Determination of the chemical compositions of heavy, medium, and light crude oils by using the Distillation, Precipitation, Fractionation Mass Spectrometry (DPF MS) method

Abstract

We have previously demonstrated that the Distillation, Precipitation, Fractionation Mass Spectrometry (DPF MS) method can be used to characterize crude oil at the molecular level by fractionating the crude oil into six different chemical classes and analyzing each class by using an optimized mass spectrometry method. In this study, the chemical compositions of five crude oil samples, including heavy, medium, and light crude oils with different API gravities, were determined using the DPF MS method. The gravimetric weight percentages of the six compound classes within each crude oil sample were also determined. In general, the heavier crude oils contained more asphaltenes and heteroaromatic compounds and less heavy, alkyl aromatic and volatile saturated hydrocarbons than the lighter crude oils. However, the only correlation found for API gravity and the gravimetric weight percentages of the entire compound classes was for the heteroaromatic compound class: crude oils with lower API gravities contained more heteroaromatic compounds. Determination of the total nitrogen content by using a controlled combustion experiment revealed only a poor correlation between the total nitrogen content and API gravity. No obvious correlations were found between API gravity and the amounts of the compounds with different elemental compositions in the whole crude oils. However, consideration of differences in the chemical compositions within the different compound classes revealed trends related to the API gravity. For example, a linear correlation was found for the decrease in the API gravity and the increase in the gravimetric weight percentage of the most abundant nitrogen-containing compounds (with the CcHhN elemental composition) in the heteroaromatic compound class, the class that contains most of these compounds. On the other hand, as the API gravity decreases, the nitrogen content outside the aromatic cores (or in another aromatic core) of the compounds in the heteroaromatic compound class increases. Further, as the API gravity decreases, the extent of unsaturation increases for the compounds in the heteroaromatic compound class. These findings correlate the API gravity of crude oil to the chemical composition of the heteroaromatic compound class. No correlations were found between API gravity and the chemical compositions of the other compound classes or the whole crude oil.