The Upper Devonian Nisku Formation, Upper Ireton, and Grosmont Formation (collectively referred as the “Grosmont reservoir”) form a giant bitumen/heavy oil reservoir in Alberta, Canada with about 406 billion barrels of original oil in place. Because of the low quality of the hydrocarbons and the complex geology, the Grosmont reservoir was not deemed commercially viable until relatively recently. The Grosmont is now under consideration for commercial development by several companies and consortia due to technological advances and higher oil prices.
One part of the work leading up to new pilot sites is the characterization of the degree of biodegradation in the Grosmont reservoir, which could be used as a proxy of the petrophysical properties of bitumen, specifically viscosity. In order to characterize the biodegradation in the Grosmont reservoir, a total of 18 samples selected from the Nisku, Upper Ireton, and Grosmont along a strike cross-section were examined by gas chromatography-mass spectrometry. Biomarker analysis indicates the Grosmont reservoir was subjected to extreme stages of biodegradation, demonstrated by the loss of regular steranes, strongly biodegraded hopanes, and reduced abundances of tricyclics and diasteranes. The rare geological marker compound 28, 30-bisnorhopane is also highly biodegraded but does exist in some Grosmont bitumen samples examined, supporting the notion that bitumens in the Devonian carbonates and the Cretaceous oil sands have a common origin.
Based on the distribution characteristics of hopanes and tricyclics, several biodegradation parameters are identified and the samples are categorized into three distinct biodegradation stages. The data shows that the stage of biodegradation is likely most severe around stratigraphic boundaries and generally increases from southeast to northwest.
Several recognized biodegradation parameters, such as 17α(H)-hopane/17α(H)-norhopane, are correlatable with the bitumen viscosity. Such parameters can be used as proxies for in situ viscosity, which is highly useful in aged samples (older than a few years) that have lost volatiles while in storage and thus no longer have the same viscosity as when retrieved from the reservoir.