Geochemistry of Tri- and Tetracyclic Terpanes in the Palaeozoic Oils from the Tarim Basin, Northwest China

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• 作者：Haiping Huang ; Shuichang Zhang ; Jin Su
• 刊名：Energy & Fuels
• 出版年：2015
• 出版时间：November 19, 2015
• 年：2015
• 卷：29
• 期：11
• 页码：7014-7025
• 全文大小：547K
• ISSN：1520-5029

文摘

Tri- and tetracyclic terpanes in oils and gas condensates from the cratonic region of the Tarim Basin have been characterized by gas chromatography鈥搈ass spectrometry to investigate their geochemical applications in organic source input and maturity assessment. Most oils show typical marine source input characterized by the dominance of C₂₃ tricyclic terpane (TT), low C₁₉TT, and low C₂₄ tetracyclic terpane (TeT), while some oils/condensates show typical terrigenous input signatures as indicated by the dominance of C₁₉TT and C₂₄TeT, which is obviously in conflict with early Palaeozoic geological settings. Commonly used organic source input parameters, such as C₁₉TT/C₂₀TT, C₂₄TeT/C₂₆TT, and C₂₀TT/C₂₃TT, show complicated correlation to maturity parameters. C₁₉TT/C₂₀TT ratios are positively correlated to n-C₁₈/phytane (Ph), while C₂₄TeT/C₂₆TT and C₂₀TT/C₂₃TT ratios increase with n-C₁₈/Ph in a narrow maturity range and then decrease at an extremely high maturity level. Unusually high C₁₉TT/C₂₀TT, C₂₀TT/C₂₃TT, and C₂₄TeT/C₂₆TT ratios occur only in samples with very low summed tricyclic terpane concentrations, suggesting extensive thermal cracking. An abnormal heating influence may cause an elevated C₂₀TT/C₂₃TT ratio when summed tricyclic terpane concentrations remain high. Experimental results from gold tube pyrolysis of two heavily biodegraded oil sands illustrate that ratios of C₁₉TT/C₂₀TT, C₂₄TeT/C₂₆TT, and C₂₀TT/C₂₃TT increase with the heating temperature at a low temperature and then decrease at a high temperature. The longstanding controversial issue regarding the relative thermal stability of tricyclic terpane components might be solved because previous studies have only covered part of thermal evolution history. Nevertheless, the distribution patterns of tri- and tetracyclic terpanes are no longer necessarily valid as an oil鈥搒ource correlation indicator. The so-called Cambrian-sourced end member or unknown source rocks in the Tarim Basin are most likely derived from the thermal alteration of normally distributed tri- and tetracyclic terpanes.