Geology of coalbed methane rervoirs in the Southeast Qinshui
Basin of China
Xianbo Su a,T ,Xiaoying Lin a ,Shaobo Liu b ,Mengjun Zhao b ,Yan Song b
a
Henan Polytechnic University,Jiaozuo,Henan 454000,China
donna summerb
Rearch Institute of Petroleum Exploration and Development,PetroChina,Beijing 10083,China
Received 10September 2004;received in revid form 20January 2005;accepted 30January 2005
Available online 8April 2005
学龄前Abstract
The No.15coal am of the Pennsylvanian Taiyuan Formation and the No.3coal am of the Permian S
hanxi Formation in the Southeast Qinshui Basin are relatively thick (N 3m),laterally continuous,and are of anthracite rank (R o =2.2–4.5%).The depth of the coal ams is generally between 200and 700m.A tectonic-thermal event during the Jurassic to Cretaceous Yanshanian Orogeny controlled coalification and thermogenic hydrocarbon generation.Becau the generation of late-stage bacterial gas,coalbed methane in the Southeast Qinshui Basin occupies two zones,which are the mixed thermogenic and late-stage bacterial gas zone near the basin margins,and the dominantly thermogenic gas zone in the structurally deep part of the basin.Basin hydrodynamics caus low gas content in the shallow subsurface (b 20m 3/t)and high gas content at the deep part of basin (N 20m 3/t).Becau cleats are rare and are mostly filled with calcite,the permeability of the coalbed methane rervoir is controlled by shear fractures.The recharge and flow of groundwater not only control gas content,but also control rervoir pressure.Abnormally high pressure rervoir in the deep stagnant zone is a favorable area for coalbed methane exploration and development.The current coalbed methane wells are generally completed in the deep stagnant zone.D 2005Elvier B.V .All rights rerved.
Keywords:Coalbed methane;The Southeast Qinshui Basin;Hydrodynamics;Gas generation;Coalification
1.Introduction
During the last three decades,the characteristics of coalbed methane rervoirs were investigated from six
mputs of variables:tectonics and structure,stratigraphy and dimentology,coal rank,permeability,gas content,and hydrodynamics by many rearchers (Pashin et al.,1991;Pashin,1998;Ayers and Kair,1994;Kair et al.,1994;Ayers,2002;Liu et al.,1998;Zhang et al.,2000;Scott,2002).
gmail是什么The coalbed methane resource is extremely large (3.28Â1012m 3)in the Qinshui Basin of Shanxi province,China.Becau of this large resource,the
0166-5162/$-e front matter D 2005Elvier B.V .All rights rerved.doi:10.al.2005.01.002电气工程专业排名
T Corresponding author.Tel.:+863913987981;fax:+863913987961.
E-mail address:suxianbo@263 (X.Su).International Journal of Coal Geology 62(2005)197–
210
/locate/ijcoalgeo
Southeast Qinshui Basin is a favorable district for coalbed methane exploration and development.More than70coalbed methane wells have been drilled by China United Coalbed Methane Co.Ltd.and China National Petroleum Corporation in this district.The highest coalbed methane production is16,000m3/d per well,and the average is2000–4000m3/d per well. Exploration confirms that coalbed methane rervoirs with high permeability exist in the Southeast Qinshiu Basin,which is the first basin to be developed commercially in China.
The data obtained from the exploration and development of petroleum,coal,and coalbed meth-ane,form the foundation for this investigation,many investigations have been published on the geology of coalbed methane rervoirs in the Qinshui Basin (Chen,1998;Shan,1998;Sun et al.,1998;Wang et al.,1998;2001;Li et al.,2000;Hu et al.,2001;Wei et al.,2002;Ye et al.,2002,Qin et al.,1997;Su and Zhang,2002).The investigations treat specific aspects of coalbed methane potential,and few broad, multidisciplinary investigations have been conducted in this area.The geological characteristics of coalbed methane rervoirs in the Southeast Qinshui Basin are discuss
ed comprehensively in this paper.It is hoped that the investigation will be a guide to the further exploration and development of coalbed methane in South East Asian coal basins.2.Results and analys
bold是什么意思2.1.Tectonic and structural tting
In the Shanxi Province,the Qinshui Basin is bounded by the uplifts of Taihang Mountain,Huo Mountain,Wutai Mountain,and Zhongtiao Moun-tain.It is one of the Mesozoic basins evolved from the Late Paleozoic North China Craton Basin.Uplift and erosion during the Triassic Indosinian Orogeny and the Jurassic to Cretaceous Yanshanian Orogeny parated the Qinshui Basin from the North China Craton Basin.The Qinshui Basin is a complex syncline striking NNE–SSW. The study area in this paper is the Southeast Qinshui Basin,which is bounded by a ground-water divide at north latitude368in the north,the Sitou normal fault in the west,and coal outcrop in the east and south,and covers660km2(Fig.
weed1).The Southeast Qinshui Basin is the south-eastern limb of the syncline.The dip of the coal-bearing strata is about58NW.Structure within the study area is relatively simple;faults are few,and folds with axial traces striking NNE–SSW and near N–S are common(Fig.1).Previous inves-tigations have sho
wn that the folds in Southeast Qinshui Basin were due to NW–SE compressional stress during the Jurassic–Cretaceous
Yanshanian Fig.1.Location map of the Southeast Qinshui Basin and structural contour map of the ba of the Pennsylvanian Taiyuan Formation.
X.Su et al./International Journal of Coal Geology62(2005)197–210
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乐知英语Orogeny(Liu et al.,1998;Qin et al.,2001a;Ye et al.,2002).Coalbed methane is being developed in the folds.
2.2.Depositional tting and coal distribution
The North China Craton Basin was eroded from Silurian to Mississippian time but subsided and received diment from Pennsylvanian to Triassic time.The basin was filled with the Pennsylvanian Benxi and Taiyuan Formations,the Permian Shanxi, Xiashihezi,Shangshihezi and Shiqianfeng Forma-tions,and Triassic deposits.The Taiyuan and Shanxi Formations are averagely150m thick and are the main coal-bearing units.Total coal thickness in the formations is about15m(Fig.2).
numero
The Taiyuan Formation is50–135m thick(gen-erally less than80m)and consists of limestone, sandstone,siltstone,mudstone,and5–10coal ams. The No.15coal is the main economic coal am
and has a thickness of1–6m(Fig.3).The Shanxi Formation is20–40m thick and consists of siltstone, mudstone,and three coal ams.The No.3am is the main economic coal am with a thickness of4–7 m(Fig.4).The depth of the No.15coal am is less than700m in most areas(Fig.5),and that of No.3 coal am is20–50m shallower than that of the No. 15am(Wu,2003
).
The paleoenvironment prior to the accumulation of No.15peat swamp during the Pennsylvanian in the Southeast Qinshui Basin was a system of barrier islands,lagoons,and tidal flats (Cheng,1992;Liang et al.,2002).There are two barrier island deposits distributed in the northeast and southwest parts of the Qinshui basin,which suggest that there were two source areas on opposite sides of the basin.A marine transgression originated in the southeast part of the basin.The transgression resulted in the termination of No.15peat swamp and the development of a widespread carbonate platform.As a result,the roof of No.15coal am is mainly limestone.
In comparison to the Taiyuan Formation,the supply of terrigenous diments was more abundant when the Shanxi Formation was deposited.Before the accumulation of the No.3peat swamp,the deposi-tional environment was a deltaic plain consisting of distributary channels and flood basins (Cheng,1992;Li et al.,2001;Liang et al.,2002).The environment新航道英语培训
after the accumulation of the No.3peat swamp was generally similar to that before the accumulation of the No.3peat swamp.The thinning and pinch-outs of the No.3coal am are due to the scouring of distributary channels in the deltaic plain.2.3.Coal rank and origin of coalbed methane The Southeast Qinshui Basin contains mi-anthra-cite and
anthracite coal with vitrinite reflectance between 2.2and 4.5%(Fig.6).The distribution of coal rank is controlled by the burial history and thermal history of the basin.The Carboniferous–Permian coal-bearing quence reached maximum burial depth in the Late Triassic,when the first coalification event took place with a normal geo-thermal gradient of 2–38C/100m,and maximum virtrinite reflectance reached 1.2%(Chen,1997;Ren et al.,1999;Ren,2000;Zeng et al.,1999).Hydro-carbon generation reached its first peak in this stage.
Fig.3.Isopach map of the No.15coal am in the Southeast Qinshui Basin.
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The Carboniferous–Permian coal-bearing quence was continuously uplifted during the Jurassic.Although the basin subsided briefly during the Middle Jurassic,the coal ams did not reach the significant burial depth,and
coalification and hydrocarbon generation were not enhanced.
From the Late Jurassic to the end of the Cretaceous period,a tectonic-thermal event during the Yansha-nian Orogeny caud an abnormal geothermal gra-dient in the Qinshui Basin,and the geothermal gradient incread to 4–68C/100m (Chen,1997;Ren et al.,1999;Ren,2000;Li et al.,2000;Liu et al.,1998;Zeng et al.,1999;Ren and Zhao,1997;Qin et al.,1997).This tectonic-thermal event is associated with positive magnetic anomalies in the southeast part of the district (Fig.6).The positive magnetic anomalies indicate that igneous intrusions exist below the deep part of the basin (Guan et al.,2001).Though the basin was slowly uplifted during this pha,the tectonic-thermal event caud a cond episode of coalification and hydrocarbon generation.The current coal rank distribution was mainly caud by the cond event.Since the Tertiary period,the paleo-geothermal gradient has decread to a normal value of 2–38C/100m (Li et al.,2000;Liu et al.,1998;Qin et al.,1997).The Southeast Qinshui Basin has been uplifted and eroded since the Tertiary and coalification has cead.
Coalbed methane in the Southeast Qinshui Basin is thermogenic gas and late-stage bacterial gas.The y 13C value of methane is À26.6x to À36.7x and gradually become heavy with the increasing of the burial depth (Sang et al.,1997;Zhang and Tao,2000;Hu et al.,2001).Bad on Whiticar (1990)plot (Fig.7),this was dominantly due to the isotopic fractiona-tion during the process o
f desorption,diffusion and migration.Actually,coal rank and the late-stage bacterial genetic gas are the main factors controlling
Fig.4.Isopach map of the No.3coal am in the Southeast Qinshui Basin.
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