1. ¹School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, UK
2. ²Petroleum Reservoir Group, Department of Geoscience and Alberta Ingenuity Center for In Situ Energy, University of Calgary, Calgary, Alberta, Canada

Correspondence: J Dolfing, School of Civil Engineering and Geosciences, Newcastle University, Cassie Building, Newcastle upon Tyne, NE1 7RU, UK. E-mail: jan.dolfing@ncl.ac.uk

Received 23 August 2007; Revised 8 November 2007; Accepted 13 November 2007; Published online 13 December 2007.

Abstract

Methanogenic degradation of crude oil hydrocarbons is an important process in subsurface petroleum reservoirs and anoxic environments contaminated with petroleum. There are several possible routes whereby hydrocarbons may be converted to methane: (i) complete oxidation of alkanes to H₂ and CO₂, linked to methanogenesis from CO₂ reduction; (ii) oxidation of alkanes to acetate and H₂, linked to acetoclastic methanogenesis and CO₂ reduction; (iii) oxidation of alkanes to acetate and H₂, linked to syntrophic acetate oxidation and methanogenesis from CO₂ reduction; (iv) oxidation of alkanes to acetate alone, linked to acetoclastic methanogenesis and (v) oxidation of alkanes to acetate alone, linked to syntrophic acetate oxidation and methanogenesis from CO₂ reduction. We have developed the concept of a 'window of opportunity' to evaluate the range of conditions under which each route is thermodynamically feasible. On this basis the largest window of opportunity is presented by the oxidation of alkanes to acetate alone, linked to acetoclastic methanogenesis. This contradicts field-based evidence that indicates that in petroleum rich environments acetoclastic methanogenesis is inhibited and that methanogenic CO₂ reduction is the predominant methanogenic process. Our analysis demonstrates that under those biological constraints oxidation of alkanes to acetate and H₂, linked to syntrophic acetate oxidation and methanogenesis from CO₂ reduction offers a greater window of opportunity than complete oxidation of alkanes to H₂ and CO₂ linked to methanogenic CO₂ reduction, and hence is the process most likely to occur.