Shale Oil Mobility Evaluation and Multi-Scale Characterization of Oil Occurrence Space of Liushagang Formation in Weixinan Sag, Beibuwan Basin

In order to clarify the occurrence space and mobility characteristics of shale oil in the Liushagang Formation of the Weixinan Sag in Beibuwan Basin, comprehensive experimental techniques such as thin section identification, fluorescence observation, scanning electron microscopy, nitrogen adsorption, high-pressure mercury injection, digital core analysis, nuclear magnetic resonance, and multi-temperature pyrolysis were utilized to qualitatively and quantitatively investigate the pore structure, occurrence space, and mobility characteristics of different types of shale oil reservoirs in the Liushagang Formation of the Weixinan Sag. Results show that: (1) matrix-type shales mainly contain nanoscale pores in organic matter, intergranular pores in pyrite, interparticle pores in clay minerals, fractures at bedding interface, as well as shrinkage fractures on the edge of organic matter particles. Oil in the matrix-type shales mainly stores in pores with pore throat radius less than 10 nm. In addition to organic pores, intergranular pores and microfractures, there are intergranular and intragranular pores developed in laminated shale oil reservoirs. Oil in the laminated shales mainly stores in pore spaces with pore throat radius between 10 nm and 50 nm. The "sandwiched" shales mainly contain intergranular pores, intergranular pores, intraparticle pores, and dissolution-related pores at the nanometer to micrometer scale. Oil in the "sandwiched" shales mainly exists in pore spaces with pore throat radius ranging from 100 to 1 000 nm. (2) There are significant differences in the oil mobility of different types of shales. Among them, the "sandwiched" shales have the best mobility. The maximum movable oil content of the "sandwiched" shales is between 22.78 and 42.63 mg/g (averaging 32.71 mg/g), and the movable oil proportion is higher than 80%. The second is the laminated shales, with the maximum movable oil content of 1.78-16.35 mg/g (averaging 10.88 mg/g), and the movable oil proportion of about 60%. The matrix-type shales have the lowest movable oil content and movable oil proportion. (3) The abundance and thermal maturity of organic matter, reservoir pore space, and mineral composition control the shale oil mobility; When the thermal maturity (R_o) is around 1.0% and TOC content is between 1.5% and 4%, shale reservoirs are mostly enriched in movable oil. In addition, the larger the pore throat radius, the better the physical properties, the higher content of brittle minerals, the lower content of clay minerals, the better shale oil movability. Based on the above results, the "sandwiched" shales have the largest storage space and the best mobility, and are the preferred target for shale oil exploration in the Weixinan Sag. These are the basis for the significant breakthroughs in offshore shale oil drilling.