Innovative Standard for Graded Evaluation of Oil Enrichment of Lacustrine Shale: A Case Study of the Lower Third Member of the Shahejie Formation, Zhanhua Sag, Eastern China

Abstract

The exploration and production results of lacustrine shale oil are closely related to shale oil enrichment evaluation. However, there exists a lack of rapid and accurate oil enrichment evaluation methods for strongly heterogeneous lacustrine shale. In this study, an innovative standard for graded evaluation of shale oil enrichment is proposed. In the innovative standard, considering the light hydrocarbon loss of shale oil, the free oil (S 1) content is corrected for light hydrocarbons. Moreover, on the basis of the hydrocarbon expulsion threshold theory, and combined with the specific geochemical characteristics of the shale in the study area, the lacustrine shale oil mobility threshold OSI (the oil saturation index) is determined to replace the fixed empirical shale oil mobility threshold (100 mg/g). By combining the absolute oil content (S 1) with the relative oil content (OSI), we are able to divide the shale oil resources into four categories: enriched resources, moderately enriched resources, low-efficient resources, and ineffective resources. This standard is successfully applied to evaluate the oil enrichment degree of the lower third member (Es3 L) of the Shahejie Formation in well Luo 69 of the Zhanhua Sag, Bohai Bay Basin. The oil enrichment degrees of shales with varying kerogen compositions and distinct lithofacies are highly diverse. Shale comprising type I kerogen is the most conducive to shale oil enrichment. However, more than 15% of the tested shales composed of type II1 kerogen include enriched and moderately enriched resources, which should be considered. Organic-fair laminated calcareous shale (OFLCS) is the most favorable lithofacies type for shale oil enrichment. The development of bedding structures and a moderate organic matter content are beneficial for shale oil enrichment. This standard is suitable for graded evaluation of the oil enrichment degree of strongly heterogeneous lacustrine shale worldwide.