Relationship between the Content of Asphaltenes and Bases in Some Crude Oils

A strong correlation between the asphaltene content and the total base number (TBN) has been found for a set of 20 crude oils. The correlation coefficient, R 2, for a polynomial fit is 0.990, over a range of TBN from 0.15 to 4.14 mg KOH/g oil and asphaltene contents of 0.4−25 mg/g oil. The higher values are for biodegraded oils. Titration of asphaltene fractions and the deasphalted oil shows that the asphaltene fractions are not in themselves bases. Extraction of bases with a formic acid/water constant boiling mixture gives recoveries of 25−37% of the original TBN, and the extracted oils still contain the remaining bases. This makes it difficult to evaluate the relevance of further characterization of the extract, which consists of molecules with molecular weights of 200−600 g/mol, with UV absorbance maxima corresponding to nitrogen heterocyclic compounds. The results suggest that the petroleum bases stabilize asphaltenes in the bulk crude oil and that acid−base interaction could be an important mechanism for asphaltene stability. In-reservoir biodegradation of the oil increases the asphaltene content, TBN, and TAN, but the specific compounds may be different from the original, geologically stable crude oil components.