25-Norhopanes: Formation during biodegradation of petroleum in the subsurface

Quantitative data from petroleum systems in China (Liaohe basin) and Canada (Athabasca tar sands) support the theory that 25-norhopanes are produced during biodegradation of petroleum in the subsurface. Within a single oil column. both case histories showed increasing severity of degradation, indicated by destruction of hopanes and production of 25-norhopanes downward to the oil-water contact. In the Athabasca samples between the (Peters. K.E., Moldowan. J.M., 1993. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, New Jersey, p. 363] scale of biodegradation levels 5-9. concentrations of C-28 20S triaromatic steroids and other biodegradation-resistant compounds increased by 35%, reflecting a concentration effect as a consequence of removal of more degradable compounds. Over the same interval, the concentrations of C-28 17 alpha 25-norhopane and C-29 17 alpha 25-norhopane increased by an order of magnitude, thus requiring that the balance be met by their net production during degradation. A detailed molecular investigation of the Athabasca bitumen revealed that C-30 17 alpha hopane degrades faster than C-29 17 alpha hopane, whilst the rate of formation of both C-29 17 alpha 25-norhopane and C-28 17 alpha 25-norhopane are similar, complicating a straightforward interpretation of demethylation of hoparies to form 25-norhopanes. Hopane degradation in the Athabasca tar sand may also occur without the production of 25-norhopanes. The results show that even within a single petroleum accumulation, a number of mechanisms control changes in the abundance and composition of hopanes and 25-norhopanes. (c) 2006 Elsevier Ltd. All rights reserved.