Sediment Control Mechanisms Under the Regulation of Base-Level Cycles in Complex Rift Basins: Spatiotemporal Coupling of "Source-to-Sink" Systems —A Case Study of the Middle - Deep Zone in the Shenxian Sag

In the study of rift basins, the source-to-sink system theory offers a fundamental framework for deciphering sedimentary filling processes and sandbody distribution patterns. This system is strongly influenced by the basin’s structural architecture, paleogeomorphic evolution, and base-level cycles. The spatiotemporal dynamic coupling of three key elements—sediment supply, transport pathways, and depositional convergence—directly governs sandbody formation and distribution, providing important theoretical insights for hydrocarbon reservoir prediction. This study focuses on sediment transport processes through sandbody pathways in complex rift basins and evaluates their differential controls on sandbody convergence within braided-river delta and lacustrine depositional systems under varying base-level cycles. Data from the Shenxian Sag in the Bohai Bay Basin, including extensive core samples, well logs, 3D seismic data, and laboratory analyses, support this investigation.Based on these data, the paleogeomorphology of key sequences was systematically reconstructed. Five valley types, three major slope-break zones, and four categories of accommodation zones were identified, with their controls on sedimentary pathways analyzed in detail. Significant differences were observed between the steep-slope and gentle-slope belts in terms of sediment supply, sandbody scale, and distribution patterns. Under the guidance of the source-to-sink theory, a spatiotemporal sand-control mechanism was established, with base-level cycles acting as the primary regulatory factor and paleogeomorphic elements serving as spatial carriers. This mechanism integrates three core components: sediment supply, transport pathways, and depositional convergence.The study further systematically elucidates four coupling modes and their corresponding sedimentary effects across different tectonic stages and structural units. Sediment supply provides the material basis for reservoir sandbody development, with its volume and intensity modulated by base-level fluctuations. The transport system is mainly constrained by paleogeomorphic features such as paleo-valleys, slope breaks, and accommodation zones. Among these, valleys demonstrate high sand-transport efficiency during base-level lowstands, which diminishes considerably during rising phases. Slope breaks in the northern gentle-slope belt are jointly controlled by base-level cycles and tectonic activity, whereas composite slope breaks in the southern steep-slope belt play a redistributive role in sandbody dispersion. The classification and sand-controlling functions of accommodation zones vary with the characteristics and configuration of syndepositional faults. The convergence system is regulated by cyclic changes in accommodation space driven by base-level movements. During base-level rise, lake-level expansion and increased accommodation space promote the development of retrogradational sequences, whereas base-level fall reduces accommodation space and favors progradational sequences. The transport and convergence systems are spatially linked and interact dynamically, with their functional relationships capable of shifting under the influence of base-level cycles.These findings provide effective guidance for predicting favorable sedimentary facies belts and sandbody distributions in the study area, leading to the identification of four prospective exploration targets. Subsequent drilling results have confirmed their potential. The research outcomes offer a new theoretical foundation and practical model for hydrocarbon exploration and sandbody prediction in analogous complex rift basins.

Keywords: source-to-sink system, sand-controlling mechanism, base-level cycle, palaeogeomorphology, Shenxian Depression, Shahejie Formation