Heterogeneous characteristics of various lamina assemblage in shale of Shahejie Formation of the Zhanhua Sag, Bohai Bay Basin, eastern China

The Eocene Shahejie Formation in the Zhanhua Sag of the Bohai Bay Basin has abundant shale oil resources. However, lacustrine shale is heterogeneous in mineralogy, sedimentary structure, organic matter, pore structure, paleoenvironment, and oil content. Therefore, the laminae assemblages are divided into laminated (lamina thickness of < 0.01 m), layered (0.01~0.1 m) and massive (no layer or layer spacing of > 0.1 m) types shale to investigate the core, X-ray diffraction (XRD), thin section, field emission-scanning electron microscopy (FE-SEM), mercury injection capillary pressure (MICP), N₂ adsorption and geochemical experiments. The results show that the oil shale of Shahejie Formation in Zhanhua Sag is abundant in calcite. The TOC content ranges between 1.35% and 5.55%. Rock-Eval S₁ and S₂ values range from 0.30 to 2.56 mg/g and 0.97~15.47 mg/g, respectively. Variable kinds of nanopores, micropores, and microfractures are commonly observed. The connectivity of pores in laminated and layered shale samples is relatively better than massive shale. The massive shale was formed in a sedimentary environment with warm and humid climate, moderate salinity, weak reduction conditions, high productivity, and paleo-setting rate. Organic matter is derived from lower bacteria, algae, and terrigenous plants. Laminated and layered shale are formed in an arid and cold environment with high salinity, strong reducibility, low paleo-setting rate. Laminated shale is located on a gentle slope at the basin margin, which is the focus of shale oil exploration because of their higher hydrocarbon generation potential, reservoir quality, and advantaged horizontal fracturing conditions. The above results have implication for the sweet spots prediction in shales with similar geological setting.