Abstract:

On the overmature Wufeng-Lungmachi marine shale in the Sichuan Basin, China, we carried out a detail investigation through standard thin section observation, argon ion polishing sheet production and SEM imaging, X-ray diffraction of whole rock component, clay mineral analysis, TOC content determination, N₂-CO₂ adsorption experiment, and RG measurement. The results show that the gas-bearing shale contains organic and inorgainc pores and micro-fractures. The organic pores are bubbly or sponge shaped, have various sizes, and are mostly evenly distributed in organic matters. The inorganic pores are mainly triangular, angular or rectangular shaped and can be divided into quartz intercrystalline and dissolved pores. The micro-fractures are mostly strip-shaped and connected to the organic and inorganic pores to form a network. In the overmature marine shale samples, organic pores exceeded 97% and inorganic pores and micro-fractures were less than 3% in quantity. The overmature shale is composed of more than 80% micro and mesopores, with pore size <10 nm. For meso and macropores with pore size above 10 nm, the organic pores were more than 97% in quantity and more than 50% in plane porosity. In comparison, the inorganic pores and micro-fractures were less than 3% in quantity and less than 50% in plane porosity. The pore size of mesopores ranged from 5 to 400 nm and those with pore sizes below 20 nm exceeded 70% in quantity. Of these, the plane porosity of organic pores increased with increasing pore size and reached the maximum value with pore size between 100-400 nm. The inorganic pores are composed mainly of intercrystalline pores and dissolved pores and had the highest plane porosity (nearly 100%) with pore size between 100-400 nm. The organic pore porosity increased with increasing TOC content, reaching the 5.5% maximum value and then decreasing progressively. The plane porosity of Layer 1 of sub-Member 1 was the highest, decreasing gradually from bottom to top in the Lungmachi Formation. In addition, plane porosity varied for different areas and the Luzhou and Yuxi areas area had the highest and lowest values, respectively. In the Wufeng-Lungmachi shale, the intercrystalline porosity was positively correlated with the siliceous mineral content due to material hardness and organic origin. Besides, the dissolution porosity was positively correlated with the carbonate mineral content as a result of dissolution under burial and compaction. This pattern is possibly relating to reduced pore size caused by collapse under burial and limited resolution of FE-SEM imaging.

Key words: marine shale, pore type, pore size distribution, overmature, Lungmachi Formation, Wufeng Formation, Sichuan Basin