Lacustrine delta-offshore bar system: depositional characteristics and formative mechanism

The lacustrine delta-offshore bar system serves as a critical hydrocarbon reservoir, in which sedimentary processes and depositional characteristics reflect a complex interplay of hydrodynamic, sedimentological, and geomorphic dynamics. Notably, such systems with their full suite of morphological attributes have not been numerically simulated prior to this study. Our Delft3D simulation has successfully reproduced the delta-offshore bar system for the first time, providing a tool to quantitatively analyze its depositional processes and stratigraphic signatures. Three new perspectives are proposed: 1) Three distinct hydrodynamic reaches of the system are recognized by hourly-averaged seaward flow velocity and hourly-averaged water level: fluvial-dominated reaches (positive seaward flow velocity and water level); interaction reaches (both nearly zero); and wave-dominated reaches (negative seaward flow velocity despite sustained positive water level). Deltaic front and offshore bars develop simultaneously in fluvial-dominated and wave-dominated reaches, respectively, which are separated by an interaction reach. They exhibit distinct morphological and depositional characteristics resulted by different depositional processes. 2) Partially submerged offshore bars form near the breaker zone in the wave-dominated reaches, originating from thin and submergent longshore shoreface. Initially, two offshore bars form on both sides of the longshore shoreface through vertical accretion, then they experience elongation and merge into a large-scale bar through lateral accretion, and finally continue to widen through landward accretion. 3) Deltas are confined to the fluvial-dominated reaches, extending to a certain length with a low length-to-width ratio and a smooth shoreline, featuring rare sinuous distributary channels before the formation of the large-scale offshore bar. Once the large-scale offshore bar forms, deltas continue to prograde basinward. The results of this study have been applied to reservoir characterization, improving the identification of single sand-bodies and internal accretions in Member 2 of the Paleogene Shahejie Formation, Shengtuo Oilfield, Bohai Bay Basin, China.