The Characteristics Of Sedimentary Microfacies And Gas Content Of The Late Paleozoic Coal-Bearing Formations In The Gujiao Area

Combining field geological survey,basic data of coalbed methane(CBM)wells and mathematical geology method,the gas content,sedimentary environment and sedimentation of the Late Paleozoic coal-bearing formations in the Gujiao area,Xishan coal field,have been analyzed in this dissertation by the methods of sedimentology,petrology,CBM geology and mathematical geology.On this basis,the effects of sedimentary environment and sedimentation on gas content have been further explained.The sedimentary facies of Carboniferous Taiyuan and Permian Shanxi formations were analyzed,and the results show that,the Late Paleozoic coal-bearing formations in the Gujiao area are mainly consist of barrier coast,carbonate platform and fluvial controlled delta facies,which can be further divided into 10 microfacies.Taiyuan formation is mainly barrier-lagoon-tidal flats facies,and Shanxi formation is dominated by deltaic plain facies.The No.2 coal seam in Shanxi formation and No.8,No.9 coal seams in Taiyuan formation are main minable coal seams in the Gujiao area with average thickness are 2.2 m,5.0 m and 4.3 m,respectively.Generally,the coal seams of Taiyuan formation are thicker than those in Shanxi formation,and the gas content in coal seams of Taiyuan formation also higher than in coal seams of Shanxi formation.Compared with the No.8 and No.9 coal seams of Taiyuan formation,the No.2 coal seam of Shanxi formation has relatively sample structure,higher ash content,lower sulfur content,higher C02 concentration and lower CH4 concentration.Based on the sedimentary facies analysis,the effects of sedimentary environment on gas content have been investigated considering burial depth,thickness,physical properties and country rock features of coal.The results show that,the axial of Malan syncline splits the Gujiao area into the east and west wings,and they have totally different relation between burial depth,thickness,physical properties,country rock features of coal.(1)In the east wing,the gas content increases with increasing burial depth in shallow buried section(the critical points of the No.2,No.8 and No.9 coal seams are 550 m,640 m and 650 m,respectively).The gas content of coal seams decreases with increasing burial depth when the burial depth are deeper than these critical points.However,this phenomenon is not obvious in the west wing.(2)In both wings,the coal thickness of the No.2,No.8 and No.9 coal seams has notable correlation with gas content when burial depth is deeper than 550 m,shallower than 640 m and shallower than 650 m,respectively.(3)The ash content of the No.8 and No.9 coal seams have outstanding effects on gas content,and higher ash content is corresponding to lower gas content.(4)About the effects of country rock on gas content,the gas content are controlled by the roof lithology,roof thickness and the thick sandstone distributed upper coal seam.Larger distance of coal seam and thick sandstone is corresponding to higher gas content in the coal seam.