Depositional evolution and models for a deep-lacustrine gravity flow system in a half-graben rifted sag, Beibuwan Basin, South China Sea

The Paleogene Liushagang Formation is part of the Fushan Sag, a continental lacustrine basin located at the Southeastern margin of the Beibuwan Basin, South China Sea. Further understanding of the deep-water gravity flow deposits in this formation will be conducive to lithologic reservoir exploration in the sag. In this study, three members of the Liushagang Formation, SQEls3 SQEls2 and SQEls1, from old to young, are used with core observation, well log data, and three-dimensional seismic data to identify four deep-lacustrine gravity flow lithofacies including their vertical and lateral relationships within the depositional system. The results are then used to establish a deep-water gravity flow depositional model. Four types of gravity flow lithofacies developed in the sag: sandy debrite, turbidite, sandy slump, and bottom-current deposits. Sand-rich sub-lacustrine fan deposits with typical turbidite channels were developed mainly in the western depression, whereas distal isolated lobes formed by sandy debrite flow deposits occurred mainly in the eastern depression. The results obtained in this study will be helpful in the research of gravity flows in similar continental lacustrine environments.