四川汉源五峰期地层中烷烃的某些分布特征

SOME DISTRIBUTIVE CHARACTERISTICS OF SATURATED HYDROCARBONS FROM WUFENGIAN STRATA IN HANYUAN AREA OF SICHUAN PROVINCE

对四川汉源五峰期地层中不同层位７个样品的烷烃化合物研究显示，正构烷烃只有窄碳数范围（ｎＣ１０～ｎＣ３１）、低主峰碳数（ｎＣ１４、ｎＣ１６），高轻重组分比值１．３９～６．４１）、轻组分中偶碳占优势（ＯＥＰ＝０２９～０．７７）、以及类异戊二烯烃含量低（ｐｒ／ｎＣ１７＝０．５２～０．８０；Ｐｈ／ｎＣ１８＝０．３５～０．８２）、植烷占优势（Ｐｒ／Ｐｈ＝０．６５～０．９２）的特征．对比研究进一步表明，黑色岩系相对非黑色岩系只有较大的正烷烃轻重组分比值，且轻重组分比值与古生物类型存在明显的依赖关系，高盐度及还原环境有利于形成植烷优势，但在总的还原背景下Ｐｒ／Ｐｈ值并未显示随还原程度加强而变小的趋势。

The Ordovician wufenglan strata in Hanyuan Area of Sichuan Province are the products deposited on the western border of Yangzi Sea and are marked by alternating of black shales with non-black shales. Detailed discussions about palaeontology and stratigraphy at this age in this area have been given (Feng Hongzhen,Yu Jianhua etc.,1991). In this paper, we further study the geochemical characteristics of the strata by using molecular organic geochemical methods. Seven samples studied here were collected from different localities in the Jiaodingshan stratigraphic section,which is only 3.24 metres in thickness.The dirtribution of saturated.hydrocarbons from these spsaples shows that normal alkanes are characteristic of narrow distributive ranges of carbonnumbers (nC10-nC31),main peaks with low carbonoumbers (nC14.nC16), high ratios of light to heavy constituents =1.39-6.41) aned even to odd predominance in the light constituent (OEP=0.29-0.77). The distribution of saturated hydrocarbons also indicates that isoprenoid alkanes are characterized by low contents (Pr/ nC17=0.52-0.80,Ph / nC18 = 0.35-0.82) and predominance of phytane (Pr / Ph = 0.65 = 0.92). The comparahve study perhaps enables the authors to draw the following conclusions. The black shales have bigger ratios of light to heavy constituents in normal alkanes and pristane to phytane in isprenoid alkanes than the non-black shales. The changes in the ratios of light to heavy constituents are mainly decided by properties of biotic types.Sedimentary environments.with reduction conditions and higter salinity probably are suitable for forming of phytane predominance (Pr/Ph<1). but uhder the background of reduction Pr/Ph values don't show the trend of edcreated with the strengthening of reduction intensity.