Biodegradation and origin of oil sands in the Western Canada Sedimentary Basin

Abstract

The oil sands deposits in the Western Canada Sedimentary Basin (WCSB) comprise of at least 85% of the total immobile bitumen in place in the world and are so concentrated as to be virtually the only such deposits that are economically recoverable for conversion to oil. The major deposits are in three geographic and geologic regions of Alberta: Athabasca, Cold Lake and Peace River. The bitumen reserves have oil gravities ranging from 8 to 12° API, and are hosted in the reservoirs of varying age, ranging from Devonian (Grosmont Formation) to Early Cretaceous (Mannville Group). They were derived from light oils in the southern Alberta and migrated to the north and east for over 100 km during the Laramide Orogeny, which was responsible for the uplift of the Rocky Mountains. Biodegradation is the only process that transforms light oil into bitumen in such a dramatic way that overshadowed other alterations with minor contributions. The levels of biodegradation in the basin increasing from west (non-biodegraded) to east (extremely biodegraded) can be attributed to decreasing reservoir temperature, which played the primary role in controlling the biodegradation regime. Once the reservoir was heated to approximately 80 °C, it was pasteurized and no biodegradation would further occur. However, reservoir temperature could not alone predict the variations of the oil composition and physical properties. Compositional gradients and a wide range of biodegradation degree at single reservoir column indicate that the water-leg size or the volume ratio of oil to water is one of the critical local controls for the vertical variations of biodegradation degree and oil physical properties. Late charging and mixing of the fresh and degraded oils ultimately dictate the ¿ nal distribution of compositions and physical properties found in the heavy oil and oil sand ¿ elds. Oil geochemistry can reveal precisely the processes and levels that control these variations in a given field, which opens the possibility of model-driven prediction of oil properties and sweet spots in reservoirs.

Microbial processes in the Athabasca Oil Sands and their potential applications in microbial enhanced oil recovery

“…Rich oil sands have greater than 10% (by weight) bitumen, while moderate oil sands have 6-10% (by weight), and lean oil sands have less than 6% (by weight). The depth of the oil sands vary from 0 to about 500 m [137].…”

“…The origin of oil sands Oil sands are created through the microbial biodegradation of light oils over millions of years, resulting in a decline of oil quality through increasing viscosity, sulphur, resin, asphaltenes, and metal content [137]. The pathways of biodegradation have only recently become understood, as scientists are able to isolate and identify microbial communities and their biodegradation metabolites.…”

Roles of Thermophiles and Fungi in Bitumen Degradation in Mostly Cold Oil Sands Outcrops

“…Biodegradation of low-molecular-weight components caused the oil to become more viscous, with the viscosity ranging from 1 cP for initially formed nondegraded light oil to 10 6 cP for heavily biodegraded, eastern Athabasca oil sands bitumen, which is currently in place (5-11). Bitumen is therefore depleted of low-molecular-weight aliphatic and aromatic hydrocarbons and enriched in polyaromatic hydrocarbons (PAHs), including alkylnaphthalenes or alkylphenanthrenes, as well as in naphthenic acids (NAs), resins, and asphaltenes (12)(13)(14). The fraction of heteroatom (S, O, and N)-containing organic compounds increases from about 25% (wt/wt) in the Peace River oil sands to nearly 60% (wt/wt) in parts of the Athabasca oil sands.…”

Hyperspectral imaging for the determination of bitumen content in Athabasca oil sands core samples

“…The degree of input from various source units has been intensely debated; however, the most recent geochemical and basin modeling studies agree that the Gordondale Member of the Lower Jurassic Fernie Formation and the Mississippian Exshaw Formation are the primary source beds for the Athabasca deposit (Adams et al, 2010;Berbesi et al, 2012;Finlay et al, 2012). The precursor oil migrated hundreds of kilometers northeast to its present-day location, where it was broken down to heavy oil by biodegradation (Head et al, 2003;Larter et al, 2006;Zhou et al, 2008). The timing of oil migration and emplacement has also been the subject of debate.…”