Study On Fine Description And Main Controlling Factors Of Coal Reservoir Of Chengzihe Formation In Jixi Basin

This paper mainly studies the main control factors of coal reservoir characteristics and physical properties of Chengzihe Formation in Jixi area.The pore and fissure structure and its development characteristics of coal reservoir are described by means of low temperature liquid nitrogen,nuclear magnetic resonance and high pressure mercury injection.Combined with relevant experimental testing methods,the differences of metamorphic degree and material composition of coal and rock in the north and south depression zone are compared and analyzed.The gas content and insitu stress characteristics of coal reservoir are analyzed by isothermal adsorption experiment and in-situ stress test.Based on the above test and analysis results,the main controlling factors of gas content,adsorption and pore fracture development of coal reservoirs in this area are discussed.Long flame coal,fat coal and coking coal are developed in Chengzihe Formation.The coal metamorphism in the northern depression belt is higher than that in the southern depression belt.The test results of macerals and industrial analysis show that the content of coal vitrinite(48%～95.9%)is relatively high,and the content of inertinite(1%～16.8%)is relatively low,with a small amount of crust(0.1%～3.4%).The coal environment of Chengzihe is continental,so the ash yield of coal and rock is high and the variation range is large.The ash yield is(9.96%～40.36%),the volatile matter(5.7%～42.74%)is the second,the moisture(0.26%～3.12%)is very low,and the fixed carbon is 16.35%～69.58%.The coal rock in the northern depression zone is affected by magmatic hydrothermal metamorphism,and the coal rock has a high degree of metamorphism.Therefore,the ash yield and fixed carbon in the northern depression zone are higher than those in the southern depression zone,and the volatile matter and moisture are lower than those in the southern depression zone.The adsorption-desorption curves of coal and rock obtained by low-temperature liquid nitrogen experiment mainly have two types of A and B.Type A is represented by RH-7,RH-8,CS36 and DH35,reflecting the pore structure as "ink bottle type" hole,which is conducive to the adsorption and storage of CBM and is not conducive to the desorption and seepage of CBM.Class B is represented by PG14,PG32,DD28 and CS-3.Most of these pores are "parallel plate" and "wedge" with one end closed,which is beneficial to the desorption and seepage of coalbed methane,but not conducive to the adsorption and storage of coalbed methane.The high pressure mercury injection experiment reflects the pore combination types of coal and rock,and the different effects of various pore combinations on the desorption and seepage of coalbed methane.NMR experiments are used to identify the types of pore fractures and their proportions in coal reservoirs.The morphology,size and density of microscopic fractures of coal and rock in this area were counted by fluorescence microscope.The results show that the content of C and D type microscopic fractures is higher,and the content of A and B type microscopic fractures is lower.Finally,the microscopic pores of Chengzihe Formation coal reservoir are mineral pores and organic pores by scanning electron microscopy.The adsorption performance of coal reservoir is obviously affected by coal rank change,macerals,moisture content and ash yield.The difference of coal metamorphism in the north-south depression zone affects the gas content of coal reservoir to some extent.At the same time,the pore-fracture development characteristics,porosity and permeability of coal reservoirs are controlled by regional magmatic thermal metamorphism,which changes the physical properties of coal reservoirs in Chengzihe Formation.