Theimpactofdepositionalenvironmentandtectonicevolu

The impact of depositional environment and tectonic evolution on coalbed methane occurrence in West Henan,ChinaZhaodan Cao a,⇑,Baiquan Lin a ,Ting Liu a,ba China University of Mining and Technology,Xuzhou 221116,ChinabDepartment of Energy and Mineral Engineering,The Pennsylvania State University,University Park,PA 19019,USAa r t i c l e i n f o Article history:Received 24January 2019Accepted 24January 2019Available online 1February 2019Keywords:Coalbed methaneDepositional environment Tectonic evolution Epigenetic erosionTectonically-deformed coala b s t r a c tA deeper understanding of the mechanisms by which geological factors (depositional environment and tectonic evolution)control the occurrence of coalbed methane (CBM)is important for the utilization of CBM resources via surface-drilled wells and the elimination of coal-methane outbursts,the latter of which is a key issue for coal mine safety.Based on drill core data,high-pressure isothermal adsorption experiments,scanning electron microscopy experiments,mercury intrusion porosimetry,and X-ray diffraction experiments,the impact of the depositional environment and tectonic evolution on CBM occurrence of the II-1coal seam of the Shanxi Formation in West Henan was analyzed.Results showed that the depositional environment led to the epigenetic erosion of tidal flat coal-accumulating structures by shallow-delta distributary channel strata.This resulted in the replacement of the original mudstone-sandy mudstone coal seam immediate roof with fine-to-medium grained sandstones,reducing methane storage capacity.Epigenetic erosion by the depositional environment also increased coal body ash con-tent (from 6.9%to 21.4%)and mineral content,filling the cleat system and reducing porosity,reducing methane storage capacity.The maximum methane adsorption capacity of the coal body reduced from 35.7cm 3/g to 30.30cm 3/g,and Langmuir pressure decreased from 1.39MPa to 0.909MPa.Hence,the methane adsorption capacity of the coal body decreased while its capacity for methane desorption increased.Owing to the tectonic evolution of West Henan,tectonically deformed coal is common;as it evolves from primary cataclastic coal to granulitic coal,the angle of the diffraction peak increases,d 002decreases,and La ,Lc ,and Nc increase;these traits are generally consistent with dynamic metamorphism.This is accompanied by increases in the total pore volume and specific surface area of the coal body,fur-ther increasing the capacity for methane storage.Increases in micropore volume and specific surface area also increase the ability of the coal body to adsorb methane.Ó2019Published by Elsevier B.V.on behalf of China University of Mining &Technology.This is an openaccess article under the CC BY-NC-ND license (/licenses/by-nc-nd/4.0/).1.IntroductionFossil fuels,such as oil and coal,have satisfied the energy needs of humankind;however,they also cause severe environmental problems and are gradually depleting.Horizontal drilling and high-volume hydraulic fracturing have led to a shale gas boom in the USA,which has irrevocably impacted global energy markets and the geopolitical landscape [1–4].Unconventional oil and gas resources,including shale gas,tight oil,and coalbed methane (CBM),are being exploited at commercial scales globally,especially in the USA,Canada,Australia,and China [5–9].Coal currently meets 70%of China’s energy consumption,and the nation has some of the largest coal and CBM reserves in the world [10,11].CBM is a potent hazard for coal mining operations,owing to its potential to trigger disastrous accidents,such as coal-gas outbursts.Furthermore,the release of methane into the atmosphere also poses a severe threat,because the global warming potential of methane is 25times that of carbon dioxide [12].There-fore,the utilization of CBM in an environmentally conscious man-ner is of utmost importance;this includes the exploitation of CBM via surface-drilled wells and underground CBM extraction.In China,CBM exploitation via surface-drilled wells is mainly con-ducted in the Qinshui Basin,East Ordos Basin,and Fuxin Basin [13].China’s coal mines release approximately 19billion cubic meters of methane each year,which is both a massive waste of resources and a severe global warming threat [14].While China has large CBM reserves,these reserves are geologically complex and strongly heterogeneous,with a low gas-bearing saturation and poor permeability [15].These traits are detrimental to CBM exploitation via surface drilling and contribute to the relatively high frequency of coal-gas outbursts.More than 15,000coal-gashttps:///10.1016/j.ijmst.2019.01.0062095-2686/Ó2019Published by Elsevier B.V.on behalf of China University of Mining &Technology.This is an open access article under the CC BY-NC-ND license (/licenses/by-nc-nd/4.0/).⇑Corresponding author.E-mail address:czdcumt07@ (Z.Cao).International Journal of Mining Science and Technology 29(2019)297–305Contents lists available at ScienceDirectInternational Journal of Mining Science and Technologyjournal homepage:www.else v i e r.c o m /l o c a t e /i j m stoutbursts have occurred in over20provinces around China, accounting for over40%of global coal-gas outbursts.More than 100ultra-large coal-gas outbursts(>1000t outbursts)have occurred in China,and the largest of these events involved the release of1,500,000m3of gas and12,780t of coal rock[16].The occurrence of CBM is controlled by various geological fac-tors,including the depositional environment and tectonic evolu-tion of the region[17].The depositional environment plays an important role in determining the formation and distribution of high-quality coal seams,and to some extent,the occurrence of CBM.This occurs via coal-accumulating features,and the lithology, lithofacies composition,and spatial assemblages of the coal-bearing rock system[18].Furthermore,by controlling the compo-sition of coal-bearing materials,the depositional environment also affects the adsorption capacity,gas-bearing capacity,and physical properties of the coal seam[19].Different depositional environ-ments lead to differences in the maceral composition of coal.For example,coal seams that are formed in water-covered reducing environments will have high vitrinite contents,which are benefi-cial for the development of cracks and joints,and thus demonstrate suitable reservoir properties[20,21].The depositional environment also has an impact on the ash content of the coal seam because ash tends tofill the pores in a coal body,degrading the reservoir prop-erties of a coal seam[20].The occurrence of methane in coal seams is controlled by tec-tonic evolution[22].The coal seams of West Henan have been sub-jected to multiple periods of tectonic activity,resulting in intense deformation of the coal bodies[23].Tectonically-deformed coal (TDC)is therefore prevalent throughout this region[24].Hence, studies on TDC macerals,coal cleat systems,and the macromolecu-lar structure and chemical composition of coal bodies are important for the exploitation of CBM via surface drilling and the prevention of coal-gas outbursts in underground coal mines[25,26].West Henan is rich in coal resources but is geologically and tec-tonically complex.The coal seams in this region have high CBM con-tents,which have led to frequent coal-gas outbursts.For instance, the Daping coal mine lies in a tectonically complex region with large stress concentrations and widespread TDC distributions.An ultra-large coal-gas outburst occurred in this mine;1,894t of coal rock and250,000m3of gas were released,resulting in148deaths and 32injuries[27].Consequently,research into the control of CBM occurrence by geological factors is important for a number of rea-sons.This includes the exploitation and utilization of CBM resources via surface drilling,as well as the elimination of coal-gas outbursts, which is important for ensuring the safety of coal mining operations.This study investigated the depositional environment and tec-tonic evolution of CBM occurrence.Section2includes a discussion on the epigenetic erosion of the II-I coal seam by the depositional environment of shallow-delta distributary channel facies,which occur throughout West Henan.The mechanisms by which epige-netic erosion controls the mode of CBM storage in the seam,and the seam’s reservoir properties and gas adsorption/desorption properties,will also be discussed.Section3focusses on the mac-eral composition,crystal structure,dynamic metamorphism,and pore structure of West Henan TDC,formed by multiple periods of tectonic stress.The mechanism by which these parameters control the capacity of coal seams for CBM storage and adsorption will also be investigated.2.Control of CBM occurrence by the depositional environment 2.1.Evolution of the depositional environmentThe West Henan region is located in the southwestern corner of the North China Plains(a large downfaulted rift basin that was formed during the late Paleozoic),near the Funiu and Zhongtiao archicontinents that lie on the southwestern margins of the North China Craton.The Upper Cambrian Changsan Formation forms the basement of the basin.Middle Ordovician Caledonian orogeny led to widespread surface uplift throughout the North China Craton and a general increase in elevation.By the end of the Ordovician, land was formed by these surface uplifts,which was then sub-jected to erosional processes.Consequently,Upper Ordovician,Sil-urian,Devonian,and Lower Carboniferous strata are absent in most regions of Henan[28].The crust then began to subside,which con-tinued until the Middle-to-Late Late Carboniferous,which caused the North China Craton to gradually submerge into the sea.During the Early Permian,the subduction of the Mongolian-Siberian plates in the northern region of the North China Craton intensified,while the subduction of the Paleo-Tethys Ocean occurred in the southern region.This created an SE-inclined paleoslope in the West Henan region,and the transgression and regression of seawater largely proceeded in the NW–SE direction[29].Because these marine transgressions-regressions persisted for a short time only,a set of transitional sedimentary strata formed in this region.The crust was relatively stable during the Middle Permian;however,the ele-vation of the North China Craton was still higher in the northwest and lower in the southwest,and the subduction of the ancient Xing’an-Mongolia Ocean intensified further during this period.This led to marine regression and the formation of a shallow-water delta system in West Henan;the continued growth of this delta system in the southward direction led to the development of poly-cyclic prograding delta systems[30].During the Late Permian,the subduction of the Tethys Ocean into the North China Craton inten-sified,and the Funiu and Zhongtiao archicontinents were rapidly uplifted,increasing the elevation of the West Henan region.There-fore,the depositional environment of this region changed from a marine environment into a terrigenous environment dominated by lacustrine deposits,which essentially ended the deposition of coal-bearing sediments.Tidalflats and deltaic environments that arise from shallow seas and silt-filled lagoons are among the best depositional envi-ronments for the formation of high-quality coal seams[31].The sedimentary system of the Middle Permian Shanxi Formation in West Henan is a polycyclic shallow-water delta coal-bearing sys-tem.Coastal-deltaic and tidalflat sedimentary systems comprise the upper and lower parts of this system,respectively[29].In par-ticular,the tidalflat sedimentary system consists of sandstones, mudstones,sandy mudstones,and coal strata,and it is an excellent coal-forming environment.The coal seam in this system is thick and has a consistent distribution.The II-1coal seam of the Shanxi Formation in West Henan,which is the main minable coal seam, lies in the lower tidalflat sedimentary system.Because this sedi-mentary system is usually submerged by seawater and mainly con-sists of water-covered swamps,the depositional environment is highly reducing,and the vitrinite content of the coal seam can be as high as83.4%.Owing to the development of cracks and microp-ores in the vitrinites,a well-developed cleat system is present in the coal seam,indicative of excellent reservoir properties[21,31].2.2.Rock core characteristicsCore data obtained from nine geological exploration boreholes were analyzed in this study(see Table1).Proximate analysis (ash content)and methane content determination were then per-formed on coal samples in the cores.In Table1,the lithology of the immediate roof of the II-1coal seam can be divided into two types:Type1is mudstone and sandy mudstone,while Type II consists offine-and medium-grained sandstone.Section2.1mentioned that the original coal-bearing deposits of the II-1coal seam were tidalflat sediments.However,298Z.Cao et al./International Journal of Mining Science and Technology29(2019)297–305the depositional environment changed from a peak-marine trans-gression environment to a marine regression environment after the formation of the coal-bearing strata,and the deltaic region grew with further regression of the shore line.This led to the for-mation of a shallow-water deltaic sedimentary system mainly impacted byfluvial and tidal actions.Prodelta facies and delta-front facies are absent in the sedimentary system.However,dis-tributary channel facies and interdistributary bay-tidalflat facies are extremely well developed[32].The injection of river water from the lower delta plain led to epigenetic erosion of the original coal-bearing sedimentary strata in lowflow-rate conditions,and the development of underwater distributary channel strata[30]. These strata were in erosional contact with the underlying coal-bearing rock system,which caused widespread epigenetic erosion of the sedimentary units and structures[20].Eroded areas mainly consist of coarse clastic rocks,such as sandstone and conglomerate, and these rocks form the immediate roof of the II-1coal seam.The eroded areas had a significant impact on the development of the coal seam and induced large variations in seam thickness.For example,areas of the coal seam were very thin(thin-coal areas), while others contained either no coal or coals with a high ash con-tent.Epigenetic erosion also impacted the mode of CBM deposition and storage,as well as the reservoir properties and adsorption/des-orption characteristics of the coal seam.2.3.Effects of the depositional environment on reservoir properties and CBM storage modesCoal seam CBM storage modes are controlled to some extent by depositional effects through coal-accumulating features,and the lithological/lithofacies compositions and spatial assemblages of the coal-bearing system[20].The wall rocks of a coal seam may be divided into impervious strata,permeable strata,and semi-impervious strata according to their lithological compositions and gas permeability.In different sedimentary systems,a coal seam will occur in different regions of the stratigraphic unit(due to geologic cycling)and form various structural relationships with the roof or wall rocks above the roof(to a certain distance)[33]. The precise composition and permeability of these lithological assemblages play a significant role in determining the capacity of the coal seam for CBM storage.In terms of methane storage capacity,sandstone<carbonates<interbedded sandstone-mudstone<mudstones<coal seam<oil shale.Sandstones,con-glomerates,and limestones with well-developed pores and cracks have permeability coefficients that are thousands to tens of thousands of times greater than those of tight,crack-free rocks (e.g.,sandy shale,shale,and muddy shale).The ability of a litholog-ical assemblage to seal and store methane is inversely proportional to its permeability;the higher the permeability,the lower the CBM content.The II-1coal seam is located in a tidalflat sedimentary system, and the original sedimentary roof of the coal seam was comprised of tidalflat mudstones-sandy mudstones,which have excellent methane storage capabilities.However,the structural integrity of the original coal-accumulating sedimentary units was eroded by shallow-delta distributary channel strata.In certain sections,the coal seam is directly covered by distributary channel facies sand-stones.There are two typical modes of methane deposition and storage in West Henan:Type I and Type II(see Table1).An analysis of Type I drilled cores indicates that the immediate roof of the II-1coal seam in these sections is comprised of tidalflat mudstones and sandy mudstones.Epigenetic corrosion was either absent or very minimal,and the original coal seam occurrence was not damaged by erosion.The thickness of the coal seam at each drilled core was5.1m,5.41m,5.8m,and5.3m.Hence,the seam thicknesses of Type I sections have a small coefficient of variation, and the distribution of the coal seam is consistent and stable in these sections.The8501and8201borehole coal samples have ash contents of6.9%and7.61%,respectively.The low ash content of these samples indicates that the coal cleat system has not been filled with ash;therefore,these rocks have excellent reservoir properties and high methane storage capacities.The mudstone-sandy mudstone roof(wall rock)is largely impermeable to gas and therefore serves as a strata seal.These rocks can also act as a stable aquiclude and prevent the solvation of CBM by groundwater, contributing to the capacity of the seam to store methane[34]. High methane contents were found in the8002and8201borehole coal samples,at17m3/t and15.24m3/t,respectively.Based on the lithological characteristics of Type II drilled cores, the original tidalflat mudstone-sandy mudstone immediate roof in the II-I coal seam was damaged by epigenetic erosion in Type II sections.Here,the immediate roof of the coal seam consists of highly porousfine-to-medium grained sandstones,typical of shallow-water deltas.As compared to the original sedimentary roof,which consisted of interbedded mudstone and sandy mud-stone strata,fine-to-medium grained sandstones are highly perme-able to air,and act as channels to facilitate the escape and migration of CBM.Furthermore,the sandstone roof could act as a water-filled aquifer.Methane solvation by groundwater may then reduce the CBM content of the coal seam.The methane contents ofTable1Core data obtained from geological exploration boreholes,and proximate analysis and methane content determination.CBM storage modes GeologicalexplorationboreholesII-1coal seamLithology of theimmediate roofDepositional environment ofthe immediate roofDepositional facies ofthe immediate roofCoal seamthickness(m)Ashcontent(%)Methanecontent(m3/t)Type I8002Sandy mudstone(0.5m)Tidalflat sedimentary system Peat swamp facies 5.113.317 8201Mudstone(0.5m) 5.87.6115.24 8501Mudstone(0.6m) 5.3 6.98101Mudstone(2.3m) 5.41Type II8303Fine-grainedsandstones(3.47m)Shallow-water deltaicsedimentary systemDistributary channelfacies7.115.9714.128302Fine-grainedsandstones(6.45m)11.321.413.46 8102Medium-grainedsandstones(7.58m)4.820.418301Fine-grainedsandstones(2.9m)68502Fine-grainedsandstones(2.2m)5.2Z.Cao et al./International Journal of Mining Science and Technology29(2019)297–305299the 8303and 8302borehole coal samples were 14.12m 3/t and 13.46m 3/t,respectively.Coal seam thicknesses measured at the 8102and 8302drilled cores were 4.8m and 11.3m,respectively.This shows that epigenetic erosion has eroded the coal seam roof and a part of the seam itself,resulting in dramatic variations in seam thickness.This could lead to the formation of coal-free areas or unrecoverable coal beds in the II-1coal seam.In addition to the erosion of coal-accumulating strata,distribu-tary channel water flows also transport large quantities of terrige-nous detrital materials (suspended clay minerals and silt)to the peat swamps,which settle alongside the coal-forming plants.This increases the amount of terrigenous detrital matter in the coal-accumulating swamp,and thus the mineral and ash contents of the coal body,which can fill the coal body’s cleat system and degrade its reservoir properties.The 8302drilled coal sample has an ash content of 21.4%,whereas the 8102drilled coal sample has an ash content of 20.41%and a porosity of 4.74%.These sam-ples have higher ash contents compared to the average ash content of the II-1coal seam (14.29%)and that of the 8501drilled core sample (6.9%).2.4.Adsorption and desorption characteristicsCBM may either occur in the free state or adsorbed state.How-ever,most CBM (over 80%)exists in the adsorbed state.Therefore,the CBM storage capacity and gas production characteristics of a coal seam are determined by its methane adsorption and desorp-tion characteristics.These characteristics may be described by the Langmuir volume (V L ),Langmuir pressure (p L ),and adsorption isotherm curve.We conducted high-pressure isothermal adsorp-tion experiments (GB/T19560-2008)to measure the maximum methane adsorption capacity (V L )and Langmuir pressure (p L )of the 8102and 8501borehole coal samples,as shown in Figs.1and 2,and Table 2.Table 2shows that the V L of the 8501and 8102samples were 35.7cm 3/g and 30.30cm 3/g,respectively,indicating that epige-netic erosion has reduced the maximum methane adsorption capacity of the coal body.This is because epigenetic erosion increased the mineral and ash content of the coal body while reducing organic matter content,effectively diluting the adsorbed methane and reducing the ability of the coal seam to adsorb methane [35].p L is the pressure that corresponds to a methane adsorption of V L /2,and this parameter reflects the ease by which methane is desorbed from the coal body;a lower p L indicates that it is easier it is for the coal body to desorb methane.Because p Ldecreased from 1.39MPa to 0.909MPa,epigenetic erosion enhanced the ability of the coal body to desorb methane.3.Control of CBM occurrence by the tectonic evolution of West Henan3.1.Tectonic evolution of West HenanCoal-accumulating areas of West Henan have been subjected to multiple periods of tectonic stress;for example,Indosinian move-ments during the Late Triassic,Early Yanshanian movements dur-ing the Late Jurassic,Late Yanshanian movements during the Late Cretaceous,and Himalayan movements during the Late Eocene [28].The large tilted fault-blocks that were formed during Himala-yan movements led to basement detachment,which subsequently induced gravitational gliding tectonics throughout West Henan.The gravity-gliding structures of West Henan consist of 18rela-tively independent tectonic sub-groups;West Henan therefore contains many gliding masses,which are large and densely dis-tributed throughout the region [36].The primary gliding surface of the gravitational gliding structure is located within the Shanxi Formation,in the lower part of the Late Paleozoic coal system.The II-1coal seam is effectively the primary ‘‘lubricating”plane of the gravitational gliding structure in the West Henan coal field [36].This has driven morphological changes in the II-1coal seam,which manifest as severe alterations to the original texture and structure of the coal seam,and the widespread development of TDC [37].Hence,the distribution of TDC throughout West Henan was mainly caused by gravitational gliding tectonics [38].TDC may be divided into three deformation sequences and ten types according to tectonic deformation mechanisms and textural differences [37].The coal bodies of West Henan typically exhibit cataclastic to mylonitic textures.The Yanlong,Xinggong,Xinmi,and Dengfeng coalfields in the central part of West Henan display the most pronounced gliding tectonic-induced coal-body texture alterations,and their coal bodies generally exhibit mylonitic tex-tures [24].In particular,the entirety of the coal seam in the Yan-long and Xinggong coalfields is powder-like and highly impermeable [39].The Yuzhou,Xin’an,and Shanmian coalfieldsFig.1.Adsorption isotherm curve for 8102borehole coalsample.Fig.2.Adsorption isotherm curve for 8501borehole coal sample.Table 2Langmuir volume V L and Langmuir pressure p L .Samples Lithology of the immediate roof V L (cm 3/g)p L (MPa)8501Mudstone (0.6m)35.71 1.398102Medium-grained sandstones (7.58m)30.300.909300Z.Cao et al./International Journal of Mining Science and Technology 29(2019)297–305in the western part of West Henan mainly exhibit granular tex-tures,while cataclastic textures are observed in a few localized sections only.The Pingingshan and Linru coalfields in the southern part of West Henan mainly exhibit cataclastic textures,and some of the original(undeformed)textures and granular textures are well-preserved in these areas.Under tectonic stress,tension cracks or shear cracks were formed in the original structure of the coal body,until it shattered and underwent brittle and ductile deformations.This dynamic metamorphism altered the macromolecular structure and chemi-cal composition of the coal bodies to varying extents[30,40,41]. The TDC of West Henan coalfields generally have higher vitrinite reflectances as compared to native(undeformed)coals and also exhibit bireflectance[42].This indicates that these coal bodies have undergone dynamic metamorphism.3.2.Determination of the occurrence and geometry of the coal seam via underground through-seam boreholesThe occurrence and geometry of the II-1coal seam in the Xin-feng coal mine in West Henan were elucidated using through-seam boreholes in the gas drainage roadways of the coal mine,as shown in Fig.3.It is apparent that the II-1coal seam has undergone intense alteration due to tectonic stress.Seam thickness varies dramati-cally,and the seam contains well-defined thin-coal areas,transi-tional thin/thick-coal areas,and thick-coal areas.Under tensile stressfields,the coal bodies in the thin-coal areas have been ‘‘pulled thin”or pulled to the point of breakage in certain sections, thus forming coal streaks or extremely thin coal seams.Coal bodies in transitional thin/thick-coal areas have a greater degree of elas-ticity and easily form methane-accumulating spaces if the roof is sufficiently impervious.As coal bodies in these areas have been severely damaged and are mechanically weak,they create danger-ous coal-gas outburst prone areas.Previous studies have shown that coal-gas outbursts are extremely likely to occur in roadways that expose transitional thin/thick-coal areas[24].Thick-coal areas have a high methane storage capacity because the seam itself is a lithologically tight gas barrier with low gas permeability rates,con-ducive to CBM occurrence.3.3.SEM analysis of TDC microstructureSEM can be used to observe the microstructure of TDC,includ-ing granule morphology and size,crack development and arrange-ment,the degree of coal-body fragmentation,and the presence of wrinkling.SEM experiments were performed on the Q1and Q2 samples extracted from the25,021working face of the Xinfeng coal mine in West Henan.The Q1sample comes from a5m thick seam,and the Q2sample comes from a1.5m thin seam.Because coal samples are not electrically conductive,the surfaces of these samples were spray-coated with a goldfilm to increase their con-ductivity.Samples were then transferred to a FEI Quanta TM250 scanning electron microscope for observation.The results are shown in Figs.4and5.Fig.4shows that the Q1sample is hard,block-shaped,and has a firmness coefficient of0.25.Owing to its exposure to tectonic stressfields,many coarse and wide surface cracks have developed. Extensions of these cracks are unstable and generally curved in shape.Furthermore,smaller secondary cracks have developed in the vicinity of the primary tensile cracks(Fig.4a),and these cracks have beenfilled with clay minerals and coal granules(Fig.4b and c).Hence,the microstructure of the Q1sample is characterized by the widespread development of brittle and tensile cracks,and sec-ondary cracks in the vicinity of the primary tensile cracks.The TDC classification of the Q1sample is primary cataclastic coal.Tensile cracks and their secondary cracks are spaces that could store methane.However,the presence of compressive stress has led to the partialfilling of these cracks with clay minerals and coal granules,decreasing the porosity and permeability of these rocks. This is detrimental to methane seepage and the exploitation of CBM;therefore,measures such as hydraulic fracturing are required to increase the permeability of the coal body.Fig.5shows that the Q2sample has a seam thickness of1.5m, and the original coal seam has been pulled into a thin-coal area by tectonic stresses.The sample is loose and powder-like,very easy to crush,and has afirmness coefficient of0.15.The structure of the coal body has suffered severe damage,and very little of the original structure remains.The coal sample has a microbreccia texture with a mix of large and small breccias(Fig.5a).Larger breccias are sev-eral hundred m m in size and exhibit a porphyroclastic texture, while the smaller breccias are only a few m m long and manifest asfine detritus.The TDC classification of the Q2sample is gran-ulitic coal.Owing to the actions of tectonic stressfields,the larger breccias in the coal body contain many cracks of varying sizes on their sur-faces(Fig.5b and c).Fig.5c shows a coal particle that has devel-oped two intersecting cracks on its surface.The coal body between the two intersecting cracks will detach from the large breccia to form small breccias with distinct corners and very poor rounding(Fig.5d).The original large breccia then dissociates into small,irregular,and poorly sorted mixed breccias with a non-uniform granular distribution.The newly-formed small breccias with distinct corners will be gradually eroded and rounded by tec-tonic stress(especially shear stresses)and develop in the better-rounded direction(Fig.5e).These small breccias eventually form an equigranular texture with a uniform particle size distribution (Fig.5f).Smaller particles and minerals will eventually adhere to the surfaces of the larger particles andfill the inter-and intra-particle gaps of the larger particles(Fig.5b and g).Fig.5g presents an image of the surface morphology of a coal particle at6000times magnification.The surface of the particle is uneven andcontains Fig.3.Occurrence and geometry of the II-1coal seam via through-seam boreholes in the gas drainage roadways of the Xinfeng coal mine.Z.Cao et al./International Journal of Mining Science and Technology29(2019)297–305301。