Two-step classification of highly mixed oil in the Qikou Sag (Bohai Bay Basin, China): Implications for origin, maturity, and biodegradation

Mixing of crude oil leads to unpredictable changes in the property and composition of mixtures, bringing uncertainties to petroleum production. Therefore, it is crucial to reveal the origin, maturity, and biodegradation of mixed oil during the exploration stage. Here we address these issues using the highly-mixed oil from the Qikou Sag (Bohai Bay Basin, eastern China) after a two-step classification. Firstly, the highly-mixed oil was classified into two groups (groups I and II) according to their physical property (API gravity and sulfur content), molecular composition, and gas chromatograms of saturated hydrocarbons. Secondly, the group I oil was further divided into two sub-groups (groups I-1 and I-2) based on the principal component analysis (PCA) and hierarchical cluster analysis (HCA) of biomarkers (including n -alkanes, isoprenoids, terpenoids, and steroids). The group I oil is non-biodegraded, in which the group I-1 oil from deep reservoirs is a mixture of hydrocarbons from three sets of source rock, whereas the group I-2 oil from shallow reservoirs is mixed by hydrocarbons from five sets of source rock. Maturity parameters of biomarkers suggest that the group I oil is thermally mature. The group II oil was slightly (PM 1–2) to severely (PM 4+) biodegraded, as indicated by low API gravity, high percentages of asphaltenes and NSO, high ratios of Pr/ n -C 17 and Ph/ n -C 18 , and the assignment of UCM and 25-norhopane. The burial depth of reservoirs and secondary faults (formed after the Neogene) are the primary controlling factors for the biodegradation. Our results demonstrate that the two-step classification facilitates the grouping of highly mixed oil, which provides a basis for indicating their origin, maturity, and biodegradation. The results also provide guidance for regional petroleum exploration and production. • A two-step classification method was used to group the highly mixed oil. • Multivariate analysis was applied to indicate the origin of highly mixed oil. • Degradation level of highly mixed oil and its controlling factors were suggested. • Migration and accumulation models of highly mixed oil were established.