IMPACT OF BIODEGRADATION ON PETROLEUM EXPLORATION AND PRODUCTION: OBSERVATIONS AND OUTSTANDING PROBLEMS

Biodegradation impacts petroleum exploration and production by removing hydrocarbon compounds that are of most value and impacting the in-situ density and viscosity that determine well flow rate.

Biodegradation is insensitive to formation depth, pressure (atmospheric to > 10,000 psi), provided that the temperature field is cool enough. Large datasets show biodegraded oils can occur at temperatures up to 100 degrees C, but severe biodegradation (Peters and Moldowan 5 or greater) is rare above 75 degrees C. One cool (tertiary basin) shows a remarkably consistent approximately linear trend in the probability of severe degradation when plotted against reservoir temperature. It appears high sulfur carbonate-derived and low sulfur clastic-derived oils are degraded equally fast.

Reservoir filling history is another important factor. In many heavy oils and tars in the studied basins, fresh alkane suites co-exist with large asphaltene contents (in extreme cases reaching 85%) resulting in heavy stock-tank gravities (as low as 5 degrees API) and extremely high viscosities. Our studies suggest that reservoir filling is a long lived process, with continuous replenishment and degradation over millions of years. In extreme cases, where reservoirs become clogged with asphaltenic residue, re-charge of alkane-rich petroleum has little impact on the final petroleum properties.

The link between oil properties / value and biodegradation is often obscured by initial variations in substrate.facies/maturity., i.e. a given degree of biodegradation in a very light gassy oils or gas-condensate system has less impact on than on an already heavy low mature oil.

Many of the world’s large, biodegraded heavy, extra-heavy oils (<10,000 cP), bitumens and tars (>10,000 cP) occur in shallow, hydrostatically pressured reservoirs where influx of meteoric, oxygenated water is possible but in overpressured cases anoxic bacterial degradation must be operative. Here anaerobic biodegradation mechanisms which involve sulfate reduction, fermentation or even other oxidants such as ferric iron need evaluation. Without understanding of these processes, our predictive capability will remain probabilistic, rather than deterministic (prospect / reservoir-specific).