Oil origin and accumulation in the Paleozoic Chepaizi-Xinguang field, Junggar Basin, China

The Chepaizi-Xinguang area is a petroliferous region in the western Junggar basin that contains several Permian tight reservoirs and has a complex petroleum system and accumulation mechanism. The oil sources and accumulation mechanisms are investigated by an integrated analysis of biomarker assemblages in reservoir oil samples, microscopic analysis of rock samples, study of fluid inclusions, and simulation of the thermal evolution. The results show that the Permian oil in the Xinguang bulge is mainly generated from the Permian Fengcheng Formation with tricyclic terpane ratios C19 TT/C23 TT lower than 0.15, C23 TT/C21 TT higher than 1.0 and gammacerane/αβ C30 hopane higher than 0.6. The Permian oil in the Chepaizi area is mostly derived from Permian Wuerhe Formation with C19 TT/C23 TT higher than 0.15, C23 TT/C21 TT lower than 1.0 and gammacerane/αβ C30 hopane lower than 0.4. The Permian oils from the Xinguang bulge show higher maturity compared to the Permian oils from the Chepaizi area possibly as a result of later oil filling. The fluid inclusions and fluorescence record the main oil charging movements during the Late Triassic to Early Jurassic and the Late Jurassic to Early Cretaceous in this area, and most of the generated oils in late Permian are lost. The unconformities between Permian strata show higher porosity and more oil and gas enrichment comparatively and are the main pathways for lateral oil migration. Laumontite has had a considerable effect on reservoir properties. Laumontite dissolution creates pores that provide good oil reservoir space and favorable exploration targets.