Petrographic and geochemical characterization of microfacies in a lacustrine shale oil system in the Dongying Sag, Jiyang Depression, Bohai Bay Basin, eastern China

The Rock-Eval oil saturation index (OSI) (S1 × 100 / TOC) value is variable because of the different characteristics of these hydrocarbon residue occurrences. OSI values in microfacies A are normally high (74.8-242.8 mg HC/g TOC, average of 117.2 mg HC/g TOC), and likely much higher if microfractures develop (> 200 mg HC/g TOC). OSI values (averaging 43.7 mg HC/g TOC) are lowest in microfacies F because of adsorption on the shales, and OSI values are the highest in microfacies D (371.3 mg HC/g TOC) and E (199.9 mg HC/g TOC) since the hydrocarbon residues are stored both in microfractures and pores between the grains. Both OSI values are high in microfacies G and H because of low TOC content. The combination of petrographic and OSI analysis appear to be a promising tool for characterizing shale oil systems.