Effects of Reservoir Compartmentalization on Hydrocarbon Composition and Bitumen Quality in the Athabasca Oil Sands Deposits, Canada

Milovan Fustic¹, Barry Bennett², Ron Spencer², Steve Hubbard³, and Steve Larter^2
1University Of Calgary (presently with NEXEN Inc.), Calgary, AB
²University of Calgary, Calgary, AB
³University Of Calgary, Calgary, AB

Recent studies have demonstrated that the bitumen residing in the Athabasca Oil Sands Deposit (AOSD) shows, on both lateral and vertical scales, large variation in hydrocarbon composition and bitumen quality (e.g., API gravity ranges 6-11). In general, the bitumen located towards the base of an oil column is usually of a higher viscosity compared to the bitumen from higher up in the reservoir. Detailed compositional analysis of a number of petroleum columns have shown that physical and chemical property variation in various areas may show anomalous inverse gradients, and even off-sets in compositional trends coinciding with geological features.
A suite of methods have been employed to characterise the bitumen composition of samples obtained from AOSD in terms of viscosity, weight percentage bitumen, and level of biodegradation (employing molecular markers determined by GC-MS). The integration of geological data and bitumen properties shows that anomalous compositional gradients may be attributed to compartmentalization of the reservoir. Although biodegradation appears to be recognised as the dominant alteration process in the AOSD, we suggest that the following geological factors should be considered: 1) Petroleum charging and mixing histories; 2) Reservoir rock properties; 3) Local variations in conditions (e.g. water composition, lithology) that may lead to different biodegradation pathways.
This information is useful for indicating the range of chemical and physical property variations that may be encountered in oil sands reservoirs. The information may be used to assist in prospect evaluation, and can indicate where potential recovery problems may occur when a resource is evaluated for InSitu operations.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece