The discovery of light oil in the overlap-erosion zones of the northwestern Junggar Basin and its significance

The heavy oil and oil sand constitute the primary target of exploration in the overlap-erosion zones of petroliferous basins. Here the authors present the first discovery of light oil in Mesozoic oil sands in the overlap-erosion zones of the northwestern Jung-gar Basin, report the basic characteristics and accumulation model of the light oil and investigate their implications for exploration. The results show that the light oil fluoresces yellowish green light and coexists with heavy oil with dark fluorescence. Reservoir sequential extraction of oil sands shows that the heavy oil (grain adsorbed and/or inclusion oils) charged reservoir before the light oil (pore free oils) and the oil sources were both from the Lower Permian Fengcheng Formation in the Mahu sag. Representative bio-marker evidence includes Pr/Ph < 1, gammacerane/C₃₀ hopance=0.43~0.82 and tricyclic terpanes C₂₀ < C₂₁ < C₂₃. Inorganic geochemi-cal studies support this two-stage oil charging events. The MnO and FeO values of diagenetic calcites are 1%~1.5% and >1.5%, respectively. Before the Cretaceous, the Fengcheng-sourced mature oil charged to the overlap-erosion zones of the study area and was biodegraded, and formed oil sand. Since the Cretaceous, the Fengcheng-sourced highly-mature and light oil has migrated from the Mahu sag to the overlap-erosion zones of the study area along transverse faults. As a result, the light oil accumulated mostly in the cross points of reverse and transverse faults. Accordingly, the zones along transverse faults are potential for light-oil exploration. The exploration in overlap-erosion zones of petroliferous basins might not be merely confined to heavy oil and oil sand. Light oil and gas could accumulate if favorable reservoir forming conditions exist.