narrow and small radii of pores. Irreducibleater files between natural gas and pore walls will beFigure 2. Accumulation mechanism of source-thinned enough to embarrass the movement of gascontacting gaswhen natural gas is injected into tight reservoirs uner its generation pressure. Potential energy exchan there is no fringe water and bottom water to createges of formation waters between the top and bottom buoyancy for gas to migrate. As the results of pistonof gapleted thehlyyped expulsion of water by gas, source-contactingand immediately by seeping at liberty since the forma-gas accumulation is of prevalent existence of gas ande not continuous medium at this time. complex relationship of water and gasTherefore the expulsion of water by gas obeys the2) Source-contacting gas is characterized withpiston-typed principle Zhang, 1999), which petight reservoir, sufficient supplying of gas, commu-formances as the wholly driven of water by gas from nicated distribution among reservoir rocks, sourcedirectly its bottom, without fringe water, bottom rocks and seal rocks. Universally direct contact be-water and thus buoyancy(Fig. 2). As a result, typI tween source rocks and reservoir rocks is the essentialcal source-contacting gas reservoirs that are of watermechanic conditionsover gas are formed when gas column is high enough3) Main driving force for gas accumulation isAccording to the piston-typed driving principle, capexpansion force resulted from gas generation andillary pressure and thele pressure from overlying water their original formation pressure is overpressure.of naturalulation constitute th4)Different essentially from conventional onesmain resistances holding back gas migrating. There-source-contacting gas accumulations are always disfore, a typical source-contacting gas accumulation istributed in low altitude positions and high potentialof original overpressure. More and more data havepositions which is contrary to conventional gas accu-ndicated the close relationshinips between source- mulations and has some essential things to do withntacting gas accumulations and abnormal pressures the location of their source rocks. Of course, weigh-Putnam and Ward, 2001)ted multi-data stacking, complex correlation analysis, power equilibrium. material equipoise and energyMechanic Characteristics of Typical Source-Contactingconser中国煤化工ecast the distributionof sourCNMHGlationsAccording to the accumulation mechatures of typical source-contacting gas reservoirs are5) Source-contacting gas is transited continuously with normal trapped gas on mechanism and disthe characteristics as followsbution, which can be described by the universal(1) Natural gas is accumulated in tight-sand reservoirs with reverse water and gas relationship, since equation as the following formula. Totally, source-contacting gas is represented as dynamic characterisZhang Jinchuan, Jin Zhijun, Liu Lifang, Wang Zhixin, Xue Hui and Pu Juntics called temporary staying. However, static status gas column for normally trapped gas accumulationof source-contacting gas accumulation can be kept When Kwt equals to 0, this formula is the equation ofhen stable preservation occurredtypically source-contacting gas reservoirs. Ow and prPp+K{wg(h+hg+△h)=p+pwg(h±represent the densities of formation water and natural△h)+hP2ggas respectively i g is gravity acceleration; h is thewhere P, is expansion force of gas; t is conduction buried depth of top boundary of gas reservoirs; hg istime and Kw is the conduction rate of formation wa- gas column height: Ah is surplus water pressureter. When Kwt equals to 1. the formula turns into a head; pe is capillary pressure.commonly used equation for calculating the height ofmp’amshbarits is South China SeaFigure 3. Sketch map of source-contacting gas basins in China.DISTRIBUTION ON SOURCE-CONTACTING GAS in ting gas accumulations are discovered or speculatedCHINABasins in central China have the best geological con-Source-Contacting Gas basins in Chinaditions for the accumulation of source-contacting gas.Based on the discovered gas fields in China, two Ordos basin (Min et al., 2000; Li, 1999), Sichuanof the most important features of the petroliferous basin( Niu and Yang, 2002; Song and Hong, 2001)basins in China are the widespread existing of coal Qinshui basin and Chuxiong basin are located alongmeasures and concerned tight reservoirs. Therefore, longitude in central China constitute the ivory-shapedmany basins in China possess mechanic conditions for area for current exploration of source-contacting gasthe accumulation of source-contacting gas ( Zhang, in which related research work has been carried out2001). Discussing on regionally geologic-geographic and tot中国煤化工 on source-contactingand current gas explorations, the growing gas isCNMHGocks and tight reserof source-contacting gas can be divided into three voirs in the basins in western China are better develprospect areas Fig. 3), such as triangle belt in oped too, which are necessary for the accumulation ofNorthwest China, ivory-shaped belt in Central Chi- source-contacting gas. Turpan-Hami basin, junggarna, and scattered belt in eastern China. In China, basin, Tarim basin, Yanqi basin and Qaidam basinhere are more than 12 basins in which source-contac- etc, formed the triangle area in Northwest Chinaurce-Contacting Gas: Accumulation Mechanism and Distribution in China103yhere source-contacting gases are abundant. Basins(3) Type C: Geological conditions for the accu-in this area are the important strategic bases for gas mulation of source-contacting gas reservoirs areexploration, and perhaps further source-contacting worse and their exploration conditions are not magas exploration. The formation and evolvement era tured. The possible layers for source-contacting gasof basins in eastern China are relatively new. Layers are relatively deep and not suitable for explorationin Cenozoic and Mesozoic are target beds for explora- currently. This type of basin includes the basins intion. Tight-sand reservoirs in the bottom of Cenozo. South China and China Seasic, Mesozoic and even older strata are favorable layers for source-contacting gas accumulations. Howev- Main Source-Contacting Gas Basins in Chinaer, the conditions for source-contacting gas reservoirs Ordos basinare very limited by their deeper buried depth and disOrdos basin. which covers 250 000 km andersedly distributed. Generally, the main basins with located in the west of North China craton, whose avsource-contacting gas in East China are Songliao ba- erage thickness of sedimentary rocks is about 5 000sin, Bohai bay basin and Subei basin etcm, is a rectangle-shaped and multi-cycled forelandAccording to the accumulation conditions of basin with stable subsidence. The present structuresource-contacting gas reservoirs and the profitable of basin shows a large single slope with the characterexploration conditions, the basins with source- of higher in east and lower in west. The basin hascontacting gas in China can be divided into three ma- experienced several movements in its evolvement hisjor types as followstory, especially vertical movements. As a result(1) Type A: This type of basins has the best ac- there lack regional faults and fault activities inner thecumulation conditions for source-contacting gas res- basinervoirs and relatively favorable for explorationThe Upper paleozoic gas source rocks are a setThere developed rich source rocks, tight-sand reser- of extensive coal-bearing rocks with transition faciesoirs, regionally downwards structures and felicitous between continental and marine facies. The rocks arematches of the necessary formation factors for mainly coal-bearing strata, such as mudstone andsource-contacting gas. In these basins, major forma- carbonate rocks, which cover about 180 000 kmtions containing source-contacting gas pools are mod- Among these sedimentary rocks, the totally averageerately buried. This type of basins is located general- thickness of coal beds is 5 to 20 m and amounts to 25ly in Northwest China and central China, including to 30 m in west and northeast part of the baOrdos basin, Sichuan basin, Turpan-Hami basin, average thickness of dark mudstone amounts to 60Tarim basin, Junggar basin, Yanqi basin, Qinshui in the whole basin, of which the average organic carbasin, Chuxiong basin and so on.bon content amounts to 1.30 %-5. 24 %. The(2)Type B: There happen two cases. One is source material belongs to general humic-type andthat the buried depths of favorable layers and destro- their values of R, change from 0. 5 to 1.0 in theying conditions of faults make it difficult to accumu- northeast part to 2.5 in the south part of the balate large-scaled source-contacting gas reservoirs, sin. In eastern Ordos basin, the average organic carthough the regionally tectonic conditions, gas source bon content amounts to 1 in the carbonate rocksrocks and tight reservoirs of source-contacting gas ac- deposited in marine facies, which is perhaps thecumulation exist. B-typed basins cover Songliao ba- source rocks, too.sin, Qaidam basin, Bohai Bay basin, southern NorthThe formation and evolvement of Upper PaleoChina basin, Subei basin and so on. The other is that zoic reservoirs in Ordos basin are controlled by thethe basins are of poor conditions for the accumulation particular conditions of sedimentation, structure andof source-contacting gas. Favorable regions in these diagenesis. The high contents of detritus in clasticbasins are dispersed and limited or the aimed layers rocks中国煤化工 paction and pressureare over the economic depths, and available data are solutionCNMHGhe primary pores lostlimited too. This type of basin includes the dispersed immensely. I'he hign content of minerals with stableMesozoic and paleozoic basins with small arepropertiesNorthwest China, Mesozoic fault basins in Northeast properties restrict the development of secondary poreChina, and Mesozoic and Paleozoic fault basins inin reservoirs. During the process of burying, higheastern Chinatemperatures and pressures made the remained poresZhang Jinchuan, Jin Zhijun, Liu Lifang, Wang Zhixin, Xue Hui and Pu Jundecreased greatly. And the stable inland-cratonic mainly gas-bearing beds with less dry beds andondition restrains the development of fractures. So water-bearing beds. Shihezi Formation, locating inthe clastic reservoirs belong to tight sandstone with the relatively top position, contains gas bedlow porosity and low permeability. Most of the po- nantly while waterbeds increase upwards graduallyrosities of Upper Paleozoic sandstone are less than The regional gas-water distribution character has12 and range generally from 4 to8% while the close relationship with sedimentary facies. Northvalues of permeability are less than 1X10ards, the sand reservoirs with better property paThe accumulative thickness of this kind of tightrameters are dominantly water-bearing beds. Thesestones is much bigger and the regional changes of po- sand bodies deposited from fluvial delta systemsrosities and permeability match well with structural such as main channels, fluvial plains and north rim ofslope, which meets the need for themulation of alluvial fans, are located close to regional source are-source-contacting gasas. While southwards. the sand bodies with worseSource-contacting gas accumulations in Ordos reservoirs. deposited in deltaic plane and deltaicbasin are of the characteristfront, are generally gas or gas-bearing beds. The(1) Large areas and various layers of gas. The longitude distribution of formation fluids indicates dipresent hydrocarbon-generating areas reach to rectly the regional relationship of water over gas in180 000 km. It is speculated that the area of source- Ordos basincontacting gas accounts for 1/3 of that of the wholebasiTianhuupc(2)Abnormal pressures are prevalent all overthe basin. The formation pressures in the Upper Pa-leozoic vary differently in the 16 discovered gabearing layers in Ordos basin. Pressure coefficientsover 1.0 account for 75 and those below 1.0 acGongwusu Form. PAlfor 25 % It shows that the high preForm. Ps 2low pressures and normal pressures occur in differentTtet Fom ct L3places within Ordos basinsnhralearea(3) No bottom water and fringe water. The gen-eral structure of reservoir in this basin is a simply Figure 4. Source-contacting gas accumulation in Orwestward declining slope, which lifts eastward re- dos basinI. source-contacting gas accumulation; 2gionally. Except Yimeng uplift and west boundary gas saturated sands; 3. water-saturated sandsthrust belt, structural traps are hardly found in thegas-bearing area in central basin, where gas layers Western Sichuan depressionwere met among most of drilled wells and hardlyWestern Sichuan depression, located in thewere bottom water or edge water foundnorthwest of sichuan basin. is a foreland basin in the(4)Water over gas in Ordos basin. The major margin of Longmen Mountain thrust belt. Divided byhydrocarbon-generating period of the Upper Paleozoic Early Indochina movement, the sedimentation ofsource rocks ranges from Jurassic to Early Creta- Western Sichuan depression can be divided into twoceous. The generated gas migrated upward and accu- tectonic sedimentary periods respectively from Sinianmulated in relatively high part of structures con- to Middle Triassic and from late Triassic to eocenetrolled by tectonic outlines. Northward, the central in which the former deposited mainlyy carbonate rocksbasin and northern slope of basin are generally gas- to form platform sediments of marine facies and thebearing layers, over where there occurred water lay- latter deposited clastic rocks mainly to form forelanders(Fig. 4). Same distribution characteristics of for- b中国煤化工 hickness. The totalmation fluids happen in latitude direction. Vertically, thickneCNMH Goudstone intercalatedgas-bearing layers, dry layers and water-bearing lay- with coal beds of lacustrine facies, deltaic facies anders vary upwards gradually, too. Taiyuan Forma- fluvial facies amounts to 6 000-7 000 m. There aretion, located in the relatively lowest position, is of abundant gas sources and reservoir sandstones withgenerally dry beds and locally waterbeds. Shanxi low porosities and low permeability in the forelandFormation, located in the middle position, contains basin, which is the necessary setting for the formaurce-Contacting Gas: Accumulation Mechanism and Distribution in China105tion of source-contacting gas. Xujiahe Formation is(2) There occur abnormal pressures in the cenlocated in Upper Triassic Series, in which members tral part of Western Sichuan basin, of which the1,3 and 5 are mainly mudstones while members 2 greatest pressure coefficient reaches 2. 36 in Xujieheand 4 are mainly tight sandstones. The alternative member 2, Well Fenggu 1, while most of the presdevelopments of wide and thick sandstones and mud- sure coefficients are greater than 1.8. Pressure coef-ones constitute the regionally intercalated distribu- ficients decrease graadually upwdhichtion of source rocks and reservoir rocks for gas accu- with the mechanic variation to indicate the existing ofmulation. Dark mudstones deposited in shallow bath- source-contacting gas accumulations. Actually, theyal lake facies, dark mudstones and coal beds deposi- distribution of abnormal pressures is related closelof the favorable gto regionallyrocks, while sandstones deposited in delta front and depths of sediments, generation and supply condicoastal lake form the important reservoir layers, tions of natural gas and tight degree of reservoirswhich is the important exploration targets for source- etc, which shows directly the occurrence of sourcecontacting gas accumulationscontacting gaIntense diagenesis and weak dissolution of Trias(3)Reviewing regionally, there is an obvioussic clastic rocks in the depression resulted in the low distribution law of Upper Triassic in Western Sipermeability and limited reserve spaces, which is chuan depression that the regional water-bearing feacomposed mainly of fissure-pores and pores. While tures of reservoirs become more obvious on marginlocal reserve spaces are composed locally of pore- than those in central depression. On the contraryfissures and fissures. Tight reservoirs distribute regional gas-bearing features of reservoir becomewidely in Central Sichuan basin with average porosity more obvious in central depression than on the marless than 8 %, permeability less than 1 103 um, gin of depression (Fig. 5). Reand the reservoirs are tighter in North and northwest gional distribution of water and gas in Western SiSichuan basin. The average porosity of Triassic res- chuan depression, upper triassic layers, includingervoirs in Central and Western Sichuan basin ranges sandstone, siltstone and argillaceous sandstone, corfrom 3.5 to 5 and the average permeability of tain universal gas. The sights of gas shows in everythe same reservoirs are less than 0. 1X10 um. Mi- well and every layer constitute the assembling sectiongration of Upper Triassic gas is controlled by the ac- of gas saturation, water and gas combination, andcumulation mechanism of source-contacting gas, even water saturation upwards, which accords withamely piston-typed driven of water by gas. Source- the universal accumulation pattern of source-contacting gas reservoirs are accumulated taken the contacting gas. Towards the wide and gentle inclinesyncline model. Furthermore, Jurassic system is direction of Western Sichuan depression, water andspeculated to be of the conditions for the accumula- gas indications become complicated gradually with intion of source-contacting gas since their average po- tertexture relationship among different wells, simul-rosities are only 3. 3 % values of permeability are taneity of gas and water production in one hole, evenless than 0. 1X10 um and the possible buried depth coexistences of water and gas in one layer. Generalof aimed formations ranges from 2 500 to 4 000 mly, the dependance upon normal traps of the gas disEvidences indicating the existences of source- tribution becomes stronger and stronger upwards.contacting gas in Western Sichuan basin can be repre- Water bearing and water containing become popularsented as followsoutside the margin of Western Sichuan depression(1) There are universal gas showing in the sec- where accumulated natural gas occurs only in normaltions of deeper depression, even in siltstones. Anti- traps, such as Zhongba, Pingluoba, Hongya, Ya'ancline traps have little effects on gas-bearing areas and Guang'an. Many drilled wells indicate that water conhardly are there active water layers in depression, in tent is中国煤化工 r even under the con-hich the water appears mainly as irreducible films, ditionCNMHGch shows that theThe water saturations range from 35 to 65 tent of formaton tlulds are limiteewithin the sandstones with porosity of more than duction layers, gas showing layers or dry layers re5%. Drilling results have indicated that water con- present the typical characteristics of source-tents are little and there have no distinctly normal contacting gas accumulations. Numberless tests ofcontacts between water and gasdrills indicate the active gas shows in source rocksZhang Jinchuan, Jin Zhijun, Liu Lifang, Wang Zhixin, Xue Hui and Pu JunOu lakeantBascheng depression well konrmershan Moutain日Figure 6. distribution of source-contactingaccuFigure 5. Source-contacting gas accumulation in west- mulation in Kuche depression, north Tarimern Sichuan basinand their directly contacting tight reservoirs. ThereKunlun mountainShache depression Bachu upliftis no movable formation water between source rocksN:Qand tight reservoirs to produce buoyancy for gas migration, which is the most direct evidence showingPI310the existence of source-contacting gasTarim basinFigure 7. Distribution of source-contacting gas inLocated at the western end of Sino-Korea-Tarim Southwest Tarim basinplatform, Tarim basin is the biggest Cenozoic andMesozoic inland sedimentary basin in China with sed- High pressures and low pressures vary complithickness of 16 000 m. Covering roughly catedly and the distribution of dry layers, water lay560 000 km. Tarim basin is a large superimposed ers and gas layers intersect variously in Tarim basinbasin that is composed of Kuche depression, south- which indicate the existences of source-contacting gawest Tarim depression, southeast Tarim depressionccumulations. Middle and Lower Jurassic formaand pre-Mesozoic platform in central Tarim. Inside tions in Kuche and southwest depressions are perhapsthe basin, Cenozoic and Mesozoic grabben depres- the prospect areas for source-contacting gas exploraSions arrangeth pallifts and tion(Figs. 6 and 7)the structural strikes are mainly in latitude directionJunggar basinly in the deeper part of Kuche and southwest depres-unggar basin is a superimposed intermediarysions with the buried depth over 10 km. As a whole, massed basin with sedimentary area of 130x 103km2conditions for source-contacting gas to accumulateand the largest thickness of 14 000 m. There are setTarim basin are favorableof gas generation formations with intensity of gasTarim basin is of favorable structure and sedi- generation and abundant resources in piedmont dementary environments for source-contacting gas to pression within the basin. Natural gas has been dis-accumulate, where carbonate rock formations and covered in Carboniferous, Permian, Triassic and Juoverlying continentally clastic formations developed rassic systems with depth between 357 and 2 618 min superimposed manner. Tarim basin enriches conAs the two main sets of gas source layers indensate gas and gas reservoirs have been discovered Junggar basin, Permian and Jurassic systems haven at least 9 layers. Excluding several beneficial faci- generated much natural gas and the central supplyinges, Middle and Lowersic clastic rocks in Tarim areas of gas distributing in southwest of this basinage value of estimated porosity is 9 %. For instance, soua ar basin is the largest basin for Jurassic coal rebasin are tight universally, of which the totally aver-he average porosity value of Middle and Lower JumYHareas of Jurassic sys中国煤化工1 Juras,israssic system scales in 8.97. 36% in KuchecatCNMHG the south margin ofdepression. Coal resources of Middle and Lower Ju-he basin For instance. the accumulated coal thickrassic system account for about 95 of that of whole ness in Badaowan and Xishanyao formations reachesbasin. Resource of coal derived gas is above 742.2X 167.5 m. Except coal measure layers, average contents of toc in dark mudstone alternations amount10 m in the whole basinurce-Contacting Gas: Accumulation Mechanism and Distribution in China107to as high as 1.68 %-4.48%. Maturity degree of the Middle Jurassic and even older strata in detailChangji depression Sangequan wngwest slopecenteral depressionsoutheast upliftWell Bian 7ell Hei2ell Tuo I Well TuoPigure 8. Distribution of source-contacting gas in Figure 9. Prognosticated model of source-contactingJunggar basingas accumulation in Southern Songliao basin amendfroWang.organic matter increases gradually from 0.42.0 upwards. Favorable gas layers for theThere developed tight-sand reservoirs in the deemulation of source-contacting gas have past its vast per part of Songliao basin, such as Lower Cretaceoof source rocks, Jurassic system has supplied lots of central depression, Shan depressions and Jurassic ingas generation phases. Identified by the tracing back in Sanzhao and Yiwhere there occur sourcecontacting gas reservoirs possibly Fig. 9). TakenPiedmont depression in Junggar basin is close to South Songliao basin as an example, Jurassic systemsedimentary sources, where sedimentation velocity is is a set of coal bearing volcanic constructions intercahigh, mudstone content is high, sedimentary compo- lated with dark mudstone deposited in fault depressitions are complex, later diagenesis is relatively in- sions. Existed at the depth from 2 000 to 5 000 mtense, and tight sands distribute widely. Therefore, Jurassic is 2 000-2 500 m thick in 12 sub-basins withthere developed favorable conditions for source- a total area of 3. 38X10-6X10* km. With type Ilcontacting gas accumulation (Fig 8). Tightsand are- kerogen, Jurassic system and Denglouku Formationas are fundamentally accordant with those of the dee- of Cretaceous are of rather poor permeability andper parts of structures, thickness centers of coal dis- their general porosity is about 5 that approachestribution, central areas for gas generation and main the lower limit for gas production. Wang (2000)gas producing areas nowadays. Porosities of Permian pointed out that most of the source rocks in Upperreservoirs range from 6 to 14 and values of per- JbrassiCDenglouku and Quantou formations ofmeability of the same reservoirs vary from less than Lower Cretaceous have been in over matured stage in1X10 to 300X10 um in Mahu sag that is located the central depression of south Songliao basin be-at the north-west margin of the basin. In Urumchi cause of the high grade of temperature and greatpiedmont depression that is located at the south mar- depth. Reservoirs become tight downwards graduallygin of the basin, porosity of Middle and Lower Juras- from Quantou Formation that makes it possible forsic is 6 %-14 and related values of permeability source-contacting gas to accumulate continuously inare mostly less than 1 X10 um, while the buried central syncline within Quantou and Denglouku folepth of the same reservoirs lies in 4 000-7 000 m. mations. As well as Upper Jurassic, Denglouku Formation of lower Cretaceous has already been tightSongliao basinened with porosities of 2.3. 6 and values ofCovering 260 X10 km and developed in Middle- permeability less than 0. 5X10 um. The compari-Late Mesozoic and Cenozoic, Songliao basin is a son results of hydrocarbon source in Nongan arealarge-scaled and fault controlled inland basin in where gas was mainly derived from coal beds indicateNortheast China. In the southern part of Songliao that natural gas came from Jurassic. Moreover, thebasin, the sediments with total thickness of about high content of organic matter with TOC of 0.46 000 m are made up of continental clastic construc- 1.6%中国煤化工 asures make the cotion, where the thickness of Cretaceous is about ditionsCNMH Gale accumulations of5 500 m. Upper Jurassic that is insulated in isolated source-contacting gasfault depressions is made up of coal-bearing clasticcon PROSPECTS OF SOURCE-CONTACTING GAS INglomerate, sandstone, siltstone, carbonaceous shale ChiNaand coal beds. However. it is still certain to discussThe regions with continuously increasing naturalZhang Jinchuan, Jin Zhijun, Liu Lifang, Wang Zhixin, Xue Hui and Pu Jungas reserves and productions, and as an important generally lower, that the supplementary reserves inbasement to improve the producing-consuming struc- crease unproportionally, that the consumption corture strongly for energy resources in China, are struction needs to be improved, and that the prodmainly in central and western China, where Sichuan tions are of higher base but lower gurgitation. Abasin and Ordos basin are especially the best ones cording to the characteristics of continental basins(Wang, 2000). Results of the Second National Re- China, it is very possible for source-contacting gas tosource Estimation show that natural gas resource in accumulate and be developed remarkably, for it hasthe two basins is over 11 X 102 m, accounting for taken up half of future gas resources in China. In302 of total gas resources in China. Since the many basins of China, lots of possible evidences ofproved gas reserves account for only 3. 8 of total source-contacting gas accumulations have been disgas resources, there are still remarkable spaces for covered recently, such as the long distanced gasdeveloping Furthermore, there are better prospects shows, discovering of large amounts of dry layersfor source-contacting gas accumulation in the basins widely existing of gas at lower structural areas, lac-in Central and West Chinaking of edge water and bottom water, combinationsDuring 40 years'exploration and development, of poor gas layers to dry layers and weakly watereastern China basins, such as Songliao basin, Bohai bearing layers, which will possibly show the exist-Bay basin, have contributed tremendously to the pe- ences of source-contacting gas accumulations.troleum industry of China. However, explorationstill in generally lower level because only partial ba- CONCLUSIONSsins or sags are of high exploration degrees, of whichSource-contacting gas is a sort of gas accumulathe average exploration degree is less than 50 % On tion that contact directly to its source rock. It maythe whole, there are still two thirds of total resources objectively represent as complicated gas and water roto be discovered, especially natural gas or further lationships to show its particular mechanism. Com-source-contacting gas at deeper parts of those basins. pared with conventional gas reservoirs, which areFor example, gas proved degree is only 5 in formed in replacement driven of gas by water, typicalSongliao basin and 15 in Bohai Bay basin. With source-contacting gas reservoirs are formed in piston-petroleum exploration and technology improved, the typed migration way. Source-contacting gas accumupotential of source-contacting gas will be shown ac- lations appear a series of special mechanic characterisgly.cordingtics. Effective combination of these characteristicsAccordinlation mechanism thethe basic evidences for the foorecastingcurrences of source-contacting gas are of universali- contacting gas reservoity. The main reason for people to be unfamiliar withSource-contacting gas reservoirs are generatedthis kind of gas is its particularly occurring patterns mainly from Mesozoic and Paleozoic with obviousldue to that the theoretically whole saturation of gas different depths. Based upon the regionally geologor absolutely water over gas of source-contacting gas ical and geographic conditions, the development ofare always used to analyze indiscriminately the actu- source-contacting gas in China can be divided into 3ally complex distribution of water and gas. As a re- prospect districts as triangle belt in Northwest Chisult, source-contacting gas and normal trapped gas na, ivory belt in central china, and scattered belt inare distinguished thoroughly. Actually, develop- eastern China. Basins located in the ivory belt in cerments and existences of source-contacting gas reser- tral China are of the best conditions, which are comvoirs always have untypical features at various levels. posed mainly of Ordos basin, Qinshui basin, SichuanThere are a larger amount of formation water in the basin and Chuxiong basin. Source rocks, reservoirsdeeper part of a basin or favorable areas where and preserving conditions for source-contacting gassource-contacting gas accumulations have been IdentV凵中国煤化工 ins in weestern chinafied recently and a lot of gases accumulate in the top whichCNMHin, Junggar basinof traps as conventional accumulationTurpan raml basin and waldam basin etc.. CenozoicSource-contacting gas is an inevitable outcome of strata are generally thick and Mesozoic tight reserexploration levels. The voirs distribute scattered in the basins in eastern Chicurrent exploration situations and main problems in na. Possible source-contacting gas accumulations areChina are respectively that the exploration degree is located locally in Songliao basin, southern Northurce-Contacting Gas: Accumulation Mechanism and Distribution in China109China basin, Bohai Bay basin and so onY2002. Characteristics of Deep Ba-There are many basins in China that are of thesin gas in Tax Reservoir in Northwest Sichuan Tractconditions for the accumulations of source-contactingNatural Gas Industry, 22(2):6-10(in Chinese)gas. Source-contacting gas basins in China can be di- Putnam, P. E, Ward, G. S, 2001. The Relation betweenvided into 3 types. A-typed basins have the best conStratigraphic Elements, Pressure Regime, and Hydrocarditions for source-contacting gas, which are locatedbons in the Alberta Deep Basin(with Emphasis on SelectMesozoic Units). AAPG, 85(4): 691-714dominantly in central and western China. B-typed ba- Rose, P. R, Everett, I. R. Merin. 1. S. 1986. Possibleins are of medium conditions for source-contactingBasin-Centered Gas Accumulation in Cretaceous Trinidadgas to accumulate, including the basins in easternSandstone, Raton Basin, Colorado. In: Spencer, W. CChina and part of western China. Formation condiMast,R. F, eds, Geology of Tight Gas Reservoirtions and exploration perspectives are common inAAPG Studies in Geology, 24: 111-128yped basins, which refer mainly to the little basinsSchmoker,J. W, Rouch, T. D, Charpentier, Rin South China and China Seas. Mechanic evidences(4):95for source-contacting gas have been discovered innuous acmany basins in China. As the little known but widely Shirley, K, 1999. Rocky Mountain Succesoccurred type of gas, source-contacting gas reservoirstial-New Strategy Aids Deep Gas Hunt. Explorer, 5: 8will be possibly the important field for gas develop-ment in ChinaSong, Y, Hong, F, 2001. The Geological Conditions ofDeep Basin Gas in Western Depression of Sichuan Basin.REFERENCES CITEDPetroleun Ex ploration and Develo pment, 28(2): 11-14Jin, Z. J, Zhang, J. C, 1999. Exploration Strategy for Deep(in Chinese)Basin Gas Reservoirs. Petroleum Exploration and Devel- Spencer, C. W,1989. Review of Characteristics of Lowopment, 26(1): 4-5(in ChinesePermeability Gas Reservoirs in Western United StateLaw, B. E,, 2002. Basin-Centered Gas Systems. AAPG BulAAPG,73(5):613-629letin,86(11):1891-1919Spencer,C. W., Mast,R. F, 1986. Geology of Tight GasLaw, B. E, Dickinson, W. W., 1985. Conceptual Model forReservoirs. AAPG Studies in Geology, 24: 1-299Origin of Abnormally Pressured Gas Accumulation in Wang.J. Q., 2000. 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