Hydrocarbon generation potential of Paleogene coals and organic rich mudstones in Xihu sag, East China Sea Shelf basin, offshore eastern China

Abstract Paleogene coal and organic rich mudstones in Xihu sag, East China Sea Shelf basin in offshore eastern China were analysed to evaluate the hydrocarbon generation potential based on organic petrographic and organic geochemical methods. The results reveal that the source rocks are dominated by Type III-II2 kerogens deposited in sub-oxic to oxic environments as coal-bearing source rocks. The total organic carbon (TOC) content values for the samples range from 0.31 wt% to 75.68 wt%, suggesting fair to excellent hydrocarbon generation potential, which is also consistent with the Hydrogen Index (HI) and extractable organic matter (EOM) content results. Thermal maturity of organic matter in Western Depression Group (WDG) area are mainly in early thermal maturity stage (Rr ranges from 0.57% to 0.76%), while Central Uplift Group (CUG) area samples have relatively higher thermal maturity (Rr ranges from 0.72% to 1.41%). The characteristics of Paleogene coal and organic rich mudstones reveal that the potential source rocks in both WDG and CUG area have fair to excellent hydrocarbon generative potential, but vary in hydrocarbon phase. WDG source rocks are expected to generate both commercial liquid hydrocarbons and gaseous hydrocarbons, which is also implied by the presence of significant liptinite and exsudatinite macerals. On the other hand, source rocks for CUG area exhibit a dominant gaseous hydrocarbon generation and some oil hydrocarbon generation potential. The data exhibited in this study illustrate source rock characteristics and provide evidence for hydrocarbon generation potential of Paleogene coal and organic rich mudstones in Xihu sag, in offshore eastern China.