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

Abstract

The Paleogene Liushagang Formation is part of the Fushan Sag, a continental lacustrine basin located at theSoutheastern margin of the Beibuwan Basin, South China Sea. Further understanding of the deep-water gravityflow 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 withcore observation, well log data, and three-dimensional seismic data to identify four deep-lacustrine gravity flowlithofacies including their vertical and lateral relationships within the depositional system. The results are thenused to establish a deep-water gravity flow depositional model. Four types of gravity flow lithofacies developed inthe sag: sandy debrite, turbidite, sandy slump, and bottom-current deposits. Sand-rich sub-lacustrine fan depositswith typical turbidite channels were developed mainly in the western depression, whereas distal isolated lobesformed by sandy debrite flow deposits occurred mainly in the eastern depression. The results obtained in this studywill be helpful in the research of gravity flows in similar continental lacustrine environments.