Numerical and physical simulation of sedimentary evolution of shallow sea gravity flow in the Yinggehai Basin, South China Sea：a case study of Dongfang Ⅱ gas field

The Yinggehai Basin hosts a series of gravity flow deposits in shallow marine environments, with highly complex reservoir development mechanisms and sand body connectivity, severely constraining the efficient development of gas fields. Through sedimentary numerical and physical simulations, we investigated the dominant factors and distribution patterns of atypical submarine fans in shallow marine settings. By integrating typical core facies markers, it clarifies that the predominant sedimentary deposits in the area are sandy debris flow deposits, essentially forming a composite submarine fans shaped by the interplay of channels and lobes. Additionally, by combining paleogeomorphic features and sand body distribution characteristics, we established a sedimentary evolution model featuring continuous progradation in the restricted stage, compensation stacking in the semi-restricted stage, and diversion swing in the non-restricted stage. Combining the geological understanding with production dynamic characteristics, we comprehensively analyzed the internal structure and connectivity of reservoir sand bodies, and delineated the internal structural style of the complex submarine fan reservoirs in the Dongfang Ⅱ gas field. Results indicate that in the gas field, the early-phase sand bodies exhibit good vertical stacking connectivity, the mid-phase sand bodies show moderate lateral stacking connectivity locally affected by seepage barriers, and late-phase deposits were modified by muddy channeling, featuring isolated sand body distribution with deteriorated connectivity. These findings provided a theoretical foundation for analyzing the internal potential of the gas field.