Abstract

Three geological models characterize the bulk of McMurray Formation reservoirs in the Athabasca Oil Sands Area in northeastern Alberta. These models are open estuarine complexes, large scale estuarine point bars and fluvial/estuarine stacked channel successions. In addition, two styles of sedimentation, vertical and lateral accretion, are addressed, both of which control reservoir geometries.

Open estuarine complexes can be characterized by amalgamated, high-angle cross-stratified sands, commonly with 0.3 to 1.5-metre-thick bed sets. Associated with cross-bed sets at the MacKay and High Hill outcrop analogues are draping mudstones up to half a metre thick. These mudstones have limited lateral extent and do not appear to be major barriers to horizontal or vertical flow. Along the Ells River, amalgamated open estuarine sand successions attain thicknesses of 40 metres, separated by 1 to 5 metre-thick laterally continuous floodplain mudstones. Drill hole and outcrop data indicate that these amalgamated sand successions extend up to 10 km² in area.

Large scale estuarine point bar reservoirs are recognised in the Syncrude mine site, the UTF Phase B in-situ pilot, and in outcrop analogues along the Steepbank, Athabasca and Ells Rivers. These reservoirs are commonly between 20 and 40 metres thick.

The three-component model is characterized by a lower cross-stratified sand succession and transition zone, overlain by inclined heterolithic stratification (IHS). In the simpler one-component model, the channel deposit is dominated by the IHS unit which extends from the top to the channel base.

Cross-stratified sands of the three-component model consist of bed sets half a metre to two metres thick, with a cumulative thickness of up to 25 metres. The transition zone consists of small scale trough cross-beds and ripple laminated sands.

The IHS unit is interpreted as a lateral accretion deposit consisting of rhythmically interbedded mudstones and current rippled sand beds dipping 5° to 12° perpendicular to paleoflow and essentially horizontal, parallel to the paleoflow direction. Mudstone-dominated intervals in the IHS unit may be more than a metre thick with a lateral extent of at least 100 m × 150 m, representing significant vertical permeability barriers.

Stacked channel reservoirs contain similar lithofacies to those observed for the large scale point bar reservoir, with appropriate reductions in scale and lateral continuity of the common components. Amalgamated thickness of a stacked channel succession is commonly 40 metres, with individual partially preserved channel deposits typically 10 to 20 metres thick. Stacked channel reservoirs may consist of the three component model characterized by sandy, fining-upward successions or a one-component model characterized by interbedded sands and mudstones of the IHS unit. As with the large scale channel successions, muddy IHS units can form relatively continuous permeability barriers, likely to influence reservoir behaviour.

Closely associated with all of the McMurray reservoir types outlined are vertically accreting channel successions. Vertically accreting channels locally incise older successions and are commonly mud-dominated, creating laterally extensive permeability barriers. Alternatively, some vertically accreting successions can be sand-dominated forming reservoirs up to 300 metres wide and 20 metres thick.