Origin of oils in "Subtle pools" in the Dongying depression, Bohai Bay Basin, China

Subtle traps or oil pools have become an important exploration play in the Dongying Depression, Bohai Bay Basin, east China. Despite recent successes in exploration, the formation mechanisms of subtle traps are still not well understood. The majority of subtle oil pools in the Dongying Depression are developed in the middle interval of the E-S3 Member of the Paleogene Shahejie Formation with the subtle traps being primarily of lenticular basin-floor turbidite sands encompassed in mudstones. Oil in the subtle traps was previously thought to have migrated directly from the surrounding source rocks of the same formation (E-S3). Detailed geochemical investigation of 41 oils and 41 rock samples from the depression now indicates that the oils from the subtle traps cannot be correlated well with the surrounding E-S3 source rocks, which are characterized by high Pr/Ph (> 1), low Gammacerane/C(30)hopane, representing a freshwater lacustrine setting. In contrast the oils features low Pr/Ph (< 1) and relatively high Gammacerane content, showing a genetic affinity with the underlying E-S4 source rocks, which also have the same qualities, indicating a brackish lacustrine setting. Oils in the E-S3 subtle traps are probably derived from mixed sources with the contribution from the upper E-S4 source rocks predominating. Therefore unconventional oil migration and accumulation mechanisms need to be invoked to explain the pooling of oils from the ES4 source rocks, which probably came through a thick low interval of the E-S3 source rocks with no apparent structural or stratigraphic pathways. We suggest that the subtle oil migration pathway probably plays an important role here. This finding may have significant implications for future exploration and the remaining resource evaluation in the Dongying Depression.