Using of stratal slicing in delineating delta-turbidite systems in Eocene Dongying depression, Bohai Bay Basin: Insights for the evolution of multi-source delta-turbidite systems in a fourth order sequence

Delta-turbidite systems represent one of most significant targets for hydrocarbon exploration and development. Due to complex stratigraphic architecture of deltas and small scale of delta-fed turbidites, the prediction and characterization of them pose a challenge to successful exploration for hydrocarbons. Through seismic sedimentology approach, 3D seismic and borehole data were used to improve reservoir prediction of the delta-turbidite systems in the Eocene Shahejie Formation, eastern Dongying depression, Bohai Bay Basin, China. A highstand systems tract (HST) is recognized in the Eocene Shahejie Formation in eastern Dongying depression. It is composed of six fourth-order sequences (PSS5∼PSS10), each of which corresponds to a given period of delta progradation. Using stratal slicing of 90°-phase seismic data, mapping the fourth stage of delta development (PSS8), as an example, highlights the distribution and evolution of multi-source deltas and their associated thin-bedded turbidites within a fourth-order sequence. Interpretations of stratal slices indicate that the formation of delta-fed turbidites is closely related to base-level changes, sediment supply and syndepositional faults. Development of early larger-scale turbidites fed by north fan delta and Yongan delta suggests that enough sediment supply, rather than base-level falling, plays a more fundamental and critical role for turbidite formation. At the end period of a highstand fourth order sequence, the largest-scale turbidites developed in the front of Dongying delta are jointly driven by both rapid sediment supply and base level falling. This research also proves that the constraint of fourth-order sequences is necessary for ensuring the isochronism of stratal slices, which provide effective reservoir prediction of small-scale delta-fed turbidites and further insights into evolution of multi-source delta-turbidite systems.