Tectono‐Geomorphic Control of Sandbody Distribution in the Strong Rifting Stage: Insights From the Shahezi Formation, Lishu Fault Depression, Songliao Basin

ABSTRACT Multi‐phase rifting and its sedimentary responses have become key topics in basin research, particularly regarding the strong rifting stage and its control on sand dispersal. This study integrates 3D seismic interpretation, drilling and logging data, and core analysis to examine the tectono‐sedimentary evolution of the strong rifting stage in the Lishu Fault Depression, southern Songliao Basin. The results show that syn‐rift fault activity and associated palaeogeomorphic differentiation strongly influenced sediment routing and sand‐body distribution. During the sedimentation period of the Shahezi Formation, basin‐margin valleys developed, including identifiable V‐shaped, U‐shaped and W‐shaped valleys. The hydrodynamic force gradually weakens from the distant to near‐source areas, and some water systems begin to converge and bifurcate; variations in the activity of the basin‐bounding fault and intra‐basin faults created heterogeneous accommodation, with the second member of Shahezi showing markedly stronger fault activity than the first member; distinct sedimentary systems—including nearshore subaqueous fans, fan deltas and lacustrine deposits—were established; the key point is that different valley morphologies (V‐, U‐ and W‐shaped) directly reflect variations in topographic relief. Such relief determines the potential energy of gravity flows and controls the transport of sand bodies from high‐energy zones to lower‐energy areas. In addition, palaeogeomorphic variations caused by differential activity along individual faults result in dynamic changes in accommodation space, with larger accommodation space being more favourable for sand‐body accumulation. At fault‐tip relay zones, the reduction in palaeogeomorphic relief leads to a sharp decrease in the potential energy of gravity flows. This physical mechanism results in a weakening gradient of hydrodynamic strength, promoting the deposition of sand bodies. Inclined fault‐trough zones dipping in the same direction act as effective unloading areas. The topographic relief associated with faults causes a stepwise decrease in gravitational potential energy, reducing the transport capacity of the fluvial system and forcing sand bodies to accumulate in these topographically lower positions. Similarly, co‐directional fault‐step zones function as a series of geomorphic buffers. The stepwise reduction in palaeogeomorphic relief triggers a corresponding stepwise decline in the gravitational potential energy of the transport system. This reduction in energy diminishes the sediment transport capacity at each step, leading to the preferential accumulation of sand‐sized particles within localised accommodation spaces formed at these fault steps. Correlation analysis indicates that valley‐source parameters, particularly source‐area size and length, are closely linked to fan‐body scale; boundary‐fault activity governed sand accumulation along the steep slope, whereas intra‐basin structural belts controlled transverse and axial dispersal of fan‐delta systems. Valleys in the gentle slope enter the lake, and the lateral and axial far‐source fan delta sand bodies are controlled by structural paleogeomorphologies such as the co‐directionally inclined fault trough belts and co‐directionally inclined fault step belts formed by small intra‐basin faults. These findings provide a quantitative source–structure–geomorphology framework that improves prediction of sand‐body distribution during strong rifting and offers guidance for deep hydrocarbon exploration in the Lishu Fault Depression.