Micropore Characteristics and Genesis of Shale Reservoirs in Coal Measures in Huainan Coalfield

The micropores (pore width < 2 nm) developed in shale reservoirs in coal measures are of great significance to the adsorption and occurrence of mud shale gas. This paper studied the marine-continental Carboniferous-Permian mud shale reservoirs in coal measures in Huainan Coalfield by the experiments of R_o,max, TOC, XRD and CO₂ adsorption. The micropore structure characteristics of reservoirs were revealed and the controlling factors and genesis were discussed. The results show that the maturity of organic matters in Taiyuan Formation, Shanxi Formation and Lower Shihezi Formation is between low mature and mature stage. Lower Shihezi Formation, deposited in the delta-plain, has the highest content of TOC and clay mineral, and the micropores are the most developed one in the three formations, followed by barrier dams-lagoon facies of Taiyuan Formation. Shanxi Formation, deposited in the delta front and pro-delta facies, has the lowest content of TOC, moderate content of clay mineral, and the least developed micropores. There are three peaks in micropore width distribution, namely peak 1 (0.366 5 nm), peak 2 (0.457 7-0.627 2 nm) and peak 3 (0.821 6 nm). Micropore volume has a positive correlation with the content of TOC and clay minerals, but a negative correlation with the content of brittle minerals. Therefore, it is concluded that the reservoirs mainly develop micropores of organic matters and clay minerals. The development of different types of micropores is controlled by different content of organic carbon and mineral composition under different types of sedimentary environment and diagenetic evolution. Micropores of peak 1 are attributed to the main pores of clay minerals and a small number of organic matter pores; peak 2 are dominated by both clay mineral pores and organic matter pores; peak 3 are mainly organic matter pores and a small amount of clay mineral pores. Organic matter micropores of peak 1 + peak 2 are supposed to be corresponding to aromatic ring interlayers pores, and peak 3 are columnar pore formed by the ordered stacking of aromatic ring or others. The micropores in clay minerals are in the internal layer of kaolinite and chlorite or the micropores formed in the late diagenesis transformation. All the above micropores may also developed in the complex of organic matters and clay minerals.