In-situ calcite U-Pb ages and absolute timing of oil charge events: A case study of ultra-deep carbonate reservoirs in the Shunbei oilfield, Tarim

To accurately determine the timing of hydrocarbon charging events in petroleum systems is important but often challenging. Traditional methods for establishing the timing of these events in tectonic complex basins frequently suffer from unreliability due to uncertain burial and/or thermal histories, as well as the poor identification of inclusion assemblage. Recently developed calcite U-Pb dating has been proven to be a valuable tool for precisely determining the timing of these events, thus enhancing our understanding of the charging histories. We integrated oil-inclusion fluorescence analysis with in-situ calcite U-Pb dating, focusing on calcite fabrics hosting primary oil inclusions, to quantitatively constrain the oil charging timing of the SHB no.1 and no.5 fault zones in the Shunbei oilfield. The fluorescence spectra of oil inclusions suggested the presence of up to three discrete charging events in these fault zones. Calcite U-Pb ages provide constraints on the timing of the first charging episode in the two fault zones, at approximately 478-469.2 Ma and ca. 447.4-424 Ma, respectively. Comparing the fluorescence spectra of reservoir oils and oil inclusions reveals that the primary charging episodes in the SHB no.1 and no.5 fault zones correspond to the latest and the earliest charging episodes, respectively. Furthermore, through a comparison of the equivalent reflectance of reservoir oils with the thermal maturation history of the Cambrian source rock, the former age can be estimated at ca. 120-0 Ma. Regions characterized by weaker latestage fault activity exhibit a higher potential for hydrocarbon preservation in deeper reservoirs.