A new ternary diagram to decipher the evolution of maturity and biodegradation of crude oil using ESI FT-ICR MS

Abstract

The nitrogen-, sulfur- and oxygen-containing (NSO) compounds widely occurred in crude oil and source rock extract, which could provide useful information for maturity and the degree of biodegradation. In this paper, NSO heteroatom compounds of crude oils with a series of different levels of maturations and biodegradation were analyzed by the ESI FT-ICR MS under a negative mode, and the variations of the relative abundance of those compounds were investigated. The changes of relative content nitrogen- and oxygen-containing compounds in oils with the increasing of maturity and biodegradation degree in the ternary diagrams of DBE = 9, 12 and 15 of N1 species, DBE = 12, 15 and 18 of N1 species and DBE = 4, 5 and 6 of O1 species show similar tendency, which is restricted to solely characterizing the evolution of either maturity or biodegradation degree of crude oil. A new ternary diagram composed of N1, O1, O2 + O3 + O4 species was presented to simultaneously distinguish the evolution of maturity and biodegradation of crude oil. Data points with high maturity oil tend to shift to N1 species end-member, showing that the relative content of N1 species increases and that of O2 species decreases with increasing maturity as a consequence of decarboxylation and dehydration. Oils with high degrees of biodegradation shift to O2 + O3 + O4 species end-member, indicating that the relative content of N1 species decreases and that of O2 + O3 + O4 species increases with increasing biodegradation level which may be due to the formation of organic acids in degraded oil. Therefore, the ternary diagram with three end-numbers of N1, O1, O2 + O3 + O4 species is a useful tool to delineate the evolution of maturity as well as to estimate biodegradation degree.