Shale gas reservoir characterisation_ A typical case in the southern Sichuan Basin of China

Shale gas reservoir characterisation:A typical case in the southern Sichuan Basin of ChinaShangbin Chen a ,b ,*,Yanming Zhu a ,b ,Hongyan Wang c ,Honglin Liu c ,Wei Wei c ,Junhua Fang aaSchool of Resources and Earth Science,China University of Mining and Technology,Xuzhou,Jiangsu 221116,Chinab Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process,the Ministry of Education,China University of Mining and Technology,Xuzhou,Jiangsu 221116,China cLangfang Branch,Petro China Exploration and Development Research Institute,Lang fang 065007,Chinaa r t i c l e i n f oArticle history:Received 27May 2011Received in revised form 31August 2011Accepted 4September 2011Available online 4October 2011Keywords:Shale gasReservoir characterisation Southern Sichuan Basin Chinaa b s t r a c tThe Lower Silurian Longmaxi Formation is an organic-rich (black)mudrock that is widely considered to be a potential shale gas reservoir in the southern Sichuan Basin (the Yangtze plate)in Southwest China.A case study is presented to characterise the shale gas reservoir using a work flow to evaluate its char-acteristics.A typical characterisation of a gas shale reservoir was determined using basset sample analysis (geochemical,petrographical,mineralogical,and petrophysical)through a series of tests.The results show that the Lower Silurian Longmaxi Formation shale reservoir is characterised by organic geochemistry and mineralogical,petrophysical and gas adsorption.Analysis of the data demonstrates that the reservoir properties of the rock in this region are rich and that the bottom group of the Longmaxi Formation has the greatest potential for gas production due to higher thermal maturity,total organic carbon (TOC)enrichment,better porosity and improved fracture potential.These results will provide a basis for further evaluation of the hydrocarbon potential of the Longmaxi Formation shale in the Sichuan Basin and for identifying areas with exploration potential.Ó2011Elsevier Ltd.All rights reserved.1.IntroductionWith the severe energy shortage and high energy prices,shale gas has recently been the focus of exploration in many countries (Canada,Australia,Europe and other countries)[1],especially in China.It is also an effective way to cope with the climate change and to promote China ’s economic growth and the energy security [2,3].The global distribution of shale gas is not uniform.China is known as one of countries that is relatively rich in shale gas resources [4].The recoverable shale gas resources are predicated to be approximately 26Â1012m 3in China,which is close to the 28Â1012m 3in the USA [5].To date,the exploration and devel-opment of shale gas reservoirs has lead to success in production in the USA [6,7].This success has resulted through a combination of scienti fic study,engineering innovation,new technology and,in some cases,persistence and risk taking.The related concepts and technologies are now mature,and lessons can be drawn from the experience of similar basins worldwide.However,basin (e.g.,Texas)conditions are ideal and probably unrepeatable,and it is dif ficult to arbitrarily apply a single genetic model of productive shale gas in the USA to China.Consequently,a peculiar exploration activity is required to ascertain the viability of subsequent devel-opment projects.The development of shale gas has depended on a combination of geological,geochemical,and engineering studies [6].Geological and geochemical evaluations are the most basic tasks in the exploration and development of shale gas reservoirs projects.Geological analysis has been identi fied and,to a certain extent,has characterised the reservoir portions of shale.Geochemical data have proven essential for explaining shale potential and observed patterns of productivity.Therefore,the Longmaxi Formation,in the southern Sichuan Basin,was analysed in this study as a case of shale gas reservoir characterisation because any high quality source rock should produce high quality gas shale in theory,which often shows stratigraphic continuity and a relatively simple structural architecture.The Lower Silurian Longmaxi Formation in the southern Sichuan Basin has been studied as a source rock for many years [8e 11].This area has become a potential hotspot as a shale gas system and has been studied by several researchers in recent years [12e 16].Nevertheless,there are few authentic wells drilled for character-ising the Silurian Longmaxi shale in detail.Changxin-1is a shallow-*Corresponding author.School of Resources and Earth Science,China University of Mining and Technology,Xuzhou,Jiangsu 221116,China.Tel.:þ8613585390796;fax:þ86051683590998.E-mail address:shangbinchen@ (S.B.Chen).Contents lists available at SciVerse ScienceDirectEnergyjournal h omepage:w/locate/energy0360-5442/$e see front matter Ó2011Elsevier Ltd.All rights reserved.doi:10.1016/j.energy.2011.09.001Energy 36(2011)6609e 6616bored well completed in 2009that was the first shale gas well in China.The well is located at Shuanghe town in Changning county,which is within in the study area.China has now entered a new basic research and exploration-development stage [17,18],and some research and exploration works focussing on shale gas reservoir conditions and favourable area evaluation have been conducted in recent years.A robust characterisation of a gas shale reservoir basically starts from geochemical,petrographical,mineralogical and petrophysical analysis.The total organic carbon (TOC)content/richness,thermal maturity,gas content,and litho-logic (reservoir quality)data (mineral matter,porosity,perme-ability and thickness)are also key parameters [6,7,19].However,special wells (or production history)to assess the reservoir are still limited.Therefore,this study investigated the reservoir based on collected basset samples.2.Regional frameworkThe Changning-Xingwen region investigated in this study is located at the margin of the southern Sichuan Basin.The Sichuan Basin,located in the west of the Yangtze metaplatform tectoni-cally,is a large tectonically stable and old oil-gas-bearing super-imposed basin encompassing approximately 180thousand square kilometres in Southwest China.The strata is South China-type stratigraphic,with a complete regional sedimentary rock exposed from a Presinian system to a Quaternary system,whose sedimentary cover thickness is approximately 6000e 12,000m from the Paleozoic to Cenozoic.The total residual thickness of the Silurian layer is approximately 0e 1200m in the southern Sichuan Basin.The Sichuan Basin is a proli fic hydrocarbon region and is currently China ’s largest gas-producing region.This basin has been subject to petroleum and gas exploration for more than 50years,and several oil and gas fields have been found [20,21].Great breakthroughs have been made in oil and gas exploration in recent years [22].There are 106gas fields and 14oil fields with proven gas reserves totalling 840billion cubic metres (bcm)and an annual gas and crude production reaching 12bcm and 145thousand tons,respectively,in the basin [20].These huge reserves are a conse-quence of six different source rocks,ranging from the Cambrian and Silurian through the Permian to the Triassic:the Lower Cambrian (marine shale),Lower Silurian (marine shale),Lower Permian (marine carbonate),Upper Permian (coal measures),Lower Jurassic (lacustrine mudstone)and Upper Triassic (lacustrine and coal-bearing mudstone),which can be summarised as “four Lowers and two Uppers ”.Dark pelitic rock and argillaceous limestone constitute the “four Lowers ”,and black pelitic rock and coal rock constitute the “two Uppers ”.The former four sets are mainly source rocks in the basin [20,23,24].Of the four formations,the Lower Silurian marine shale (Longmaxi Formation)is more widespread (Fig.1).The Longmaxi Formation (shale)distribution is determined based on the depositional environment and the subsequent erosional events related to the tectonic history.A generalised Paleozoic stratigraphy for the southern Sichuan Basin is shown in Fig.2.An analysis shows that the Longmaxi Formation is formed in a neritic shelf with a biased,stagnantFig.1.Isopach map of the Longmaxi Formation in the southern Sichuan Basin (modi fied from [16]).S.B.Chen et al./Energy 36(2011)6609e 66166610marine sedimentary environment.The shale gas reservoir charac-terised in this study is at the bottom group,called the Longmaxi black shale,which is the most organically rich part of the Lower Silurian.The Longmaxi Formation is present with a range of 229.2e 672.5m in thickness in the southern Sichuan Basin.The lateral extent and thickness of the Longmaxi Formation (strati-graphic)is stable in the Changning-Xingwen area,and the thick-ness ranges from 200to 300m on the basis of field investigation [16,25].The stratigraphy consists of black shale,black and dark/grey shale,dark grey shale,and silty mudstone,mainly comprising carbonaceous and clay shale.The bottom of the Longmaxi Forma-tion is rich in carbonaceous and various graptolites,and it has widely distributed dispersions and berry-like grains of pyrite,even if obvious strati fication is spread on part of the layer.3.Materials and methods3.1.Samples and experimental protocolThere are a few wells that had been drilled previously to discover conventional hydrocarbon-bearing (sandstone)layers in the Silurian and Ordovician as the source rock,but not with the speci fic aim of characterising this unconventional shale gas reser-voir.Therefore,it is dif ficult to collect cores to better de fine the shale gas reservoir through experiments.This study,then,relied on basset samples instead of cores.The fresh basset samples arerepresentative of a great extent in the main zone of interest.The samples are necessary just to describe the unconventional reser-voirs in terms of mineralogical,geochemical and petrophysical characteristics.A relatively complete experimental program was conducted and is listed in Table 1.3.2.Experimental analysis descriptionTo evaluate this unconventional hydrocarbon system,it is important to understand the various organic and inorganic geochemical processes controlling the generation,storage and access to the trapped gas [19].Organic geochemistry is essential to de fine the amount of gas generated by the shale system.ThekeyFig.2.Paleozoic stratigraphic column of the Sichuan Basin.Table 1Experimental program.Test projectsLaboratoryMineralogy:X-ray diffraction Key Laboratory of Coalbed MethaneResources and Reservoir Formation Process,the Ministry of Education,China University of Mining and TechnologyOrganic thermal maturity Mercury (Hg)intrusion tests Methane adsorption testsPetrophysical analyses (Porosity)Organic geochemistry (TOC)Petroleum Geology Experimental Center of Research Institute of Petroleum Exploration and Development,Zhongyuan Oil field BranchS.B.Chen et al./Energy 36(2011)6609e 66166611factors affecting the total volume of hydrocarbons are the original TOC and the kerogen type.In this study,the geochemical param-eters include the determination of organic matter richness (present-day TOC),a description of the organic matter type (kerogen optical analyses-maceral composition),and determina-tion of the thermal maturity(kerogen optical analyses-vitrinite reflectance).The type of organic matter can be determined visually by means of transmitted light microscopy.The approach for the maturity determination was optical.Vitrinite reflectance is the most common optical method,and it is performed through a microscopic inspection of kerogen and an analysis of the reflec-tivity of the particles via a photomultiplier.The mineralogical composition of the bulk powder samples and the clay fraction was determined using X-Ray Diffraction(XRD).The bulk mineralogy provides a quantitative estimate of the crystalline components that are present in the shale and is related both to the petrophysical and the friability properties of the rock itself.The methodology is unable to reveal the non-crystalline components. The TOC is obtained from the geochemical analyses,and the percentages of minerals are corrected to approach100%consid-ering the TOC.To evaluate this unconventional hydrocarbon system,it is important to understand its porosity.A quantity of200e300g of gas shale material was necessary for this step.The samples were weighed and the bulk volume was measured by mercury immer-sion.The bulk density was then calculated.The sample was then crushed,and the grain volume and grain density were measured.A sorption analysis of gas shale is a critical element of reservoir analysis.The volume of gas that can be stored in the sorbed state is a function of pressure and temperature.Because shale reservoirs have microporous materials that have a small external surface area,such as organic matter in mudrocks,the Type I(Langmuir) adsorption isotherm is adaptive.The tests were conducted at the reservoir temperature and pressures.The parameter of the Lang-muir isotherm was calculated byfitting the experimental data. The Langmuir isotherm[26]can be written as:V E¼V L P/(P LþP), where V E is the volume of absorbed gas per unit volume of the reservoir in equilibrium at pressure P,V L is the Langmuir volume (based on monolayer adsorption),which is the maximum sorption (storage)capacity of the sample(absorbent)at infinite pressure,P is the gas pressure,and P L is the Langmuir pressure,which is the pressure at which the total volume absorbed(storage capacity) (V E)is equal to one half of the Langmuir volume.The gas absorption tests were conducted on samples with different amounts of TOC.4.Results and discussion4.1.Geochemical characterisationA geochemical analysis was performed on basset samples rep-resenting approximately270m of the Lower Silurian Longmaxi Formation,especially the bottom group.A relatively complete geochemical characterisation of the Longmaxi Formation has been revealed.anic carbon contentThe TOC contents for the75samples measured in this study range between0.29and5.35%for the entire formation with a mean value of1.96%.Twenty-eight values among all samples are greater than2.0%,with mean value of3.38%,which are mainly distributed at the bottom of Longmaxi Formation.Thefirst shallow well (Changxin-1)has cored a153-m-thick black shale at the bottom of the Longmaxi Formation,including a10-m-thick carbonaceous shale of the Upper Ordovician Wufeng Formation.The mean TOC of the upper110m is2%,whereas the mean TOC of110e153m is6% [27].The accumulated thickness with a TOC much more than2%is 80m,which is similar to our test results.The values increase with burial depth especially in the bottom 50m(Fig.3).A comprehensive analysis indicates that the TOC values increase from the top to bottom of the Longmaxi Formation, and the thickness is50m,at least,with2.0%TOC values vertically. The TOC value of the top layer is relatively poor.The TOC must be the loss amount in the basset samples for weathering.Ma et al.[28]conducted substantial experimental research in both well samples and Earth surface(basset)samples from Jianghan Basin in the Lower Yangtze region and the Guizhou-Guangxi area.They concluded that the loss ranged from50%to80%. According to that result,the adjusted TOC values ranged from0.44 to8.03%(mean value3.93%)by selecting a minimum of50%as the adjusted factor.Thus,the Longmaxi Formation,with its high organic carbon content,is favourable for generating shale gas.anic matter typeThe maceral and the carbon isotope of kerogen are favourable classification parameters for source rock[29].According to these methods,the organic matter was classified as four types:sapropelic (I),humic-sapropelic(II1),sapropelic-humic(II2)and humic(III) (Table2).Some studies of the organic matter type of the Longmaxi Formation have been conducted in this study area and adjacent areas.The Kerogen d13C content of the Longmaxi Formation in the southern Sichuan Basin has been reported by many scholars:À30& [26],À29.67&andÀ30.51&,respectively,in the Jianshen-1well [30];and betweenÀ29.3&andÀ29.8&with a mean value ofÀ29.6&[16]in Changning.In the southern Sichuan Basin and northern part of the Guizhou province,the Kerogen d13C content of the Longmaxi Formation ranges fromÀ28.7&toÀ30.4&,with a mean value ofÀ29.4&[31].From this perspective,the organic matter type is I.Furthermore,although there are differences between the maceral,other studies have also demonstrated the organic matter type is I[29,32,33].The source material of the organic matter was composed mainly of varieties of algae(major component),zooplankton,and fungi,forming a mostly amorphous organic maceral[26,31,33].Thus,the comprehensive analysis through pre-existing studies shows that the organic matter type of the Lower Silurian Longmaxi Formation is sapropelic(I),which hasa strong generation capability.4.1.3.Thermal maturityThe thermal maturity was measured in29samples through a microscopic inspection(by reflected light microscopy)in this study. The parameter is vitrinite-like material reflectance(R om),also called marine vitrinite,which is derived in the marine sediment below the Permian strata.The vitrinite-like material reflectance can be the Early Paleozoic maturity index[34].Zhong et al.[35]established the relationship between the vitrinite-like material reflectance(R om)and the equivalent vitrinite reflectance(R o).The relationship between R om and R o is shown in the following equations:R o¼1.042R omþ0.052(0.30%<R om<1.40%),R o¼4.162R omÀ4.327(1.40%R om<1.60%),R o¼2.092R omÀ1.079(1.60%<R om<3.0%).Using the above arithmetic relationship,the vitrinite-like material reflectance(R om)has been transferred into equivalent vitrinite reflectance(R o),which was used for the reservoir evalua-tion.The equivalent vitrinite reflectance values were determinedS.B.Chen et al./Energy36(2011)6609e6616 6612between 1.85%and 3.3%with mean value of 2.69%,which showed that the thermal maturity was at the (high)over-mature stage (R o >2.0%).The thermal maturity parameters suggest that the Longmaxi Formation is in the (dry)gas generation window in the study area.In the over-mature thermal stage,the source rock mainly generates dry gas based on the principle of methane accompanied with a small amount of gas condensate.Research [36]has shown that large quantities of gas may still potentially be generated,primarily from the secondary cracking of in-situ oils in the thermally over-mature areas.Similar to the Bar-nett Shale,the principal source of gas in the proli fic Newark East Field is considered to be the secondary cracking of oil and bitumen [7,37].Consequently,the Longmaxi Formation has the greatest potential for gas production due to the higher thermal maturity in the area.4.2.Mineralogical characterisation of the Longmaxi Formation The mineralogical analyses were performed on the same samples utilised for the TOC determination.The gas shale interval ischaracterised by a marked heterogeneity over short distances.The 270-m-thick interval can be subdivided into multiple layers.This paper employed the X-ray diffraction technique to qualitatively and quantitatively analyse 39samples collected from the southern Sichuan Basin (Changning-Xingwen area),in the hope of finding certain features of the Longmaxi shale mineral composition.The results show that the Longmaxi Formation has an extremely complex mineral composition and that clay,quartz and calcite are the main mineral compositions of the Longmaxi Formation,withTable 2Organic matter type classi fied by the maceral and the carbon isotope of kerogen [29].Organic matter type Carbon isotopeof kerogen(d 13C)(&)Maceral of kerogen (TI value)Sapropelic type (I)<À29>80Humic-sapropelic type (II 1)À29e À2740e 80Sapropelic-humic type (II 2)À27e À250e 40Humic type (III)>e 25<Fig.3.Total organic carbon distribution of the Longmaxi Formation.The TOC values are not adjusted.S.B.Chen et al./Energy 36(2011)6609e 66166613a mean content of53.39%(range16.8%e70.10%),29.15%(range16.2%e75.2%)and 5.46%(range0.2%e19.3%),respectively.The other mineral compositions are feldspar,dolomite,gypsum,pyrite, siderite,chlorite,halloysite and traces of undistinguished amor-phous substances,of which the average content was less than5%.It is suggested that all samples contain clay minerals composed of illite,kaolinite,illite and a montmorillonite mixed layer.Illite is the most common of the clay minerals,and it has the highest content, ranging from10.3%to41.3%with a mean value of24.49%.The samples also contain kaolinite,illite,illite and a montmorillonite mixed layer with ranges of1.1%e18.1%(11.11%),2.5%e10.4%(3.92%) and3.92%(2.5%e10.4%),respectively.Clay minerals were also favourable for the formation and development of the shale gas reservoir.The bottom of the Long-maxi Formation,especially,which had a low content of illite and chlorite and high porosity,is an important horizon for exploration and development.Because a reduced sedimentary environment favours organic matter enrichment and preservation,it provides good deposition conditions for shale gas.In addition,pyrite,in the form of diagenetic crystals,was quite abundant,and pyrite abun-dance is closely related to the presence of organic matter.The formation is a lithologically complex interval of low permeability that requires artificial stimulation to produce,which is closely correlated with the brittle mineral content.The thickness of the bottom group of the Longmaxi Formation is more than30m, and the quartz(brittle mineral)content is more than50%.Therefore,the bottom group of the Longmaxi Formation is the ideal horizon for the exploration and development of the shale gas.The mineral composition of a shale gas reservoir has important effects on the shale gas,such as its adsorption and storage,fracture eval-uation,seepageflow migration,and fracture-made seams.The overall composition is similar to that of the Barnett shales[37],witha lower quartz content.4.3.Petrophysical characterisation(porosity)The integration of total porosity data(free gas potential)with sorbed gas capacities provides a measure of the maximum poten-tial gas capacity,which is used to determine the economic feasi-bility of the shale gas reservoir[7].A mercury porosimetry analysis revealed that total the porosity ranges between1.71%and12.75% for the16samples(Table3)with a mean value of4.3%.In general,compaction reduces mudrock porosity with greater burial depth and its associated higher temperatures and subse-quent diagenesis[38].However,the porosity increases with increasing burial depth(Fig.4),which must be related to the mineral composition of the samples.The porosity has a significant positive correlation with the brittle mineral(including quartz and calcite)content(R2¼0.82,p<0.01,Fig.5)and a significant negative correlation with the clay mineral content(R2¼0.81, p<0.01,Fig.6).This correlation indicates that the brittle minerals are primarily responsible for the porosity of the mudrocks and shales.The organic matter secondarily contributes to the porosity because the microporosity associated with the organic fraction could not be measured(<2nm diameter),as the smallest pore diameter measurable using Hg porosimetry is3nm.Table3Skeletal density,bulk density and total porosity of the Longmaxi Formation (measured by Hg porosimetry).Sample#Basset location/depth(m)TOC(%)Skeletaldensity(g/cm3)Bulkdensity(g/cm3)Porosity(%)sx98400.54 2.2342 2.1386 4.2806 sh1382 1.07 2.2272 2.1435 3.7583 sh0894 1.18 2.2082 2.1636 2.023 sh0599 1.11 2.2160 2.1745 1.8753 sh03108 1.28 2.2134 2.1378 3.4134 sx60142 1.17 2.1902 2.1528 1.7116 sx58145 1.21 2.2142 2.1522 2.8007 sx56147 1.37 2.1895 2.1244 2.9717 sx48158 2.33 2.1399 2.1023 1.758 sx47162 2.35 2.0414 1.9645 3.7666 sx44165 2.46 2.1683 2.1078 2.7892 sx36170 4.67 2.0107 1.9105 4.9798 sx22177 4.15 1.9900 1.81648.7228 sx13184 4.39 2.1049 1.836512.751 sx07188 3.82 1.9542 1.81577.0906 sx01197 5.35 1.8711 1.69029.6644Fig.4.The porosity of the16samples with burialdepth.Fig.5.Correlation between porosity and brittle mineral(quartz and calcite)content.Fig.6.Correlation between porosity and clay mineral content.S.B.Chen et al./Energy36(2011)6609e6616 66144.4.Gas sorption capacityTo estimate the gas sorption capacity in situ,this study con-ducted a methane adsorption experiment from10different depth samples.The TOC of the10samples ranged from1.07%to4.67%. Methane adsorption was measured at equilibrium pressure at approximately7MPa and under reservoir temperature(30 C)at moisture equilibration.The results show that the high-pressure methane adsorption capacities(the adsorbed gas amount)for the moisture-equilibrated Longmaxi shale samples varied from0.42cm3/g in the organic-poor sample(sample sx58,TOC1.21%)to1.13cm3/g for the organic-rich sample(sample sx36,TOC4.67%) (Table4).The Longmaxi had lower sorbed gas capacities,with a mean value of0.637cm3/g.The methane isotherms showed a Langmuir-type isotherm up to8MPa.From the isotherms,the gas sorption increased rapidly at relatively low pressures,whereas the sorption sites were continuouslyfilled.The steep initial slope of the isotherm was caused by the overlapping adsorption potential between the pore walls,with the adsorbate gas molecule diameter only slightly smaller than the pores[39].The isotherm features suggest the pores were accessible at lower pressures,although micropores are the majority in Longmaxi.The analysis also indicates that the sorption capacity increases from the top to the bottom of the Longmaxi Formation,with a maximum value of1.13cm3/g(Fig.7).It is suggested that the reservoir has a more promising sorption capacity at the bottom of the Longmaxi Formation.Fig.8shows a positive correlation(linear relationship)between the TOC concentrations and the Langmuir volume,which represents the volume of adsorbed gas capacities (R2¼0.58,p<0.05).The results indicate that the organic matter was in part responsible for adsorbing the gas,a conclusion similar to that obtained by Ross and Bustin[7].The greater sorbed gas capacities of the moisture-equilibrated samples with increasing TOC are due to micropores associated with the organic fraction onto which the gas can adsorb.(the shale porosity of microporous materials is classified according to size using the IUPAC classifica-tion:micropores(<2nm),mesopores(2e50nm)and macropores (>50nm)).5.Conclusions and future developmentsThe organic matter type of the Lower Silurian Longmaxi Formation is sapropelic(I),which has strong generation potential.A comprehensive analysis indicates that the TOC values increase from the top to the bottom of the Longmaxi Formation,and there is a50-m-thick layer with high organic carbon(TOC values of2%),which favour the generation of shale gas.The Longmaxi Formation has the greatest potential for gas production due to the higher thermal maturity in the region.Therefore,the Longmaxi Formation(espe-cially the lower bench)has good shale gas potential in the southern Sichuan Basin due to its high organic content,sapropelic quality and high thermal maturity.The mineral composition of the shale gas reservoir has impor-tant effects on the storage and development of shale gas.A more than30-m thickness at the bottom of the Longmaxi Formation has a quartz content of more than50%,which is an ideal geologic horizon for the exploration and development of shale gas.The porosity increases with the burial depth.The porosity has a signif-icant positive correlation with the brittle mineral content and a significant negative correlation with the clay mineral content. Analysis also indicates that the sorption capacity of the shale gas reservoir is strong at the bottom part of the Longmaxi Formation. Consequently,the bottom group of the Longmaxi Formation is the ideal horizon for the exploration and development of shale gas in the early stage.Now that an integrated methodology to characterise a shale gas (unconventional)reservoir has been settled and tested,the work-flow can be employed at other regions to produce an assessment of the potential in an area of interest characterised by a substantial vertical and lateral heterogeneity.Shale gas represents a potentially enormous amount of unconventional gas resources in China.It is suggested that research efforts should be enhanced and strategic surveys and optimisation should be conducted as soon as possible. More wells should be drilled to acquire more accurate and detailed data of the reservoir.AcknowledgementsThe authors would like to sincerely thank various organisations for the continuous supply of funds;this work was jointly supportedTable4Sorbed gas capacities,TOC,and equilibrium moisture contents of Longmaxi Formation samples taken from the study area,Langmuir volumes determined at 7MPa.Sample#TOC(%)Moisture(%)Langmuir volume(cm3/g)sh13 1.07 1.790.47sh08 1.18 1.960.51sh05 1.11 2.170.43sh03 1.28 1.750.84sx58 1.21 1.810.42sx56 1.37 1.930.52sx48 2.33 2.450.95sx47 2.35 2.080.52sx44 2.46 2.020.58sx36 4.67 3.871.13Fig.7.Sorption capacity variation of the Longmaxi Formation invertical.Fig.8.Correlation between sorption capacity and TOC content.S.B.Chen et al./Energy36(2011)6609e66166615。