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<p><strong>Abstract：</strong>Base on the identification of rock flakes, XRD, physical property analysis and field EM, the characteristics of the reservoir, the types of the reservoir are systematically analyzed. The results show that the main rock types of the upper sandstone reservoir of Cangdong depression are chip quartz sandstone (52%), followed by feldspar quartz sandstone (32%) and quartz sandstone (16%). Because the average burial depth of the upper sandstone reservoir is greater than 2900m, the existence of native pores is rarely found under strong compaction, and the reservoir space type of this reservoir is mainly secondary dissolved pores and intercrystalline micropores, but there are few cracks in local areas. This reservoir has a mean porosity of 7.71% and mean permeability of 0.59 mD. The types of diagenesis in the upper sandstone reservoir mainly include compaction, cementation, dissolution and metasomatism. Combined with the law of the combination and evolution of Ro and clay minerals and the structural characteristics of the microscopic particles, we can know that the upper sandstone of the second section in the hole of Cangdong depression is in the early stage.</p> <p> </p> <p><span><strong>Keywords: </strong>Cangdong depression; 2</span><sup>nd</sup><span> member of Kongdian formation; upper sandstone; reservoir; diagenesis</span></p>

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<p><span>The second member of the Kongdian Formation (Ek</span><sub>2</sub><span>) 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 Ek</span><sub>2</sub><span>, 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 Ek</span><sub>2</sub><span> 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 II</span><sub>1</sub><span> kerogens. Laminated felsic shales and laminated mixed shales had obvious advantages in the thermally extractable hydrocarbon content (S</span><sub>1</sub><span>) 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.</span></p>

The lacustrine shale in the second member of the Kongdian Formation (Ek2) is the most significant target of shale oil exploration in the Cangdong Sag, Bohai Bay Basin, China. To investigate the occurrence mechanisms and to reveal the influencing factors of shale oil mobility in Ek2, a series of analyses (X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), total organic carbon (TOC) analysis, Rock-Eval pyrolysis, low-temperature nitrogen physisorption (LNP), mercury intrusion porosimetry (MIP), and multiple isothermal stage (MIS) pyrolysis) were conducted on samples collected from well cores in the Cangdong Sag. The results show that the lithofacies can be categorized as laminated felsic shales, laminated and massive mixed shales, and laminated and massive carbonate shales. The shales were characterized by a 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, compared with other lithofacies, had clear advantages in the amount of free hydrocarbon that can be volatilized from the rock (S1), the oil saturation index (OSI) value, and the free oil and movable oil content. LNP, MIP, and MIS pyrolysis analyses show that the residual shale oil mainly occurred in pores with diameters smaller than 200 nm, and the pore diameter when residual oil occurred in some laminated shale samples could reach 50 μm. The lower limits of the pore diameter where free oil and movable oil occurred were 7 and 30 nm, respectively. The mobility of shale oil is controlled by the shale oil component, thermal maturity, TOC content, and pore volume. The results herein provide a basis for the evaluation of optimal shale oil intervals.