Characteristics of Hydrocarbon Fluid Inclusions and Their Significance for Evolution of Petroleum Systems in the Dabashan Foreland, Central China

Field investigation combined with detailed petrographic observation indicate that abundant oil, gas, and solid bitumen inclusions were entrapped in veins and cements of sedimentary rocks in the Dabashan foreland, which were used to reconstruct the oil and gas migration history in the context of tectonic evolution. Three stages of veins were recognized and related to the collision between the North China block and the Yangtze block during the Indosinian orogeny from Late Triassic to Early Jurassic (D1), the southwest thrusting of the Qinling erogenic belt towards the Sichuan basin during the Yanshanian orogeny from Late Jurassic to Early Cretaceous (D2), and extensional tectonics during Late Cretaceous to Paleogene (D3), respectively. The occurrences of hydrocarbon inclusions in these veins and their homogenization temperatures suggest that oil was generated in the early stage of tectonic evolution, and gas was generated later, whereas solid bitumen was the result of pyrolysis of previously accumulated hydrocarbons. Three stages of hydrocarbon fluid inclusions were also identified in cements of carbonates and sandstones of gas beds in the Dabashan foreland belt and the Dabashan foreland depression (northeastern Sichuan basin), which recorded oil/gas formation, migration, accumulation and destruction of paleo‐reservoirs during the D2. Isotopic analysis of hydrocarbon fluid inclusions contained in vein minerals shows that δ13C1 of gas in fluid inclusions ranges from –17.0‰ to –30.4‰ (PDB) and δD from –107.7‰ to –156.7‰ (SMOW), which indicates that the gas captured in the veins was migrated natural gas which may be correlated with gas from the gas‐fields in northern Sichuan basin. Organic geochemical comparison between bitumen and potential source rocks indicates that the Lower Cambrian black shale and the Lower Permian black limestone were the most possible source rocks of the bitumen. Combined with tectonic evolution history of the Dabashan foreland, the results of this study suggest that oil was generated from the Paleozoic source rocks in the Dabashan area under normal burial thermal conditions before Indosinian tectonics and accumulated to form paleo‐reservoirs during Indosinian collision between the North China block and the Yangtz block. The paleo‐reservoirs were destroyed during the Yanshanian tectonic movement when the Dabashan foreland was formed. At the same time, oil in the paleo‐reservoirs in the Dabashan foreland depression was pyrolyzed to transform to dry gas and the residues became solid bitumen.