羌塘盆地二叠系展金组烃源岩生物标记化合物特征及意义

Characteristics and significance of the biomarkers in the source rocks from the Permian Zhanjin Formation in the Qiangtang Basin,northern Xizang

关键词:  羌塘盆地  二叠系  展金组  生物标记化合物特征

Key words:  Qiangtang Basin  Permian  Zhanjin Formation  biomarkers

摘要:

本文对羌塘盆地二叠系展金组烃源岩的岩心样品进行了烃源岩基本特征及生物标志化合物特征研究,重点探讨了烃源岩生烃母质来源、沉积环境特征及成熟度特征。研究显示,展金组烃源岩富含正构烷烃、类异戊二烯烷烃、萜类化合物和甾类化合物。其中,正构烷烃多为前高后低的单峰型,具有低碳数优势,nC₁₅、nC₁₆或nC₁₇为主峰碳;规则甾烷中C₂₇-C₂₈-C₂₉呈"V"字型分布,C₂₇占优势;萜烷具有三环萜烷>五环三萜烷>四环萜烷的相对丰度,三环萜烷/五环三萜烷值在1.1~1.96之间;生烃母质主要来源于海相低等水生生物。Pr/Ph分布范围在0.53~1.16,平均值为0.85,植烷优势较明显;五环三萜烷的Ts/(Tm+Ts)值表明烃源岩形成于还原环境。而伽马蜡烷的含量以及伽马蜡烷/αβ-C30藿烷比值,证明烃源岩沉积时水体盐度为正常盐度。甾烷的C₂₉ ααα20S/(20S+20R)和C₂₉ αββ/(αββ+ααα)、萜烷的C₃₁αβ22S/(22S+22R)、Ts/(Tm+Ts)值等反映烃源岩成熟度的参数显示,烃源岩处于成熟阶段。

Abstract:

The present paper deals with the characteristics of source rocks and biomarkers in the core samples of the source rocks collected from the Permian Zhanjin Formation in the Qiangtang Basin, northern Xizang, with the emphasis on the origin, sedimentary environments and maturity of the source rocks. The findings have disclosed that the source rock samples collected from the Zhanjin Formation are rich in n-alkanes, isoprenes, terpanes and steranes. The n-alkanes are characterized by a high and low single peak, respectively, and a dominant peak of low carbon numbers including nC₁₅, nC₁₆ and nC₁₇. The C₂₇-C₂₈-C₂₉ series display the "V-shaped" distribution in the regular steranes, with C₂₇ as a dominant peak. The terpanes have the relative abundance of tricyclic terpane > pentacyclic triterpane > tetracyclic terpane, and the ratios of tricyclic terpenes/pentacyclic triterpenes range between 1.1 and 1.96. The primary sources of organic matter are interpreted to come from the lower marine aquatic organisms. The Pr/Ph ratios range from 0.53 to 1.16, with an average of 0.85, suggesting aphytane dominance. The distribution patterns of pentacyclic triterpanes and the ratios of Ts/(Tm + Ts) indicate that the source rocks were formed in the reduction environments. The contents of gammaceranes and the ratios of gammaceranes/αβ-C 30 hopanes have proved that the source rocks were formed in the seawater with a normal salinity. The maturity parameters such as C₂₉ααα20S/(20S + 20R) and C₂₉αββ/(αββ + ααα) ratios for the steranes, C₃₁αβ22S/(22S + 22R) ratios for the terpanes, and Ts/(Tm + Ts) ratios all show that the source rocks are still in the mature stage, which is consistent with the Ro values indicated by the selected samples.