Microscopic characteristics of pore structure and evolution in the coal-bearing shale

Coal-bearing shale reservoirs are widely distributed in China, which mostly deposited in a large time span from Carboniferous to Neogene and were formed in continental and marine-continental environment.Coal-bearing shale reservoirs have relatively small monolayer thickness but large accumulated thickness and are horizontally varied.The organic matter abundance is large with the total carbon content (TOC) range of 0.5%-30%, the organic maturity has a large range with the vitrinite reflectance (R_o) of 0.5%-2.5%, and the kerogen is mainly humic type.The mineral composition is complex with high content clay minerals (average> 50%) and low content brittle minerals (average< 40%).In this study, some fresh core shale samples were taken from the Longtan formation in Sichuan basin, the Shanxi formation in Ordos basin and the Badaowan formation in Junggar basin.The different characteristics of morphology, spatial distribution and origin were analyzed by the experiment of field emission scanning electron microscopy (FE-SEM).In addition, a comprehensive and effective method was established to clarify the pore structure and evolution characteristics in coal-bearing shale reservoir by the combination of high pressure mercury injection, CO₂ and N₂ adsorption-desorption experiments.Coal-bearing shale develops with various types of pores, including organic matter pores, inter-particle pores, intra- particle pores and mirco-fractures.The inter-particle and organic matter pores are most common in the pore system.Different pore types have the distinct genetic mechanism, morphology and pore size distribution.The main diameter distribution ranges of the nanoscale pores are < 6.5 nm and 80-200 nm, in which the pores with the size below 10 nm play a dominant role in the nanoscale pore system of coal-bearing shale.Pore shapes of slits, wedges and inkpots are mostly developed in coal-bearing shale, and these pores have strong heterogeneity.Effected by both diagenetic evolution and thermal evolution, the evolution process of pore in coal-bearing shale can be divided into five stages, including the unripe stage of rapid compaction (R_o< 0.7%), mature hydrocarbon dissolution stage (0.7%< R_o< 1.3%), high mature pore sealing stage (1.3%< R_o< 2.2%), high-over mature pore secondary cleavage stage (2.2%< R_o< 2.7%), and over mature slow compaction stage (R_o> 2.7%).