Study On Coalbed Methane Accumulation Model Of The Middle Of Kuqa-Bay Coalfield Locating In The Northern Margin Of The Tarim Basin

The Kuqa-Bay Coalfield in the northern margin of the Tarim Basin is rich in coalbed methane(CBM)resources and has great exploration and development potential.The coal seam of the Taliqike Formation has a medium degree of thermal evolution and steep inclination,which is significantly different from other mining areas of Xinjiang province and the Qinshui Basin with a high degree of CBM development in China.Taking the steep inclined coal reservoir in the middle area of Kuqa-Bay coal field as the research object,with the basis of measurements on coal rock,CBM drainage water sample,and combining with the analysis of the geological characteristics of the coal field,this paper analyzes the physical characteristics and gas bearing characteristics of the coal reservoir,aiming at revealling the main factors controlling CBM enricnment and establishing CBM accumulation mode in this area.The main achievements are as follows:(1)In the middle of Kuqa-Bay Coalfield,the main type of coal rock is semi-dark coal,followed by semi-bright coal;the main structure of coal body is fragmentary structure,followed by primary structure,with a small amount of fragmentary structural and mylonite structural coals,and the integrity of coal body is good.Micro-pores and transition pores are most developed,followed by meso-pores,which are favorable for methane adsorption and CBM migration;Medium-and high permeability reservoirs with good physical properties are developed in the study area.(2)CH₄ is the main gas component of CBM in study area.As burial depth increases,the CBM concentration tends to increase.The gas saturation is mainly controlled by hydrogeological conditions,and gas content and gas saturation increases as burial depth increase,indicating that gas content of deep coal is good.The coal seam Langmuir volume(V_L)is ranged between 15.00 and 30.00m³/t.The gas saturation of coal reservoir is ranged between 59.43%and 91.52%,with an average greater than80%,The gas saturation condition can reach good to excellent level,which is conducive to the development of CBM in study area.(3)Based on the results of gas composition and isotope analysis,combined with the sedimentary geological conditions of CBM,it is considered that CBM of the Kuqa-Bay Coalfield is thermal in origin.R_O values are ranged between 0.60%and1.00%,locating in a large amount of gas generation stage,providing rich gas sources for coal reservoirs.Most of the reservoirs in the area are medium permeability and high permeability,which provide favorable conditions for high production of coalbed methane.The increase of coal reservoir pressure is conducive to the adsorption of coalbed methane,which is favorable for increasing gas content of the coal reservoir.The main coal seam has good surrounding rock conditions,which play a role of lateral sealing for coalbed methane enrichment.The stagnant water in the burnt rock area plays a role of hydraulic sealing and hydraulic plugging for coalbed methane in shallow coal.In summary,the accumulation model of steep inclined coalbed methane in the middle area of Kuqa-Bay coalfield has been established,which has been proved by exploration practice.