Molecular characterization of petroleum acids from diverse geological origins via n-Butylsilane reduction

Abstract

Petroleum acids are widely present in crude oil and are important indicators of generation and preservation of the crude oil. However, it is difficult to analyze these compounds using gas chromatography due to their strong polarity and low volatility. In this work, we established a method to reduce petroleum acids into corresponding hydrocarbons using n-butylsilane as the reducing agent. The reduced products of the acids from three crude oils with different geological origin were analyzed by gas chromatography-mass spectrometry and high resolution Orbitrap mass spectrometry coupled with electrospray ionization. Various hydrocarbons with specific biomarker carbon skeletons were identified in the reduced products and compared to the corresponding compounds in the saturates of the crude oils, which provides molecular clues for the understanding of the geochemical formation mechanism of petroleum acids. The results show large difference in molecular composition of petroleum acids in different crude oils. The Liaohe acids were composed of fatty acids which should be primary acids in source rock or derived from low-maturity kerogen. In contrast, the Xinjiang and Shengli acids were dominated with naphthenic acids. Xinjiang naphthenic acids showed good consistency in carbon skeleton and carbon number with the saturates. This indicates that these naphthenic acids originate from the bio-oxidation and degradation of cycloalkanes, which can also be further decarboxylated and degraded to generate cycloalkanes. Abundant 8,14-secohopanes and 25-norhopanes and corresponding acids were detected in Shengli crude oil. The compositional differences between the acids and corresponding hydrocarbons suggests that biodegradation occurs preferentially on large molecules with long side chains and has structural selectivity on the isomers. The results implies that 25-norhopanic acids seems to be the intermediates for the biodegradation of hopanes to 25-norhopanes.