Sedimentary Facies Analysis of the Third Eocene Member of Shahejie Formation in the Bonan Sag of Bohai Bay Basin (China): Implications for Facies Heterogeneities in Sandstone Reservoirs

Abstract

The middle sub-member (Es3z) within the third member (Es3) of the Eocene Shahejie formation is the main source of the generation and accumulation of hydrocarbons in the lacustrine deltas of Bonan depression. Exploration and research work in different blocks is carried out separately. Types of sedimentary facies, and their vertical and lateral evolution in Es3z are not studied in detail. To fill this knowledge gap, we did a detailed analysis of facies and lithological characteristics through integrative studies of cores, well logs and seismic data. Identification of sedimentary structures and lithology of the reservoir zone from cores are calibrated with high-quality well logs and seismic data. Depositional facies in Es3z reservoirs are identified through analysis of sedimentary structures, grain size, log’s trends and seismic sections. Es3z was deposited in the fan delta front setting where five facies associations are found, among them distributary channels consisting of MCS, CSg, PCSs, MS, RCL, WCS, PBSs, RCS and GBS lithofacies, natural levee containing DFs, and furthermore, sheet sand are associated to CBS and SSM lithofacies. GM, GGM and DGM lithofacies are related to inter-distributary deposits, whereas mouth bars consist of PLS, CS and CFS. Depositional history, flow direction of the sediments, and facies distribution are investigated through detailed facies mapping and cross-section profiling to show that the sediments were sourced from southeast to northwest. We found thicker succession of sedimentary profiles towards north and north-west directions. Belt distributary channel deposits, covering a wide range of areas, act as potential reservoirs along with mouth bar deposits, while mudstones in interdistributary channels act as a good source and seal rocks. The methodology adopted has great potential to explore the reservoirs of fan delta front in lacustrine deltas.