Exploring the sedimentary evolution processes and hydrodynamic mechanisms of typical mouth bars

Mouth-bar sand bodies serve as significant hydrocarbon reservoirs in the deltaic sedimentary system. However, due to the limitations of the dimensions, scales, and resolutions of data on outcrops, modern deposits, and underground reservoirs, the systematic characterization of both the internal sedimentary evolution processes and the architectural characteristics of the mouth-bar sand bodies remains challenging. By integrating sedimentary numerical simulations and modern sediment observations, we analyze the sedimentary evolution of typical mouth bars, reveal their internal architectural patterns, and thoroughly explore the hydrodynamic mathematical models and mechanisms in their development areas. The results indicate that the sedimentary evolution of typical mouth bars can be divided into five stages: vertical aggradation, progradation, lateral aggradation, stabilization, and the formation of composite mouth bars. These mouth bars exhibit varying morphological characteristics across these evolutionary stages, appearing small elongated tongue-shaped, nearly rhombus, triangular or V-shaped, mid-channel bar-shaped, and long ellipse-shaped sequentially. The sedimentary evolution of mouth bars is governed by hydrodynamic variations. The combined action of jets, basin-floor friction, and water buoyancy, results in the formation of varying aggradational styles and diverse planar morphologies of the existing mouth bars. Additionally, variations in hydrodynamic conditions lead to differentiation in the internal architectural patterns of mouth bars. Accordingly, their internal aggradational styles can be classified into vertical aggradation, progradation, and lateral aggradation. For an individual mouth bar, the developmental model of its internal architecture shows spatial differentiation, featuring vertical aggradation-predominated central part, progradation-dominated front end, and lateral draping and aggradation-prevalent side wings. This study provides a theoretical basis for the fine-scale characterization of the internal architectures of mouth bars in fluvial-dominated deltas while also serving as a guide for analyzing the subsurface architectures of mouth-bar reservoirs.