Reservoir Physical Properties And Logging Identification Of Different Macrolithotypes Lignite In Erlian Basin

In recent years,China has achieved success in the development of coalbed methane in bituminous coal and other medium-and high-grade coal,such as Qinshui,Ordos.Erlian Basin is one of the key areas of lignite coalbed methane exploration in China,which has the characteristics of large coal-bearing depression,large coal seam thickness and biogas recharge,but the development progress of lignite CBM resources is still slow.There are great differences between lignite and bituminous coal in reservoir physical properties,so it is particularly important to study the reservoir physical properties of lignite systematically.This paper systematically collects the coalfield geological exploration and coalbed methane measured data of lignite in Erlian Basin,and comprehensively uses the theories and methods of coal petrology and coalbed methane geology.This paper systematically analyzes the geological background,coal accumulation mode and reservoir physical characteristics of macrolithotypes lignite in Erlian Basin,and probes its logging identification marks,so as to provide for the exploration and development of lignite CBM resources in Erlian Basin.The coal accumulation period of Erlian Basin is mainly Mesozoic Jurassic and early Cretaceous.There are three coal accumulation models in the basin:semi-graben type,graben type and depression type.The macrolithotypes lignite in the study area are mainly woody brown coal,clastic coal and silky coal.The main macerals are humic group and inertinite group,and the contents of three types of lignite stable group are relatively low.The water content of lignite in the basin is low and the fixed carbon content is high.Coal facies analysis shows that lignite is mainly formed in wet forest swamp and water forest swamp.Low-temperature CO₂ adsorption experiments show that the adsorption of isothermal clastic coal on lignite samples is fastest and the adsorption capacity is the highest.According to the low-temperature nitrogen adsorption curve,the pores of lignite can be divided into three types:type A is represented by clastic coal,pore type is mainly micropore,type B is represented by silk coal,pore is mainly represented by small pore and micropore,type C curve is represented by woody brown coal,pore type is semi-open pore.The mercury injection curve shows that the macropore content of lignite from silk coal to woody brown coal and clastic coal decreases gradually,while the content of mesopore and transition pore increases gradually.Low-field NMR experiments show that the pore development of lignite in Erlian Basin is concentrated and the connectivity is good.The isothermal adsorption experiment shows that the adsorption capacity of lignite decreased gradually from silk coal to woody brown coal and clastic coal.Based on the comprehensive analysis of the experiment,the woody brown coal has the potential of CBM resources.Different logging methods have different responses to different macroscopic coal and rock types of lignite in the study area.The logging value of deep-lateral resistivity increases gradually from silk coal to clastic coal and woody brown coal,the interval transit time difference from silk coal to woody brown coal and clastic coal decreases gradually,and the compensated density from woody brown coal to clastic coal increases gradually.the Compensated Neutron is getting higher and higher from silk coal,woody brown coal to clastic coal.Furthermore,the logging identification standards of different macrolithotypes lignite in this area are summarized based on a variety of logging data.Taking well Jimei 3 and well Jimei 4 as examples,the macrolithotypes lignite are identified.Among them,woody brown coal is widely developed in coal seams,and the coal seams are thick,while clastic coal and silky coal are thin.