The ISME Journal (2008) 2, 442–452 & 2008 International Society for Microbial Ecology All rights reserved 1751-7362/08 $30.00 www.nature.com/ismej ORIGINAL ARTICLE Thermodynamic constraints on methanogenic crude oil biodegradation Jan Dolfing1, Stephen R Larter2 and Ian M Head1 1School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, UK and 2Petroleum Reservoir Group, Department of Geoscience and Alberta Ingenuity Center for In Situ Energy, University of Calgary, Calgary, Alberta, Canada 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. The ISME Journal (2008) 2, 442–452; doi:10.1038/ismej.2007.111; published online 13 December 2007 Subject Category: geomicrobiology and microbial contributions to geochemical cycles Keywords: anaerobic oil degradation; methanogenesis; syntrophy; window of opportunity; syntrophic acetate oxidation; hydrocarbons Introduction basis of known mechanisms of hydrocarbon degra- dation, conventional wisdom among petroleum The largest deposits of petroleum on Earth are not, geologists has for some time, been that biodegrada- as conventionally assumed, in the Middle East. The tion in oilfields was driven by oxygen delivered to vast Saudi Arabian and Kuwaiti oilfields of Ghawar petroleum reservoirs in meteoric water. This para- (2.6 Â 1011 barrels (bbl) in place) and Burgan 10 digm has been questioned in light of the discovery of (7.0 Â 10 bbl in place) are dwarfed by the trillion a range of bacteria capable of coupling the oxidation bbl deposits of western Canada (Athabasca tar of aliphatic or aromatic hydrocarbons to the reduc- sands; 1.7 Â 1012 bbl) and Venezuela (Orinoco heavy 12 tion of nitrate, iron and sulphate (Widdel and Rabus, oil belt; 1.2 Â 10 bbl). These so-called super-giant 2001) and microbial consortia capable of linking heavy oil fields are the result of biodegradation of aliphatic hydrocarbon oxidation to methane genera- the lighter, more readily produced and valuable oil tion (Zengler et al., 1999; Anderson and Lovley, fractions over geological time. Biodegraded oil fields 2000; Townsend et al., 2003). are more difficult to produce and the oils more Evidence is emerging to support the notion that difficult to refine than oil from conventional fields in-reservoir petroleum biodegradation is caused by and are thus less economically attractive. On the anaerobic hydrocarbon degrading bacteria. Reduced naphthoic acids, metabolites characteristic of anae- robic hydrocarbon degradation have been detected Correspondence: J Dolfing, School of Civil Engineering and in biodegraded petroleum reservoirs, but not in non- Geosciences, Newcastle University, Cassie Building, Newcastle degraded reservoirs (Aitken et al., 2004). Geochem- upon Tyne, NE1 7RU, UK. E-mail: [email protected] ical and isotopic evidence also suggests that in many Received 23 August 2007; revised 8 November 2007; accepted 13 cases the end product of hydrocarbon degradation in November 2007; published online 13 December 2007 petroleum reservoirs is methane (Scott et al., 1994; Thermodynamic constraints J Dolfing et al 443 Larter et al., 1999; Sweeney and Taylor, 1999; reports in the literature provide strong evidence of Pallasser, 2000; Boreham et al., 2001; Masterson methanogenic degradation of aliphatic hydrocar- et al., 2001). Compositional gradients in oil columns bons or crude oil. Zengler et al. (1999) report towards the underlying water leg in biodegraded degradation of pure hexadecane by an enrichment petroleum reservoirs suggests that the oil water culture; Anderson and Lovley (2000) documented transition zone is the primary site of biodegradation rapid mineralization of 14C-labelled hexadecane in in petroleum reservoirs (Head et al., 2003; Larter sediments from a crude oil-contaminated aquifer; et al., 2003). and Townsend et al. (2003) observed methanogenic On the basis of these and other data, a new transformation of crude oil in sediments from a gas conceptual model of in-reservoir petroleum bio- condensate contaminated aquifer. degradation has been developed (Head et al., 2003). In this paper, we evaluate the thermodynamics of In this model, anaerobic degradation of petroleum five possible routes of methanogenic hydrocarbon occurs most actively at the oil water transition zone. degradation, viz (with hexadecane as example): Electron donor, mainly hydrocarbons, is delivered to (i) complete oxidation of alkanes to H2 and CO2, the oil water transition zone by diffusion from the oil linked to methanogenesis from CO2 reduction: column, with inorganic nutrients such as ammonium 4C16H34 þ 128H2O ! 64CO2 þ 196H2 ðreaction 1Þ ions provided from the water leg (Head et al., 2003; Manning and Hutcheon, 2004). This is consistent 196H2 þ 49CO2 ! 49CH4 þ 98H2O ðreaction 2Þ with reports from other deep subsurface environ- sum 4C H þ 30H O ! 15CO þ 49CH ðreaction 3Þ ments that microbial activity is stimulated at geo- 16 34 2 2 4 chemical interfaces (Parkes et al., 2005). When the (ii) oxidation of alkanes to acetate and H2, linked water leg contains low levels of sulphate, hydro- to acetoclastic methanogenesis and CO2 reduction: carbon degradation is driven by methanogenesis; À þ indeed, many biodegraded petroleum reservoirs 4C16H34 þ 64H2O ! 32CH3COO þ 32H þ 68H2 contain isotopically light methane indicative of a ð reaction 4Þ mixed secondary biogenic and thermogenic source 32CH COOÀ þ 32Hþ ! 32CO þ 32CH ðreaction 5Þ (Scott et al., 1994; Larter et al., 1999; Sweeney and 3 2 4 Taylor, 1999; Pallasser, 2000; Boreham et al., 2001; 68H2 þ 17CO2 ! 17CH4 þ 34H2O ðreaction 6Þ Masterson et al., 2001; Head et al., 2003). sum 4C H þ 30H O ! 15CO þ 49CH The significance of methanogenic crude oil de- 16 34 2 2 4 gradation in petroleum reservoirs goes beyond its (iii) oxidation of alkanes to acetate and H2, linked potential role in the biodegradation of petroleum to syntrophic acetate oxidation and methanogenesis reservoirs; it may ultimately be crucial for processes from CO2 reduction: that can enhance the recovery of residual oil. 4C H þ 64H O ! 32CH COOÀ þ 32Hþ þ 68H Typically, over 60% of the oil in place in a 16 34 2 3 2 À þ petroleum reservoir remains unextractable follow- 32CH3COO þ 32H þ 64H2O ! 64CO2 þ 128H2 ðreaction 7Þ ing standard production procedures and the possi- 196H2 þ 49CO2 ! 49CH4 þ 98H2O bility that methanogenic degradation of this residual sum 4C H þ 30H O ! 15CO þ 49CH oil can re-pressurize a petroleum reservoir, has some 16 34 2 2 4 potential for enhancing oil recovery. Furthermore, (iv) oxidation of alkanes to acetate alone, linked to the volumetrics of gas recovery are far better than for acetoclastic methanogenesis: oil (typically 70% of gas in place can be recovered) À þ and methanogenic conversion of non-recoverable 4C16H34 þ 30H2O þ 34CO2 ! 49CH3COO þ 49H residual hydrocarbons to recoverable gas may be an ðreaction 8Þ economically viable way of extending the opera- À þ tional life of petroleum reservoirs (Parkes, 1999; 49CH3COO þ 49H ! 49CO2 þ 49CH4 ðreaction 9Þ Larter et al., 1999; Head et al., 2003). In addition, sum 4C16H34 þ 30H2O ! 15CO2 þ 49CH4 methanogenic hydrocarbon degradation may be a significant process in the attenuation of contami- and (v) oxidation of alkanes to acetate alone, linked nated anoxic sediments and aquifers (Weiner and to syntrophic acetate oxidation and methanogenesis Lovley, 1998; Anderson and Lovley, 2000; Bekins from CO2 reduction: et al., 2005). 4C H þ 30H O þ 34CO ! 49CH COOÀ þ 49Hþ Because of the potential importance of methano- 16 34 2 2 3 genic crude oil biodegradation and our limited ðreaction 10Þ À þ knowledge of the organisms and mechanisms in- 49CH3COO þ 49H þ 98H2O ! 98CO2 þ 196H2 volved, it is important that we learn more about what governs the microbial conversion of oil to methane. ðreaction 11Þ Quantitatively, the most important component of 196H2 þ 49CO2 ! 49CH4 þ 98H2O crude oil is the saturated hydrocarbon fraction and ðreaction 12Þ little is known about the methanogenic degradation of long-chain aliphatic hydrocarbons; only three sum 4C16H34 þ 30H2O ! 15CO2 þ 49CH4 The ISME Journal Thermodynamic constraints J Dolfing et al 444 The effects of temperature, pH, acetate and H2 From these data the threshold concentrations of 0 concentration on each of these processes is deter- products and reactants, which result in DG o0 were mined and conditions under which each process is calculated as follows. The example given is for the likely to be most favourable are identified and threshold H2 concentration for hydrogenotrophic related to conditions typical of petroleum reservoirs. methanogenesis.