Shale oil in saline lacustrine systems: A perspective of complex lithologies of fine-grained rocks

Fine-grained sedimentary rocks deposited in saline lakes are an important target for shale oil exploration worldwide. The complex lithologies of such rocks control their shale oil potential, and thus it is essential to study their lithofacies to better understand these controls. In this study, we investigated the fine-grained sedimentary rocks of the Eocene Shahejie Formation in the Dongpu sag, Bohai Bay Basin, eastern China. The mineralogy, sedimentary features, and organic matter content can be used to divide the fine-grained sedimentary rocks into three types and six sub-types: siliceous limestones (organic-rich and laminated, organic-rich and massive, organic-bearing and laminated, and organic-bearing and massive), calcareous siltstones (organic-bearing and massive), and mixed lithologies (organic-bearing and massive). The organic-rich and laminated siliceous limestones (with dissolution fractures between laminae) and organic-bearing massive calcareous siltstones (with interparticle pores between quartz and feldspar) show the highest porosity and shale oil potential. For the organic-rich laminated siliceous limestones with organic matters in the main hydrocarbon generation stage, the high shale oil potential is mainly due to the favorable reservoir quality. In comparison, there are few organic matters present in the organic-bearing massive calcareous siltstones and, as such, the oil potential of this type of lithofacies depends on whether there is a good source rock close to the thin silty interbeds. These results will guide shale oil exploration in the Dongpu sag. Our data suggest the complexity of shale oil accumulation in saline lacustrine systems as controlled by the different lithofacies with various factors.