Geochemical identification of marine and terrigenous condensates-A case study from the Sichuan Basin, SW China

摘要

Condensates are extensively produced from the marine Lower Permian Maokou Formation and the Lower Triassic Jialingjiang Formation (the main marine condensate-producing strata) in southern Sichuan Basin, and also in the terrigenous Upper Triassic Xujiahe Formation in the central and southern Sichuan Basin. Based on the analyses of light hydrocarbons, biomarkers and stable carbon isotopic compositions of marine and terrigenous condensates and the light hydrocarbon composition of natural gases in the Jialingjiang Fm and Xujiahe Fm in the Sichuan Basin, the following conclusions are drawn: (a) condensates produced from the carbonate reservoirs of Jialingjiang Fm and the Maokou Fm (marine condensates) belong to a single oil family, and the condensates from the terrigenous Xujiahe Fm (terrigenous condensates) belong to another family; (b) marine condensates, were generated from sapropelic organic matter in the late stage of high maturation, and the biomarker composition indicates that they were derived from the Lower Silurian shale and the Lower Permian carbonate, while the terrigenous condensates were generated from mature humic source rocks; (c) most of the marine condensates and gases encountered in the Lower Permian Maokou Fm and Lower Triassic Jialingjiang Fm were generated from the same source, and a few of the gases in the Jialingjiang Fm contain admixtures of coal-derived gas, while terrigenous condensates and gases were generated from the Xujiahe Fm humic source rocks; (d) effects that could be related to evaporative migration and biodegradation are not evident in the examined condensates.