--> Abstract: Petroleum Biodegradation: What is the Problem with 25-Norhopanes? by Barry Bennett, Paul Farrimond, Carolyn M. Aitken, Martin D. Jones, and Steve R. Larter; #90039 (2005)

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Petroleum Biodegradation: What is the Problem with 25-Norhopanes?

Barry Bennett1, Paul Farrimond2, Carolyn M. Aitken2, Martin D. Jones2, and Steve R. Larter1
1 University of Calgary, Calgary, AB
2 The University of Newcastle, Newcastle upon Tyne, England

The origin of 25-norhopanes is controversial. They have been detected in source rocks but are typically found in high abundance in biodegraded oils. A quantitative balance between the removal of hopanes and production of the corresponding 25-norhopane suggests they may arise from processes occurring in petroleum reservoirs.

In this presentation we examine quantitative data for a series of hopanes, hopanoic acids, 25-norhopanes and 25-norhopanoic acids isolated from a suite of variously biodegraded oils. We discuss the inter-relationship of these compounds by considering the following chemical reactions including oxidation (e.g. hopane to hopanoic acid), demethylation (e.g. hopane to 25-norhopane) and decarboxylation (e.g. hopanoic acid to a hopane). Evidence for parallel demethylation proceeding in both the hopane and hopanoic acids to their respective 25-norhopane and 25-norhopanoic acid counterparts is presented. It was noted that the relative abundance of 25-norhopanes display an inverse relationship with the hopanes but rarely with a 1:1 molar relationship. Quantitative data suggests that decarboxylation and oxidation reactions may be ruled out as significant reactions and in addition, the identification for the first time, of a novel series of 17b(H), 21b(H) 25-norhopanoic acid and the presence of 17b(H), 21b(H) hopanoic acids supports the likelihood of demethylation in reservoir as the dominant process and the possibility even of hopanoid bearing organisms as active participants in degradation. We suggest that 25-norhopanes do indeed represent products linked to the petroleum biodegradation process and their presence or absence may be indicative of specific microbial processes active in heavy oil reservoirs.

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