Fracture development in shale and its relationship to gas accumulation

GEOSCIENCE FRONTIERS 3(1)(2012)97-105availableatwww.sciencedirect.comGEOSCIENCEFRONTERSChina University of Geosciences(Beijing)GEOSCIENCE FRONTIERSGEOSCIENCEFRONTIERSjournalhomepagewww.elsevier.com/locate/gsfORIGINAL ARTICLEfracture development in shale and its relationship to gasaccumulationWenlong Ding ", Chao Li, Chunyan Li, Changchun Xu, Kai Jiu, Weite Zeng,Liming WuSchool of Energy, China University of Geosciences, Beijing 100083, ChinaReceived 20 October 2010: accepted I September 2011Available online 9 November 2011KEYWORDSAbstract Shale with high quartz, feldspar and carbonate, will have low Poisson's ratio, high Young'sShale.modulus and high brittleness. As a result, the shale is conducive to produce natural and induced fracturesFractureunder extermal forces. In general, there is a good correlation between fracture development in shale andDominant factor.the volume of brittle minerals present. Shale with high ToC or abnormally high pressure hasGas accumulation:well-developed fractures. Shale fracture development also shows a positive correlation with total gasGas productionaccumulation and free gas volume, i.e., the better shale fractures are developed, the greater the gasaccumulation and therefore the higher the gas production. Fractures provide migration conduits andaccumulation spaces for natural gas and formation water, which are favorable for the volumetric increaseof free natural gas. Wider fractures in shale result in gas loss. In North America, there is a high successratio of shale gas exploration and high gas production from high-angle fracture zones in shale. Goodnatural gas shows or low yield producers in the Lower Paleozoic marine organic matter-rich rocks inthe Sichuan Basin are closely related to the degree of fracture development in brittle shales.C 2011, China University of Geosciences(Beijing) and Peking University. Production and hosting byElsevier B V. All rights reserved.1. IntroductionCorresponding author. Tel: +86 10 82320629.E-mailaddress:dingwenlong2006@126.com(wDing)Recent successful exploration for marine shale gas in North1674-9871@ 2011, China University of Geosciences( Beijing)and PekingAmerica(Hill and Nelson, 2000: Curtis, 2002 Warlick, 2006 LiUniversity. Production and hosting by Elsevier B V. All rights reservedXJ, et al, 2009; Nie et al. 2009b: Tan, 2009), found that lowporosity and permeability shale, rich in organic matter, withPeer-review under responsibility of China University of Geosciences sufficient fractures or a significant fracture systems formed bymicro-fractures and nano-pores/fissures may be an effectivedoi:l0.10161sf2011.l0.001natural gas reservoir (Sun et al., 2008). Natural fracture development will affect not only the recovery potential of a shale gasreservoir, but also determine the quality of shale gas reservoir andProduction and hosting by Elseviergas production (Montgomery et al., 2005: Bowker, 2007; Nieet al., 2009b). Fracture development contributes to the volu-FLSFVIERmetric increase of free中国煤化工 ptve gasCNMHG98W. Ding et aL. Geoscience Frontiers 3(1)(2012)97-105and the increase of total gas accumulation in shale( Curtis, 2002; calcic-siliceous shale has a clay mineral content of 27%, mainlyChen et al., 2009 Li, D H et al., 2009; Nie et al., 2009b). Many consisting of illite with minor montmorillonite, and a quartzstudies have been made on the origin of fractures and their content of 35%-50%, averaging about 45%. The Devonian Ohiodistribution in shale gas reservoirs as well as gas accumulation shale in the Appalachian Basin has a quartz content of 45%-60%conditions. However, analysis of the dominant factors controlling In North America, the Devonian-Carboniferous shale gas resershale fracture development and research on the relationship voirs have high content of biogenetic organic silica(e. g, radio-between shale fracture development and gas accumulation needs laria), and high quartz, mostly >40%, some up to 75%. Thefurther study. Based on analyses of dominant factors affecting siliceous matrix is mainly clay-silt-sized quartz, and the shale isfracture formation in shale gas reservoirs and statistical analyses finely laminated. The shale gas fields under development haveof gas accumulation data from China and elsewhere, this paper well-developed natural fracture systems normally due to very highdescribes the dominant factors controlling shale fracture devel- quartz content which increase the brittleness of the shale.opment and the relationship between shale fracture developmentFig. 1B illustrates mineral compositions of the Lower Paleozoicand gas accumulation in order to provide constraints for expe- Cambrian and Silurian gas-bearing marine dark shale from wellditing strategic investigation and target selection of shale gas Changxin-I at the Changning structure in the Weiyuan area of theresources in ChinaSichuan Basin, China. Based on the mineral composition of 225shale samples taken from 3 wells penetrating the Lower Cambrian2. Shale fracture developmentQiongzhusi dark shale in the weiyuan area, the clay content rangesfrom 15% to 21%(av. 18.5%): quartz content between 59% and69%(av. 629): plagioclase 19%0-25%(av. 22%); carbonateThere are many factors influencing fracture development and (calcite and dolomite)7%-13%(av. 9.5%) Based on test results ofdistribution in shale gas reservoirs. Compared with other types of 64 graptolitic dark shale samples taken from 4 wells in the Lowerreservoir rocarbon ToC) have several common features and also special 41%(av. 34%) quartz 45%-76%(av. 61. 5%) plagioclase is rarefeatures in terms of fracture development. These features consist of and carbonate content(calcite and dolomite)is in the range of 7%-non-tectonic and tectonic factors which are intrinsic and extrinsic 20%(av. 13.5 %). In this area, the Cambrian shale is rich infactors, respectively, that control fracture development in shaleplagioclase and has similar quartz content, much lower clay andslightly lower carbonate compared to the Silurian shale(Table 1)2. 1. Non-tectonic factorsIn the lower Silurian Longmaxi Formation of the Changningtructure in the southern Sichuan Basin, dark shale from wellNon-tectonic factors influencing fracture development in shale gas Changxin-l has 20%-30% of quartz, 10%-25% of carbonatereservoirs include lithology and mineral composition, rock (calcite dolomite), sometimes up to 35%, and 1%-4%ofmechanics, TOC, abnormal high pressure, shale thickness, dehy- pyrite. Drilling results show that, fractures are well developed indration and ductile properties of clay minerals, compaction and the shale interval in well Changxin-I due to its high carbonatepressure solution during diagenesis, thermo-contraction, differential content. However, the presence of micro-fissures in shale layers inerosion, and weathering denudation. Under the same stress, lithology the Weiyuan area is attributed to the high quartz. content andand mineral composition, rock mechanics, TOC and abnormal high moderately abundant carbonate and feldspar(Table I). From thispressure are the major factors influencing fracture development(Hill evidence, it is concluded that the brittle nature of the shale is theand Lombardi, 2002; Ding et al., 2003; Zhao et al., 2008)result of high contents of quartz, carbonate and feldspar. Therefore, natural and induced fractures tend to be formed under2.1.1. Lithology and mineral compositionexternal forces and fracture development is favorable for gasShale lithology and mineral composition are the main intrinsic migration and accumufactors controlling fracture development in shale. Shale asDark shale rich inis brittle, leading to better develop-a general lithologic terminology includes dark siliceous, calcar- ment of fractures compared to the more " plastic"gray shale rich ineous,carbonaceous, ferruginous, sandy shale varicties and oil calcite(Hill and Lombardi, 2002; Nie et al., 2009a; Li, D.H.,shale(Li et al., 2007). Shale has complex mineral composition. et al., 2009). Feldspar and dolomite also increase the brittlenessBeside clay minerals such as kaolinite, montmorillonite and illite, of dark shale(Nelson, 1985). If shale has less swelling clayit contains many clastic and authigenic minerals, such as quartz, minerals but more silica, carbonate and feldspar, the rock is highlyfeldspars, mica, calcite, dolomite, phosphosiderite, siderite and brittle and likely to be fractured. In shales with identical mineralpyrite. The mineral content can be determined using elemental compositions, the finer the grain size, the more conducive tocapture spectroscopy (ECS), X-ray diffraction(XRD) and scan- fracture development it is, and vice versa(Zeng and Xiao, 1999;ning electron microscope(SEM) techniques(Table 1)Li, D H, et al., 2009). Fractures normally develop where thereFig. 1A shows plots of mineral compositions of the Devon- is a change in lithologyian-Carboniferous marine shales under development in theDuring artificial fracturing, silica-rich shale is more prone toUnited States. The data can be conveniently divided into two fracturing than clay-rich shale(Li et al., 2007; Tan, 2009). Silt-mineral composition fields. In the overlapping area of mixed stone, fine sandstone or sandstone interbeds as well as the pres-logies such as bossier shale sandstone and siltstone, the ence of open or incompletely filled natural fractures in shale cancontents of quartz, feldspar and pyrite are less than 40%, enhance the permeability of shale reservoirs. Shale gas has highercarbonate is >25%, and clay minerals are <50%, In the Ohio, permeability in faulted and fractured rocks(Li, X., et al., 2009)Woodford and Barnett shales, the contents of quartz, feldspar andpyrite are 20%-80%0, carbonate is <25%, and clay minerals are in 2. 1.2. Rock mechanicshe range from 20% to 80%. In the Barnett Formation of Mis- Fractures are the results of rock rupture and their development hassissippi System in the Fort Worth Basin, gas-producing dark long been deba中国煤化工CNMHGStatistics of relationship between fracture development and lithology/mineral content in shale reservoirs.FormationShale lithology andQuartzFeldspar Carbonate Fracturesedimentary environmentcontent(%) content(%) content(%) content(%) developmentaccunulationUnited West TexasDevonianMixture of shaleDevelopedShalesandstone and siltstoneFort Worth abyssal Mississippi3550(*)Micro-fissuresforeland basinBarnett Fmcalcareous mudstonecontaining clay; deepslope in stillwater-basin faciesAppalachianDevonianCarbonaceous shaleMany groups ofForeland basin Ohio Fm天high-angle fracturessedimentary environmentMichiganDevonianDark shale, gray andTwo groups of neShaleCraton BasinAntrim Fmgreen shale interbeddedNw orthogonal nearlywith carbonatevertical naturaldeep water sedimentsractures developeAkama PassiveDevonianSiliceous shale, sedimentaryFracture networkShale gascontinental margin Woodford Fm. environment in still water15-217-13ChinaCraton basinDark gray-dark carbonaceousin Sichuan Basin. Cambrianshale, silty shale and siltstone,18.562295Micro-fissures developed; Rich showing 5basically filled withof shale gasChinaiongzhusi Fm. neritic continental shelf faciesecondary calcitedolomite and quartzLower Silurian Rich in graptolite dark gray-dark6-41RarelyActive shows NLongmaxi Fm. silty shale, carbonaceous shale615of shale gassiliceous shale intercalated withargillaceous siltstone: neriticabyssal continental shelf faciesCraton basinLower Silurian Rich in graptolite dark gray-dark 30-6020-300-25Well developedin ChangningLongmaxi Fm. silty shale, carbonaceous shale,structure,siliceous shale intercalated withSichuan basinargillaceous siltstone: neriticabyssal continental shelf faciesMinimum-Maximum中国煤化工CNMHGW. Ding et al. Geoscience Frontiers 3(1)(2012)97-105▲ Weiyuan area shale0.1.00Ohio shale. Well Changxin-1 shaleWoodfordBarnett silice-ous shale0.250.750.750.251.0001.000.250.500.751000250500.751.00Quartz, Feldspar, PyriteQuartz, Feldspar, PyriteAStates(after u X.J. et al., 2009)B Sichuan Basin, ChinaFigure 1 Ternary diagrams showing mineral compositions of Paleozoic marine shale reservoirs.formation have been suggested: single shear strength, dual shear potential tensile fractures(including tension-shear fractures)maystrength, triple shear strength, strain energy density and maximum be produced. a positive value for 8(6>0)refers to the tensiletension stress strength. The most widely accepted of the five strength of a rock.conditions is the Coulomb-Mohr generalized single shearAbove discussion on fracture generation in rocks shows thatstrength principle and Griffith generalized maximum tension shale would rupture when the stress reaches the ultimate strengthstress strength principle(Price, 1966). The main parameters used of the shale. Shale rupture can be divided into two types, tensileto describe elastic deformation of rocks include the Youngs rupture and shear rupture. Under identical stress field, fracturemodulus, shear strain modulus, volumetric elastic modulus and development from shale rupturing is closely related to thePoissons ratio, which reflect rock tensile strength, shear strength, parameters of rock mechanics of different lithologies, such ascompressive strength and lateral relative compressibility, respec- Youngs modulus (E), shear strain modulus (G), volumetrictively. Rock shear rupture is not only related to shear stress on the elastic modulus(K), Poisson ratio (u), cohesion(C), internalrupture surface, but also normal stress on the rupture surface. The friction angle ()and rock rupture strength under differentstress condition at each point is determined by the tectonic stress confined pressures. These parameters may be obtained fromfield. In order to determine whether rock rupture has occurred at HTHP three-axis tests for rock mechanics.any point in the tectonic stress field or determine the developmentBased on experimental analysis of rock mechanics parametersof fractures, the concept of rupture value ()is introduced, which for different lithologies from four reservoirs with dominant darkis defined as I=TT, where Tn is the shear force on surface and argillaceous shale and siltstone interbedded with fine sandstonelr] is the ultimate shear stress For [TI=C+intan C is rock and medium sandstone in the Huoshaoshan oilfield in the easterncohesion, on is the normal stress on the shear rupture surface, is Junggar Basin, China (Xu et al., 2000), the infuence of rockinternal friction angle and tan o is the internal friction coefficient. mechanics parameters of various argillaceous shale on fractureIf /< l, then, no fractures are generated; if I> 1, fractures are development can be illustrated. In three-axis rock mechanicsgenerated Based on the value of l, fracture development can be experiments under various confined pressures, 34 cores fromidentified (Xu et al., 2000). When a rock is subjected to a gener- different depths in 5 wells were tested, including 12 argillaceousalized tension, tension-shear or tension rupture may occur. In this siltstone, 10 silty mudstone, 9 pure mudstone and 3 dolomiticcase, the Coulomb-Mohr principle is not applicable. The plan mudstone samples. Parameters such as rupture strength, elasticrupture principle based on the Griffith strength theory can be used modulus, Poisson's ratio, cohesion and internal friction angleto identify the rupture(Yu, 1998; Zan et al., 2002). This principle indicate that: (1)different argillaceous shales have differentis based on tension rupture, and is virtually an equivalent rupture strengths. Specifically, dolomitic mudstone has the highestmaximum tension stress theory suitable for the determination of rupture strength of 215-239 MPa, followed by silty mudstone attension rupture. If the tensile strength of a brittle material is 110-207.6 MPa, argillaceous siltstone at 105.5-196.5 MPa, andidentified, the effective tension stress aE can be used as a charac- pure mudstone has the lowest value at 99.75-171.5 MPa;( 2)teristic parameter to describe the development of tensile fractures mechanics parameters of different argillaceous shales such asand tension-shear fractures in the rock. This is defined as the Youngs modulus, Poisson's ratio, cohesion and internal frictiontension stress condition in a rock. The greater ae is, the higher is angle are different. Dolomitic mudstone has the largest values ofthe possibility of tension rupture. This is especially the case in all parameters and very high cohesion, indicating the highesthomogeneous carbonate rocks. The tensile strength of rocks resistance to tension, shear and compression. Poisson's ratio anddiffers slightly, and oE tends to have a positive correlation with the Youngs modulus of argillaceous siltstone, silty mudstone anddevelopment of tensile fractures or tension-shear fractures. dolomitic mudstone in four reservoirs(Fig. 2)show a negativeNamely, higher ag indicates greater rupture potential and the correlation. Dolomitic mudstone with high rupture strength hasdevelopment of a larger number of fractures, i.e., when oe >8, a higher Poisson中国煤化工odulus, while siltyCNMHGw. Ding et al./Geoscience Frontiers 3(1)(2012)97-105carbon during hydrocarbon generation, which would increase thepore volume of the shale by 4.9%(Jarvie et al., 2003). Based on this30.20deduction, higher Toc will produce more ultra-micro pores in theshale matrix and a greater number of micro-fissures, correspondingto higher abundance of the shale gas reservoir.2.1.4. Abnormal high pressureH Poisson's ration (u)o Abnormal high fluid pressure is an intrinsic factor of rock rupture.Youngs modulus E(10°MFor dark laminar thick argillaceous shale rich in organic matter,rapid deposition results in under-compaction. In a closed state,Argillaceousabnormally high fluid pressure would be created because of thecombination of clay mineral conversion and dehydration, hydro-Figure 2 Variation of mechanical parameters of different carbon generation, hydrothermal pressurizing and cementation.mudstone, Huoshaoshan oilfield in Junggar Basin, Nw ChinaAbnormally high-pressure fractures might be produced whenexcessive fluid pressure(> hydrostatic pressure)equals to 1n or 1/of matrix pressure(Ding et al., 2003). Fractures tend to be closemudstone and argillaceous siltstone with less rupture strength have when fluid pressure in pores is less than that in fractures. Generally,a lower Poisson's ratio but higher Young's modulus. Therefore, the opening and closing of fractures under abnormally high pressureunder identical tectonic stress conditions, argillaceous shale with is a multi-cycle process. During this process, small fractures formedlow rupture strength is more brittle and prone to produce fractures. early are continuously extended by later rupture, hence formingThis type of brittle shale rich in organic matter is the preferredtarget for strategic screening of shale gas resources.arger vertical tensile fractures and a large number of microfractures as well as some shear fractures. Therefore. a certaisized fractured shale gas reservoirs could easily form in the distri2.1.3. Total organic carbon(TOC)TOC not only controls the overall gas accumulation in shale, butition zone of high-pressure massive shale(gas generating zone)also influences fracture development. Generally, shale in thefracture zone has a high exploration success ratio and high gas 2. 2. Tectonic factorsproduction corresponding to high TOC (Xu et al., 2000). Underidentical geodynamic conditions, rock mineral composition and Tectonic factors are also an extrinsic factor for rock rupturemechanical properties, TOC is an important factor affecting Primary tectonic effects related to fracture development include: (1)fracture development in shale(Hill and Lombardi, 2002). Shale under local or regional tectonic stress, a shale zone with higherwith high contents of organic matter and quartz is more brittle, plasticity experiences ductile shear rupture and produces tectonicless tensile strength, and is prone to produce natural and induced fractures. These are mainly high-angle shear fractures and tensionfractures under extemal forces(Pan et al., 2009), which is in turn shear fractures, usually associated with faults or folds and form infavorable for the desorption of shale gas, free gas accumulation groups nearly perpendicular to the bedding plane, with well-definedand flow. In North America, most of dark shales have high Toc orientation and a smooth fracture surface. In shale, these ki(ToC 2%)as well as high biogenic organic siliceous matter high-angle tectonic fractures are mainly developed and some might(normally >30%). This type of siliceous shale rich in organic extend through the shale into a sandstone reservoir as trans-layermatter is very brittle and easily develops fractures or micro- fractures; (2)under extensional or compressional conditions, a fewfracture systems. For example, the Barnett shale in the Fort low angle decollement fractures parallel to bedding planes might beWorth Basin has TOC content in the range of 1.0%-13.0%(av. produced by shear stress acting along the low permeability shale4.5%)and quartz contents of 35%-50%(av. 45%).Micro-fissures bedding planes. These fractures are mainly distributed at the topare abundant. The abundance of organic matter is attributed to and bottom of the shale, and generally have highly variable atti-a high sea level at the beginning of shale deposition when nutrient- tudes. Obvious features of scratch and mirror plane structures arerich upwelling occurred. As a result, deep slope basin facies developed on the fracture plane (Zeng and Xiao, 1999);(3)undersediments accumulated in a strongly reducing environment that the action of horizontal compression and pressure solution, tectonicpreserved the organic matter. The shale protolith sediments mainly sutures may be formed which are perpendicular or nearly perpen-included bathyal mud(from shallow water continental shelf) and dicular to bedding surfaces. When the suture column is parallel orbio-skeleton remains. Burial of the siliceous biosome(such as nearly parallel to the bedding plane, it is referred to as a parallelradiolaria) resulted in high content of siliceous matter in the Toc- suture; (4)vertical load fractures may be formed when the verticalrich shale (Li, x.J., et al., 2009)load exceeds the compression strength of shale; (5) verticalBased on the relationship between TOC and fracture develop- differential load fractures may be generated when shale is rupturedment in shale under exploitation for shale gas worldwide(Table 2), due to uneven load from the overlying strata;(6) a significantthe higher the TOC in shale, the greater the total gas accumulation, amount of extensional and compressional fractures may be formedcorresponding to a high volume of free gas and better development by the mechanical rupture of confined rocks as a result of theof fractures. The relationship between TOC and fracture develop- invasion of magma under high pressure(Ning, 2008);(7)saltment can be divided into four levels: (1)TOC <2.0%0, poor fracture domes are conducive for shale fracture development. due todevelopment; (2) TOC at 2.0%-4.5%, moderate fracture develop- basement uplifting and faulting, many small folds and irregular flowment;(3)TOC at 4.5%-7.0%0, good fracture development; (4)TOC directions exist in salt rocks(Zhang and Yuan, 2002).7%, very good fracture development( Fig. 3). Experimental resultAs a consequence of the above mentioned features, tectonicof Jarvie et al. (2003)also provided evidence for the data listed in stresses control fracture development. Tectonic fractures are formedTable 2. Shale with TOC of 7.0% would consume 35% of organic during the concentratioYH中国煤化工CNMHGTable 2 Data showing relationship between ToC and fracture development in shaleCountry BasinTOC(%)Total gasAdsorptive gasFree gasFracture developmentaccumulation volume (mt)volume(m t-)ed AppalachianDevonian ohio Fm0.5-23.01.70-2830.85-1.42(50%)0.85-1. 42(50%) Many groups of high-angle fracturesStates foreland basinwell developedMichigan Cratonic Devonian Antrim Fm. 0.3-24.01.13-2830.79-1.98(70%)0.34-0.85(30%) Two groups of ne NW orthogonalnearly-vertical natural fractures moderatelyIlinois cratonicDevonian New Albany 1.0-25.01.13-2640.57-1.32(50%0.56-1.32(50%) Fracture system well developedFort Worth abyssal Mississi10-13.0849-9914.25-5.00(50%)4.24-4.91(50%) Micro-fissures very developedforeland basinBarnett FmSan Juan forelandCretaceous0.5-3,0037-1,270. 28-0.95(75%)(*s) 0.09-0.32(25%o) Fracture network moderately developedLewis FmCratonic basinLower Cambrian04-11.07(*)027-1.03225(85)Micro-fissures developed; basically filledWeiyuan area,Qiongzhusi Fm0.51-4.45with secondary crystals such as calcite,Sichuan BasinLower silurian209(61)dolomite and quartzCratonic basin inLower Silurian293(153)Fractures very developedSichuan BasinAverage(sample quantity**:(75%)-Gas accumulation percent中国煤化工CNMHGW. Ding et al./Geoscience Frontiers 3(1)(2012)97-1051039fractures can effectively improve the performance of a shalereservoir and could particularly increase permeability of a shalereservoir. Since tectonic fractures are mainly developed within thebreaking points of fold structures as well as nearby fault zones,high-yield areas of shale gas reservoirs tend to be distributed in2008)3. Relationship between shale fracture and gasaccumulationOn the basis of previous discussion, it is clear that shale fracture0Poor Moderate Good Very goodare closely related to gas accumulation and gas production. AsFracture developmentshown in Table 2, there is a positive relationship between fracturedevelopment in shale, total gas accumulation and free gas volumeFigure3 Relationship between TOC and fracture development in This relationship is mainly attributed to the special gas generationechanism in shaleervoused to conventional lowpermeability gas reservoirs. Shale gas diffuses from micropores inthe shale matrix to the large pores and fractures, following Darcysa period with equivalent stress variation, the area with the greater law, but adsorptive gas on the pore surface in the matrix may bestress variation gradient would have higher probability to produce desorbed under certain pressures, and Darcian How is not applifractures. For example, at locations where there is a marked change cable(Zhang et al., 2004). Fracture development promotes thein the stratigraphy, such as in a basin structure, a fault convergence volumetric increase of free gas and desorption of the adsorptivezone, the apex of a tight anticline and the transitional zone between gas in shale( Curtis, 2002). Fracture development also determinesthe slope and base of sag, the stress variation gradient is high and the quality of a shale gas reservoir. In general, if fractures are wellshale deformation is severe. In such areas, shale fractures are developed, shale gas reservoirs are of good quality, and if fracturesusually very well developed (Xiang, 2008).or micro-fractures are well developed in shale, shale gas accuUnder similar geological conditions, fracture development in mulations are highly enriched.brittle shale is closely related to faults. In a shale zone close tofractures have dual contributions to the formation of a shalefault zone, fractures are well developed with high density. Fracture gas reservoir. On the one hand, they provide migration channelsdensity is also significantly affected by fault scale and intensity of and accumulation space for natural gas and formation water, andfaulting. Under the same rock facies, fractures are more easily therefore increase of total free gas accumulation in shale. Norproduced when the fault size is larger and faulting activity is more mally, shale has very low initial permeability. If natural fractureintensive in the shale zone. Although the area of shale fracture are not well developed, artificial fracturing is necessary todevelopment may be closely related to a fault zone, or a particular generate additional fractures so as to communicate with the wellfault, the zone of fracture development might not always exist near bore and provide more pressure drawdown and a larger area fofaults, reflecting the complicated nature of the relationship gas desorption For storage and development of shale gas, espe-between faults and fracture development(Ning, 2008). Tectonic cially during the initial production period of a single well,B4-31 wellA2-31 well6877m3dC4-31 well14150m3dFracture numbers12,169m3dFigure 4 Rose diagrams comparing fracture orientation and abundance with gas production Antrince et aL., 1992)中国煤化工CNMHGW. Ding et al. Geoscience Frontiers 3(1)(2012)97-105fractures play a significant role. On the other hand, if fracture sizethe toC, the more ultra-micro pores are present in the shaleis too great, natural gas may be dissipated.matrix, the more micro-fractures are generated, and the higherfractured shale gas, as a non-conventional natural gas, maythe gas enrichment.have a thermal cracking, biogenic or hybrid origin. Shale gas ( 2) Fractures have dual contributions to the formation of shalewith a thermal origin is mainly adsorbed by organic matter, thengas reservoirs. They provide migration conduits and accu-expelled via fractures, or occupies pore spaces in shalemulation space for natural gas and formation water, and they( Pan et al., 2009). Thermal origin shale gas reservoirs arehelp to increase of total free gas accumulation and desorptiondependent on gas diffusion and accumulation via micro-fractureof adsorptive natural gas. a greater number of fractures andwhereas faults and macro-fractures may destroy the gas reservoirmore dispersed distribution correspond to a higher gasUnder abnormal high pressure conditions generated by gasproduction. However, if the fracture size is too large, loss ofgeneration from thermal cracking, fractures are produced alongnatural gas will occur. If large fractures are well developedthe stress concentration face and lithological contact/transitionalgas production will be lower.face, which provide the threshold porosity and permeability (3)Shale fractures are closely related to shale gas accumulationnecessary for gas accumulation(Zhang and Pan, 2009). Theand production. There is a positive correlation betweenforming of gas reservoirs with a biogenic origin is closely relatedfracture development, total gas accumulation and free gato the active exchange of fresh water. Fractures act as conduits ofvolume. The better the fractures are developed in shale, theformation water. In places where faults are well developed,greater the gas accumulation and therefore production. Thereformation water is active and also the physiological activity ofis a high success ratio of shale gas exploration and high gasmethanogenesis microorganism so that more gas can be gener-production in fracture zones in North America. Good gasated. Fractures provide conduits for gas diffusion and accumushows or low yield producers from the Lower Paleozoiclation, and tectonic stress may play a positive role( Martini et almarine shale rich in organic matter in the Sichuan Basin in2003; Li, D.H., et aL., 2009). Areas under development usuallChina are closely relatedthe degree of fracturehave well-developed fracture systems. For instance two groupsof Nw and NE near vertical natural fractures are mainly devel-oped in the shale gas recovery tracts in the Antrim Formation ofthe northern Michigan Basin. Shale gas production is related tomicro-fracture development in the Barnett shale in Newark East Acknowledgmentsgas field in the Fort Worth Basin. Economic recoverable reservesin the New Albany shale in the Illinos Basin are also associated The study was sponsored jointly by the National Natural Sciencewith a fracture system. Fractures are gas accumulation spaces Foundation Project(41072098, 41002072), National Specialand gas flow conduits, and are necessary for the migration of Project of Investigation and Evaluation on Strategic Screening forshale gas from matrix pores to the well bore. RecoverablNational Oil Gas Resources -"Potentials of Shale Gasreserves of shale gas ultimately depend on the fracture occur- Resources in Key Chinese Areas and Optimization of Favorablerence, density, combination feature and openings in the reservoir Areas"(No. 2009GYXQ-15), Major Spccial Project for National(Hill and Lombardi, 2002; Li et al. 2007; Li, D H, et al 2009;cience and Technology (2008ZX05031-001-005HZ. 973Zhang and pan, 2009)Project of Development Plan for National Key Fundamentaloverall major geological factor controlling shale entation is the Studies( 2006CB202302), CNPC Science Technology Innova-The dispersivity of fracture density and orproductivitytion Foundation Project(2008D-5006-01-06).More fractures and a high dispersed trend correspond to higher gasproduction (Decker et al., 1992)(Fig. 4). Open, mutually Referencesperpendicular fractures or numerous sets of natural fractures willincrease shale gas production(Hill and Nelson, 2000). All high- Bowker, K.A., 2007. Barnett shale gas production, Fort Worth Basinyield gas wells in the eastern United States are located in frac-issues and discussion. AAPG Bulletin 91(4). 523-533ture zones, while wells located in areas with poorly developed Chen, G.S., Dong. DZ Wang. S.Q. Wang, L.S., 2009. A preliminaryfractures have low or no gas production. For example, the higheststudy on accumulation mechanism and enrichment pattern of shale gas.yield gas wells are mostly distributed along NE-trending zone ofNatural Gas Industry 29(5), 17-21(in Chinese with English abstract)high-angle fractures in the Devonian shale of the Big Sandy gasCurtis, J B, 2002. Fractured shale-gas systems. AAPG Bulletin 86(11),field in the Appalachian Basin. Although natural fracture system is Decker,AD. Coates, J.M. Wicks, D.E. 1992. Stratigraphy, gas occurrence, formation cvaluation and fracture charactcrization of the antrimobstruct artificial fractures and hence reduce shale gas recoveryshale, Michigan Basin. 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