Occurrence characteristics of lacustrine shale oil in the second member of the Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, China

The second member of the Kongdian Formation (Ek₂) in the Cangdong Sag has become an important field of shale oil exploration in the Bohai Bay Basin. To investigate the occurrence characteristics and discuss the controlling factors of shale oil mobility in the Ek₂, the research presented in this study is based on core and thin section observations, XRD analysis, total organic carbon (TOC), Rock-Eval pyrolysis, multiple isothermal stages (MIS) pyrolysis, low-temperature nitrogen physisorption (LNP), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). The results show that the Ek₂ shales can be classified into five types of lithofacies, including laminated felsic shales, laminated mixed shales, massive mixed shales, laminated carbonate shales, and massive carbonate shales. The shales were characterized by high organic matter abundance and moderate thermal evolution with good to excellent hydrocarbon generation potential and contained a high abundance of Type I and II₁ kerogens. Laminated felsic shales and laminated mixed shales had obvious advantages in the thermally extractable hydrocarbon content (S₁) value, oil saturation index (OSI) value, free oil, and movable oil content with other lithofacies. Analysis of LNP, MIP, and MIS pyrolysis show that the residual shale oil mainly occurred in the pores with diameters smaller than 200 nm, and the occurrence pore diameters of residual oil in some laminated shale samples could reach 50 μm. The lower limits of the occurrence pore diameter of free oil and movable oil were 7 nm and 30 nm, respectively. The mobility of shale oil is controlled by the shale oil component, thermal maturity, TOC content, and pore volume.