Huo Hongliang1,2 , Liu Chenglin1,2 , Guo Zehui1,2 , Qiao Tong1,2 , Tian Anqi1,2 , Huang Yiyang1,2
(¹State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum (Beijing), Beijing 102249, China
2College of Geosciences, China University of Petroleum (Beijing), Beijing 102249, China)
Abstract: Deep high-maturity natural gas accumulations represent important exploration targets in the central-southern Xihu Sag; however, the genetic relationships among gas sources, thermogenic evolution, charging history, and migration pathways remain poorly understood. The Structure C anticline, a representative deep gas-bearing structure in the central-southern Xihu Sag, provides an ideal natural laboratory for deciphering these complex processes. Here, we integrate regional geological information with comprehensive gas geochemical and petroleum system analyses, including source-rock geochemistry, hydrocarbon biomarkers, natural gas molecular compositions and stable carbon isotopes, fluid inclusion analysis, gold-tube pyrolysis, basin modeling, and seismic interpretation, to investigate the origin, charging evolution, and accumulation mechanisms of deep high-maturity natural gas. The results indicate that Eocene Pinghu Formation coal-measure strata are the dominant source rocks, characterized by mixed mudstone, carbonaceous mudstone, and coal lithologies, type II1-II2 kerogen, and significant terrestrial organic matter input. Biomarker characteristics and light-hydrocarbon compositions further confirm that associated hydrocarbons were mainly derived from local Pinghu coal-measure source rocks, whereas the contribution from Huagang Formation source rocks was limited. Natural gases from the Structure C anticline exhibit typical coal-derived thermogenic signatures, with δ13C1 values ranging from −36.24‰ to −32.78‰. Thermal evolution modeling suggests progressive maturation of Pinghu source rocks, with source-kitchen maturity increasing northward beneath the study area. Fluid inclusion analysis and burial-history modeling reveal a multi-stage charging history involving an early oil-dominated phase followed by late gas invasion. Oil charging mainly occurred during 14-11 Ma, whereas gas charging occurred during 13-10 Ma and was enhanced after 5 Ma. Integrated isotopic and seismic evidence suggests that deep high-maturity gases migrated predominantly through steep, source-connected faults and accumulated within favorable fault–sandbody configurations. These results demonstrate that deep gas enrichment in the central-southern Xihu Sag was controlled by the coupling of coal-measure source kitchens, multi-stage thermogenic gas generation and charging, and fault-controlled migration pathways. This study provides new insights into the origin and accumulation mechanisms of deep high-maturity natural gas systems in complex marginal marine basins.
Keywords: deep high-maturity gas; coal-measure source rocks; hydrocarbon charging history; carbon isotopes; fault-controlled migration; Xihu Sag
Author Profile (first author):
Huo Hongliang, Male, PhD, mainly engaged in petroleum geology and hydrocarbon accumulation mechanisms. E-mail: 15010065116@163.com
