Impact of marine incursions on lacustrine source rocks: organic matter quantity, quality, and kinetics in the Paleocene South Yellow Sea Basin, offshore eastern China

Abstract

The impact of marine incursions during transgression, on the quantity and quality of organic matter in lacustrine source rocks, is critical to understanding their source rock properties and therefore on their hydrocarbon generation potential, but is also challenging and controversial. Here we examine this issue based on a case study in the South Yellow Sea Basin (SYSB) offshore eastern China, addressing organic matter (OM) quantity, quality and petroleum generation kinetics. A total of 139 lacustrine source rock samples was collected from the fourth member of the Paleocene Funing Formation (E1f4) in the SYSB, and investigated for organic petrographic and geochemical characteristics. The samples are immature to early oil window mature and the OM in all samples is dominated by terrigenous material, as shown by the organic petrography, relatively low hydrogen indices and biomarker distributions. The sediments are classified as inert to gas-prone. However, the presence of marine fossils (red and brown algae, coccolithophores and probably Neomonoceratinabullata) and the biomarker results, suggest that marine incursions occurred during the deposition of the lower unit of the E1f4 (E1f4L). The samples from this unit display higher HI values and more depleted δ13Corg ratios, but lower TOC and S1 + S2 contents, than the upper unit of the E1f4 (E1f4U) source rocks without marine influence. This suggests that marine incursions primarily improved the quality, rather than quantity, of organic matter. Bulk kinetics indicated that marine incursions also led to a more homogenous OM and compositional kinetics and phase properties suggest that black oil is generated throughout the process of kerogen conversion. Thus, our data suggest that marine incursions had a positive effect on the OM quality and oil generation potential of these lacustrine source rocks. Therefore, the impact of marine incursions should be fully considered during source rock evaluation of coastal lacustrine basins, especially during high sea-level stages. In addition, the greatest hydrocarbon exploration potential is observed in the South 4th Sag of the Southern Depression and thus this study provides valuable data for shaping regional exploration strategy in the SYSB.