西湖凹陷古近系地层水中烃类的地球化学特征

Geochemical characteristics of hydrocarbons in Palaeogene formation water in Xihu Depression

作为油气中有机组分残留在地层水中的直接显示,研究地层水中烃类的地球化学特征对分析油气的沉积环境、生源母质、成熟度等具有重要参考意义。通过对西湖凹陷古近系平湖组和花港组地层水萃取烃类分析后发现:烃类来源于氧化环境下的陆源有机质输入,现均已达到成熟-高成熟阶段。其中黄岩构造带、天台构造带和西次凹的烃类以来自黏土矿物含量颇为丰富的泥岩为主,而平湖斜坡带的源岩缺乏黏土矿物,更可能以煤为主。天台构造带的地层水中烃类很特殊,其姥植比异常高,二萜类具有蕨类植物和裸子植物双重贡献的特征,不同于其他构造带的二萜类以裸子植物来源的特点。进一步研究发现天台构造带原油的姥植比和二萜类特征与其地层水一致,同样具有特殊性,且多种芳烃地球化学参数也均不同于其他构造带,因此推断天台构造带的原油是不同于其他构造带的特殊类型。天台构造带地层水中烃类的特殊性是推断其原油为特殊类型的有力入口,对进一步研究天台构造带的特殊原油提供了指示意义。

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.