Identification of effective source rocks in different sedimentary environments and evaluation of hydrocarbon resources potential: A case study of paleogene source rocks in the Dongpu Depression, Bohai Bay Basin

Source rocks in lacustrine rifting basins have great heterogeneity, which leads to imprecise assessments of hydrocarbon resources. Previous studies evaluated the source rocks of different sedimentary environments in lacustrine basins or depressions using the same lower limit of effective source rocks (i.e., the minimum total organic carbon (TOC) at which the source rocks are capable of expelling hydrocarbons). In this study, the source rock samples were selected from five formations in the Dongpu Depression for organic geochemical experiments and quantitative simulations of hydrocarbon expulsion. According to salinity differences, every formation was divided into a saline area, transition area and freshwater area. The results show that the maximum thickness of source rocks developed in the saline and transition areas is 700 m. Additionally, source rocks with a higher TOC and oil-prone organic matter types (II1 and II2) also develop in the saline and transition areas, and most have entered the mature stage. The TOC values corresponding to the lower limit of effective source rocks in the saline, transition, and freshwater areas are 0.3%, 0.4% and 0.5%, respectively. Furthermore, the hydrocarbon expulsion ratio (amount of hydrocarbon discharged per unit of TOC from the source rocks) for the freshwater area is 253.1 mg/g, while the ratios in the saline and transition areas are relatively higher at 439.4 mg/g and 369.3 mg/g, respectively. The hydrocarbon expulsion rate (change in the expulsion ratio per unit thermal evolution degree) and hydrocarbon expulsion efficiency (ratio of cumulative hydrocarbon expulsion amount to generation amount) in the saline, transition and freshwater areas present maxima when the Ro (vitrinite reflectance) values are 0.95%, 0.85% and 0.82%, respectively. The hydrocarbon expulsion quantities in the saline, transition and freshwater areas account for 24%, 49% and 27% of the total, respectively. Therefore, the main hydrocarbon expulsion area is the transition area of the Dongpu Depression associated with the thickest source rocks. Es3M and Es3L source rocks in the transition area with high organic matter abundant and type II kerogen are more promising than the other formations. This study is the first to report on effective source rocks in different sedimentary environments in lacustrine basins and can provide guidance for future exploration in the Dongpu Depression and a reference for future research on source rocks in other lacustrine basins.