Characteristics and accumulation mode of volcanic rock reservoir of the Fengcheng Formation in the southern of Mahu Sag, Junggar Basin

Objective To clarify the oil and gas accumulation patterns and main controlling factors of the thin weathered crust volcanic rock reservoir at the top of Member 2 of the Fengcheng Formation in the southern Mahu Sag(Manan Sag). Methods This paper provides a comprehensively analyses of the volcanic rock characteristics, reservoir properties, and oil and gas geological features. Furthmore, an oil and gas accumulation model for volcanic rocks in the slope area is established. Results Research has shown that Member 2 of the Fengcheng Formation in the southern Mahu Sag is mainly composed of basalt and tuff. The basalt with the volcanic overflow phase developed high-permeability reservoirs abundant in oil and gas. Overall, the crude oil is medium-high viscosity and light-medium waxy. Its distribution pattern of C20, C21, and C23 tricyclic terpanes resembles a mountain peak, showing a significantly higher relative content of tricyclic terpanes. In contrast to crude oil, the distribution of tricyclic terpanes in in-situ source rocks demonstrates an upward pattern, with maturity parameters of biomarkers also displaying lower maturity characteristics. Based on the consistent research results of the carbon isotope characteristics of crude oil in the study area and in the Fengcheng Formation source rocks, it is infered that the oil reservoir in the study area has undergone multistage oil and gas charging. Conclusion The volcanic rocks in the study area exhibit strong vertical and horizontal heterogeneities, with crude oil density decreasing with depth. It is believed that oil and gas mainly migrate from low to high parts along permeable strata below volcanic rocks and enter volcanic rock reservoirs under buoyancy. They accumulate reservoirs in structures or lithological traps with relatively good reservoir properties and preservation conditions. This is a typical slope area-structure lithology-multistage volcanic rock oil and gas reservoir formation model.