River patterns transition of subaqueous distributary channels in delta front and its significances for petroleum geology: an insight from Paleogene Yabus Formation in the Sag A of Melut Basin, South Sudan

Subaqueous distributary channel sandbodies within delta fronts are crucial reservoirs in continental petroliferous basins. Understanding the spatiotemporal transition of river patterns in these channels is essential for accurate evaluation and prediction of oil and gas reservoirs, as well as for providing direct evidence of basin evolution. In the Yabus Formation of the Sag A of Melut Basin, a comprehensive analysis involving sequence division, sedimentary characteristics, seismic facies, highresolution reservoir inversion, and sand body distribution revealed significant insights. During the Yabus Formation deposition, three intermediate base-level cycles were identified, each showing transition phenomena in the river patterns of subaqueous distributary channels within the delta front. Clear identification criteria for different river patterns were established. Braided subaqueous distributary channels exhibited dominant vertical accretion, high sand content, significant sandstone thickness, and continuous-strong amplitude seismic reflections. While the braided-meandering transition pattern showed a combination of vertical and lateral accretion, medium sand content, moderate sandstone thickness, and medium continuous-medium strong amplitude seismic reflections. Meandering subaqueous distributary channels were characterized by lateral accretion, low sand content, minimal sandstone thickness, weak continuous-weak amplitude seismic reflections, and mud-rich inversion features. The primary control factor influencing the transition of river patterns in these channels was identified as the long-term base-level cycle, shaped by paleotopography and sediment supply. Braided subaqueous distributary channels emerged as the main exploration interval for structural prospects, serving as lateral high-speed migration pathways. Dendritic braided and meandering transition intervals were deemed favorable for both structure-lithologic prospects and the expansion of new exploration fields and layers.