Lithofacies and pore characterization of continental shale in the second Member of the Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, China

Abstract

Abstract The shale of the second Member of the Kongdian Formation (Ek2) in the Cangdong Sag is continental shale with vitrinite reflectance values (%, Ro) ranging from 0.45% to 0.84%. This study adopted systematic microscopic observation and experimental analysis to examine lithofacies and pore structure of the Ek2 shale. Four primary lithofacies are identified based on mineral compositions and sedimentary structures, namely laminated siliceous shale, laminated argillaceous shale, laminated mixed shale and laminated calcareous shale. Scanning electronic microscopy (SEM) images reveal that organic matter pores are less abundant than inorganic mineral pores. Inorganic mineral pores are mostly developed between rigid minerals, such as quartz, feldspar, calcite and dolomite. Organic matter pores are mainly presented in high mature shales and some are under protection of mutual support from mineral grains. Results of low temperature N2 adsorption analysis show that micropores are less developed than mesopores and macropores. For shales with Ro ranging from 0.45% to 0.60%, pore volumes decrease with increased Ro; while for shales with Ro ranging from 0.60% to 0.84%, pore volumes increase with increased Ro. Total organic carbon (TOC), analcime, plagioclase and calcite contents all have positive correlations with pore volumes for shales with Ro ranging from 0.60% to 0.84%, and dissolved pores make greater contribution to pore volume and permeability than organic matter pores in Ek2 shale. The geochemical analysis results show that laminated argillaceous shale has highest average TOC content of 5.20%, followed by laminated siliceous shale of 3.94%, laminated mixed shale of 3.04% and laminated calcareous shale of 1.27%. Considering study above, it is inferred that laminated siliceous shale and laminated mixed shale with higher thermal maturity are favorable for petroleum exploration in Ek2 shale.