A novel method for identifying oil content and moveable thresholds in heterogeneous shales

Abstract

The exploitation of shale oil is one of the most crucial strategies for alleviating energy shortages, and identifying high-quality oil content sweet spots is a top priority. The common oil content evaluation models typically apply a relatively fixed oil rich threshold and an oil moveable threshold as constraints. However, the high degree of heterogeneity of terrestrial shales means that the accuracy and practicality still require improvement. Here the ΔQ value was introduced to distinguish whether the shale is receiving or expelled hydrocarbon, and validated the comparison using geologic and geochemical data from these two types of shale. Based on the realization that hydrocarbon-accepting shales have high oil content and mobility, first-level and second-level oil quality thresholds are divided and identified, which represent the boundaries of oil quality from poor to good. The application of this model was demonstrated using 225 continuously cored shale samples from the Shahejie Formation of the Dongpu Depression in the Bohai Bay Basin. The results showed that three resource quality tiers were defined: high-quality (S1 ≥ 3.77 mg/g & OSI ≥ 235 mg/g TOC, 12 % of total resources), medium (0.75 mg/g ≤ S1 < 3.77 mg/g & 94 mg/g TOC ≤ OSI < 235 mg/g TOC, 32.44 % of total resources) and poor (S1 < 0.75 mg/g & OSI < 94 mg/g TOC, 55.56 % of total resources). ΔQ can be used as a constraint to quickly and accurately identify the oil quality threshold for strongly heterogeneous shale, thus avoiding the complex process of analyzing the influencing factors. In addition, this study found that high-quality shale oil is generally found in received hydrocarbon shale, which mainly exhibit laminated-carbonate lithofacies, and a high frequency interlayering or fracture development. In particular, the expelled hydrocarbon shale’s hydrocarbon generation potential and expelled hydrocarbon efficiency together determine oil quality. Overall, this study is expected effectively reduce potential shale oil exploration risks.