Energy Reports (Nov 2022)
Migration directions of crude oils from multiple source rock intervals based on biomarkers: A case study of Neogene reservoirs in the Bodong Sag, Bohai Bay Basin
Abstract
Identifying oil origins and reconstructing oil migration directions are vital for oil exploration in continental petroliferous basins, but they become difficult and complicated when multiple source rock intervals are encountered. This is the case of the Neogene fluvial reservoirs in the Bodong Sag, Bohai Bay Basin, east China, which show significant spatial heterogeneity in oil charging. Here, we take the LK9-2 Structure in the southeastern Bodong Sag as an example to determine the potential factors controlling the oil migration directions from multiple source rock intervals. Total organic carbon contents, Rock-Eval pyrolysis results and biomarkers were applied to distinguish the major oil sources and trace the oil migration pathways. The first–second and third members of the Eocene Shahejie Formation and the third member of the Oligocene Dongying Formation in the Bodong Sag all showed high potential of generating oils for the overlying Neogene reservoirs. However, oil–source correlation by biomarkers revealed that the Neogene oils of the LK9-2 Structure were mainly derived from the third member of the Dongying Formation. Oils generated from the underlying Shahejie Formation were, however, mostly blocked by the thick mudstones of the Dongying Formation. The dimethyldibenzothiophenes and benzo[b]naphthothiophenes parameters, indicative of oil migration directions, showed the failure of lateral oil migration within the Neogene reservoir layers, which had been cut by active shallow normal faults. Therefore, the uppermost thick source rock intervals and abundant active shallow normal faults in the way of lateral oil migration were the two major factors that controlled the oil migration directions from multiple source rock intervals and probably accounted for the many uncharged traps in the Bodong Sag.
