Kang Shilong, Shao Longyi, Qin Lanzhi, Li Shuxia, Liu Jinshui, Shen Wenchao, Chen Xiaodong, Eriksson Kenneth A, Zhou Qianyu
State Key Laboratory of Coal Resources and Safe Mining and College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing Campus, D11 Xueyuan Road, Beijing 100083, People's Republic of China.
China National Offshore Oil Corp-Shanghai, 388 Tongxie Road, Shanghai 200335, People's Republic of China.
ACS Omega. 2020 Dec 8;5(50):32267-32285. doi: 10.1021/acsomega.0c04109. eCollection 2020 Dec 22.
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·HC/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 °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 homohopane (0.59-0.62), and 22S/(22S + 22R) for C 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 ∑-C/∑-C (0.38-3.62), Pr/Ph (3.33-9.23), Pr/-C17 (1.91-14.88), Ph/-C18 (0.35-1.83), 22S/(22S+22R) of C homohopane (0.59-0.62), 22S/(22S + 22R) of C 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 and . 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.
东海陆架盆地西湖凹陷是油气勘探开发的重点区域。西湖凹陷的油气主要来源于古近系平湖组煤（简称平湖煤）。本研究通过光学显微镜分析、有机地球化学分析和分子地球化学分析等手段，对平湖煤的生烃潜力、有机质来源及沉积环境进行了分析。结果表明，平湖煤的显微组分以镜质组为主（73.91% - 96.13%），壳质组含量相对较高（1.47% - 23.02%），惰质组含量较少（0 - 5.18%）。Ⅱ - Ⅲ型干酪根以及较高的总有机碳（TOC）含量（8.24% - 56.77%）、可提取有机质（EOM）含量（14601 - 112259 ppm）和氢指数（HI）（178.76 - 291.18 mg·HC/g·TOC）表明，平湖煤既具有产气潜力又具有产油潜力，不仅能够生烃，还能够排烃。镜质体反射率（0.54% - 0.90%）、Tmax（421 - 453℃）以及生物标志物相关参数，包括碳优势指数（CPI）（1.10 - 1.61）、奇偶优势（OEP）（1.09 - 1.49）、C30藿烷的22S/(22S + 22R)（0.59 - 0.62）和C29藿烷的22S/(22S + 22R)（0.57 - 0.60），表明这些煤系烃源岩已进入生烃门限，其中大部分已进入排烃门限。生物标志物相关参数∑-C/∑-C（0.38 - 3.62）、Pr/Ph（3.33 - 9.23）、Pr/C17（1.91 - 14.88）、Ph/C18（0.35 - 1.83）、C30藿烷的22S/(22S + 22R)（0.59 - 0.62）、C29藿烷的22S/(22S + 22R)（0.57 - 0.60）、1,2,7 - TMN/1,3,7 - TMN（0.74 - 14.39）和1,2,5 - TMN/1,3,6 - TMN（2.22 - 21.07）表明，平湖煤中的有机质是在相对氧化的泥炭地环境中沉积的，以陆源高等植物输入为主，尤其是富含树脂的植物。燃烧源多环芳烃（PAHs）的缺失表明，在泥炭形成过程中，植被火灾和泥炭火灾均不太频繁发生。西湖凹陷中煤厚度较大的区域在未来油气勘探中应予以特别关注。从全球角度来看，新生代煤系烃源岩因针叶植物的出现而具有较高的脂肪族组分含量，其产油倾向应予以特别关注。
