Study on Sedimentary Facies of the Lower Eocene Basal Deposits in the Southeastern Part of the Jiyang Depression, Bohai Bay Basin, China

Abstract

Based on the analysis of cores, logging, and seismic data, this paper identifies the sedimentary facies of the basal deposits from the lower Eocene in the southeastern part of the Jiyang Depression and studies its depositional law. On this basis, a sedimentary model of the basal deposits is established. The basal sediments of the lower Eocene in the Jiyang Depression are mainly red, brown, gray, and variegated clastic rocks, which are mainly divided into Kongdian and lower Shahejie-4 Formation. It is considered that the climate during the depositional period was relatively arid, and the depositional process was greatly affected by floods. Three types of sedimentary facies were identified in the study area: proluvial fan, fan delta, and beach-bar deposition. Proluvial fans are mainly developed at the exit of the paleo gullies in the south area, and fan deltas are mainly distributed in the large gentle slope belt in the northwest, while beach bars are isolated and scattered along the lake shoreline in shallow lakes. The fan-delta and beach-bar clastic rocks are well sorted, with high compositional and textural maturity and can be used as good oil and gas reservoirs. The basal deposits in the southeast of Jiyang Depression are mainly flood-controlled sedimentary models. During the flood period, a large amount of detrital material brought by the flood was deposited in the gentle slope zone, forming the deposition of proluvial fan and fan delta, and the sand bodies at the front of the fan delta formed beach-bar deposits on its front under the transformation of waves and coastal currents. However, during the inter-flood period, only small-scale proluvial fan deposits are formed, fan delta and beach-bar sand bodies are rare.