There are abundant shale oil and gas resources in the Gulong Sag, Songliao Basin. However, so far, the petrological characteristics of the Gulong shale oil reservoir, especially the types and association of accumulation space, and the state of the Gulong shale oil are not well understood. The core of the Upper Cretaceous Qingshankou Formation in the Gulong Sag is described and analyzed by thin section, electron microscope observation and 3D CT. The reservoir space of the shale oil in the Qingshankou Formation in the Gulong Sag is diverse and mult(i)scale. In addition to nm and μm pores, there are lamellation fractures. Based on the size of lamellation fractures and their relationship with oil and gas, they can be divided into: (1)Nano fissures, width in 10~50 nm, length in 50~100 nm, or longer; at both ends thins out, the center of the tip is wide, and the micro curve is worm- like. It is also an important reservoir space and an important part of nano- level oil element, which is closely related to nano- level pore oil element and micron oil base.(2)Micron fissures, 0. 1 μm to several microns in width, tens of microns to hundreds of microns in length, is closely related to nanometer oil element and micron oil base. (3)Medium micro fissures, several microns to dozens of microns in width, several hundred micrometers in length, and micron oil base close relationship.(4)Large micro-fissures, tens of microns to 100 μm in width, hundreds of microns to a few millimeters in length, and micron oil base and micro- joints and micro- joints are closely related to large lamillation fissures (hundreds of microns in width, visible to the naked eye). As these bedding seams develop, they are often treated as bedding. It is concluded that the lamellation fractures were mainly formed during the tectonic inversion folds at the end of the late Nenjiang Formation and late Mingshui Formation, Upper Cretaceous. In addition, a large number of bedding calcite veins are also developed. The sizes of calcite veins may be generally devided into three kinds: ① small sizes: 0.1 mm to 1.0 mm; ② medium sizes: 1.0~5.0 mm; ③ the largest is between 0.5 and 1.0 cm, and the maximum width is 2.5 cm. The larger calcite veins are composed of fibrous columnar calcite that is vertically foliated. conjugated fractures and extrusion structures are commonly developed in large calcite,which is the result of paleostress inversion at the end of late Nengjiang and late Mingshui Formation in the Gulong Sag, and also an important basis for paleo- stress recovery. After recovery, it is believed that the maximum stress in the late Negjiang and late Mingshui Formation (possibly extending to the Yi’an Formation) came from the horizontal direction (possibly east—west direction), and the maximum horizontal extrusion stress at 1500 m depth and 2500 m depth can reach 139.16 MPa and 204.27 MPa respectively. The vertical minimum stresses are only 35. 44 MPa and 59. 07 MPa, respectively. Therefore, a large number of bedding lamellation fractures and fibrous columnar calcite veins are developed under this stress state. In addition, there are a series of friction mirrors, scratches, steps, smooth surfaces, scales and fragments on the foliation surface, indicating that there is strong bedding shear along the foliation. The densely developed micro- fractures and large fractures form a spatial network of fractures in the bedding direction, which results in good or very good permeability of shale in the bedding direction. The spatial network of fractures and micro-nano pores together form a three dimensional special fracture- pore body, which is similar to carbonate fracture pore body. Nano pores, micron pores and fractures are of great significance for shale oil exploration and development in the Qingshankou Formation in the Songliao Basin.