Reservoir characteristics and enrichment conditions of shale gas in the Carboniferous-Permian coal-bearing formations of Qinshui Basin

Qinshui Basin, as one of the most important coal-bearing basins in China, not only has plenty of coal and coal-bed methane resources, but also has a lot of shale reservoirs. However, there is little research on the characteristics and potential evaluation of shale gas reservoirs in this basin. In this paper, we studied the characteristics of shale gas reservoirs in the Upper Paleozoic of Qinshui Basin, China. Comprehensive experimental methods, including X-ray diffraction, NMR, FIB-SEM, microscopic identification of thin sections and nitrogen adsorption etc. were applied to analyze the characteristics of organic geochemistry, rock and mineral composition and pores evolution of organic-rich shale gas reservoirs. On this basis, the exploration and development potential of shale reservoirs in the study area is evaluated. The results show that different types of pores and micro fractures developed here, which provide enough spaces for the storage of shale gas. Mineral pores, mainly including intergranular pores and intercrystalline pores in shapes of plate, triangle or irregular are well developed, whereas only a small amount of organic pores in shapes of dot or occasional ellipse developed. Porosity has a large specific surface area, which has a range from 2.84 m2/g to 6.44 m2/g with an average of 4.26 m2/g. The average value of pore size distribution is between 3.64 nm and 10.34 nm, which means mainly meso-pores developed. The appropriate ratio of mineral composition, which is composed of 57.5% of clay minerals and 41.3% of brittle minerals, is pretty good for the development of mirco-pores, gas absorption and fracturing. High value of TOC and Ro, caused by abnormal thermal gradient in Mesozoic, provided favorable conditions for shale gas formation and storage. On the whole, though the burial depth is shallow, there is great exploration and development potential for shale gas in the C-P period in the Qinshui Basin because the organic chemical conditions, mineral composition and reservoir characteristics are quite suitable for the formation and storage of shale gas. © 2016, Editorial Office of Earth Science Frontiers. All right reserved.