Lithofacies and depositional model of organic-rich lacustrine mudstone: a case study of the second member of the Eocene Liushagang Formation in the Weixinan Sag, Beibuwan Basin, China

The mudstone in the second member of the Eocene Liushagang Formation is the most important oil and gas supply interval in the Weixinan Sag. Natural gamma-ray curves, SEM, XRD and major and trace element analyses were used to investigate the lithofacies, tectonostratigraphy, palaeoenvironment and influence of environmental factors on the lithofacies and to create a depositional model for the second member of the Eocene Liushagang Formation in the Weixinan Sag. The results show that organic- and clay-rich siliceous mudstone dominated (LA1A) (in the bottom) and organic- and siliceous-rich clay mudstone dominated (LA1B) (in the top), whose deposition thicknesses were respectively around 50–150 and 40–80 m. For both deposition stages LA1A and LA1B, the overall trends were similar in that they were mainly characterized by an upward half cycle. The hidden mechanisms of organic matter enrichment for both stages can be summarized as high fault activity rate and tectonic subsidence rate, deep lacustrine facies, extreme heat and anoxic environment. However, these stages differ in the organic matter productivity conditions (LA1A > LA1B). In this study we propose a model to explain the depositional processes of the second member of the Liushagang Formation, which provides theoretical support for shale oil exploration in the exploration area and is crucial for understanding the lithofacies deposition mechanism of lacustrine mudstones in other areas. Also, it may serve as evidence in the study of the Eocene carbon cycle and extreme heat events in terrestrial and low-latitude regions.