Identifying oil sources in the Wen’an Slope of the Baxian Depression, the Bohai Bay Basin, North China

Abstract With accumulation of geochemical data of oils and source rocks in mature basins, oil source re-identification is important for petroleum system analysis and subsequent exploration. In the Baxian Depression of the Bohai Bay Basin, the main source rocks are in the Sha-3 (Es3) and Sha-1 (Es1) Members of the Paleogene Shahejie Formation. These cannot be differentiated using the standard mass chromatograms and bivariate cross-plots of biomarker parameters obtained from the representative samples of the source rocks. In this paper, discriminant analysis (DA) was employed to highlight the difference between these two sets of the source rock samples and to identify the sources of the oils in the multivariate space. Based on a comprehensive literature review, we selected twenty four candidate parameters that may reflect the oil sources, and excluded ones which are affected by the thermal maturity and biodegradation in the study area. Eight informative parameters were chosen with stepwise DA from these candidate parameters to establish a discriminant model for the oil-source correlation. The model achieved a high correct discrimination rate using leave-one-out cross-validation (90.7%) and original validation (94.4%). We found that these 8 parameters can describe the characteristics of the main sources and effectively differentiate these sources. The high posterior probabilities in the oil-source correlation results indicate that the oils in the Wen’an Slope were mainly sourced from the source rock in Es3. These results, together with geological mapping of reservoir sand-bodies, suggest that more oil derived from the Es3 source rocks may exist below the thick shale and/or mudstone sequence in the Es1 than previously thought.