A PETROLEUM GEOLOGICAL PERSPECTIVE ON THE BIODEGRADATION OF OIL IN THE DEEP SUBSURFACE AND ITS IMPLICATIONS FOR THE DEEP BIOSPHERES OF EARTH AND MARS

Biodegraded oil occurs in petroleum reservoirs at depths from surface to 5km where it has been produced by biodegradation under anaerobic and oligotrophic conditions over time periods of millions to tens of millions of years with large fractions of the oil being mineralized. Methane is the common terminal product. We have combined petroleum reservoir geochemical characterization techniques and molecular biology of petroleum reservoir cores samples to examine the nature and activity of organisms in the deep subsurface. Examination of geochemical compositional gradients in biodegrading oilfields allowed us to tentatively establish rates of biodegradation active at the base of oil columns. Comparisons of observed degradation rates with the maintenance energy requirements of bacteria allow us to make a broad assessment of the maximum biomass size. Recent advances in biodiversity theory are used to estimate the diversity of the subsurface biosphere.

From the distribution of biodegraded oils worldwide it seems that reservoirs buried to 80oC or higher are effectively sterilized with regard to hydrocarbon degraders and are not reinoculated with near surface organisms during uplift, indicating the deep biosphere is a stable isolated biosphere derived from long period evolution of surface derived organisms during burial. Biodegradation of hydrocarbons occurs under anaerobic conditions and reservoir studies suggest that the actual fluxes of hydrocarbons being destroyed in oilfields around 40-70 C are around 10^-4 kg/m^2/year of oil water contact area. The biota appear to live predominantly at the interface between the oil and the water and isolation of genetic material from ANME organisms in petroleum reservoir cores suggests that methane cycling may be important. We discuss how the low biomass, deep biosphere of oil reservoirs can adapt to such long timescales and slow process rates and examine conditions under which modes of living needed to survive such an environment would be established. We suggest it is nutrients, not electron acceptors or donors that are limiting in Earth but in Mars (if life exists subsurface), we suggest electron donor supply must often be limiting and be related not to burial of surface derived organic matter but to supply of either hydrogen or methane from below !