Genesis and Geological Significance of the Black Oil Reservoir with a Condensate Gas Cap in the Baijiantan Formation of Hefeng 3 Block, Junggar Basin, NW China

Abstract The Triassic Baijiantan Formation in the Hefeng 3 block, Junggar Basin, hosts an unusual hydrocarbon accumulation characterized by the coexistence of conventional black oil and a condensate gas cap within a single reservoir. This study integrates geochemical data, PVT simulations, and fluid inclusion analysis to unravel its genesis. Results show that the reservoir is a medium-porosity, ultralow-permeability system. The black oil originated from mature Fengcheng Formation source rocks, whereas the gas cap comprises mixed-source kerogen-cracking gases from both the Fengcheng Formation and Carboniferous sources. We propose a two-stage charging model: early accumulation of Fengcheng-derived black oil, followed by late injection of highly mature Carboniferous-derived dry gas. Evaporative fractionation induced by gas invasion transformed the original volatile oil into the present-day black oil–condensate gas cap configuration. This model challenges conventional single-source interpretations and provides a new genetic framework for complex fluid-phase reservoirs in the Junggar Basin. Our findings offer practical exploration guidance for similar composite reservoirs in superimposed basins worldwide.