Main hydrocarbon accumulation period and process of deep reservoirs in the Tahe oil field, Tarim Basin, northwestern China: Insights from molecular geochemistry, fluid inclusions, and U-Pb and Re-Os geochronology

The deep Ordovician hydrocarbon reservoirs in Tahe oil field, Tarim Basin, are characterized by a complex geological setting involving multistage hydrocarbon generation and charging. Uncertainties in the thermal evolution and burial history of the basin have restricted the reliability of conventional methods for establishing hydrocarbon charging chronology, fueling debates over the charging history and main accumulation period of the reservoirs. Earlier studies have determined some periods of hydrocarbon charging in the Tahe oil field, but the main hydrocarbon accumulation period of the present Ordovician reservoirs has long been controversial due to the lack of critical evidence. Recent advances in the in situ calcite U-Pb dating and crude oil Re-Os dating methods have provided precise chronological constraints on hydrocarbon charging and generation, thereby enhancing the understanding of the hydrocarbon accumulation history. In this study, traditional molecular geochemistry, fluid inclusion analyses, and the developed U-Pb and Re-Os dating methods were combined to reconstruct the hydrocarbon accumulation process and identify the main accumulation period of the Ordovician reservoir in the Tahe oil field. Fluid inclusion analysis integrated with basin modeling results suggest the presence of three charging events in the study area, during the late Caledonian to early Hercynian orogeny, late Hercynian orogeny, and late Yanshan-Himalayan orogeny, respectively. The calcite U-Pb ages provide constraints on the timing of the first two charging events, at ca. 443.5−417 Ma and ca. 298 Ma, respectively. The Re-Os dating of crude oils indicates that the present crude oil in the reservoir was mainly generated during the early Permian (292 ± 29 Ma). The integration of crude oil Re-Os isotope dating, maturity parameters, and source rock thermal evolution modeling indicates that the late Hercynian period was the main accumulation period of the present reservoir in the study area.