Geochemical and geological characteristics of the Es3L lacustrine shale in the Bonan sag, Bohai Bay Basin, China

Although extensive studies have been conducted on unconventional shale oil resources in recent years, little work has been performed on the resource potential of lacustrine shales. The Es3L (lower sub-member of the third member of the Eocene Shahejie Formation) shale of the Bonan Sag, a set of thick lacustrine-deposited sediments, offers a good example for furthering the understanding of these rocks. Geochemical and mineralogical investigations were carried out on a large sample set from the 220 m thick sequence of well Luo69. In total 200 core samples were analyzed for total organic carbon (TOC) and Rock-Eval pyrolysis, and 421 core samples were analyzed for mineral compositions. The Es3L shales have a very high TOC content, with Type I lacustrine oil prone kerogen in the early oil generation stage. Rock-Eval Tmax suppression is evident in the lower part of the Es3L member associated with a rise in PI (production index). With respect to fracability or brittleness, the content of brittle minerals in the Es3L shale is generally higher than that of marine shales in the USA, with higher calcite mineral (averaging 51.82 wt.%) and lower clay content (averaging 18.86 wt.%). Interparticle pores (in minerals grains or clay aggregates), intraparticle pores (intercrystalline dolomite and pyrite framboid) and four types of fractures (tectonic fractures, abnormal pressure fractures, clay shrinkage fractures and inter-laminar fractures) are developed in the Es3L shale. Organic matter pores are not present in the studied samples probably due to lower maturity. Tectonic fracture may play an important role in hydrocarbon migration and accumulation. The results of this study suggest that the Es3L shale in the Bonan sag has very good shale oil potential, especially in the lower section.