Sedimentary characteristics of large-scale lacustrine beach-bars and their formation in the Eocene Boxing Sag of Bohai Bay Basin, East China



The beach-bar reservoir play has become an important exploration target within the Bohai Bay Basin, especially in the Boxing Sag within the Dongying Depression, where a large-scale lacustrine beach-bar oil pool has been discovered recently. The sedimentary characteristics, distribution patterns and formation mechanisms of beach-bar sand bodies in the upper fourth member of the Eocene Shahejie Formation (Es4s) in the Boxing Sag were studied in detail based on seismic, well log data and core data. The Es4s in the Boxing Sag is composed of a third-order sequence consisting of three systems tracts, i.e. a lowstand systems tract, a transgressive systems tract and a highstand systems tract. Beach-bar sand bodies were deposited widely in the basin during the lowstand systems tract period. The sandy beach-bars are characterized by siltstones, fine-grained silty sandstones interbedded with thin mudstone units. The presence of well-developed sedimentary structures, such as swash bedding, parting lineation, parallel bedding, ellipsoidal mud clasts, ripples, terrestrial plant debris and vertical burrows, suggests that beach-bars were deposited in a relatively shallow water environment under the influence of strong hydrodynamics. Laterally, the sandy beach facies occurred as a more continuous sheet-like body around the sandy bar in most parts of the sag. Stratigraphically, beach-bars were distributed mainly in the lowstand systems tract and they were less well-developed in the transgressive systems tract and highstand systems tract. Several factors were probably responsible for the occurrence of the large-scale beach-bars during the lowstand systems tract period, including: (i) a gentle palaeoslope and relatively weak structural activities; (ii) a shallow-water condition with a strong hydrodynamic environment; (iii) high-frequency oscillations of the lake level; and (iv) an abundant terrigenous clastic feeding system with multiple-point and linear sediment sources.