New biodegradation degree proxies based on acids and neutral nitrogen- and oxygen-containing compounds characterized by high resolution mass spectrometry

Abstract

Heavy oil is one of the most significant unconventional petroleum resources and one of its formation mechanisms is biodegradation. The characterization of the molecular compositions of petroleum is crucial for determining the various degrees of biodegradation. In this work, detailed investigations of acids and polar NSO compounds in different levels of biodegraded oils from the Bongor Basin (Chad) were carried out by negative ion electrospray ionization (-ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The present findings show that the heteroatomic compounds of biodegraded crude oils are predominantly composed of N1, N1O1, O1, O2, O3 and O4 species. The relative content of N1, N1O1 and O1 species tend to decrease and those of O2, O3 and O4 species appear to increase with the increasing of biodegradation. The ratio of acyclic and cyclic acids (A/C ratio) of the O2 species increase with an increase of biodegradation, indicating the increasing of O2 class during microbial alteration. A new parameter, i.e. the ratio of (O2 + O3 + O4)/(N1 + O1) species, is presented to quantificationally define biodegradation levels. This parameter is inversely proportional to API gravity and has a favorable positive correlation with total acid number (TAN), suggesting acidic compounds are formed by the microbial alteration of N1 and O1 class and are more presented in advanced biodegradation. A modified ternary diagram including N1, O1 and O2 + O3 + O4 species describe the detailed changes of nitrogen- and oxygen- containing compounds. Data points with a high level of biodegradation tend to shift to O2 + O3 + O4 species end-member and retreat from N1 species end-member, showing the relative content of O2 + O3 + O4 species increase and those of N1 species decrease with increasing biodegradation which may due to the formation of organic acids in degradation oil. The ratio of (O2 + O3 + O4)/(N1 + O1) species and the modified ternary diagram provide new parameters to estimate the biodegradation degree and TAN in crude oil.