Sedimentary characteristics and development mechanism of sublacustrine fan in the Second Member of Liushagang Formation of eastern Wushi Sag, Beibuwan Basin

Sublacustrine fans are an important reservoir in petroliferous basin and the analysis of sublacustrine fans will guide hydrocarbon exploration. A sublacustrine fan was recognized in the second member of the Liushagang Formation in the eastern Wushi Sag and it has significant exploration potential. However, previous research on its sedimentary characteristics and development mechanisms are relatively limited, and the patterns of hydrocarbon enrichment remain unclear. By combining the core, logging, and seismic data, we conducted an in-depth investigation into the sedimentary characteristics, internal architecture, and development mechanisms of sand bodies in the sublacustrine fan. Results indicate that the sublacustrine fan is composed of predominantly mudstone and siltstone deposits, and its sand bodies had undergone repeated winnowing and transportation processes. The sand bodies of the sublacustrine fan could be divided into four stages based on high frequency sequence. The four stages of sand bodies exhibited a retrogradational stacking pattern in vertical direction. In horizontal direction, the sublacustrine fan distributed in the north-northeast (NNE) direction, which was controlled by hydrodynamic intensity and sediment source direction. The main controlling factors for the development of sublacustrine fan deposits include: ① Paleostructure: intense tectonic activity formed slope-break zones controlled by syn-sedimentary faults, showing slope-like characteristics in morphology favorable for the sedimentation of sand bodies in the sublacustrine fan; ② Paleo-source: a significant drop in lake level created favorable conditions for sediments originated from the Qixi Uplift to move toward the center of the sag; ③ Paleoclimate: the warm and humid climate conditions favored denudation in the provenance, thereby providing abundant sediment supply for the formation of sublacustrine fans; ④Paleogeomorphology: the denudation zones and troughs developed along the northern margin of the study area served as the primary transport pathways for sediments supply of the sublacustrine fans.