Holba, Albert G.¹, Lisa Wright², Leon I. Dzou³, B. J. Huizinga¹, Jim J. Hickey⁴, Stephen G. Franks⁵, Mark H. Scheihing²
(1) ConocoPhillips IGA, Houston, TX (2) ConocoPhillips Alaska, Anchorage, AK (3) BP America, GoM Deepwater Exploration, Houston, (4) Applied Reservoir Petrology, LLC, Dallas, TX (5) RockFluid Systems, Inc, Dahran, Saudi Arabia

ABSTRACT: Effects and Impact of Early and Late Stage Anaerobic Biodegradation

Anaerobic processes have only recently been widely recognized as an important mechanism in the biodegradation of crude oils. Two case histories illustrate the differing impact of anaerobic degradation on oil and reservoir quality as a function of the contrast in organic and inorganic geochemistry in these two reservoirs. 
Portions of South Pass 61 field in the Gulf of Mexico have experienced extensive biodegradation by sulfate-reducing bacteria with associated precipitation of late-forming pyrite and biogenic gas. The supply of sulfate from salt dome anhydrite has facilitated the biodegradation. Also, reservoir quality has been degraded by the associated formation of pore-filling pyrite. 
In contrast, Kuparuk River Field ( North Slope, Alaska) has experienced more areally and stratigraphically restricted occurrences of asphaltics and viscous oil (< 20 degrees API). Depletion of n-alkanes and other paraffins, indicative of biodegradation, is not obvious in Kuparuk viscous oil samples, depletions in select aromatic hydrocarbon families (LCAA’s – long chain alkyl aromatics) indicate first stage anaerobic biodegradation. 
The contrast in inorganic geochemistry between these two fields is responsible for the contrast in degree of oil degradation. In the case of South Pass 61, oil degradation is severe because of the abundance of sulfate available to sulfate-reducing bacteria. In contrast, oil degradation at Kuparuk Field has been less severe because of limitations in required compounds necessary to sustain anaerobic microbial activity. However, subtle alteration has economic impact as well. It controls oil stability with respect to precipitation of tar and in the distribution of oil quality across the

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.