Study On Geological Controlling Factors Of Methane Weathering Zone In Fukang Mining Area,Southern Margin Of Junggar Basin

In order to study the distribution characteristics and control factors of methane weathering zone of low rank coalbed methane in Fukang mining area of Xinjiang,avoid exploration risks and reduce development difficulties,according to the characteristics of low methane content and concentration of low rank coalbed methane,using the test data of methane content and methane concentration in coal seam,this paper summarizes the standards used by domestic and foreign scholars to divide methane weathering zone,and puts forward the multi-parameter division method applicable to methane weathering zone in this mining area:methane content>1m~3/t and methane concentration>70%are used for wells with low degree of thermal evolution(Ro<0.65%),while methane content>1.5m~3/t and methane concentration>70%are used for wells with high degree of thermal evolution(Ro?0.65%).Further,the standard is used to divide the depth of the weathering zone at each point in the mining area,The effects of structure,sedimentation,hydrogeology and coal seam burning degree on the depth of the methane weathering zone are discussed,the reasons for the depth difference of the methane weathering zone at each point are analyzed,and the generation and enrichment mode of coalbed methane are summarized.The results show that the depth of methane weathering zone in Fukang mining area varies from 350m to 700m,showing the characteristics of overall depth of weathering zone and great difference in depth of each point.In terms of structural form,the depth of the compound fold methane weathering zone is less than that of the monocline methane weathering zone and less than that of the anticline weathering zone,which indicates that the compound syncline is better than the monocline and better than the anticline in terms of inhibiting the deepening of the weathering zone;in terms of the stratum dip angle,the south wing of Fukang syncline has a greater angle than the north wing,and the depth of the weathering zone is 530m greater than 350m of the north wing,so,the greater the stratum dip angle is,the deeper the weathering zone is.From the perspective of structural complexity,Dingjiawan syncline has simple structure,with 470m of weathering zone,complex structure at the updip end of Fukang syncline,densely distributed faults,and the depth of weathering zone is 600m.The more complex the structure is,the deeper the weathering zone is.From the perspective of the sealability of the coal seam roof and floor,in the Fukang syncline,the depth of the weathering zone of the Xishanyao Formation is shallower than that of the Badaowan Formation.The mud content in the coal roof of the Xishanyao group is higher than that of the Badaowan group.It can be seen that the tightness of the cap layer under the control of sedimentary evolution has an inhibitory effect on the deepening of the weathering zone.From the perspective of hydrodynamics,the hydrodynamic conditions of Fukang Syncline in the western region are stronger than those in the eastern region,and the weathering zone in the western region is deeper than that in the West Bank of Baiyang River,which indicates that the point with strong hydrodynamics will bring the methane in the shallow coal seam to the deep and deepen the depth of the weathering zone.In terms of the extent of biogas replenishment,the proportion of biogas in Fukang Syncline in the west area is less,and the depth of the weathering zone is 600m,while that in Baiyanghe in the east area is more,and the point of biogas replenishment is more,so the depth of the weathering zone is also shallow.In terms of coal seam burning degree,the stronger hydrodynamic force in the fire zone is easy to drive methane to escape.In addition,the head pressure formed in the fire area plays a good role in sealing the deep CBM.Therefore,the fire depth determines the upper limit of the depth of the weathering zone,that is,the depth of the weathering zone is always deeper than the depth of the fire.Finally,the generation and enrichment mode of coalbed methane in Fukang mining area is summarized,which is bounded by 800m and 1000m,followed by biogenic gas belt,mixed belt and thermogenic gas belt from top to bottom.