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

Abstract

Tri- and tetracyclic terpanes in oils and gas condensates from the cratonic region of the Tarim Basin have been characterized by gas chromatography–mass 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, a...