Controlling effect of slope break zone on sedimentary systems in the slope area of foreland basin: A case study of the 4th Member of Xujiahe Formation in central Sichuan basin

To clarify the control effect of slope break zones on sedimentary systems in the foreland basin slope area and determine the distribution pattern of these systems, the fourth member of the Xujiahe Formation in the central Sichuan basin was used as a case study. Ancient landforms were reconstructed using the impression method, and slope break zones were identified. Subsequently, core observation, logging facies analysis, and seismic facies analysis were carried out todelineate sedimentary microfacies in the foreland slope area. By integrating sedimentary microfacies and reconstructed ancient landforms,this research proposes that large-scale slope break zones exert significant control over the distribution of sedimentary systems and associated sand bodymicrofacies. Results indicate that the sedimentary period of the fourth member of the Xujiahe Formation in the central Sichuan basin was characterized by a foreland slope setting, marked by the presence of large slope break zones. In this background, a far-source braided river delta sedimentary system developed, mainly composed of braided river delta front subfacies. Further differentiation reveals distinct inner and outer delta front belts,encompassing four sedimentary microfacies, including mouth bars, underwater distributary channels, interdistributary bays, and sheet sand. Above the slope break zone, underwater distributary channel microfacies are prevalent in the inner delta front zone. Conversely, below the slope break zone, composite sand bodies composed of underwater distributary channels and river mouth bars dominate the outer delta front zone. The Hekou dam, situated below the slope break zone, exhibits favorable lateral shielding conditions and superior roof and floor conditions, which are conducive to the formation of lithologic traps. Its proximity to hydrocarbon source rocks in the western Sichuan depression renders it a highly prospective target for future exploration endeavors. © 2024 Geological Society of China. All rights reserved.