Depositional systems and sequence architecture of the Oligocene Dongying Formation, Liaozhong depression, Bohai Bay Basin, northeast China

The sequence-stratigraphic framework and depositional systems of an Oligocene lacustrine rift succession in the Liaozhong depression of Bohai Bay Basin in northeastern China were investigated using seismic profiles, complemented by well logs and cores. Seven sequences are identified on the basis of unconformities on basin margins and correlative conformities in the basin center. Depositional systems are associated with systems tracts within sequences. Lowstand systems tracts are dominated by sublacustrine fans; transgressive systems tracts are mainly composed of beach-bar deposits; and highstand systems tracts are characterized by deep lacustrine fan or braid-delta deposits. Episodic tectonic movement was the principal factor that controlled the development of the sequences. Lake-level changes resulting from climatic oscillations may have controlled the development of these sequences. The balance between tectonic subsidence and sediment supply controlled the type of lake basin. The Bohai Bay Basin was a balanced-fill basin in the early and middle stages of the Dongying Formation and evolved into an overfilled basin at the end of deposition of the Dongying Formation. This study enhances the understanding of depositional system configuration and systems tracts in a typical rift basin and proposes that the most favorable sandstone reservoirs are developed in sublacustrine fan deposits. Relative lowstand sublacustrine fan deposits capped by relative highstand prodelta or deep lacustrine mudstones form the highest potential lithostratigraphic traps.