Petroleum origin and accumulation in the rift basin: a case study from the Nanpu sag, Bohai Bay basin, China

The origin of oil reservoirs in four structural belts in the Nanpu sag, Bohai Bay basin, was studied on the basis of the results of Rock-Eval pyrolysis on 140 samples and biomarker analysis on 30 source rock samples and 43 oil samples. The three possible source rock intervals have different biomarker associations and were deposited in different environments. The third member of the Shahejie Formation (Es-3) is characterized by relatively high gammacerane/alpha beta C30 hopane (>0.09), high extended tricyclic terpane ratio (ETR) ([C-28+ C-29]/[C-28+C-29+Ts]) (>0.4)), low C-27 diasterane/sterane (<1.0), and low C-27/C-29 steranes (<1.13), and was deposited in saline-water, anoxic conditions with high contribution from terrestrial organic matter (TOM). In contrast, the first member of the Shahejie Formation (Es-1) and the third member of the Dongying Formation (Ed(3)) were characterized by low gammacerane/alpha beta C-30 hopane (<0.09), low ETR (<0.4), and were deposited in freshwater-dominated, anoxic conditions, but have different C-27 diasterane/sterane (0.73-1.08 and 0.72-2.24, respectively) and C-27/C-29 steranes (0.94-1.47 and 1.12-1.82, respectively). Three oil groups have been identified in the study area of the Nanpu sag. Oils mainly originated from paleo-reservoirs during the period of Neotectonism not from the source rocks directly. Such a petroleum origin and accumulation pattern has important implication for petroleum exploration. The residual paleo-reservoirs may occur in Es-3 of Nanpu 1 and Nanpu 5 structural belts, Es-1 of the Nanpu 4 structural belt, and Es-3 and Es-1 of the Nanpu 2 structural belt in the Nanpu sag, Bohai Bay basin.