Tectonic Evolution And Its Control On Coalbed Methane Reservoiring In Western Guizhou

Western Guizhou located at the connection of Tethys tectonic domain andmarginal-Pacific tectonic domain, the process of tectonic evolution was extremely complex,which is the most important controlling factor for the generation, migration, andaccumulation of coalbed methane （CBM）. Thus, taking the the regional tectonic evolutionas basic approach, based on the field geology, the analyses of coalfield exploration dataand remote sensing data, combined with the microscopic deformation, test calcite veininclusions, and petrofabric analysis of tectonic rock, this paper studied the structuralcharacteristics and evolutionary history in western Guizhou deeply, discussed thestress-deformation environment and dynamic mechanism of structural deformation inmid-late Yanshan Epoch. On that basis, combined with the laboratory tests of reflectivity ofvitrinite, mercury injection, isothermal adsorption, and scanning electronic microscope（SEM） of coal samples, applied basin modeling technique, the characteristics of coalbedmethane reservoiring under tectonic control in western Guizhou were profoundly revealed.Major research results as follows:First, based on the systematic division of tectonic layers, the tectonic evolution wasdivided into6Stages in western Guizhou: crystalline basement formation stage （Pt2-Pt3）,passive continental margin stage （Z-S）, intra-continental taphrogenesis stage （D-P）, stableplatform stage （T₁-T₂）, continental depression stage （T₃-J₂）, and fault-folded uplift stage（J₃-Q）. The continental-marine alternating facies developed along NE-SW direction fromNW to SE in late Permian in western Guizhou controlled by NE and NNE syndepositionalflaults and the paleotopography were characterized by being high in the northwest and lowin the southeast formed in Dongwu movement. The original coal basins were damagedstrongly after coal-forming period, especially the movement in mid-late Yanshan Period,and the most important coal-controlling structures are large syncline and synclinore.Second, the structural deformations were largely determined in mid-late YanshanEpoch in western Guizhou, characterized by being strong when close to the basementfaults and being weak when far away from the basement faults. The middle Yanshan Epochis key period for building the tectonic framework in western Guizhou, the research area hasexperienced the tectonic stress evolution of NE-SW compression—nearby NScompression—NW-SE compression, so, the sequence of structural deformation is NW foldand thrust fault—nearby EW fold and thrust fault—NE-NNE fold and thrust fault; In lateYanshan Epoch, regional extension was formed in western Guizhou influenced by rapid uplifting of Xuefeng mountain, reversion and left-slipping of some faults arised. Throughthe microscopic deformation, test calcite vein inclusions, and petrofabric analysis oftectonic rock, the structural deformations mainly were brittle to brittle-ductile formed atmiddle-low to low temperature in the upper crust.Third, after identified the preceding conception that the coal rank raised by theregional magmatic thermal metamorphism on the basis of plutonic metamorphism, thispaper emphasized that structural differentiation of middle Yanshan Epoch decided theburied depth and further controlled the metamorphic grade of coal, metamorphism patternwas the established in western Guizhou. The viewpoint was put forward and different fromthe preceding conception that the basement faults were not the channel of magma, theHercynian rifting has not been developed to western Guizhou, Shuicheng-ziyun fault wasthe boundary fault of the aulacogen internal South China plate in later Paleozoic, thetaphrogenesis much smaller than the south region of the Nanpanjiang fault. The basementfaults had not cut the crust, and they were not the the channel of magma increasing the coalmetamorphic grade.Fourth, the middle Yanshanian Epoch is the key period for CBM reservoiring, oneside, the structural differentiation decided distributional characteristics of coal rank, whichis so important to physical property of coal reservoir; On the other side, the tectonicthermal event had not only increased the thermogenic gas, but also promoted the growth ofthe pore-fracture system, the permeability and adsorbability of coal reservoir wereimproved available.Fifth, the characteristics of CBM reservoiring under tectonic control in westernGuizhou was profoundly revealed:1) The CBM reservoir occurrence in the open syncline,and characterized by syncline-controlled gas;2) The scope of tectonic uplift is large inMesozoic-Cenozoic, and the depth of CBM reservoir is small;3) Multiple phases ofhydrocarbon generation, and high CBM content;4) The physical properties are greatlyvaried, significantly controlled by coal rank;5) The tectonic thermal event improved thephysical properties of coal reservoir available;6) The procession of CBM reservoiring iscomplex and the fractionation in different zone is clear.