Coal petrography characteristics and coal facies constraints of Kezilenur Forma-tion coal seams in the northern tectonic belt of Kuqa Depression

LI Ling; TANG Shuling; WANG Bin; TANG Dazhen; ZHAO Tiantian; ZHI Yuanhao; XIE Yani; SHEN Lin; LIU Tuanfei

doi:10.12438/cst.2024-1836

Abstract：Jurassic coal seams are widely developed in the northern tectonic belt of Kuqa Depression,Tarim Basin.Exploration and devel-opment practices in recent years show that the exploration potential of coalbed methane(CBM)resources in this area is huge,and it is ex-pected to become an important guarantee for increasing natural gas reserves and production.Through macroscopic observation and micro-scopic coal composition,industrial analysis,and elemental geochemical analysis,the coal petrography and coal quality characteristics of the Jurassic Kezilenur Formation in the northern tectonic belt of the Kuqa Depression were identified.The coal facies types was identified,and the constraining effect of coal facies on coal petrography and coal quality was revealed.The research results indicate that the coal of the Jurassic Kezilenur Formation has the characteristics of medium-high vitrinite content,low minerals,ultra-low moisture,low-medium ash,medium-high volatile matter,and medium fixed carbon.Vertically,the coal petrography and coal quality of the upper No.0,No.2,and No.3 coal seams are well developed,while No.1 coal seam has strong heterogeneity.The mineral sources in coal are mainly terrestrial debris,supplemented by endogenous debris,and most coal seams were formed in weak oxidation-weak reducing and reducing environ-ments,with only parts of No.l and No.5 coal seams in the YT1H well formed in an oxidizing environment.The coal facies of the Kezilen-ur Formation are divided into four types,from land to lake,the coal facies are orderly distributed in the form of wet forest swamp facies,shallow water-covered forest swamp facies,wetland mixed vegetation marsh facies and low position herbaceous marsh facies.The coal-forming plants in YT1H well are woody plants,and the depth of water-covered of swamp changes greatly,and the wet and shallow water-covered forest swamp facies appear alternately.The coal-forming plants in DT1 well are mainly aquatic herbaceous plants,which have large water-covered depth and strong water retention,and the coal facies belongs to low position herbaceous marsh facies of eutrophic type.The coal-forming plants in MQ1 well are mainly herbaceous plants,supplemented by woody plants,and the coal facies is wetland mixed vegetation marsh facies,which has medium degree of water-covered and is a medium-eutrophic swamp.The development charac-teristics of coal petrography and coal quality are subject to the coupled constraints of the location of coal-forming swamps,the degree of water-covered,the type of coal-forming plants and hydrodynamic condition.The low position herbaceous marsh and shallow water-covered forest swamp facies with high degree of water-covered,strong reducibility and weak water mobility are the most favorable coal fa-cies types in the study area,which can be used as the basis for the selection of advantageous coal development zones.