THE COMPOSITION OF BIODEGRADED OILS IN THE DEEP SUBSURFACE-IMPLICATIONS FOR PREDICTION OF DEGRADATION LEVEL AND THE ACTIVITY OF THE DEEP BIOSPHERE

Four first order factors control molecular, bulk and commercially significant physical properties of deep(>ca 500m) reservoired biodegraded petroleums in any petroleum system:- (1) Current reservoir temperature and charge time; (2) maximum reservoir temperature pre final oil-charge; (3) mixing of early degraded charge and later fresh oil, and (4) water leg size/ nutrient supply. Temperature is the primary control on the rates of subsurface anaerobic biodegradation with degradation rates increasing for decreasing reservoir temperatures below about 80°C and reservoir temperatures above ca 80-90°C effectively pasteurising a reservoir for biodegradation even if it is subsequently uplifted and cooled (Wilhelms et al). Degradation rates in reservoirs around 60-80C, estimated by numerical examination of geochemical gradients in real oilfields, suggests alkane degradation rates of around 10^-6 to 10^-7 Ma^-1. These low degradation/ metabolic rates (a largobiosphere), and implied low cell counts, result from nutrient rather than electron donor (oil) availability and we show oil field scale degradation times are similar to field charging times with n-alkane removal taking from 5-10Ma for a typical N.Sea oil column. Thus mixing of fresh and earlier degraded oil charge will have perhaps the dominant role on oil degradation level observed and oil physical properties. As diffusive hydrocarbon supply is not rate limiting in subsurface biodegradation, diffusive supply of nutrients to the site of degradation is adequate and will be the overall rate controlling factor. Field data and a theoretical mass transport/ kinetic model of the effect of water leg thickness on biological degradation rate suggests a water leg effect on net degradation rate. Using conservative assumptions about the maintenance energy requirements of deep petroleum reservoir biota, and our inferred biodegradation rates, we suggest that near the base of the deep biosphere in petroleum reservoirs (ca75-80°C) cell counts may well be below current microbiological detection limits (ca 10^4cells/cc) and alternative strategies to those suitable for shallower sediments will be needed to study the base of life in the crust. We suggest some possible approaches.