Are vanadyl porphyrins products of asphaltene/kerogen thermal breakdown?

Abstract

The idea was examined that during early stages of thermal maturation of source rocks or petroleums, the release of bound vanadyl (VO 2+) porphyrins (P) from the immature asphaltenes or kerogen could contribute to an increase in extractable VO 2+-P. To simulate such a change in the laboratory, the Dead Sea ‘floating’ asphalt and total organic (bitumen + kerogen) fractions of the La Luna and Serpiano carbonaceous sedimentary rocks were heated at controlled temperatures (100, 200, 300 and 400°C) for different periods of time. Residual (and altered) asphaltenes or kerogens were then separated from the heated products and analysed for VO 2+-P content by electron spin resonance. Three conclusions were reached: (1) thermal degradation of immature asphaltenes or kerogen generated no or very little (<1 wt%) extractable VO 2+-P during simulated maturation of petroleums or source rocks and thus by inference during natural maturation; (2) petroleum VO 2+-P by far are those that were saved from incorporation into asphaltenes or kerogen precursors (or young asphaltenes or kerogen) during diagenesis; and (3) a high VO 2+-P content of a petroleum is not an indication of its immaturity.