PALEOPASTEURIZATION AND THE BASE OF THE BIOSPHERE - A PETROLEUM GEOCHEMICAL VIEW

While organisms can maintain life support at temperatures as high as 113°C near the earths surface in nutrient rich environments (Blöchl et al., 1997), the limits of the base of the deep crustal biosphere remain more poorly defined. Petroleum reservoirs provide a unique portal into the base of the biosphere where standard biological techniques may prove inadequate due to low metabolic rates and low predicted cell counts (Larter et al., 2000). While geochemists are able to describe the sequence of subsurface oil degradation, geological controls provide the main constraint on oil degradation (Larter et al-this meeting). Reservoir temperature, late oil charging and water leg size being important controls on overall level of degradation. While recent studies speculate that the limit of the deep biosphere may be at temperatures as high as 150°C (Parkes et al., 1994), petroleum geologists use a rule of thumb limiting effective petroleum biodegradation over geological timescales to reservoir temperatures below 80°C. However non-degraded oils are also found in reservoirs below this temperature. We suggest that these shallow (500-2000m), cool, non-biodegraded oil occurrences in uplifted basins and in many onshore inverted basins (Barent Sea, Wessex basin, USA, North Africa) exist because the petroleum reservoirs were pasteurised by deeper burial to a temperature above approximately 85°C prior to oil charging. Even when such pasteurized reservoirs are subsequently uplifted to much cooler regimes and filled with oil, degradation does not occur, infering that the sediments are not recolonised by bacteria.

We suggest that the base of the biosphere in nutrient depleted oligotrophic deep petroleum basins is not in the realm of hyperthermophilic bacteria but is around 80°C due to the inability of organisms in deep oligotrophic reservoirs to synthesise thermally labile metabolites such at ATP fast enough.

Blöchl, E., et al. Extremophiles 1, 14-21. (1997). Larter, S.R., et al (2000) Proc. GeoCanada 2000-3pp. Parkes, R. J. et al. Nature 371(6496,29 September), 410-413 (1994).