Pyrolysis product characteristics and hydrocarbon generation-retention-expulsion model of alkaline lacustrine organic-rich shale

• Fengcheng alkaline lacustrine shale represents a distinctive Type I kerogen. • Alkaline lacustrine shale exhibits an early bitumen generation stage. • Massive oil generation and expulsion are synchronous processes at the main oil stage. • Substantial oil originating from previously retained oil is expelled at the late oil window. • Alkaline lacustrine kerogen exhibits high efficiency in converting into oil. Alkaline lacustrine shale, deposited in high-salinity alkaline lakes, represents a unique organic-rich shale. The Fengcheng Formation of the Junggar Basin is the oldest alkaline lacustrine sediments producing substantial petroleum. However, the understanding of hydrocarbon generation, retention, and expulsion characteristics of alkaline lacustrine shale remains limited by existing knowledge and methods. Based on an improved semi-open pyrolysis experimental procedure, this research investigated the hydrocarbon generation-retention-expulsion mechanism of Fengcheng alkaline lacustrine shale. The Fengcheng Shale has a high amount of retained oil (∼ 280 mg/g TOC) dominated by adsorbed oil during the early oil stage, characterized by the early bitumen generation. Kerogen efficiently transforms into liquid hydrocarbons within the main oil window, where total generated and retained oil yields also reach peak levels at the Easy% R o of around 1.0 %. Abundant hydrocarbon generation and expulsion occurred synchronously during the main oil window. During the late oil stage, the oil generation capacity of kerogen diminished, and adsorbed oil cracked into free oil, which was further expelled. Comparison with global organic-rich shales reveals that although the Fengcheng Shale has low organic carbon content, it exhibits a high efficiency of kerogen conversion to oil, likely due to unique haloalkaliphilic plankton and alkaline minerals. Overall, the alkaline lacustrine shale exhibits distinctive Type I source facies, characterized by efficient and continuous oil generation during the early and main oil windows, followed by extensive oil expulsion during the main and late oil windows. These findings significantly advance the knowledge in alkaline lacustrine organic-rich shale, and the insights may serve as a template for other alkaline lacustrine basins. Moreover, this study provides an effective experimental procedure for determining the quantity of hydrocarbons generated, expelled, and retained within organic-rich shale.