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

Abstract

Three potential source rock intervals were evaluated based on the results of Rock-Eval pyrolysis on nearly 700 rock samples and the origin of the twenty-two oil-fields was studied by the biomarker analysis on 62 source rock samples and 93 oil samples in the Raoyang Sag and Baxian Sag, Bohai Bay Basin. The first member of the Shahejie Formation (Es) was deposited in an anoxic and salted water column with the dominant contribution of algal organic matters, reflected by its rock samples with high gammacerane/C30-hopane, high ETR, high C35/C34 hopane, high steranes/hopanes, low pristane/phytane, low C27-diasterane/C27-sterane and low C19/C23 tricyclic terpane. Characterized by higher C19/C23 tricyclic terpane, higher pristane/phytane, lower steranes/hopanes and lower C35/C34 hopane than the Es1 rock samples, the biomarkers of rocks suggest that the third and forth members of the Es were deposited in the sub-oxic to anoxic environments with a significant terrestrial organism input. Further studies suggest that the best source rock interval for oil-generation is the Es1 source rock interval in the Raoyang Sag and is the Es3 source rock interval in the Baxian Sag by organic facies analysis of source rocks. 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.