Hydrocarbon generation potential, geochemical characteristics, and accumulation contribution of coal-bearing source rocks in the Xihu Sag, East China Sea Shelf Basin

The Xihu Sag is an important petroliferous area in the East China Sea Basin. Petroleum in this sag was generated from coal-bearing source rocks, i.e., coal, carbonaceous mudstone, and mudstone. The lack of detailed studies on the source rocks restricts petroleum exploration. Hydrocarbon generation potential, geochemical characteristics, and relative contribution were assessed using 38 source rock samples and 13 oil/oil sand samples. Results show that source rocks have different hydrocarbon generation potentials. The total organic carbon (TOC), production yield (S1+S2), extractable organic matter (EOM), and total hydrocarbon content suggest that coal has the highest organic matter abundance, followed by carbonaceous mudstone and mudstone. All source rocks are dominated by Type III-II2 kerogen and are, therefore, gas-prone. Biomarker assemblages indicate that the organic matter of coal is mainly derived from terrigenous higher plants with little contribution from phytoplankton, accompanied by strong bacterial modification, which was deposited in an oxidizing depositional environment with a poorly stratified water column. Mudstones, in contrast, have less terrigenous higher plants but higher phytoplankton contributions and were deposited in a more reducing depositional environment. Almost all biomarker parameters of carbonaceous mudstones are between coal and mudstones, suggesting that the organic matter sources and depositional environments for carbonaceous mudstones are between that of the coals and mudstones. The greater phytoplankton contribution to the mudstones is verified by the higher liptinite content. However, the lower organic matter abundance and the lower hydrogen index indicate that vitrinite rather than liptinite is the major hydrocarbon generation maceral. Although coal and carbonaceous mudstone have significantly higher organic matter abundance, more oil-prone kerogen types, and more gold-tube pyrolysis yields, oil-source correlation employing biomarkers and carbon isotopes suggest that mudstones are the major source rocks in the Xihu Sag. This apparent contradiction may be due to the thin layer thickness and limited distribution of the coal. The fact that more contributions of coal and carbonaceous mudstone for oils are found in the western slope belt where coal and carbonaceous mudstone were deposited seems to support this viewpoint. This contradiction may also be caused by the inability of coal to effectively expel the generated high molecular weight hydrocarbons.