Sedimentary conditions for good coal reservoir development in the Carboniferous and Permian, Qinshui

ARTICLESChina"(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 2005ARTICLESare well developed, and are also outcropped around the relatively much sulfur(sulfur content over 1%), and thebasin. Within the basin, the Carboniferous and Permian pyrite is commonalso encounter in wells(Fig. 1). According to the twofold(2)Marshes resulted from gradual filling of interchadivision scheme of the Carboniferous and the threefold nel bays or abandoned channels on delta plains. The floordivision scheme of the Permian which are widely adopted rocks of coal beds are interchannel bay dark gray mudin the world, the coal beds mainly occur in the upper Car- stones or distributary channel sandstones. The interchan-boniferous and lower Permian. In the ascending order, the nel bays commonly are brackish or fresh water environupper Carboniferous is divided into the Benxi Formation ments, and the mudstones deposited are different fromand Jinci Formation, and the lower Permian is divided into those deposited in the restricted seas. They do not containthe Taiyuan Formation and Shanxi Formation(Fig. 2). The normal marine fossils, and their B content is <100X10Jinci Formation is equivalent to the Jinci Member of the (commonly less than 60X10-6)and Sr/Ba is <1, reflectingTaiyuan Formation in the previous traditional division brackish or fresh water environments. The mudstones de-scheme. The Benxi Formation overlies disconformably on posited in the restricted seas contain relatively more Band commonly >100X 10(commonly less than 100Xother formations is conformable. The coal-rich upper 10-6-140x 10)and Sr/Ba is >l. The sulfur content ofCarboniferous and lower Permian are studied in this paper. coals formed on the delta plains is generally low(<1 %),In the study area, the upper Carboniferous-lower Penian is 100-200 m thick, and consists of sandstones and and this also indicates fresh water coals. The depositionalmudstones intercalated with limestones and coal beds. sequence consists of distributary channel sandstones, inThese rocks are deposited in the alternating marine and or channel sandstones and coals(Fig. 4)nonmarine environments.3. 4. Based on the outcrop andIn summary, coals in the study area mainly form in theborehole data, there are mainly 16 major coal beds in the tidal flats and delta plains. Statistics shows that such enupper Carboniferous andnd lower Permian, which are num- vironments can form thick widespread good coal reserbered 1,2,3,4,5,6,7,8,9,10,11,12, 13, 14, voirs(such as 3" and 15" coal beds), and can also form15, 16. The total thickness of these major coal beds is thin and restricted poor coal reservoirs(such as I"and 11*2-18 m, commonly 6-12 m(Fig. 1)coal beds). The distribution area and thickness of coal2 Control of sedimentary environments on favorable beds mainly depend on the area of marshes and the time toI reservoirlast. The marshes with large area and lasting a long timecan form widespread thick good coal reservoirs. The areaA lot of studies have been done on the sedimentary en- and lasting time of marshes are closely related with thevironments of coal-51. But studies on the relation be- sea level fluctuation(see discussion below). Generallytween the distribution area and thickness of coal and speaking, due to the cutting by distributary channels, thesedimentary environments are few. However, determina- coals formed on delta plains are less continuous than thosetion of such relation is very important to predict good coal on tidal flatsreservoirs3 Control of sea level fluctuation on development ofBased on the analysis of sedimentary characteristics of coal reservoirsfield profiles, during the late Carboniferous and earlyPermian, carbonate platforms, restricted seas, barrier isIn the late Carboniferous and early Permian, marinelands, lagoons, tidal flats and deltas were developed in thetransgression and regression occurred many times, andstudy area(Fig. 3). On the tidal flats and deltas, coal beds According to previous studies4), the upper Carboniferousdepositedand lower Permian can be divided into two sequencesOn the basis of analysis of the lithology and sedimen- The bottom boundary of Sequence I is the discon-tary environments of floor rocks of coal beds, marshes formity between the upper Carboniferous and the Ordoviform in the following sedimentary environmentscian, and the top boundary is the bottom of the Miaogou1)Marshes resulted from gradual filling of the re- Limestone of the Taiyuan Formation. This sequence constricted seas or lagoons during the descending of sea level. sists of the Benxi Formaiton Jinci formation and theSuch marshes occur on intertidal and supratidal flats, and lower part of the Taiyuan Formation, and is about 50-90belong to peat flats. The floor rocks of coal beds are in-m thickterchannel bay dark gray mudstones or coastal beachThe bottom boundary of Sequence ll is the bottom ofsandstones. The depositional sequence consists of carbon- the M中国煤化工 Ian Formation, and theate platform limestones, restricted sea mudstones and peat top bdten the lower and midflat coals from bottom to top, or carbonate platfodle pCNMHGS of the upper part ofstones, restricted sea mudstones, barrier island sandstones the Taiyuan Formation and the Shanxi Formation, and isand peat flat coals(Fig 4). Such coals commonly contain about 60-100 m thick(Figs. 2 and 5)Chinese Science Bulletin Vol 50 Supp. December 2005ARTICLES90 kmXinzhuang mineshian mineMang countyo ShoutSy003YangquanABorghole4 XIyu minesy004Nanzhuang mineHuoshan mineShuiyuguanHuangyanhui⊙ Qf countyhEshundayanfEnyang/uyao minefrist mineushe▲ wencheng mineWuxianlizhuang minemazhuangXucun minehenyu mineKhabaijiazhuangXiangyuantankaXiwang-mine▲ Touhe mine▲ Sadiaohe mineHuokou moGu countyT1ooz welr,jingfang minewangzhuang minexianghe、 Fengs bin4bhn ell Wangyu mine aShonjazhuiang minezhuang mine⊙ HoumaBorehole 3tXinjiangghnghongthinLocation of cross-sectionOutcrop and mining pitFig. 1. Isoline map of coal thickness of the upper Carboniferous and lower Permithe cross-section of Fig. 5.)YHa中国煤化工 -he map indicates the location cCNMHGChinese Science Bulletin Vol 50 Supp. December 2005ARTICLEShange of sea levelStratigraphy Marker bed nessI lithologicsandstoneBeichagou sandstQiligou sandstoneXiedao limestoneWujiayu limestoneOrdovicianFine sandstoneSandy shaleSiltsandstoneYH中国煤化工meFig. 2. Division of sequences of the upper CarboniferouCNMHGChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESTidal flatDistributary channelbeach baTidal flatcoastal beach barCarbonate platformRestricted clastic shallow-seFig 3. The sedimentary model of the upper Carboniferous and lower Permian, Qinshui Basin0mPeat flatRestricted clastic shallow-sea∴] Coastal beach barRestricted clastic shallow-sea9The Congziyu section of Taiyuan groupThe Qiligou section of Taiyuan groupPeat marsh3 mInterchannel baySubsea distributary channel 6mDistributary channelThe Congziyu section of Taiyuan groupThe Qiligou section of Shanxi groupPeat flat Lagoon3mBarrier islandRestricted clastic shallow-seaCarbonate platformThe Nanyaotou section of Taiyuan groupFig. 4. Evolution sequence of coal sedimentary environments in the upper Carboniferous-lower PermianEach sequence consists of many parasequences. Each Permian of the North China Platform". In Sequence Iarasequence represents a small rising and falling cycle of there are 4 limestone beds. The top one, i.e. the Wujiayusea level. Each parasequence is commonly 5-15 m thick, Limestone, is the largest in area, and thus represents theand mostly starts from the carbonate platform limestones largest flood surface. This limestone bed and strata belowor restricted sea mudstones to deltaic or tidal flat coals(Fig. compose the transgressive system tract, and the strata2), and belongs to the shallowing-upward cyclesabove it compose the highstand system tractAccording to the stacking style of parasequences, eachthere are also 4 limestone beds. The topsequence can be further divided into the marine transgres中国煤化工 is largest in area4.Thissive system tract and highstand system tract( Fig. 2), andthe lowstand system tract lacks. This is a characteristic ofCNMHGnPoSe the transgressiveit compose the highstandsequence development in the late Carboniferous and Early system tractChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESQin 4 wellongitudinal scale020304050m、Qin3wellCongzhoustemStrati-borchole/Transgressive systFig. 5. Sequence framework and distribution of coal beds in the upper Carboniferous and lower Permian, Qinshui Basin.c According to outcrop observation and drilling profiles, early period of sea level fall, water begins to shallow butpal reservoirs are not uniformly distributed in the upper not shallow enough for marsh to develop widely; (3)durCarboniferous and lower Permian. In some intervals, coal ing the late period of sea level fall, water becomes veryreservoirs are concentrated, while in others they are sparse shallow, and silting and filling result in large flat areas(Figs. 2 and 5). Laterally, some coal reservoirs are largewhere marshes widely develop and form coal beds. Thearea. while others are small in area and even lenticularlonger this situation lasts, the thicker and wider the coalThis study shows that the area and vertical differential bed isdistribution of coal reservoirs are controlled by sea level In the late Carboniferous-early Permian, the sea in thefluctuationtudy area belonged to an epeiric sea, and the sea floorIn each shallowing-upward parasequence, coal beds was very flat. Thus the marine transgression and regresoccur at the top(Fig. 2), suggesting that coal beds develop sion were planar. The planar marine regression can formin the late period of sea level fall. Similarly, in each se- broad marshes and depositcoal bed, such as 15quence, coal beds are few, thin and locally distributed in coal(Fig. 5). Similarly, the planar marine transgressionthe marine transgressive system tracts, but they are in can rapidly flood broad areas, and deposit stable limestonelarge numbers, thick and widely distributed in the high- beds. For example, the top rock of 15" coal is very stablestand system tracts, especially in the late highstand system limestone which covers most of the study area( Fig. 5)tracts(Figs. 2 and 5). For example, 15 coal which is the This indicates that the broad marshes are flooded rapidlythickest and largest in area in Sequence I occurs at the top The sea level fluctuation also indirectly controls the re-of the highstand system tract, and is formed in the late servoiring performance. Statistics shows that the broaderhighstand system tract. 3"coal which is the thickest and the coal reservoir is, the lower the ash production rate islargest in area in Sequence Il occurs in the upper part of For example, in the Xishan Mine of Taiyuan City, thethe highstand system tract, and also is formed in the late most stable coal beds(3, 14 15"coalsproduce least ash,highstand system tract(Fig. 5). These two sequences rep- and the ash production rate averages 20%; for the relaresent two large sea level rising-falling cycles in the late tively stable coal beds, the ash production rate averagesCarboniferous and early Permian, which results in two 27%; and for the unstable coal beds, the ash productionlarge coal accumulation cycles In the upper part of each rate av3 A ch mainlu farms from terrestrial finecycle, coal beds are abundant, but are few in the lower clastic中国煤化工 or flooding water.part(Fig. 5). That is to say, the late period of the sea level marsheCNMHG Suspended fine che o?fall is favorable to coal formation. The reasons are as fol- ticsanca UI uIc flashes is. the more com-lows:(I)During the sea level rise, water deepens, which plete the filtering is. Thus the larger the area of marshes isis not favorable to marsh development; (2)during the the lower the ash production rate of coal reservoirs is. The22Chinese Science Bulletin Vol. 50 Supp. December 2005ARTICLESamount of ash directly affects the reservoiring perform- 2. Han Dexin, ed, Coal Petrology of China(in Chinese with Englishance. According to statistics, the lower the ash productionabstract), Xuzhou: China Mining Technology Press, 1996, 462rate is, the higher the porosity of coal reservoirs is(Fig. 6),and the better the reservoiring performance is. Therefore, 3. Shang Guanxiong. Sequence Stratigraphy of the Late Paleozoic ofthe reservoiring performance of coal reservoirs formedNorth China(in Chinese with English abstract), Beijing: Geologi-late highstand system tracts is the best among the coalal Press,1993,1l7-151reservoirs in the sequence.4. Chen Shiyue, Liu Huanjie, Sequence Stratigraphy and Coal Accumulation of the Late Paleozoic of North China (in Chinese with-0.6538x+14.7641800English abstract), Dongying: China Petroleum University Press,R2=0.54482000,18-455. Zhong Rong, Fu Zeming, Relation of marine transgression and re-14gression in the Late Carboniferous-Early Permian with thick coalones in North China Platform, Acta Geologica Sinica(in Chinesewith English abstract), 1998, 72(1): 64-7510006. Fu Xuehai, Qin Yong, Jiang Bo et al, Compression test of cleat andumeric simulation of permeability, Coal Bull. (in Chinese with6.00English abstract), 2001, 26(6): 573-5774.007. Fu Xuehai, Qin Yong, Li Guizhong, Major controlling factors ofpermeability of coal reservoir in central and southern Qinshui Ba-sin, Coalfield Geology and Exploration(in Chinese with English5.00abstract,2001,293):16-198. Jiang Bo, Qin Yong, Fan Binghuan et al., Reservoiring perform-Porosityance of coal reservoirs and coal bed gas exploration in Huaibei area.Fig. 6. The crossplot between ash production rate and porosity of coalChina Mining Technology Press(in Chinese with English abstract),bed in Qinshui Basin.2001,30(5):433-4379. Qin Yong, Fu Xuehai, Ye Jianping et al, Mechanism and charac-4 Conclusionsteristics of control of coal reservoir petrophysical factors in China,Following conclusions are drawn from this studyChina Mining Technology Press(in Chinese with English abstract),(1)Good coal reservoirs are mainly formed in marsh1999,28(1):14-19environments on tidal flats and delta plains. Coal reser- 10. Zhang Hui, Coal porosity genetic types and its study, Coal Bull.(invoirs formed on tidal flats are more continuous laterallythan those on delta plains11. Zhang Jianbo, Wang Hongyan, Zhao Qingbo, eds, China Coal Bed(2)The fluctuation of sea level shows important controlGas Geology(in Chinese), Geological Press, 2000, 89over the vertical and lateral distribution of coal reservoirs. 12. Zhang Yousheng, Qin Yong, Chen Jialiang, Permeability heterogeThe coal reservoirs formed in the highstand system tractseity model of coal reservoirs, China Mining Technology Press(inare more abundant, thicker and more widespread, and thusChinese with English abstract), 1998, 27(1): 43-46the highstand system tracts are more favorable to the for- 13. Harpalani, S, Schraufnagel, R. A, Shrinkage of coal matrix withmation of good coal reservoirs. The coal reservoirs formedrelease of gas and its impact on permeability of coal, Fuel, 1990, 6in the transgressive system tracts are lower in abundancethinner and much less widespread, and thus transgressive 14. Hucka, B.P. Analysis of cleat in Utah coal seams, Utah Geologicalsystem tracts are not favorable to the formation of goodand Mineral Survey Open-file Report 154, 1989coal reservoirs15. Laubach. S. E. Marrett, R. A. Olson, J. E, et al., CharacteristicsAcknowledgements During the study, Professor Tang Dazhen, Pro-and origins of coal cleat a review, Intemal Journal of Coal Geology,fessor Liu Dameng and Dr. Tang Shuhuan from China University ofGeology helped a lot. Professor Tang Xiuyi from Huainan Mining Col- 16. Levine, J.R, Model study of the influence of matrix shrinkage onlege gave constructive suggestions. Here we sincerely thank them forbsolute permeability of coal bed reservoirs, Geological SocietyPublication,1996,(199):197-212.References17. Puri, R, Evanoff, J C, Brugler, M. L, Measurement of coal cleat1. Institute of Coal Science, Sedimentary environments ofporosity and relative permeability characteristics, SPE 21491, 1991coal-bearing strata in Xishan, Taiyuan(in Chinese with English ab-TH中国煤化工stract), Beijing: Coal Industry Press, 1987, 8-203CNMHG, accepted July 29, 2005)Chinese Science Bulletin Vol 50 Supp. December 2005