Geochemical Characteristics and Hydrocarbon Expulsion of Lacustrine Marlstones in the Shulu Sag, Bohai Bay Basin, Eastern China: Assessment of Tight Oil Resources

In recent years, tight oil exploration has made significant progress in the lower part of Shahejie Formation (\( {\text{Es}}_{3} ^{{\text{L}}} \)) in the Shulu Sag, Bohai Bay Basin, Eastern China, which shows good exploration prospects for tight oil. However, accurate evaluation of tight oil resource potential is influenced by a lack of studies and an incomplete understanding of hydrocarbon expulsion from marlstone source rocks. This study investigated the geological and geochemical characteristics of marlstone source rocks, their hydrocarbon generation and expulsion, and the tight oil resource. Results show that the marlstone source rocks were deposited in a reducing to weakly oxidizing lacustrine environment with low-middle salinity, distributed widely in the central and southern troughs, with the maximum thickness greater than 700 m. The marlstone source rocks have relatively high organic matter abundance (0.06–7.97% of total organic carbon content with an average value of 1.51%), are dominated by type II and I kerogen, and are at the immature to mature stage (0.3% < vitrinite reflectance (VR) < 0.8%), which reveals fair–good source rocks for the marlstones. The threshold and peak of hydrocarbon expulsion for marlstone source rocks are at 0.51% VR and 0.6% VR, respectively. The amounts of generation and expulsion from marlstone source rocks are 19.72 × 108 t and 8.53 × 108 t, respectively, with an expulsion efficiency of 43%. The total tight oil resource in place is 10.9 × 108 (5.8 × 108 t within the carbonate rudstone reservoir and 5.1 × 108 t within the marlstone reservoir), indicating a significant tight oil potential and promising exploration prospect in the Shulu Sag, Bohai Bay Basin, Eastern China.