Control of complex lithofacies on the shale oil potential in ancient alkaline lacustrine basins: The Fengcheng Formation, Mahu Sag, Junggar basin

The study of ultra-deep (>4500 m) oil and gas reservoirs in hydrocarbon-bearing basins is a hot spot for oil and gas exploration in the world today, the heterogeneity of lithofacies is a key factor influencing the potential of deep-buried shale oil. It is highly important to better understand the control of complex rock on the shale oil potential for shale oil exploration and development. The Fengcheng Formation (P1f) in the Mahu Sag is a type of ancient alkaline lacustrine shale oil, which was chosen as a research example in this paper. Four lithofacies were identified based on mineralogy: felsic, lime, dolomitic and mixed shale lithofacies. P1f exhibits a very high shale oil potential, and the oil saturation index (OSI) can reach 176 mg HC/g TOC, far exceeding the lower limit of effective benefits. The shale lithofacies of P1f is complex, and the complex lithofacies has a controlling effect on shale oil potential, specifically in terms of hydrocarbon generation, storage and hydrocarbon migration. The different lithofacies differ in various aspects and are controlled by different factors. Felsic shale is an enriched oil shale with a high hydrocarbon generation potential and high storage capacity that can receive foreign hydro-carbons. Mixed shale is a self-generating and self-accumulating oil shale with a low hydrocarbon production potential and low storage capacity. Lime shale is a partial reservoir-type oil shale with a low hydrocarbon production capacity and satisfactory storage capacity that can receive foreign hydrocarbons. Dolomite shale is a partial source rock-type oil shale exhibiting a high hydrocarbon generation capacity and low storage capacity, which can expel hydrocarbons. Studies have indicated that felsic shale exhibits a notable hydrocarbon generation potential, high storage capacity, easy foreign hydrocarbon migration, high brittle mineral content, and easy fracture production. This shale type is the most favorable shale oil exploration target in P1f within the Mahu Sag. In addition, this study reveals the relationship between rock lithofacies and shale oil potential, which provides a new idea for the potential evaluation of similar lacustrine shale oil.