Even carbon number predominance observed in C50-C110 n-alkanes and monocyclic alkanes in the highly mature source rock

Abstract

Carbon number predominance of n-alkanes and monocyclic alkanes reveals essential geological information such as biological source, sediment maturity, and paleoenvironmental conditions. The carbon number range of the special distribution pattern is reported to lower than C40, prominantly in the range of C24-C34. Generally, this special distribution pattern fades away with the increase of maturity. However, these well-accepted beliefs are challenged by our investigations. In this work, C10-C125 n-alkanes and monocyclic alkanes were identified for the first time in the highly mature source rock of Well Luntan-1 in Tarim Basin (China). Both series exhibited significant even carbon number predominance observed in C50 and C110, but there was no carbon-number predominance observed before C50. The similar distribution patterns of n-alkanes and monocyclic alkanes indicated that they might have the same biological precursors. The discovery of higher alkanes with ultra-long carbon chains and their extraordinary display of carbon number predominance provided us with a new perspective, which may open up the era of macromolecular biomarkers exploration.