Geochemical characteristics of hydrocarbons in Palaeogene formation water in Xihu Depression

As a direct indication of residual organic components in oil and gas in formation water, it is of great significance to study the geochemical characteristics of hydrocarbons in formation water for the analysis of sedimentary environment, source and maturity of oil and gas.The hydrocarbons in formation water are obtained and analyzed based on the extraction experiments of Palaeogene Pinghu Formation and Huagang Formation formation water in Xihu Depression.The hydrocarbons are derived from terrigenous organic matter input under oxidative environment, and are found to have reached the mature-high mature stage.The hydrocarbons in the Huangyan structure, Tiantai structure and west sub-sag are mainly mudstones with abundant clay minerals, while the source rocks in Pinghu slope lack clay minerals and are more likely to be dominated by coal.The hydrocarbons in the formation water of Tiantai structure have some features of abnormally high Pr/Ph and the double contributions of diterpenoids from ferns and gymnosperms, which is different from other structures whose diterpenoids come from gymnosperms.Further study shows that the Pr/Ph and diterpenoids characteristics of crude oil in Tiantai structure are similar with hydrocarbons in formation water, and have the particularity. Moreover, many geochemical parameters of aromatic hydrocarbons of crude oil in Tiantai structure are different from other structures.Therefore, it is inferred that the crude oil in Tiantai structure is different from other structures.The particularity of hydrocarbons in the formation water of Tiantai structure is a powerful entrance to infer its crude oil as a special type, which provides an indication for the further study of the special crude oil in Tiantai structure.