Characteristic of source rocks and origin of crude oils in the Raoyang Sag and Baxian Sag, Bohai Bay Basin, China: Insights from geochemical and geological analyses

Three source-related oil groups have been identified. There are two oil groups for the discovered oils in the Raoyang Sag. Group I oil, dominated by the Es1-derived oils, has been found in all the oil-fields in the Raoyang Sag, and Group II oil, mainly derived from the Es3 source rock interval, only distributes in the north-east area and in the south end of the Raoyang Sag. However, the three oil groups have been found in the Baxian Sag, but the Es3-derived oils are dominant in them and have been found in the major oil-fields in the east area and the Wen'an slope in the west area. Furthermore, the Es4-derived oils (Group III oil) mainly distribute in the Baxian Sag, without any findings in the Raoyang Sag because of the absence of the Es4 source rocks. As our studies, oils almost accumulate near their source rocks without a long-distance migration except the slope areas in the Raoyang Sag and Baxian Sag. Therefore, the source rock distribution is a key factor to oil accumulations in the Raoyang Sag and Baxian Sag, especially the efficient source rock distributions, which mainly developed in the organic facies A and B based on the geochemical and geological researches.