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

Abstract

Petrology and Rock-Eval analyses have been conducted in the lower third and upper fourth members (lower Es3 and upper Es4) of the Shahejie Fm source rock intervals in well Fanye1 in the Dongying Sag, Jiyang Depression, Bohai Bay Basin, eastern China. The organic matter in rocks intersected by this well is dominated by lamalginite and bituminite, and most samples contain Type I or II kerogen. Microfacies A–H has been identified according to their organic-mineral composition and structure using polished thin sections under microscopy. The form and location of hydrocarbon residue occurrences (mainly as solid bitumen with fluorescence) are different in those microfacies: microfacies A, which is dominant in well Fanye 1, is calcareous mudstone without lamination, and the hydrocarbon residue is dispersed through the rock, with some being present in subtle microfractures. Microfacies B and F are mostly composed of shale, with adsorbed hydrocarbon indicated by fluorescence along mineral grains. Microfacies C is laminated argillaceous carbonate, with hydrocarbon residue mainly occurring in thin microfractures along the bedding plane. Microfacies D and E are mainly carbonate, and microfacies D is laminated with micrite and sparite, and hydrocarbon residue occurrence is not only in the microfractures along the bedding, but also in the micropores among the grains in the sparite layers, which is the main place for hydrocarbon residue occurrence in microfacies type E without lamination. Microfacies G and H are both micrite. Microfacies G shows bioturbation and hydrocarbon residue occurs within small, irregular microfractures along the bedding planes and pores. Only a little solid bitumen without fluorescence has been observed in microfacies H.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 (74.8–242.8mgHC/g TOC, average of 117.2mgHC/g TOC), and likely much higher if microfractures develop (>200mgHC/g TOC). OSI values (averaging 43.7mgHC/g TOC) are lowest in microfacies F because of adsorption on the shales, and OSI values are the highest in microfacies D (371.3mgHC/g TOC) and E (199.9mgHC/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.