Phenolic compounds in oil-bearing fluid inclusions: Implications for water-washing and oil migration

Abstract

In comparison to genetically related crude oils, oil-bearing fluid inclusions are often relatively enriched in polar compounds. Phenolic, carbazolic and benzocarbazolic compounds, which are part of the polar fraction, are characterised by elevated solubilities in water, rendering them potentially useful indicators for oil migration and entrapment, as well as water-washing of oil-bearing fluid inclusions. These compounds were investigated in a set of crude oils and fluid inclusions, and methods were modified to allow for the direct and simultaneous analysis of phenolic and carbazolic compounds. By avoiding chromatographic column fractionation, this new analytical approach allows for faster analyses and higher throughput. Phenolic and carbazolic compounds were found to be present in all investigated crude oil samples, yet they represent only minor constituents of oil-bearing fluid inclusions. A newly described phenol–cresol index (PCI) is systematically elevated (> 0.7) in oil-bearing fluid inclusions, while crude oils are characterised by significantly lower values (< 0.5). Supported by observations of increasing PCI during progressive water-washing experiments, our data suggest oil-water interaction as a possible reason for the elevated PCI in oil-bearing fluid inclusions. Moreover, some of the phenolic compounds detected in oil-bearing fluid inclusion samples may derive from co-occurring aqueous fluid inclusions, whose simultaneous co-extraction cannot be avoided on this spatial scale. Our study highlights the importance of oil-water interaction in the subsurface and offers new techniques to aid understanding of this commercially relevant phenomenon.