Hydrocarbon Generation Potential and Depositional Setting of Eocene Oil-Prone Coaly Source Rocks in the Xihu Sag, East China Sea Shelf Basin

The Xihu Sag in the East China Sea Shelf Basin is a focus for hydrocarbon exploration and development. Hydrocarbons in the Xihu Sag are believed to have mainly originated from coals in the Paleogene Pinghu Formation (shortened as Pinghu coal). In this study, the hydrocarbon generation potential, origin of organic matter, and depositional setting of the Pinghu coal were analyzed by means of optical microscopic analysis, bulk organic geochemistry, and molecular geochemistry analysis. The results reveal that the maceral compositions of the Pinghu coal are characterized by a predominance of vitrinite (73.91-96.13%) with relatively high contents of liptinite (1.47-23.02%) and only minor amounts of inertinite (0-5.18%). Type II-III kerogen and high values of TOC (total organic carbon) (8.24-56.77%), EOM (extractable organic matter) (14 601-112 259 ppm), and HI (hydrogen index) (178.76-291.18 mg-FIC/g-TOC) indicate that the Pinghu coal is both gas- and oil-prone and could not only generate but also expel hydrocarbons. The results of vitrinite reflectance (0.54-0.90%), Tmax (421-453 degrees C), and biomarker-related parameters, including CPI (carbon preference index) (1.10-1.61), OEP (odd-to-even predominance) (1.09-1.49), 22S/(22S + 22R) for C-3(1) homohopane (0.59-0.62), and 22S/(22S + 22R) for C-32 homohopane (0.57-0.60), suggest that these coaly source rocks have entered the hydrocarbon generation threshold, most of which have entered the expulsion threshold. Biomarker-related parameters of Sigma n-C21-/Sigma n-C-22(+) (0.38-3.62), Pr/Ph (3.33-9.23), Pr/n-C17 (1.91-14.88), Ph/n-C18 (0.35-1.83), 22S/(22S+22R) of C-31 homohopane (0.59-0.62), 22S/(22S + 22R) of C-32 homohopane (0.57-0.60), 1,2,7-TMN/1,3,7-TMN (0.74-14.39), and 1,2,5-TMN/1,3,6-TMN (2.22-21.07) suggest that organic matter in the Pinghu coal was deposited under relatively oxic peatland conditions characterized by a predominance of terrestrial higher plant input, especially the resin-rich Pinaceae and Taxodiaceae. The absence of combustion-derived PAHs indicates that neither vegetation fire nor peat fire occurred very frequently during the formation of peat. Areas in the Xihu sag with considerable thicknesses of coal should be paid particular attention for future hydrocarbon exploration. From a global perspective, Cenozoic coaly source rocks, which are characterized by a relatively high content of aliphatic components, should be paid special attention for their oil-prone properties related to the advent of conifer plants.