Pore characteristic analysis of a lacustrine shale: A case study in the Ordos Basin,NW China |
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| Institution: | 1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;2. PetroChina Turpan-Hami Oilfield Company, Xinjiang 839009, China;3. MLR Key Laboratory of Genesis and Exploration of Magmatic Ore Deposits, Xi''an Center of Geological Survey, China Geological Survey, Xi''an 710054, China;4. State Key Laboratory of Petroleum Resource and Prospecting, College of Geosciences, China University of Petroleum, Beijing 102200, China;5. College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049, China |
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| Abstract: | Organic shales deposited in a continental environment are well developed in the Ordos Basin, NW China, which is rich in hydrocarbons. However, previous research concerning shales has predominantly focused on marine shales and barely on continental shales. In this study, geochemical and mineralogical analyses, high-pressure mercury intrusion and low-pressure adsorption were performed on 18 continental shale samples obtained from a currently active shale gas play, the Chang 7 member of Yanchang Formation in the Ordos Basin. A comparison of all these techniques is provided for characterizing the complex pore structure of continental shales.Geochemical analysis reveals total organic carbon (TOC) values ranging from 0.47% to 11.44%, indicating that there is abundant organic matter (OM) in the study area. Kerogen analysis shows vitrinite reflectance (Ro) of 0.68%–1.02%, indicating that kerogen is at a mature oil generation stage. X-ray diffraction mineralogy (XRD) analysis indicates that the dominant mineral constituents of shale samples are clay minerals (which mainly consist of illite, chlorite, kaolinite, and negligible amounts of montmorillonite), quartz and feldspar, followed by low carbonate content. All-scale pore size analysis indicates that the pore size distribution (PSD) of shale pores is mainly from 0.3 to 60 nm. Note that accuracy of all-scale PSD analysis decreases for pores less than 0.3 nm and more than 10 μm. Experimental analysis indicates that mesopores (2–50 nm) are dominant in continental shales, followed by micropores (<2 nm) and macropores (50 nm–10 μm). Mesopores have the largest contribution to pore volume (PV) and specific surface area (SSA). In addition, plate- and sheet-shaped pores are dominant with poor connectivity, followed by hybrid pores. Results of research on factors controlling pore structure development show that it is principally controlled by clay mineral contents and Ro, and this is different from marine systems. This study has important significance in gaining a comprehensive understanding of continental shale pore structure and the shale gas storage–seepage mechanism. |
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| Keywords: | Lacustrine shale Pore structure characteristics All-scale pore size analysis Mercury intrusion Gas physisorption The Ordos Basin |
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