Coal-forming Environment Of The Extra-thick Coal Seam And Its Control Of Reservoir Pore Characteristics In Shengli Coalfield,Erlian Basin

The research on coal-forming environment and its control of reservoir pore characteristics in the low-rank and extra-thick coal seams is of great significance for CBM exploration and development.This article takes the No.5 coal seam of the Lower Cretaceous Saihantala Formation in Shengli coalfield,Jiergalangtu depression,Erlian Basin as the research object.By means of coal rock and coal quality analysis,organic carbon isotope testing,sporopollen analysis,and X-ray fluorescence,coal-phase parameters and elemental geochemical analysis methods,combined with its geological evolution background,the coal-forming environment of extra-thick coal seams was studied from multiple perspectives including plant community,paleoclimatic change,and the construction of coal-forming micro-environment.The pore structure of coal reservoir was analyzed by mercury injection test,cryogenic liquid nitrogen adsorption,and low-temperature carbon dioxide adsorption test to further explain the evolution rule of pore structure under the control of coal-forming environment.The results show that the No.5 coal seam is characterized by large humic groups,less inert group and mineral group,and few stable groups.It is mainly medium-low ash coal with high moisture content and high volatile matter yield.The sulfur content is low,mainly organic sulfur,and inorganic sulfur is mainly pyrite.The coal-forming plants are mainly gymnosperms and ferns,containing a small amount of bryophytes and algae,and the climate is dominated by warm and humid climate during the coal-forming period.The sedimentary environment is dominated by the shoreline shallow lake.The water body is alkaline,and the swamp is subject to long-term supply of terrigenous clastic material.Combined with the geological evolution background,the coal-forming period is divided into the initial sedimentary period,terrigenous recharge period and the accumulation of extinction period.The large pores,mesopores,and small pores are well developed in the study area,which are mainly open pores.The pore types are mainly wedge-shaped holes and ink bottle holes,and the pores have good connectivity.The pore structure is mainly affected by the vegetation type,the preservation degree of the plant structure and the incorporation of foreign substances.The lower the degree of gelation,the worse the fluidity of the water,the smaller the fragmentation,the less the injection of terrigenous clastic material,the better the preservation of the plant structure.The lower the ash yield,the greater the pore volume,the pore structure more reasonable configuration.