Oil content and resource quality evaluation methods for lacustrine shale: A review and a novel three-dimensional quality evaluation model

Abstract

Lacustrine shale oil represents an important opportunity for unconventional petroleum exploration and has received considerable attention from petroleum geologists. However, the assessment of oil content and resource quality of shale oil reservoirs with substantial heterogeneity and the determination of favorable exploration targets challenge geologists and petroleum explorers. Numerous methods and models have been proposed to evaluate the oil content and resource quality of shale oil accumulations. However, the scope of applying different methods and models varies. This study reviews the methods and models for assessing oil content and resource quality of lacustrine shale and discusses their advantages and disadvantages. Subsequently, a three-dimensional model for evaluating the resource quality of shale oil plays based on multistep pyrolysis parameters is proposed and applied to a case study of the shales of the Jurassic Daanzhai Formation (J1zD) in the Langzhong area of the Sichuan Basin, SW China. To improve the model applicability, a simple algorithm for the total oil yield (TOY) is proposed, which can be used for a rapid and accurate evaluation of the resource quality of a shale play for which only routine pyrolysis data are available. The relative proportions of enriched, potential, less efficient, and invalid resources of the studied Daanzhai shales are estimated to be 66.7%, 14.1%, 0.5%, and 18.7%, respectively. Compared to the evaluation models based on routine Rock-Eval pyrolysis, the new model proposed in this study exhibits the following advantages: (1) the ability to characterize the resource quality of a shale reservoir in terms of geochemical characteristics and oil movability; (2) the consideration of the influence of thermal maturity on the resource quality of shale oil plays; and (3) the use of multistep pyrolysis for establishing the quality evaluation method. The proposed model can allow the determination of favorable areas for shale oil exploration in other basins in the world, improve exploration efficiency, and reduce risks.