Petroleum Biodegradation in West. Africa and Western Canada-Contrasts and Similarities. A Review of Biodegradation, a Key Process in the Origin of Deep Water Oil and Gas Accumulations

Steve Larter¹, Femi Jokanola¹, Jennifer Adams¹, Haiping Huang¹, Barry Bennett¹, Cindy Riediger¹, and Martin Jones^2
1 University of Calgary, Calgary, AS
² NRG, CEGS, University of Newcastle, Newcastle, United Kingdom

Petroleum biodegradation has affected most of the oil and much of the gas on the planet. The vast heavy oil deposits of the foreland basins of the Americas dominate world oil reserves while biodegradation is a key process reducing oil quality in reservoirs that have always been cooler than 80C in most current deepwater provinces including those of West Africa. We review recent published advances in our understanding of subsurface biodegradation and present a theory of subsurface petroleum alteration suitable for prediction of oil properties in deep water settings and elsewhere. Petroleum biodegradation proceeds under anaerobic conditions with oil destruction rates comparable with field charging rates. The dominant end products of biodegradation are methane and carbon dioxide as well as acidic altered oils and approximately 50% of an oil has been destroyed when API gravities of around 20 are reached in marine oil systems. The rates of destruction of oil and gas are reservoir temperature and topology related. Self consistent models of biodegradation have been developed that allow pre drill prediction of fluid properties but are highly dependant on the modelled oil charge histories. We present a review of published techniques for reservoir scale simulation of biodegradation in deepwater and other reservoirs comparing simple biodegradation index approaches (BDI, OPI) and more complex 2 and 3d simulations. We contrast the quite different situations of early charge and long residence time found in the tar sand and heavy oil deposits of the foreland basin of Western Canada and active recent charging and degradation found in the major deepwater exploration prospects of the Atlantic margin of Africa. We conclude that oil charge histories are the crucial element in pre drill prediction of oil quality and reflect on the inadequacy of current geochemical and modelling techniques for assessing these at appropriate levels of accuracy.