Micro-pore Structure and Gas Accumulation Characteristics of Shale in the Longmaxi Formation, Northwest Guizhou

For further study on characteristics of micro-pores in the marine shale reservoirs in Northwest Guizhou, shale samples in the Lower Silurian Longmaxi Formation were selected to investigate effect of micro-pores on the characteristics of gas accumulation in shales, through methods of the argon broad ion beam-field emission scanning electron microscope and the nitrogen adsorption/desorption, analysis on the characteristics of micro-pores and related geochemical analysis. Results show that there are seven types of pores in the Longmaxi Formation shale, including interparticle pores, intraparticle pores, intercrystalline pores, dissolution pores, fossil pores, organic pores, and microfractures, among which the interparticle pores and organic pores are best developed. According to the nitrogen adsorption/desorption curves, the pore structures can be classified into three types, including the cylindrical pores with both opening ends, narrow parallel-plate pores and tapered parallel-plate pores with four opening sides. Diameter of micro-pores ranges from 2 to 64 nm, mainly at 2-6 nm. The diameter of micro-pores (less than 2 nm) ranges from 0.4 to 1.8 nm, The micro-pores with a diameter of 0.4-1.0 nm contribute most to the pore volume. Pore volume is dominated by meso-pores (2-50 nm) with a proportion of 83.1%. The micro-pores and meso-pores make major contribution to the specific surface area of pores with proportions of 20.1% and 79.3%, respectively. The total organic carbon (TOC) is the major factor controlling development of nanopores. Different pore types have different characteristics of gas occurrence and migration, indicating that nanopores provide favorable conditions for the occurrence and microscopic migration of gas in shales.