Sedimentary characteristics and controlling factors of beach-bar sandstones of the upper section of the fourth member of Shahejie Formation on the southern slope of Dongying Sag,Bohai Bay Basin

The sedimentary characteristicsof beach-bar sandstones of the upper section of the fourth member of Shahejie Formation on the southern slope of Dongying Sag were analyzed based on logging, coring and testing data. The controlling factors for the sedimentary distribution of beach-bar sandstones were discussed according to palaeogeomorphology, sediment source, paleobathymetry and paleo-hydrodynamics restoration. The results showed that the gentle slope zone in the rift lake basin is themain factor controlling the development of large-scale beach-bar sandstones. The main beach-bar of this area has a topographic gradient between 0.5° and 2°. The micro ancient terrains and their periphery are favorable for the deposition of thick beach-bar sandstones. Sediment sources provide material for the deposition of beach-bar sandstones. When the sediment supply index is between 0.4-0.6, sandstone beach-bar is developed. While the source supply index is between 0.2-0.4, mixed beach-bar is developed. By combining elemental analysis with logging and core observation, the paleobathymetry of the study area was determined, and the depositional environment was a shallow lake environment. Palaeo water force controlled the lateral differentiation of the sand bodies.