Sedimentary development and main controlling factors of beach-bar sand bodies in Haizhong Depression, Beibuwan Basin

Multiple wells drilled in the marine depression of the Beibuwan (Beibu Gulf) Basin in South China Sea have achieved high-yield hydrocarbon flows, confirming its favorable exploration potential. Therefore, studying its sedimentary development characteristics and dominant controlling factors is of great significance. Therefore, investigating the sedimentary evolution patterns and influential factors in this region is crucial. By examining core and well loggings, we analyzed the attributes of beach bar sand bodies and identified sedimentary facies based on seismic attributes and profile interpretation, and ancient geomorphological reconstruction. Results indicate that the beach bar sand bodies had different facies, showing cross-bedding, parallel bedding, storm sedimentary sequences in well core specimens, and “four-sector” pattern in the probability grain size accumulation curves. The well loggings demonstrated continuous funnel-shaped patterns with multiple reverse cycles, while seismic profile show micro-hump reflections and strong amplitude wave seismic reflections. The beach-bar sand bodied were developed at the southwest-dipping end of the low uplift to the southwest of Weizhou Island and the western side of the gentle slope zone to the south of Weizhou Island. The longitudinal upper beach bar sand bodies were superposed each other, indicating frequent migration and multi-stage overlapping of the sand bodies in response to fluctuations of lake levels. The positioning and distribution of the sand bodies were influenced by terrain relief, material sources, water depth, and lake level fluctuations in the past. Ancient trough or channel cradled the sand bodies, while underwater low rise or ridge positioned the sandbars. Material sources determined the size of beach bar sand bodies. Additionally, the depth and the direction of local river heading, lake waves, and coastal lake flow determined the depositional range and depth of sedimentary facies from shore to deep lake. This study offered valuable data for future exploration of oil and gas reserves in beach bar reservoirs of the study area and those with similar geological settings.