Pyrolysis Kinetics of Lacustrine Shales from the Yanchang Formation: Revealing the Role of Kerogen Type in Shaping Hydrocarbon Generation and Expulsion Pattern

The Yanchang Formation in the Ordos Basin is a key target for continental shale oil exploration in China. Due to its complex geological background and diversified organic inputs, the hydrocarbon generation and accumulation in the lacustrine basin remain to be fully understood. Unlike marine shales rich in Type I kerogen, this lacustrine system is dominated by Type II and III kerogens. In this study, Rock-Eval pyrolysis was performed on lacustrine shales with Type IIa, IIb, and III kerogens to investigate the effect of kerogen type on their hydrocarbon generation and expulsion characteristics. The results reveal that the hydrocarbon generation potential of the Yangchang Formation shale generally follows the order of Type IIa > Type IIb > Type III. Pyrolysis kinetic calculations of the kerogens demonstrate a clear hierarchy of hydrocarbon generation and expulsion among the kerogen types, of which Type II kerogen has better hydrocarbon generation potential, earlier generation timing, and narrower generation window than Type III kerogen. The discrepancy in hydrocarbon generation potential and pyrolysis kinetic behavior is largely attributed to the kerogen components and types, which manifests as a kerogen-type constraint on the hydrocarbon generation and expulsion of shale. Based on the geological mapping of the lacustrine shales in the study area, we propose a “kerogen type-specific” exploration strategy that prioritizes Type IIa-rich intervals in moderate-maturity areas for shale oil exploration, Type IIb as secondary prospects, and Type III in high-maturity areas for shale gas exploration. This study provides a systematic investigation of pyrolysis simulation and hydrocarbon generation and expulsion kinetics on the Yanchang Formation shale, as well as a practical framework for optimizing exploration in analogous lacustrine basins.