Mechanisms of petroleum accumulation in the Bozhong sub-basin, Bohai Bay Basin, China. Part 1: Origin and occurrence of crude oils

摘要

The origin of the fourteen major oil fields in the Bozhong sub-basin, Bohai Bay basin was studied based on the results of Rock-Eval pyrolysis on more than 700 samples and biomarker analysis on 61 source rock samples and 87 oil samples. The three possible source rock intervals have different biomarker assemblages and were deposited in different environments. The third member of the Oligocene Dongying Formation (E3d3, 32.8-30.3 Ma in age) is characterized mainly by high C_(19)/C_(23) tricyclic terpane (>0.75), high C_(24) tetracyclic terpane/C_(26) tricyclic terpane (>2.5), low gammacerane/αβ C_(30) hopane (<0.15) and low 4-methyl steranes/ΣC_(29) steranes (<0.15) ratios, and was deposited in sub-oxic to anoxic environments with significant terrigenous organic matter input. The first (E2S1, 35.8-32.8 Ma) and third (E2S3, 43.0-38.0 Ma) members of the Eocene Shahejie Formation have low C19/C23 tricyclic terpane and low C_(24) tetracyclic terpane/C_(26) tricyclic terpane ratios and were deposited in anoxic environments with minor terrestrial organic matter input, but have different abundances of 4-methyl steranes and gammacerane. The hydrocarbon-generating potential and biomarker associations of these three source rock intervals were controlled by tectonic evolution of the sub-basin and climate changes. Three oil families derived from E2S3, E2S1 and E3d, respectively, and three types of mixed oils have been identified. All large oil fields in the Bozhong sub-basin display considerable heterogeneities in biomarker compositions and originated from more than one source rock interval, which suggests that mixing of oils derived from multiple source rock intervals or multiple generative kitchens, and/or focusing of oils originated from a large area of a generative kitchen, is essential for the formation of large oil fields in the Bozhong sub-basin. E2S3- and E2S1 -derived oils experienced relatively long-distance lateral migration and accumulated in traps away from the generative kitchen. E3d3-derived oils had migrated short distances and accumulated in traps closer to the generative kitchen. Such a petroleum distribution pattern has important implications for future exploration. There is considerable exploration potential for Dongying-derived oils in the Bozhong sub-basin, and traps close to or within the generative kitchens have better chance to contain oils generated from the Dongying Formation.