Petrophysical Properties of Bitumen from the Upper Devonian Grosmont Bitumen Reservoir, Alberta, Canada

Zhao, Yi ¹; Machel, Hans G.^1
1 Earth & Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.

The Grosmont is an Upper Devonian carbonate platform complex in Alberta, Canada. It is about 150 x 600 km in size and recognized as the single largest carbonate bitumen reservoir in the world, with an estimated 318 billion barrels OIP. Present depths range form about 200 m in the NE to about 1000 m in the SW. The bitumen is the result of extensive biodegradation.

A complex diagenetic history, especially dolomitization and karstification, has made the Grosmont a very heterogeneous and unusual reservoir. The reservoir bitumen is characterized by extremely high viscosities of more than 1 million cP at the current reservoir temperature of about 11°C, with API gravities of 5° to 9°. This study investigates various petrophysical parameters in order to develop the most suitable thermal recovery technology.

In order to map out the variations in bitumen viscosity, 49 samples from 15 wells were selected for viscosity measurements, elemental analysis, thermal gravimetric analysis, and simulated distillation tests. The results show that the bitumens from legacy wells (drilled 20-30 years ago) and fresh wells (drilled 1-2 years ago) are quite different. For example, the viscosities of legacy samples are about one order of magnitude higher than those of fresh samples (in the range of 10⁶ cP to 10⁷ cP at 20°C for fresh and legacy samples, respectively). This finding strongly suggests a distinct aging effect in the legacy samples. Viscosity variations with depth are cyclic and related to stratigraphic level. However, no significant variations have been found along the regional strike, suggesting similar original fluid properties and a similar aging history of those samples.

These patterns are strikingly different from those commonly found in the neighboring Cretaceous oil/tar sand deposits, which have relatively low viscosities above the oil-water contacts and marked increases at or below the contacts. The reason(s) for the observed patterns in the Grosmont are under further investigation. Possibilities include (1) oil-water contacts migrating up and down over time; (2) oil migration and/or biodegradation controlled by aquitards that divide the reservoir into stratigraphically separated units; and (3) differences in microbial activity, i.e., aerobic versus anaerobic, possibly controlled by the level of oxygenation over time.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009