Geochemistry and heterogeneous accumulation of organic matter in lacustrine basins: A case study of the Eocene Liushagang Formation in the Fushan Depression, South China Sea

Determining the spatial distributions of organic-rich mudstones in hydrocarbon-bearing lacustrine basins is crucial for targeting exploration wells and discovering commercially viable petroleum accumulations. However, the strong heterogeneity of sedimentary systems and organic matter(OM) input and accumulations make it difficult to identify OM-rich mudstones in lacustrine deposits. This paper takes the Eocene Liushagang Formation, in the Fushan Depression, South China Sea, as an example to clarify the relationship between the geochemical characteristics of OM and sedimentary facies. A practical approach is proposed for predicting the spatial distribution of good-quality source rocks with high OM contents. The method used is a combination of logging-based surface fitting and the mudstone to stratum thickness ratio(M/S ratio) of different depositional systems. It was found that this approach improved TOC content prediction both vertically and laterally. The results suggest that semi-deep lacustrine mudstones with predominantly aquatic organism input, deposited in suboxic-anoxic saline stratified water, contain more preserved OM than other mudstones. Using this method, limited areas on the southern slope of the lower Liushagang sequence(SQls3), the central part of the Huangtong Sag in the middle Liushagang sequence(SQls2), and the northern part of the Huangtong Sag in the upper Liushagang sequence(SQls1) are determined to be the best-quality source rocks in the development area.