Abstract:

The main factors controlling the enrichment and high production of alkaline lacustrine shale oil in the Permian Fengcheng formation in the Mahu sag of Junggar Basin remain unclear, which limits theoretical understanding and efficient development. Based on the petrological, organic geochemical and physical characteristics of the Fengcheng shale oil reservoirs, the effects of source rock and reservoir lithology on shale oil enrichment in the Fengcheng formation were analyzed. Combined with the production profiles obtained from the key wells, the influences of shale oil enrichment, fractures and formation overpressure on the production of shale oil were identified. The findings are obtained in five aspects. First, in the Fengcheng shale oil reservoirs, the free hydrocarbon content increases with the increase of total organic carbon content (TOC), and the oil saturation index increases with the burial depth, indicating that the abundance, type and maturity of organic matter are the key factors determining the enrichment of shale oil. Second, siltstone exhibits the best storage space, followed by mudstone and endogenous rock, suggesting that the reservoir lithology controls the storage space and thus affects the enrichment of shale oil. Third, the enrichment is fundamental to the high production of shale oil. The shale oil production per meter from the Fengcheng reservoirs increases with the increase of TOC and feldspar mineral content. Fourth, the relationship between fracture orientation and present-day maximum horizontal principal stress direction affects the flow and production of the shale oil. Fifth, formation overpressure plays a role in retaining porosity, increasing permeability, reducing viscosity, and enhancing production/recovery of shale oil. The study clarifies the main factors controlling the enrichment and high production of shale oil in the Fengcheng formation, providing a scientific basis for further exploration and development.

Key words: Junggar Basin, Mahu sag, Fengcheng formation, shale oil, enrichment, high production, main controlling factor