Distribution and significance of C40+ alkanes in the extracts of Cretaceous source rocks from the Songliao Basin

• Published in: Science China. Earth sciences no. 10; pp. 1510 - 1520
• Main Author: FENG ZiHui FANG Wei ZHANG JuHe LI ZhenGuang HUANG ChunYan WANG Xue ZHAO QinLing HUO QiuLi
• Format: Journal Article
• Language: English
• Published: 2007
• Subjects: Basin; chromatography; chromatography-isotope; chromatography-mass; gas; GC-IRMS; high-temperature; HTGC; HTGC-MS; ratio-mass; rock; Songliao; source; spectrometry
• ISSN: 1674-7313; 1869-1897

Source rock extracts and crude oils from the Songliao Basin were analyzed by high-temperature gas chromatography (HTGC), gas chromatography-mass spectrometry (HTGC-MS) and gas chromatography-isotope ratio-mass spectrometry (GC-IRMS), for high molecular-weight alkanes. The distributions of n-alkanes in the Nenjiang Formation extracts are in the C14―C63 range; a bimodal distribution occurs in the and C21―40 regions. The C30―C37 n-alkanes are accompanied by C29―C35 hopanes, whereas the high molecular-weight C45―C47 n-alkanes co-occur with abundant isoalkanes, alkylcyclohexanes and alkylcyclopentanes. The high δ13C values of the n-alkanes and the microscopic maceral compositions indicate a highly diversified organic source input for the Nenjiang Formation source rocks, ranging from aquatic plants, blue alge-bacteria, to land plant material. In contrast, n-alkanes in the rock extracts of the Qingshankou Formation are characterized by a single modal distribution, with relatively low abundances of C29―C35 hopanes, but high molecular-weight isoalkanes, alkylcyclohexanes and alkylcyclopentanes. The relatively low δ13C values of C22―C44 n-alkanes and organic material compositions indicate that the source rocks in the Qingshankou Formation contain dominantly type I algal organic matter. The relative abundance of compounds in source rocks changes little at low maturity stage, but decreases drastically at higher maturity levels, with a concurrent reduction in the odd/even carbon predominance. In crude oils, in contrast, the relative abundance of compounds appears to relate closely with the oil source and oil viscosity.