Geochemical Characterization of the Eocene Coal-Bearing Source Rocks, Xihu Sag, East China Sea Shelf Basin, China: Implications for Origin and Depositional Environment of Organic Matter and Hydrocarbon Potential

Abstract

Eocene coal-bearing source rocks of the Pinghu Formation from the W-3 well in the western margin of the Xihu Sag, East China Sea Shelf Basin were analyzed using Rock-Eval pyrolysis and gas chromatography–mass spectrometry to investigate the samples’ source of organic matter, depositional environment, thermal maturity, and hydrocarbon generative potential. The distribution patterns of n-alkanes, isoprenoids and steranes, high Pr/Ph ratios, abundant diterpanes, and the presence of non-hopanoid triterpanes indicate predominant source input from higher land plants. The contribution of aquatic organic matter was occasionally slightly elevated probably due to a raised water table. High hopane/sterane ratios and the occurrence of bicyclic sesquiterpanes and A-ring degraded triterpanes suggest microbial activity and the input of microbial organisms. Overwhelming predominance of gymnosperm-derived diterpanes over angiosperm-derived triterpanes suggest a domination of gymnosperms over angiosperms in local palaeovegetation during the period of deposition. The high Pr/Ph ratios, the plot of Pr/n-C17 versus Ph/n-C18, the almost complete absence of gammacerane, and the distribution pattern of hopanes suggest that the samples were deposited in a relatively oxic environment. Generally, fluctuation of redox potential is coupled with source input, i.e., less oxic conditions were associated with more aquatic organic matter, suggesting an occasionally raised water table. Comprehensive maturity evaluation based on Ro, Tmax, and biomarker parameters shows that the samples constitute a natural maturation profile ranging from marginally mature to a near peak oil window. Hydrogen index and atomic H/C and O/C ratios of kerogens suggest that the samples mainly contain type II/III organic matter and could generate mixed oil and gas.

Geochemical Evaluation of Pinghu Formation Source Rocks for Hydrocarbon Potential in East China Sea of Xihu Depression

“…This is of great signi cance, especially in regions with limited exploration activities, because it provides additional geochemical evidence for further comparisons and analyses. Moreover, previous studies have not de nitively established which constituent of the Pinghu Formation serves as the predominant hydrocarbon source rock [1][2][3][15][16][17]20 . However, there is a prevailing consensus that coal plays this role.…”

“…The source rock thickness of the Pinghu Formation in the Xihu Depression is large, with a wide distribution area and deep burial depth. Their organic matter abundance and type of hydrocarbon source rock are high with great potential to be one of the major source rocks 1,11,17 . Based on a comprehensive analysis of organic geochemical characteristics, petroleum system correlation, basin formation history, sedimentary evolution and depositional environments, this study suggests that the Pinghu Formation has high hydrocarbon generation potential in this deep area (> 3500 m) within Xihu Depression.…”

“…From the perspective of hydrocarbon generation potential, carbonaceous mudstone and coal both perform well 17,34 . Figure 8a shows that the TOC values of coal samples vary with different mineral contents, ranging from 31.7-72.9%, with an average of 51.3%.…”

“…The Pinghu Formation is widely recognized as a gas-rich interval 1,12 , which is renowned for its rich hydrocarbon resources, with estimated oil and gas production of approximately 2.3×10 9 t and 2.377×10 12 m 3 , respectively 13 . Its coalbearing rocks are the primary source rock of the Xihu Depression [13][14][15][16] , which are widely distributed and have a high thickness 1,3,17 .…”

“…Previous studies have focused on the coal-interval source rocks and conducted research extensive analyses, indicating their increasing importance 3,17 . However, previous studies are still insu cient and require more geochemical evidence to supplement and improve them 18 .…”