Molecular and isotopic evidence for mixed-source oils in subtle petroleum traps of the Dongying South Slope, Bohai Bay Basin

Subtle traps have become an important petroleum play type for exploration in the Dongying Depression, Bohai Bay Basin, eastern China. The majority of the oil pools are in the middle interval of the Es3 member (Es3M) of the Eocene–Oligocene Shahejie Formation, with the subtle traps being primarily in the lenticular, basin floor turbidite sands enclosed within basin floor mudstones. Detailed geochemical investigation of 68 oil and rock samples from the depression reveals that in contrast to previous models, the oils from the subtle traps correlate poorly with the surrounding Es3M source rocks. While the high pristane/phytane (Pr/Ph > 1) and low gammacerane/C30-hopane ratios in these source rocks represent a freshwater lacustrine setting, the oils feature low Pr/Ph ratios (<1) and relatively high gammacerane contents similar to the upper Es4U source rocks deposited in a brackish lacustrine setting. The compound-specific isotope ratios of n-alkanes in the oils have intermediate values between those observed in extracts from the lower Es3(Es3L) and upper Es4(Es4U) source rocks. Consequently, the oils in the Es3M subtle traps were most likely derived from mixed Es3L and Es4U sources, with the contribution from the Es4U source rocks being predominant. The oils generated from the Es4U source rocks must have migrated through a thick interval of the Es3L source rocks to reach the subtle traps in the Es3M, through which there are no apparent structural nor stratigraphic pathways. The traditional migration and accumulation model, i.e. indigenous crude oil with short migration distance to lenticular oil pools, should be revised to account for the charging of oils from Es4U source rocks.