Estimation of vanadyl porphyrin concentration in sedimentary kerogens and asphaltenes

Abstract

We describe a new, rapid method for determining the concentration of vanadyl porphyrins (VO 2+–P) associated with the kerogen of bituminous sedimentary rocks using electron spin resonance (ESR). The method is simple, straightforward and inexpensive. Several concentrations of a vanadyl (VO 2+) standard dissolved in glycerol–lignite mixture were prepared. The VO 2+ concentrations ranged from 100 to 1000 ppm. The anisotropic ESR spectra of both the standards and kerogen samples were recorded at room temperature and the integrated areas of the pre-selected ESR line (attributed to nuclear spin m I =−5/2) were computed. The concentrations of VO 2+ found in the kerogen samples were calculated using the relative ratio of the integrated areas for the standards and the kerogen samples. The VO 2+–P concentrations of the kerogen materials were then calculated using 450 as the mean molecular weight of these species. Quantitative determination of VO 2+–P in the kerogen fractions in the range of 800–8000 ppm and higher is feasible by the method reported. The method of analysis was also extended to the asphaltene samples (enriched with VO 2+–P) and a coal sample containing non-porphyrin VO 2+ associated with its organic fraction.