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

Shilong Kang; Longyi Shao; Lanzhi Qin; Shuxia Li; Jinshui Liu; Wenchao Shen; Xiaodong Chen; Kenneth A Eriksson; Qianyu Zhou

doi:10.1021/acsomega.0c04109

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

ACS Omega. 2020 Dec 8;5(50):32267-32285. doi: 10.1021/acsomega.0c04109. eCollection 2020 Dec 22.

Authors

Shilong Kang¹, Longyi Shao¹, Lanzhi Qin², Shuxia Li¹, Jinshui Liu², Wenchao Shen³, Xiaodong Chen², Kenneth A Eriksson⁴, Qianyu Zhou³

Affiliations

¹ 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.
² China National Offshore Oil Corp-Shanghai, 388 Tongxie Road, Shanghai 200335, People's Republic of China.
³ School of Earth Science and Engineering, Heibei University of Engineering, Taiji Road 19, Handan, Hebei 056038, People's Republic of China.
⁴ Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States.

Abstract

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 C₃₁ homohopane (0.59-0.62), and 22S/(22S + 22R) for C₃₂ 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 ∑n-C_21-/∑n-C₂₂₊ (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 C₃₁ homohopane (0.59-0.62), 22S/(22S + 22R) of C₃₂ 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.