Coal-Bearing Organic Shale Geological Evaluation of Huainan–Huaibei Coalfield, China

Abstract

Organic shale in coal-bearing strata contains a large amount of shale gas resource in China. The organic shale is typically interlayered with coal beds and tight sandstone, and thus, joint development of the three unconventional gases, coal bed methane, tight gas, and shale gas, is becoming a frontier topic. In this paper, an integrated characterization of coal-bearing shale was implemented through a systematic series of measurements to assess the potential of shale gas in the Huainan–Huaibei coalfield. According to X-ray diffraction, quartz, kaolinite, and mixed illite/smectite (I/S) are the major minerals and the brittleness index for most samples is low to moderate (30–60). The risk of water swelling during hydraulic stimulation may be low in terms of the low concentration of smectite and mixed I/S. The efficiency of acid solution is limited by the low concentration of calcite and dolomite. The pore system has been well-developed on the basis of the thin section, scanning electron microscopy, mercury intrusion, and N2 physisorption. Microfractures and large macropores (1–15 μm) were observed in many samples. Slit shape is the predominant geometry of pores, followed by eclipse. Meso- and macropores are the best developed pores in coal-bearing organic shale, with two peaks at 2–3 and 100 nm. Specific surface area and clay minerals can significantly affect the gas adsorption capacity, but total organic carbon (TOC) content is the primary controlling factor. Furthermore, the high TOC content and appropriate thermal maturities imply high potential of gas generation with possible contribution from biogenic gas. These analyses indicate the Permian coal-bearing organic shale in the Huainan–Huaibei coalfield being a promising player in shale gas development.