Division and formation mechanism of coalbed methane reservoir in Huainan Coal field, Anhui Province,

ARTICLESChinese Science Bulletin 2005 Vol 50 Supp 7-17of CBM, the gas is self-generation and self-storage, and itDivision and formationoccurs in the coal seam mainly as the adsorbed state. Bsically, it is accumulated as the reservoir in-situ, and themechanism of coalbed methgas-water boundary does not exist; the distribution of thegas pool(accumulation) in the coal seam is uninterruptedane reservoir in huainan Coal- and discontinuous. Therefore, the division of the CBmreservoir should be differentiated from that of the conven-field. Anhui province. China tional natural gas reservoir, and it has the necessity toconduct the special intensive researchZHANG Xinmin Ll Jianwu. HAn BaoshanThe Huainan coalfield is located in the southern part ofDONG Mintaothe Huaibei plain in Anhui Province, and extends acrossboth banks of the middle reaches of the huaihe River; theXi'an Branch, China Coal Research Institute, Xi'an 710054. chinaPermo-Carboniferous coalfield lies concealed under theCorrespondence should be addressed to Zhang Xinmin (email: xmzhang Huaihe River alluvial plain, with the area of about 2120km. The coal resources are abundant, and it is an impor-Abstract The coalbed methane(CBM) reservoir is the tant coal production base in East China area. Within thebasic geological unit of CBM storing and CBM resource de- coalfield, the CBM resources also are abundant Based onveloping. The forming conditions of the CBM reservoir in- estimation, the CBM resources quantity is 2973x10 mclude coal thickness, coal rank, seam buried depth, caprock, and the resources density attains to 1.4x10 m/km2;inand geological structure The division of a cBM reservoir in addition, the economic geographic location is superiorhe Huainan coalfield is mainly based on the geological Since 1980s. the CBM researches and resources explorastructure form and seam buried depth. According to theFufeng nappe and the secondary structures of Huainan syn. tion and development works have been conducted here byclinorium, seven CBM reservoirs are divided in the Huainan pertinent intemal and external constitutions, and a lot ofCoalfield,such as Caijiagang, and the dynamic mechanism CBM geological phenomena have been exposed. The au-of CBM forming is analyzed. The structural position where thors have been lucky to conduct the CBM geologicalthe CBM reservoir is located has the important controlling research, resources assessment, and exploration and de-role on the features of CBM reservoir.velopment in Huainan coalfield since the late epoch ofKeywords: coalbed methane(CBM) reservoir, dividing, Huainan 1980s. Taking the Huainan coalfield as an example, basedoilfield, nappe, synclinorium, formation mechanism of CBM res- on the CBM research, the exploration and developmentresults of many years, this paper concretely conducts theDOI:10.1360/98zk0027dissection and analysis on the division and formationmechanism of CBM reservoir. Some preliminary recogniIn recent years, in the coalbed methane(CBM) geo- tion concems the problems related to the determination oflogical research and resource exploration and develop. the CBM reservoir, boundary condition, feature descrip-ment, the researchers have progressively paid attention tothe problems concerning the basic geological unit of CBMtion and formation mechanism and so on, and we hope toaccumulation and its division, and the researches and dis-scuss with the people of the same fieldcussions related to the CBM reservoir are increased. 1 Formation conditions and division of CBM reser-However, the existing research results are all the concept voirof the CBM reservoir and the CBm reservoirs are divided In the nature the distributions of coal seams and gas ingenerally into several types4, while the research results seams are uneven. Due to the comprehensive action andon the concrete division and feature description of CBM restriction of various geological factors, the CBM naturalreservoir are few. This is the weak ring in the CBM geo- accumulation(occurrence)units with different scales andlogical research, and is unfavorable for guiding the prac- categories are formed. Due to the existence of this kind oftices of the CBM exploration and development.natural units, the people can concretely recognize andWith respect to the accumulation of the conventional grasp the CBM resources. The CBM reservoir is both theatural gas, under the action of the buoyancy and capillary basic unit of CBM accumulation in nature, and the basicforce and through different passages, the gas as free state geological unit for recognizing CBM and conducting(or/and dissolved state) is moved to various traps and ac- CBM resources exploration and development. The re-cumulated as the reservoir. The gas reservoir"floats"on searches on the formation mechanism and types of thethe water column, and its scope is limited by the gas-water CBM reservoir are the core problems of the CBM founda-boundary and gas reservoir height, and it is relatively tionaH中国煤化工 pect to the definition ofconcrete and obvious. The distribution of the gas pool theplars have different(accumulation) in the reservoir is disconnected and dis- ogniCNMH Ge Cbm reservoir givencontinuous. Oppositely, with respect to the accumulation by the authors is that the CBm reservoir is the coal-rockChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESbody of the same coal-bearing strata in which a defined basic geological unit for planning the CBM resources dequantity of gas is preserved under the action of strata velopment, calculating CBM reserves, and conducting thepressure(water pressure and gas pressure). It has the in- CBM production. In particular, under the situations withdependent structural form; the CBM reservoir is the basic manifold coal geological conditions and complicated andgeological unit for conducting the CBM exploration and varied geological structures in coalfields of our countrydevelopment. According to this definition, the forming its action and significance are more outstanding. This alsoconditions of the CBM reservoir include both aspects: is the fundamental objective for us to suggest the conceptstorage and preservation. The storage conditions refer of CBM reservoir, analyze the formation condition ofmainly to the seam thickness and coal rank, while the CBM reservoir, and conduct the division of CBM reserpreservation conditions mainly suffer the impacts from the voircaprock and geological structural action. In addition, due As to the division of the CBM reservoir, it is startedto the controlling action of the seam buried depth on the from the definition and formation conditions of the CBMgas component, the reservoir pressure and the permeabil- reservoir, and based on various definite boundariesity of the coal reservoir, it also is an important factor for concretely define the space scope of every CBM reservoir.the CBM reservoir forming. To sum up, the formation In the practical work, it takes the distribution features ofconditions of the CBM reservoir include five aspects: the the coal-bearing strata and coal seam(seam pinch-outseam thickness, coal rank, seam buried depth, caprock and zone and thickness limit, etc. ) structural lines(fold axisgeological structure4. The concrete significance and acand fault), gas weathered zone, seam buried depth limit,tion mechanisms of five formation conditions of the CBM and coal rank limit and so on as the boundaries. to definereservoir are discussed in detail in ref. [4), and it is un- the boundary of the single CBM reservoir. Among thesenecessary to go into detailsboundaries, the distributions of the coal-bearing strata andA CBM reservoir should have the independent struc- coal seam and the structural line are the main divisiontural form, and the coal reservoir is basically continuous bases. Fig. I reflects the conceptual knowledge on thein sediment and structure in order to guarantee that the CBM reservoir. In the following sections,lateral flowing of the formation fluid (water and/or gas)in the definition and the conceptual knowledge on the CBMseam is connectedreservoir, and based on various boundaries to conduct theStarting from the definition of the CBM reservoir, and discussions on the problems such as the division and for-according to the formation conditions of the CBM reser- mation mechanism of the CBM reservoir in Huainan coalvoir, to conduct the division and description of the CBM fieldreservoir, it has the important theoretical value and practical value. It is the base for analyzing the enrichment fea. 2 The division of CBM reservoir in Huainan Coalfieldtures of CBM, researching the reservoir-forming mecha-mecha- In the regional geology, the Huainan coalfield is locatednism, establishing the CBM reservoir-forming mode, se- in the southeastern margin of the North China platform, inlecting CBM exploration and development pattern, and the transitional zone on the boundary between Northguiding the CBM resources development. It is also the China plate and South China plate, and belongs to theSeam outcropRiver valleyLower limitof gasweatheredSand body2000CBM reservior 3CBM reservior 2CBM reservior 1Fig. I. The CBM reservoir mode map. The map illustrates schematically the geological and hydrological backgrounds and variouboundaries of the CBM reservoir. In one coal-bearing stratum, the minable seavoir; the seams with various ranks from brown coal to anthracite -2 all can form中国煤化工 form the CBmreservoir due to the too lower adsorption capacity. The seams above the underlow the underground water level are saturated by water. In the gas weatheredCNMHd in the gas compo-nent the methane concentration is lower than 80% In the seam pinch-out zone the seam thickness becomes smaller and even disappearsWhen the seam buried depth is larger than 2000 m, the gas has not the economic exploitation value under the current technical conditionsChinese Science Bulletin Vol. 50 Supp. December 2005ARTICLESforeland location of the Qingling- Dabieshan orogenic belt. of which, five seams(Nos 13-1, 11-2, 9-2, 8 and 1 seams)Its eastern boundary is the Tanlu fault, and the western are universally developed in the entire Huainan coalfield,boundary is the Fufeng deep fault. In the north it is adja- with the average thickness all exceeding 2 m, and they allcent to the Bengbu uplift, and in the south it is near to the can be taken as the main seams for CBM developmentDabieshan orogenic belt; in the middle part the Wudian The coal rank is from gas coal to coking coal, and belongsfault divides the Huainan coalfield as both easterm section to low-medium metamorphosed degree. Therefore, theand western section. In the eastern section the Permo- distribution scope of coal-bearing strata and seams, andCarboniferous coal-bearing strata are deeply buried in the the coal rank boundary cannot constitute the boundariesnortheasterm margin of the Mesozoic-Cenozoic Hefei ba- for the CBM reservoir division. In Huainan coalfield thesin, with the buried depth exceeding 2000 m, and the division of the CBM reservoir is mainly based on thework and knowledge are few currently. In the western geological structural form, gas weathered zone and seamsection the buried depth of the Permo-Carboniferous coal- buried depthbearing strata is shallower, and it is suitable for the present Structurally, the Huainan Coalfield is a ramp nappecoal mining technology, and the generally indicated structural system constituted by the allotochthonous sysHuainan coalfield just refers to this part. In this paper the tem, main decollement surface, branch thrust fault anddiscussion scope also is limited in the western section of autochthonous system. Its southern margin is the Fufengthe Huainan coalfield in a broad sense(see Fig. 2)nappe, the northern margin is the Shangyao-MinglongshanIn the Huainan Coalfield the Permo-Carboniferous nappe, and its main body is the Huainan synclinoriumcoal-bearing strata have the total thickness of 910 m; the placed in between both nappes, and it is the commonminable and locally minable seams are only developed in autochthonous system of both nappes. The Huainan synthe permian Shanxi Formation, Lower Shihezi Formation clinorium is distributed in E-w direction. Totally, it isand the lower part of the Upper Shihezi Formation, and is raised westward and is plunged eastward, and in the inte-distributed in the strata with the thickness of about 370 m. rior a series of secondary folds and faults are developedThere are 19 layers of minable and locally minable seams, (Fig. 2). The special structural background and compliC-PShunqiaoouzElFOIF2〖K6愿310白3[7囫n白4「8圖120510kmShouxianBagongshanXieqiao synclineA③④km20 kmFig. 2. The brief geological structure map of Huainan coalfield, 1, Stratigra2, anticline; 3, syncline; 4, normal fault; 5,reverse fault; 6, Cretaceous; 7, Jurassic; 8, Triassic; 9, Permo-Carboniferoulchaeozoic. OShangyao-Minglongshan thrust fault; @)Fu中国煤化工per Proterozoic: 12. arFuli thrust fault;⑤Shouxian-Laorencang normal fault;Wudian fault; (Guzhen-ChangfenCNMH Gault: @OJiangkouji fault① Wanghutong fault:② Kouziji fault.① Zhuji-Tangji anticline;①smargun syncline: (wy Chenqiao-Panji anticline:OXieqiao-Gugou syncline: VLutang anticlineChinese Science Bulletin Vol. 50 Supp. December 2005ARTICLEScated structural style are the most important geological shan thrust fault(or Fufeng thrust fault)(Fig 3)factors having the influences on the division and features In the Huainan synclinorium a series of secondary foldsof the CBM reservoir in Huainan coalfieldare developed. They are successively from south to north:As stated above, the Huainan coalfield is constituted by Lutang anticline, Xieqiao-Gugou syncline, Chenqiao-three systems, that is the Fufeng nappe, the Shangyao- Panji anticline, Shangtang-Gengcunji syncline and ZhujiMinglongshan nappe, and the Huainan synclinorium Tangji anticline, of which, the Chenqiao-Panji anticlineplaced between both nappes. These three systems consti- and Xieqiao-Gugou syncline have the largest scale. Intute three independent CBM accumulating units. The particular, the Chenqiao-Panji anticline has the largeststrata involved in the Shangyao-Minglongshan nappe are uplifted amplitude, and the wavy undulation of the hingemainly the archaeozoic, Neoproterozoic and Lower Pa- also is most obvious. Accompanying with the secondaryleozoic; the Permo-Carboniferous coal-bearing strata only folds, a series of near E-w-directional faults, both normalremain in the local positions, and do not possess the con- fault and reverse fault, are developed. In the raising posiditions for forming the CBM reservoir. The strata involved tion in the western section of the synclinorium(west ofin Fufeng nappe also include the Permo-Carboniferous Guqiao), a series of normal faults with the NNE directioncoal-bearing strata besides the Archaeozoic, Neoprotero. is developed, and it is cut transversally into several faultedzoic and Lower Paleozoic. As the constituent of the up- blocks. The fault surface dips to west commonly, constifaulted thrust rock sheet of the Fufeng thrust fault, the tutes the step structure from east to west, and destroys thePermo-Carboniferous coal-bearing strata are placed be- continuity of the coal- bearing strata(seams). These foldstween Fufeng thrust fault and Shungengshan thrust fault, and faults are the main bases for dividing the CBM reser-nd is distributed around Fengtai to Jiulonggang It is the voir in the Huainan synclinorium. Thus, the eastern sec-distribution scope of the production mines in the Huainan tion of the Huainan synclinorium is divided into Xiji CBMmining district, and there are mainly the coalmines of Li-1, reservoir, Panji CBM reservoir, Zhuji CBM reservoir andXie-1, Kongji and Balitang, etc. The Huainan syncli- Madian CBM reservoir. The western section is dividednorium is the main body of the Huainan coalfield, and it into Liuzhuang CBM reservoir and Zhangou CBM reserconsists mainly of the Permo-Carboniferous coal-bearing voir. Its western end(around Laomiaoji) is located in thestrata and underlying strata. It is divided into the Xieli tilted end of Huainan synclinorium, the coal-bearing strataleep area, Panxie mining district and Xieji mining district, have suffered the severe denudation, and the seam buriedand the modern large-scale coalmines such as the Pan-1, depth is shallow. In addition the transverse faults are densePan-2, Xieji, Huajiahu and Zhangji and so on are estab- the seam continuity is very poor, and the value of CBMlished. Therefore, the Fufeng nappe and Huainan syncli- resources is lesser, so the CBM reservoir is not dividednorium are the geological units conducting the division of The locations and scopes of various CBM reservoirs in thethe cbm reservoir in huainan coalfieldHuainan synclinorium refer to Fig. 4Around Fengtai to Jiulonggang, the shallow PermoCarboniferous coal-bearing strata belong to the Fufeng3 Main geological features of CBM reservoirs innappe, the strata strike is roughly in the E-W direction, dip Huainan Coalfieldto north, and the dip angle is largely varied. Locally, itOn the basis of dividing the CBM reservoirs, taking thevertical or inverted. The coal-bearing strata of the gas reservoirs as the objective and dissecting their geo-Bagongshan thrust rock sheet in Fufeng nappe can be di- logical features, this is the base for studying the formationvided as a CBm reservoir, and is named as the Caijiagang mechanism of gas reservoir and assessing the resourcesCBM reservoir. Its upper boundary is the lower limit of development potential. The geological features of thethe gas weathered, and the lower boundary is the inter- CBM reservoir mainly refer to the structural position,secting line between coal-bearing strata and Shungeng. boundary condition, seam development situation, occurC-PFufeng faultE-O中国煤化工CNMHGFig 3. The Fufeng nappe and Caijiagang CBM reservoir( The section location refers to rig. Z)(Atter the data of No. 147 Anhui coalgeological Crew, revised). O Bagongshan thrust rock sheet; @2 Shungengshan thrust rock sheet; @Shoushan thrust rock sheetChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESC-P2圖6P9Fengtai3历7E-10Huainan10 kmFig. 4. The division of CBM reservoir in Huainan synclinorium(The names of the faults and folds are the same in Fig. 2). 1, Strati-graphical boundary; 2, anticline: 3, syncline; 4, normal fault; 5, reverse fault; 6, archaeozoic: 7, Upper Proterozoic; 8, Cam-brian-Ordovician; 9, Permo-Carboniferous; 10. gas weathered zone. A, Xiji CBM reservoir(in lower wall of Fufeng thrust fault it is stillxisted); B, Panji CBM reservoir; C, Zhuji CBM reservoir; D, Liuzhuang CBM reservoir; E, Zhangou CBM reservoir; F, Madian CBMrence, structural deformation features of the coal seam 4.1 The sedimentary features of coal-bearing strata(reservoir), coal metamorphosed situation (metamorphosed degree and metamorphism), gas content in seam, The Huainan coalfield is located in the southern marginpermeability of seam fracture(cleatsystem, and reservoir of the North China Permo-Carboniferous coal-accumu-ressure etclating basin, and the sedimentation of the coal-bearingUtilizing various research results and practical produc- strata belongs to the southern North China area. Under theon data, to conduct the integrated analysis, the main control of moving regularity of the coal-accumulation ingeological features of various CBM reservoirs in the the North China basin from north to south with time, inHuainan coalfield and the development situations of cbmhe huainan coalfield main minable seams all are develresources are listed in Table 1oped in Permian Shanxi Formation, Lower Shihezi ForIn Table I, in the Caijiagang CBM reservoir located in mation and Upper Shihezi Formation, and it is an imporFufeng nappe, the seam dip angle is varied largely, the tant constitution of the coal-rich area in these three periods,seam suffered the strong compression, the gas content in Shanxi Formation, Lower Shihezi Formation and Upperseam is high, and the permeability is low. While in the Shihezi Formation, in the whole North China coal-accuXinji CBM reservoir and Panji CBM reservoir located in mulating area4. The Permian coal-bearing strata are theautochthonous system, the seam dip angle is gentle, the sedimentary sequence consisted by the clastic coastal sysprimary texture of the coal matrix is preserved better, andthe seam permeability is better. It reflects that the struc- tem and offshore deltaic system, with the thickness of 670tural location of the CBM reservoir located in Huainan m and coal seams of 32-40 layers, and the minable andcoalfield has the most important controlling action on the locally minable seams are distributed in the strata with thefeatures of cbm reservoirthickness of about 370 m, of which, main seams, Nos13-1... 8 and 1 seams. are formed in the tidal flat4 CBM reservoir-forming mechanism in the Huainan or in the transitional zone of upper and lower deltaicCoalfieldplains. The seam intervals are smaller, the lateral variaIn the Huainan coalfield, the geological features, dis- tion is lesser, appears as sheet or blanket, and is stablytribution location and their relationships of various CBM distributed in the range of hundred or tens square kilomereservoirs are the results of the comprehensive action and ters. It reflects the inherent features of the seam formed inimpacts of multiple geological factors such as the sedi- the中国煤化工: sedimentary environmentation, thermal evolution, structural deformation and mentCNMHGating basin The develunderground water movement and so on in the geological opnlarge thickness, stablehistory.level, wide distribution and relatively concentrated on theChinese Science Bulletin Vol 50 Supp. December 2005Table 1 The brief geological features of various CBM reservoirs in Huainan CoalfieldZhangCBMCBM reCBM reservoirCBM reservoiCBM reservoirCBM reservoirCBM reservoirCBM reservoirrmStructural location Nappe, upper wall of system, southern both flanks of Panji anti-Autochthonous system, Autochthonous system, Autochthonous system, Autochthonous system, Autochthonous system,Fufeng thrust faulflank of xieqiao-eastern section ofwestern section of southern flank of Chen- northern flank of Chen-Fugou synclinelineZhuji-angii anticline Zhuji- Tangji anticlineqiao anticlineUpperGas weathered zone Gas weathered zone Gas weathered zoneGas weathered zoneLower wall of Guqiao Gas weathered zone Gas weathered zoneFufeng thrust faultXieqiao-GugouSouth: Xieqiao-GugouSouth: Fo faultyncline hingeNorth: Minglongshanwithpth of Fufeng thrust faultMinglongshan thruNorth: For faultthrust fault surfacfault surfaceI Eastern side Guzhen. Changfeng Guzhen, Changfeng Guzhen-Changfeng fault Guzhen- ChangfengGuqiao faultChenqiao faultChenqiao faultEstern side Turning zone of KongjiChenqiao faultGuqiao faultGugiao faultChenqiao faultJiangkouji faultJiangkouji faultness of minable22Seam occurrenc25°-60°, locally vertical or inverted15°—25°10°-15°10°-15°8°-15°10°-15°8°-15°Anticline, small faults areSeam structureand structural coal is structural coal isore. and fracture is de-AnticlineMonoclineMonoclineDeep-seatedDeep-seated metamor-Deep-seatedmagmatic thermal meta- Deep-seated metamor- Deep-seated metamor. Deep-snetamor-Deep-seated metamor-Coal metamorphism I phism, gas coal and fat morphism.phism. gas coalphism, gas coal and fat phism, gas coal and faand fatmorphism, gas coalGas content in seamcm/g)61-14m/19(12)29-10.7/64(37)26-14169(67)UnexploredUnexplored4.2-6.114.9(3)UnexploredSeam permeability0.03(1)004-0.390.18(6)001-1.20042(3)9.7-112/10.37)9.5-128/11.0(5)中国煤化工adient( kPa/m)CBM developmentGas suction in coal-as suction in coal-CNMHGminemine, one surface Gas suction in coalmNon-conductedNon-conductednductedNon-conductedNote: 1)It is represented by No 13-1 seam, as dry and ash-free base; 2)the permeability by well testingARTICLESsection,provides the good reservoir basis for the forma- suffers the deep-seated metamorphism, and the coal ranktion of various cbm reservoirsis the gas coal stage. Therefore, it can be considered thatUnder the control of the tidal flat and deltaic coal- in Huainan coalfield before the Jurassic (205 MaBP)theforming environment, the surrounding rock of the coal Permian seams already attained the maximum buriedreservoir is the thin interbedded combination of mud- depth. According to the comprehensive analysis by utilizstone-fine sandstone commonly, of which the piebald ing apatite fission track data combined with the deposiIdstone bed developed in the middle part of the Upper tion-burying evolution, after the coal-bearing strata wereShihezi Formation has the features of large thicknessburied three structure-thermal events were successivelgood continuity and wide extended range, and it has the occurred roughly 240-220 MaBP, 160-120 MaBP andbetter sealing capacity on the gas in underlying seams 80-60 MaBP, and in every event the crust elevation andsuch as Nos 13-1 seam and so on. In addition, some sand- denudation were accompanied". Of them, the first strucstone bodies in coal-bearing strata often occur as the seam tural-thermal event was equal to the early epoch of Indosroof and floor, appear as the sheet, finger-dendritic on inian movement. It was inferred from the regional geo-plane, and they are mainly the deltaic sandstone complex logical conditions and coal metamorphosed features thatconsisted by the distributary channel sand. These sand- in this epoch the seams had attained the maximum buriedstone bodies sometimes are the reservoirs of the accompa- depth. The second structural-thermal event was equal tonied"sandstone gas"(or aquifers), and there is the gas the middle epoch of Yanshan movement. It was related toshow in the CBM exploration (e.g, in Xinji XS-02 well). the magma intrusion in the Panji area, and had the obvious4.2 The history of coal-bearing strata burying and CBM impact on coal metamorphism. The third structural-ther-mal event was equal to the late epoch of Yanshan movement, although it also was very strong. The impact on coalThe geological age for the formation of the Permian metamorphism was unobvious. In the Huainan coalfieldseams in the Huainan coalfield is roughly 295-253 the main elevation occurred 120-23 MaBP (Fig. 5).TheMaBP. They are covered by the Shiqianfeng Formation different buried and structural-thermal evolution stagesand Lower Triassic, and all are the conformity contact had different control actions on the CBm generation andrelationships. Because above the coal- bearing strata the preservationMiddle-Late Triassic, Jurassic, Cretaceous and late strataIn 250 MaBP. the Permian seams were formed soon af-re lacked, the deposition-buried history of these periods ter and the buried depth was shallow. The paleo-geotem(241-23 MaBP)cannot be directly recovered. Based on perature was lower than the threshold of the thermogenicthe regional geological data, in the Huainan coalfield and gas generation. Under the biological-chemical action, itits adjacent areas, the Jurassic and Cretaceous strata are generated the biogas. In general, this kind of biogas allthe scattered distributed alluvial and diluvial facies sedi- was escaped in the late geological history. About 240ments and volcanic rocks with unstable thickness, and 220 MaBP, the coal-bearing strata were deeply buriedamong various strata it is the unconformity contact rela- (with the maximum buried depth of about 3200 m).Thetionship. It indicated that at that time the structural coal occurred the deep-seated metamorphism. Thereafter,movement was dominated by the block differential vibra- although the basin was slowly elevated, the buried depthtory movement", and had not possessed the structure- of the main Permian seams all still maintained betweensedimentary conditions to produce thick-layered andwidely distributed deposits. In addition, in the research 2500-3000 m, and the paleo-geotemperature maintainedprocess of the Permo-Carboniferous coal metamorphismbetween 80C-130C until 120 MaBP. Let the coalificain North China, the famous Chinese coal geologist, Prof. tion could be continuously conducted, the coal rank at-Yang Qi et al. (1998)indicated that the Mesozoic was the tained the gas coal and fat coal stages, at the same time, astage in which the Permo-Carboniferous coal in North lot of thermogenic gas was generated In the thermal eventChina suffered the deep-seated metamorphism, except for between 160-120 MaBP, in Panji and adjacent areas, dueindividual situations in Jiyuan-Houma in Shanxi Province. to the thermal dynamics brought by magmatic intrusion,The deep-seated metamorphism only can let the Permo- the seams in these areas were superimposed onto theCarboniferous coal in North China metamorphosed to magmatic thermal metamorphism, and the coal rank wasHuainan coalfield, except for Panji and its surrounding formed). It not only raised the generation quantity of thearea in which the coal is metamorphosed to the coking pyrolysis gas of the coal organics in this area, but also wasstage due to the magmatic activity. In other areas the coal very中国煤化工m adsorption ca1)Changsha Geotectonics Research Institute of Chinese Academy of Sciences,CNMH Gral simulation and predictionon assembly technology and special equipment for Xinji shallow coalbed methane demonstration development", 2000, 97-22 y project:"Researchon favorable block of coalbed methane development, the sub-topic report of the"Ninth Five- Year Plan"state scientific-technical keChinese Science Bulletin Vol 50 Supp. December 2005ARTICLES30℃60T3000Fig. 5. The structural-sedimentary evolution map of XS-02 well in Xinji area of Huainan coalfield since Late Paleozoic(After ChangslGeotectonics Research Institute of Chinese Academy of Sciences, 2000, simplified)and cleat development degree. Up to date the formwork of with thickness of 50-700 m, and the Permian seams werethe coal metamorphism in entire Huainan coalfield has buried again. Because the buried depth was shallow, in thebeen basically determined( the measured vitrinite Rmax is locations where main seams were located in, the geotemranging in 0. 72%0-1.48%, and the thermogenic gas gen- perature all were lower than 45 C, the secondary ther-eration of coal was terminated in this periodmogenic gas generation could not beDue to the strong uplift and long-term denudation oc- However, due to the filtration downward of atmosphericcurred 120-23 MaBP, the overlying strata and coal- water along the outcrop, the fresh water and methane bac-beating strata itself were unroofed and pressure-released, teria were brought into the seams and surrounding rocks,so that the buried depth of main seams are less than 1000 the organic components such as heavy hydrocarbons inm,and in some places the seams have exposed on surface. CBM and the normal paraffin hydrocarbons in seams ocThe buried depth of coal-bearing strata is shallower, the curred the biological degradation, and generated the secformation pressure is declines, and the vast amount of gas ondary biogas/%, 0). It had been confirmed by the CBmin seams was escaped through the modes such as desorp- component in Xinji area, as well as multiple isotope datation, diffusion and filtration and so on, at the same time, and the coal-rock organic geochemical analysis results".the matter exchange was conducted with atmosphere and The existence of the secondary biogas had a certain of roleunderground water; it was very unfavorable for the CBM to raise the gas content in CBM reservoir in Huainan coal-reservoir formation Under the control of the vertical dis- field, and formed the coexistence phenomenon of thetribution regularity of CBM, within a certain of depth be- thermogenic gas and secondary biogaslow the coal outcrop the gas weathered zone was formedAfter undergone the strong uplift and long-term denu- 4.3 Structural deformationdation since 23 MaBP the Huainan coalfield was settledagain, received the Cenozoic and Quaternary deposits the中国煤化工 vent multiple reforms ofCNMHG1)Lanzhou Geological Research Institute of Chinese Academy of Sciences, The coalbed methane geochemical research, the sub-topic report of the"Ninth Five- Year Plan"state scientific-technical key project: " Research on assembly technology and special equipment for Xinji shallow coalbedmethane demonstration development"(97-224), 2000.Chinese Science Bulletin Vol 50 Supp. December 2005ARTICLESthe Indosinian, Yanshan and Himalayan movements, of 4.4 Hydrodynamic conditionshich. the reform of the yanshan movement was thestrongest one. This coalfield is located in the southenIn accordance with the analysis of the hydrogeologicalconditions. in the huainan coalfield there are four mainls n gin of the north China plate, and its eastern boundarye Tanlu fault zone. In the south it is adjacent to the aquifers: Quaternary porous water aquifer, Permian sand-Qingling- Dabeishan mega-orogenic belt, and the geotecstone fracture water aquifer, Taiyuan Formation karst-fracture water aquifer, and Ordovician karst water aquiferrecent structural deformation framework of the huainan They are superimposed onto each other, and have the hy-coalfield was formeddraulic connections. Through different hydraulic connec-On both southern and northern sides of the coalfield. tion modes, such as the direct filtration downward ofthere are two thrust nappes distributed in near E-w diredQuaternary porous water, the lateral recharge of the karstwater in piedmont, the vertical leakage recharge of thetion. They constitute the regional barriers and boundaries floor Ordovician karst water, and the coal-bearing stratafor CBM distribution and preservation, and are the struc- contacted with Ordovician limestone in lateral due to thetural condition for forming the Caijiagang CBM reservoir. faulting and so on, the coal-bearing strata have obtainedThe Huainan synclinorium with near E-W direction placed the recharge of underground water. Totally, the permeabil-between both nappes is an independent unit of CBM oc- ity coefficient of fracture water in coal-bearing stratacurrence. It has developed a series of secondary anticlines small, and the dynamic reserve of underground water isand synclines with near parallel arrangement. In the inte- lesserrior of the synclinorium, there are two sets of main faults. In the low-mountain and hilly area in both southern andOne set is the strike reverse fault with Nww direction and northern sides of the coalfield, a part of bedrock isa few of normal fault; another set is the cross-cutting posed. The underground water is directly recharged by thenormal fault with NNE direction and roughly parallel to atmospheric water quickly it transfers to the lateral runoff,the Tanlu faulting zone. It cuts the folds and fault struc- and flows to the basin center direction, and the runofftures with Nww direction, and the sealing property on condition is good. In the middle plain area of the coalfield,sulted in that the coal reservoirs had the different struc- a catchment area, and the hydraulic gradient of the shaltural forms and deformation features, destroyed the conti- low water is only one ten-thousandth; the deep under-nuity of coal reservoir. Some of them are the boundaries ground water is in the stagnant status. This kind of hyof the Huainan coalfield, and some of them constitute the draulic condition is favorable for maintaining the forma-natural boundaries of CBM reservoirstion pressure in seam. Under the situation that the coalIt should be noted that under the overall compressivemining is not conducted in coalmines in Xinji and Panjiareas, the average formation pressure gradient of the CBMsystem of the Huainan synclinorium, there are many reservoir attains 10.7 kPa/m. It is slightly higher than theelongational structures constituted by E-w-directionalnormal faults, antithetic faults and rolling anticlineshydrostatic pressure gradient.These elongational structures in the thrust structural sys- 4.5 The geological modes of CBM reservoir formationtem were the "necessary results of the elongational action in the Huainan Coalfieldappeared by the continental crust upwarping, which wasBased on the above-mentioned analyses, the geologicalresulted from the large-scale bending deformation of the modes of the CBm reservoir formation in the huainanlithosphere in the foreland position when the Qinling- coalfield are summarized as followsDabieshan orogenic belt was formed"ll. Due to the elonIn the southern part of the North China cratonic coalational action, the tensional fractures in coal-bearing accumulating basin, in the Permian, the offshore plainstrata were more developed, and it was favorable for rais- coal-accumulating environment steadily appeared, and theng the permeability of CBM reservoir. The authors con- Huainan coalfield was located in the center position of thedered that in the Xinji CBM reservoir and Panji CBM coal-rich area in three periods, i. e, Shanxi period, Lowereservoir, the relatively higher and overall not so lower Shihezi period and Upper Shihezi period. Five main mieam permeability were measured (refers to the related nable seams such as No. 13-1 and so on formed in themeasured values of permeability in Table 1); in XS-02 test tidal flat or the transitional zone environment of upper andwell of the Xinji CBM well set the highest gas production lower deltaic plains, are stably distributed in the scope ofattained to 3278 m/d, and after 3 years it still maintained the whole coalfield, and become the main seams of variat about 1000 m'ld; these phenomena have the obvious ous中国煤化工 ling rock of the seams iscausality with the improvement of the permeability of theCNMHion of mudstone firCBM coal reservoir in this tensional structural environthe sheetshaped, and sometimes it is the reservoir of the accompa-Chinese Science Bulletin Vol. 50 Supp. December 2005ARTICLESnied"sandstone gas"(or aquifer). The piebald mudstone coal reservoirs have the different structural forms and debed developed in the middle and upper parts of the coal- formation features, the continuity of the coal reservoirsbearing strata has the features of large thickness, good suffered the destruction, and they constituted the naturalcontinuity and wide extended range, and it has the better boundaries of various CBM reservoirs. Under the com-sealing capacity on CBM in large scope. These sedimen- pressive structural environment in the Huainan syncli-tary backgrounds allow that the CBm in the entire coal- norium, due to the existence of the elongational structurefield has the identical generation, storage and cover basic formed with the extension in the early epoch, in theconditionscoal-bearing strata, the tensional fractures were more deAfter the Permian seams were formed, they were buried veloped, and then, the permeability of the coal reservoirby the continuously deposited overlying strata, and the was raised. The structural deformation of the Yanshanmaximum buried depth was about 3200 m. Until 120 period determined the forms, distributions and main fea-MaBP, the buried depth of main seams all maintained be- tures of various CBM reservoirs in the Huainan coalfieldtween 2500-3000 m, the deep-seated metamorphism ofIn the low-mountain and hilly areas in both southerncoal was steadily conducted, and the coal rank attained the and northern sides of the coalfield, the underground watergas coal and fat coal stages. a lot of thermogenic gas has is actively alternated, the runoff condition is good, and thebeen formed, and this thermogenic gas is the main gas sealing capacity of the hydrodynamic condition on CBMsource of the CBM reservoir in Huainan coalfield. The is very poor. In the middle plain area of the coalfield, i.ethermal event occurred in 160-120 MaBP resulted in that the distribution area of the Huainan synclinorium, it is athe seams in Panji and its surrounding areas were super- catchment area, and the hydraulic gradient of the shallowimposed by the magmatic thermal metamorphism. The underground water is gentle. The deep underground watercoal rank was raised to the coking coal stage(locally had is in the stagnant status, and it allows the seams maintainformed the natural coke), and the adsorption capacity of the higher hydrostatic pressure, and produces the effectiveseams and the cleat development degree were superior to hydraulic sealing on CBMthose in other areas5 Research signifiIn the period of 120--23 MaBP, in the Huainan coal-within the scope of gas-bearing zones in seams orfield, the strong uplift and long-term denudation occurreoalfield, to conduct the division of CBM reservoirsso that the buried depth of the main seams was less than study their features and the formation mechanism, and to1000 m, and some seams were exposed on surface. The establish the concrete reservoir formation modes, it canoriginal balance condition of the CBM occurrence was enhance our understanding of the CBM reservoir forma-destroyed, andgas was largely escaped; due to the tion and enrichment regularities, and is favorable for con-material exchange with the atmosphere and underground ducting the selection and assessment of CBM resourceswater, within a certain depth below the seam outcrop the development area and planning the CBM exploration andgas weathered zone was formed. These variations were development formaton in Huainan coalfield, and com-very unfavorable for the CBM reservoir formation. Sinceased onabove-mentioned geological mode of23 MaBP, with the settlement of Huainan coalfield again, CBM reservoirthe Permian seams were buried again. Because the buried bined with the analysis on Table l, it can be seen that indepth is shallow, the seams do not generate the secondary the Huainan coalfield, the reservoir condition of the panjithermogenic gas. However, due to the downward filtration CBM reservoir is the best The second ones are the Xinjiof the atmospheric water, the fresh water and methaneCBM reservoir and Liuzhuang CBM reservoir. The Panjibacteria were brought into the seams and their suround- CBM reservoir is located in the center of the huanng rocks. The secondary biogas was formed, and allowedlinorium, and occupies the main body of both flanksthe CBM reservoir in Huainan coalfield become the com- of the Panji anticline. Its structural position is superior, theund CBM reservoir where the thermogenic gas andstrata are gentle, the tensional fractures are more develThe strong compression and napping occurring with oped, and the seam thickness is larger. In the entiresecondary biogas were coexisted.Huainan coalfield, the coal rank is the highest, up to theYanshan movement resulted in the deep formation of the coking coal stage; the CBM exploration results have indi-Huainan coalfield, and the entirety of the Permian coal- cated that the gas content and permeability are the highestaring strata was destroyed. The space location and rela- in whole coalfield. The gas reservoir has the largest areative relationship of the coal-bearing strata( seams)were and the CBM resources are the most. Therefore, the Panjimoved and varied, and these all had the deep influence on CBMthe CBM occurrence and migration. The Fufeng nappe sourd中国煤化工Coalfieldand Huainan synclinorium formed by the compression andCNMH Gisnapping were both independent units of CBM occurrence. Aey prof, Tang Xiuyi of the Anhui Technical tThe secondary anticline, syncline and main faulting struc- supported by the state 973 Plan"The basicn the formationtures developed in the Huainan synclinorium allowed the mechanism and economic exploitation of coalbed methane reservoir inChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESChina"(Grant No. 2002CB211700).Chinese Science Bulletin 2005 Vol 50 Supp. 17-23ReferencesSedimentary conditions forloration and Development in Chinese, Being: Geological Pub. good coal reservoirlishing House, 1996, 14-18.development in the2. Qian, K- Zhao Q B, Wang Z.C., Coalbed Methane Exploration Carboniferous and permiand Development Theory and Test Measuring Technology(in Chinese), Beijing: Petroleum Industrial Press, 1996,Qinshui Basin3. Zhao, Q. B, Li, w.Z., Wang, Y. B. et al., Present situation of coal-JIN Zhenkui& WANG Chunshengbed methane exploration and key target selection, in Oil and Gaience, China Petroleum University, Beijing 102249Exploration Road of Oil Companies(in Chinese)(ed. Zhao, Z. Z ) ChinaCorrespondence should be addressed to Jin Zhenkui (emailBeijing: Petroleum Industrial Press, 1998, 203-2144. Zhang, X M, Zhuang, J, Zhang S. A, Coalbed methane geology Abstract In the Carboniferous and Permian of the Qinnd resource evaluation in China (in Chinese), Beijing: Science shui Basin, coal reservoirs are well developed. But differentPres,2002,51-64.coal reservoirs are obviously different in distribution, thickness and reservoir performance, etc. This paper studies the5. Tan, M. Y, Tao, M. X- Jia, H. Y et al, Geological structural factors controlling the development and distribution of goodframework evolution and oil and gas condition analysis in Hefei coal reservoirs from the viewpoint of sedimentology and se-quence stratigraphy. The study shows that good coal reserbasin (in Chinese), Coal Geology and Exploration, 2003, 31(4): voirs are mainly formed in marsh environments on tidal flatsand delta plains. The coal reservoirs formed on tidal flats aremore continuous laterally than those on delta plains. The6. Shang,GQ-, Research on Late Paleozoic Coal geology in North fluctuation of sea level shows important control over the verChina Platform(in Chinese), Taiyuan: Shanxi Science& Technical tical and lateral distribution of coal reservoirs. Generallyspeaking, the coal reservoirs formed in the highstand systemtracts are more abundant, thicker and more widespread, and7. Cheng, Y. Q, An introduction to Chinese Regional Geology (in thus the highstand system tracts are more favorable to theformation of good coal reservoirs. The coal reservoirs formedChinese), Bejing: Geological Publishing House, 1994, 90-152. in the transgressive system tracts are lower in abundance8. Yang,Q, Pan, Z. G, Weng, C. M et al, Discuss on Metamor. thinner and much less widespread, and thus the transgressivephosed Features and Geological Factors of Permo- carboniferous system tracts are not favorable to the formation of good coalCoal in North China(in Chinese), Beijing: Geological PublishingKeywords: Qinshui Basin, Carboniferous-Permian, good coal reserHouse,1988,53-59voirs, sea level fluctuation, controlling factors.Scott, A.R., Kaiser, w.R Ayers, W. B et alogenic and DOl: 10.1360/98zk0005econdary biogenic gases, San Juan Basin, AAPG Bulletin, 1994,In this paper, "good coal reservoirs"refer to the coal78(8):1186-1209beds that are thick, widespread and possess relatively high10. Zhang, Z H, Wang Q Tao, M X et al. CBM geochemical fea- porosity and permeability. The formation of good coalreservoirs is controlled by many factors, including seditures and their significance in reservoir-forming research, in CBM mentary, diagenetic and structural ones-. This paper,Ex- with the Carboniferous and Permian of the Qinshui Basinploitation(in Chinese)(eds Song, Y, Zhang, X M. ) Beijing: Scias an example, discusses the control of sedimentary conditions over the coal reservoir thickness and distributionnce Press, 2005. 27-35scope from the viewpoint of sequence stratigraphy, andanalyzes the cause leading to the obvious difference ofcoal beds of different horizons in thickness and distribu-structure under the Fufeng nappe and its formation mechanismtion scope. The sedimentary conditions and distribution ofHuainan coalfield, Anhui province(in Chinese), Coal Geology and thic中国煤化工ded, so that the distriExploration, 2003. 31(3): 1-4be predictedCNMHGReceived March 15, 2005; accepted June 30, 2005)he Carboniferous and Permian in the Qinshui BasinChinese Science Bulletin Vol 50 SIDecember 2005