Petroleum charge history of deeply buried carbonate reservoirs in the Shuntuoguole Low Uplift, Tarim Basin, west China

Abstract

The newly discovered Shunbei Oilfield marks a major breakthrough in deep marine carbonate oil and gas exploration in the Tarim Basin. Understanding the petroleum charge history of the deep reservoirs will greatly benefit further petroleum exploration and development in this area. In this paper, we reconstruct the petroleum charge history of the reservoirs by integrating molecular geochemical correlations of reservoir oils and oil components (free oils and quasi-inclusion oils) from reservoir rocks, fluid inclusion analysis and basin modelling results. Molecular parameters derived from steranes, terpanes and aromatic compounds indicate that the reservoir oils, free oils and quasi-inclusion oils were all derived from the same source rock, deposited in a highly reduced marine environment with organic matters being mainly contributed by algae and bacteria. Utilizing the correlation between methylated aromatic compounds and vitrinite reflectance, the calculated vitrinite reflectance (Rc) values for the reservoir oils, free oils and quasi-inclusion oils fall into two distinct maturity ranges, with one spanning from 0.80%Rc to 0.96%Rc, and the other ranging from 1.15%Rc to 1.24%Rc, respectively. The timing of oil charge in the Shunbei Oilfield is constrained by combining fluid inclusion homogenization temperature (Th) with burial history modelling. Two oil charge events occurred in the reservoirs with the first marked by the relatively “low-maturity” oil, occurring in the latest Silurian around 426 Ma; whereas the second represented by the relatively “high-maturity” oil, occurring during the early Carboniferous around 330 Ma. This study highlights the significance of stable tectonic setting and excellent sealing capability of cap rocks for reservoir preservation. The finding unravels the petroleum charge and accumulation history of the Shunbei oil reservoirs and provides new insights for future exploration.