Abstract:

To clarify the occurrence space and mobility of the shale oil in the Fengcheng formation of the Mahu sag, Junggar Basin, the data of rock thin section, SEM and NMR, and experiments such as total scanning fluorescence were used, together with 2D NMR logging data, to systematically characterize the microscopic pore structure and crude oil occurrence characteristics of the shale reservoir, and identify the factors controlling oil mobility. The storage space of the shale reservoir of the Fengcheng formation in the study area is mainly composed of intergranular pores, intercrystalline pores, dissolution pores, organic pores, and microfractures, with dissolution pores and fractures in dominance. The mobility of shale oil varies significantly in reservoirs with different lithofacies. The best mobility is found in the felsic shale rich in terrigenous clastic silt-sand bands, followed by the dolomitic shale with well-developed dolomitic laminae, and the worst mobility is found in the mixed shale rich in clay minerals. Organic matter abundance, depositional fabric, and pore structure are key factors controlling the mobility of shale oil in the Fengcheng formation. When total organic carbon (TOC) content of the shale in the study area ranges from 0.5% to 1.5%, the oil saturation index reaches its maximum range, indicating good mobility of the shale oil. In thin-bedded felsic shale and laminated dolomitic shale, pores (mainly residual intergranular pores and dissolution pores) and microfractures are developed, with a high proportion of large pores, which facilitates the formation of favorable occurrence space and flow channels for shale oil, promoting the enrichment of mobile oil.

Key words: Junggar Basin, Mahu sag, Fengcheng formation, shale, reservoir, crude oil occurrence state, shale oil mobility, controlling factor