--> Abstract: Progress and Problems in Assessing Subsurface Petroleum Biodegradation- a Key Process in the Origin of Deep Water Oil and Gas Accumulations and the Volumetrically Important Tar Sands of Foreland Basins, by Steve Larter, Haiping Huang, Barry Bennett, Martin Jones, Ian Head, Jennifer Adams, Thomas Oldenburg, Cindy Riediger, Martin Fowler, and Dennis Coombe; #90039 (2005)

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Progress and Problems in Assessing Subsurface Petroleum Biodegradation- a Key Process in the Origin of Deep Water Oil and Gas Accumulations and the Volumetrically Important Tar Sands of Foreland Basins

Steve Larter1, Haiping Huang1, Barry Bennett1, Martin Jones2, Ian Head2, Jennifer Adams1, Thomas Oldenburg1, Cindy Riediger1, Martin Fowler3, and Dennis Coombe4
1 University of Calgary, Calgary, AB
2 NRG, CEGS, University of Newcastle, Newcastle, United Kingdom
3 Geological Survey of Canada, Calgary, AB
4 Computer Modelling Group, Calgary, AB

Petroleum biodegradation has affected most of the petroleum 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. We review our recent advances from the Bacchus 1 program in our understanding of subsurface biodegradation and present a grand unified 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 sweet biodegradation are methane and carbon dioxide as well as acidic altered oils and approximately 50% of 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 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 margins, GOM and elsewhere.

The Bacchus consortium is acknowledged for support and encouragement.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005