关键词: 芦草沟组, 吉木萨尔凹陷, 成藏模式, 油源对比, 页岩油

Abstract: This study focuses on the Lucagou Formation of Jimusaer sag in Junggar Basin. Based on the lithological and reservoir pore characteristics, the migration and enrichment mechanisms of shale oil were determined using geochemical analysis data. The analysis of biomarkers shows that the lower section of sweet spots and the central thick mudstone interval of Lucagou Formation are independent petroleum systems without the occurrence of mixed shale oil source in the longitudinal direction, and the crude oil in reservoir sandbodies is derived from adjacent hydrocarbon source rocks, and characterized with near-source migration and in-situ accumulation, displaying a self-generation, self-reservoir and near-source accumulation model. Besides originating from nearby hydrocarbon source rocks, the shale oil in the upper section of sweet spots is mainly derived from deep hydrocarbon source rocks, and characterized with lateral migration and enrichment of shale oil, displaying a mixed accumulation model based on long-distance lateral migration and vertical near-source migration. The analyses of well profiles, rock thin sections, and high-pressure mercury injection experiments show that the upper section of sweet spots has the characteristics of low crude oil density, large reservoir thickness (single sandbody with the thickness of more than 1.6 m), well-developed intergranular pores, and good horizontal permeability, providing favorable conditions for the lateral migration of shale oil. By contrast, the crude oil density is higher, the reservoir thickness is thinner, and intergranular pores are not well developed in the central mudstone interval and the lower section of sweet spots, which is unfavorable for the lateral migration and enrichment of shale oil. The research results have identified two favorable depth intervals for the development of sweet spots of shale oil in the upper section of Lucagou Formation, i.e., the shallow depth section with high porosity and the deep depth section with developed secondary dissolution pores. The lateral migration distances of shale oil during enrichment in these two sections are 7300 m and 4100 m respectively. The shale oil enrichment in the lower section of sweet spots and the central thick mudstone interval needs to consider the hydrocarbon generation intensity and expulsion amount of adjacent source rocks. The favorable exploration interval in the lower section of sweet spots is below the peak oil generation depth (over 3 300 m). The research results have certain guiding significance for the discovery of interbedded shale oil sweet spots and the shale oil exploitation.

Key words: Lucaogou Formation, Jimusaer sag, accumulation model, oil-source correlation, shale oil