Architectural characteristics of beach-bar reservoirs in the lower submember of the 2^nd member of the Paleogene Dongying Formation in block SC7, Huanghekou Sag, Bohai Bay Basin

Beach bars in block SC7 of the Huanghekou Sag, Bohai Bay Basin are identified as important reservoirs in the shore-shallow lacustrine area. Figuring out the internal structural characteristics of the beach-bar sand bodies can offer significant guidance for hydrocarbon exploitation. Based on the examination of modern analogs and data on cores and logs, we carry out architectural dissection for sand bodies in the lower submember of the 2^nd member of the Dongying Formation (the Dong 2 lower submember) in block SC7, segmenting the beach-bar facies featuring thick mudstones and thin sandstones to five sedimentary microfacies:bar center, bar lateral margin, inner marginal beach, outer marginal beach, and lacustrine mud. By analyzing the origin and internal structural characteristics of the thick-mudstone and thinsandstone beach bars, we investigate the sedimentary characteristics and pattern and growth process of the beach bars. Consequently, the beach-bar architectures are categorized into three levels, namely composite sand bar,single bar, and accretionary body, along with the exploration of the spatial distributions of these architectural units of beach bars. To identify single bars,we propose four indicators:inter-bar mudstone, logging curve characteristics, relative elevation difference, and lateral phase change of a single bar. Using these indicators, we identify single bars in the area with a dense well pattern, analyzing their scales, evolution process, and interconnectivity. Furthermore, four phases ofaccretionary bodies within single bars are determined, finally ascertaining the effects of 4^th-level architectural interfaces, single-bar lateral phase change, and accretionary bodies within single bars on hydrocarbon distribution.