Analysis Of The Three History Of Source Rocks And Gas Controlling Characteristics Of Coal Structure In Jixi Basin And Hegang Basin

In this paper,the main mining coal seams and source rocks in Jixi Basin and Hegang Basin are taken as research objects.A combination method,including geological data analysis,field investigation,underground description,experimental test analysis,and software simulation,is used to analyse the variation of coal structure in different coal mines,and to study the relationship between coal structure and coal reservoir characteristics,CBM reservoir.The types of CBM reservoir in different coal seams is classified too.Beisides,through the experimental test,the source rocks are evaluated,and the three histories of Jixi Basin and Hegang Basin are simulated based on the basic data of CBM wells,and then,the influence of the burial history,thermal history,hydrocarbon generation history on the CBM reservoir are analyzed.The coal structure is dominated by fragmented coal with a small amount of granulated coal and mylonitized coal distributing in Jixi Basin.The coal consistence coefficient is relatively high,and the coal reservoir is mainly pressure-type.Coal body is seriously damaged in Hegang Basin where the coal structure are mainly fragmented coal structure and granulated coal structure with mylonitized coal distributing in parts of the basin.The consistence coefficient of coal body in Hegang Basin is higher than that in Jixi Basin,and coal reservoir are mainly stress type and transition type.Coal reservoirs in Jixi Basin,including NO.14 coal seam in Pinggang Mine,NO.3 coal seam in Chengshan Mine,NO.3 coal seam in Lishu Mine,NO.27 coal seam in Zhengyang Mine and NO.3 coal seam in Zhangxin Mine,are all belong to pressure type reservoir.Coal reservoirs in Hegang Basin,including NO.15 and NO.18 coal seam in Yixin Mine,NO.11 and NO.23 coal seam in Xingan Mine,NO.11 coal seam in Nanshan Mine,NO.11 coal seam in Niaoshan Mine,are all belong to the stress-type gas reservoir;and NO.23 coal seam in Junde Mine belongs to the transition-type gas reservoir.The reservoir pressure,permeability and gas saturation of pressure-type gas reservoir is larger than stress-type gas reservoir,and the coal structure is less damaged.The stress-type gas reservoir is in a high stress closed state,which is strongly affected by tectonic action,relatively low permeability and low gas saturation.Three histories have influenced the current gas reservoirs in the study area from the aspects of structure,metamorphism of coal,effective burial depth,and roof and floor of coal seam lithology,which are the reasons for the differences in the current CBM reservoirs.Two large-scale syncline geological structures,with deep buried depth of syncline axis and high reservoir pressures,is one of the factors affecting the gas content and gas saturation of the coal seams in Jixi Basin.The strong tectonic action in Hegang Basin causes high-stress-closed reservoir state and seriously damaged coal body structure,which is one of the reasons for the low permeability and gas saturation of CBM reservoirs in Hegang Basin.The main hydrocarbon-generation source rock in Jixi Basin and Hegang Basin is coal which belongs to great hydrocarbon source rocks and III-type kerogen and has strong gas generation ability.The degree of coal maturity in Jixi Basin and Hegang Basin all reach the mature stage,contributing to high gas content in the CBM reservoirs.The buried depths of the main coal seams in the study area range from 400 to 1200 meters.The roof and floor rocks of the main coal seam are silty mudstone,siltstone or mudstone and mudstone interbed.The large buried depth and the good sealing cover are also the reasons for the preservation and aggregation of coalbed methane.