Geochemical quantification of mixed marine oils from Tazhong area of Tarim Basin, NW China

Located in the center of the Tarim Basin, NW China, Tazhong area was found to be enriched in oil and gas resources, and the resources are mainly originated from two sets of marine source rocks, namely, the Cambrian-Lower Ordovician (ϵ-O1) source rocks and the Middle-Upper Ordovician (O2+3) source rocks. Due to complex post-depositional alterations such as multiple episodes of hydrocarbon generation, migration and accumulation, most oil reservoirs in this area occur as mixed oils.Results of geochemical studies of crude oils, combined with bulk stable carbon isotope data of 108 oil samples, indicate that most marine oils from Tazhong area are mixtures sourced from both the ϵ-O1 and O2+3 strata. Based on bulk stable carbon isotope data of the oils, quantification studies were made to evaluate the relative contributions of different source rocks to the mixed oils, and the results show that the Middle-Upper Ordovician source rocks are the main contributor for the oils in the Tazhong area, despite fewer exceptions that some oil reservoirs were mainly originated from the Cambrian-Lower Ordovician source rocks. However, for the oil reservoirs occurring in different subunits in the Tazhong area, the relative contributions from different source rocks do not show regular distribution patterns, implying that Tazhong area was subject to multiple episodes of hydrocarbon generation, migration and accumulation in geological history.

► Occluded compounds inside asphaltenes verified the mixed features of Tazhong oils. ► Stable carbon isotope ratios were used to quantitatively evaluate the mixed oils. ► Most Tazhong marine oils were contributed from both the ϵ-O1 and O2+3 source rocks. ► The O2+3 source rocks were the main contributor for the Tazhong marine oils.