Geochemical, Mineralogical, and Lithofacies Characterizations of Lacustrine Shale: A Case Study From the Cangdong Sag, Bohai Bay Basin, China

The study of lithofacies identification and its distribution characterizations of shales are essential for the geological evaluation of shale oil exploration and development. In this study, core description, optical microscope, total organic carbon (TOC), whole-rock X-ray diffraction (XRD), Rock-Eval pyrolysis, X-ray fluorescence (XRF), scanning electron microscope (SEM), and well logging data were used in the qualitative analyses of the geochemical and mineralogical characteristics of shales. Based on TOC, sedimentary structure, and ternary diagram of siliceous minerals, carbonate minerals, and clay minerals, eight predominant lithofacies were identified in the shale of the second member of the Kongdian Formation(Ek2): ①Low to moderate organic matter content ( referred to as "Low to moderate TOC") massive carbonaceous mudstone (C-1); ②Low to moderate TOC laminated carbonaceous mudstone (C-2); ③Low to moderate TOC layered mixed carbonaceous mudstone (C-3); ④Moderate to high organic matter content (referred to as " Moderate to high TOC ") laminated carbonaceous siliceous mudstone (S-1); ⑤Moderate to high TOC massive mixed siliceous mudstone (S-2); ⑥Moderate to high TOC laminated clayey siliceous mudstone (S-3); ⑦Low to moderate TOC massive mixed mudstone (M-1); ⑧Moderate to high TOC layered carbonaceous/ siliceous mixed mudstone (M-2). The favorability of lithofacies was investigated based on TOC, pyrolysis parameters, pore structures, and specific mineral contents. S-1 was currently considered as the most favourable lithofacies with excellent hydrocarbon potential, high amount of free hydrocarbon, and abundant organic pores, S-2, S-3, and M-2 were considered as favourable lithofacies, while C-1 and M-1 were ranked as the least-promising lithofacies in the research area. The lateral distribution of the shale is quite different, and Ek23 contains the most favourable lithofacies for shale oil exploration.