Abstract:

In order to improve the success rate of penetrating sweet spots of shale oil reservoirs and to realize the efficient producing and development of shale oil, it is necessary to study the reservoir space, pore structure and oil content of shale oil reservoir. Taking the Middle Permian Lucaogou formation in the piedmont belt of Bogda mountain in the southeastern Junggar basin as the research object, using the reservoir characterization technologies such as rock thin section analysis, X-ray diffraction whole-rock mineral analysis and field emission-environment scanning electron microscope and combining with the organic geochemical experiments, the paper analyzes the reservoir spaces and shale oil occurrences of different lithofacies. The results show that the Lucaogou formation is dominated by mixed depositions of shale, sandstone and carbonate rocks. The reservoir spaces and pore structures of different lithofacies control the oil-bearing properties of the reservoir. Free-state shale oil mainly occurs in the pores with the pore diameters more than 30 nm. The reservoir spaces of the shale mainly include intercrystalline pores in clay minerals, intercrystalline pores in carbonate minerals, pores in organic matters, bedding fractures, structural fractures and overpressure fractures. Adsorbed shale oil occurs on the surfaces of inorganic minerals and kerogen, while free-state shale oil occurs in the intergranular pores, organic matter pores and bedding fractures. The reservoir spaces of sandstone and carbonate rock in the interlayers are dominated by intergranular (intercrystalline) pores, intergranular (intercrystalline) dissolved pores, and intragranular (intracrystalline) pores. Free-state shale oil mainly occurs in all kinds of nano-pores. Organic matter enriches in the lithofacies of matrix. As the abundance of the organic matter increases, the amount of adsorbed hydrocarbons in kerogen increases, and the organic carbon content shows a power function relationship with the contents of movable hydrocarbons. The lithofacies in the interlayer have relatively low organic carbon contents which show linear relationships with the contents of movable hydrocarbons. Qitaizhuang area and the central and northern areas of Chaiwopu sag are favorable shale oil reservoir development areas, and the shale oil in the lithofacies of the interlayers in the third member of Lucaogou formation have high contents of movable hydrocarbons, which can be exploited easily and should be considered as the key target for exploration.

Key words: Junggar basin, Bogda Mountain, Lucaogou formation, shale oil, occurrence, lithofacies, reservoir space, oil-bearing property