PREDICTING PETROLEUM BIODEGRADATION IN UNDRILLED PROSPECTS: HOW FAR CAN YOU GO?

The topic of petroleum biodegradation lay largely dormant for around a decade from the early 1980's, not least perhaps because it was felt that the process was well understood. The historical view of biodegradation in the reservoir was that it takes place under oxic conditions, which requires a flow of meteoric water to replenish the molecular oxygen needed by the aerobic bacteria involved. Aerobic oxidation was believed to be dominant since it had not been demonstrated that anaerobic bacteria could degrade saturated hydrocarbons in the laboratory. Low reservoir temperature has always been regarded as critical, although it has never been clear whether this was a cause or simply a correlation.

Work on petroleum biodegradation has, however, undergone a gradual resurgence in the 1990's, not least due to the publication of several papers demonstrating that anaerobic bacteria can indeed oxidise saturated hydrocarbons, albeit more slowly than aerobes. But will an improved basic understanding of this and other mechanisms of biodegradation allow prospect-level prediction of petroleum biodegradation, since this is what explorationists are interested in when evaluating the economics of undrilled structures?

We have adopted several approaches in order to see how far we can push prospect-level prediction at the present time, and these will be the subject of this presentation:

A statistical approach on a regional scale: this can take you far, but it can lead to uncertainties over a certain temperature range, as exemplified by one field on NOCS.

Local case studies: i) a restricted geographical area with reservoirs of the same age at similar shallow depths, but containing petroleum which is variably biodegraded; ii) an area with uplift, where shallow, cool (<60°C) reservoirs contain everything from heavily biodegraded to non-biodegraded petroleum.

The outcome of these approaches will be used to discuss whether or not a greater understanding of fundamental mechanisms could actually lead to improved prediction of presence of biodegraded petroleum at the prospect level.