Levels of Biodegradation in Oils—Causal Relation to Meteoric Oxygenated Waters
Biodegradation of commercial quantities of oils was traditionally considered to be constrained to low temperatures (<70-80oC) and oxic environments. Still, during the latter years mounting evidences seem to suggest biodegradation of reservoired petroleum to be mainly an anoxic process where electron acceptors other than oxygen are the main agents and act as both promoters and limiting factors. Bacterial reproduction seem also to forwarded as a limiting factor whilst the traditional view (with parallels to medicine) was that bacterial growth is always an exponential function if the bacteria at all are able to survive and reproduce. Finally, it has been suggested that reservoirs could become “pasteurized” and protected against further biodegradation by burial and later uplift.
A case from the Mid-Norwegian shelf, representing four named different traps in regional proximity and which seem to have been part of the same petroleum system and migration regime, illustrates “text-book” examples of progressive biodegradation including removal of specific aromatic isomers and nor-hopanes. The levels of degradation seem to be directly linked to and controlled by a) low temperatures, b) proximity to land and hence access to oxic land-derived waters and c) intra-reservoir proximity to oil-water contacts. The overall degradation is suggested linked to glacial sea-level fluctuations and “head” of meteoric water influx and evidences suggest that biodegradation is “turned on” or activate as semi-open reservoirs of the Sales type-3 category reservoirs become influenced by oxygenated waters influx during sea-level lowering.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005