Earth Sciences Report 2000-07

Author(s) Date 2000-05-31

Detailed sedimentological and stratigraphic analysis of 45 outcrops, 140 cores and stratigraphic picks from over 4 000 well logs, along with preliminary palynological analysis, allow for a better understanding of the preserved stratigraphy and depositional settings of the Athabasca oil sands deposit, hosted primarily by the McMurray Formation, in northeastern Alberta. Facies analysis indicates that the Lower Cretaceous McMurray Formation was deposited and reworked as fluvial, estuarine and coastal plain sediments within an evolving landscape that changed form incised-valley fills carved on a regional unconformity, to broad estuaries, lakes and bays on a coastal plain. Locally, Devonian weathered limestone basement highs pierced the landscape. Sedimentation on top of bedrock highs was typically low, resulting in localized condensed sections. Elsewhere, within incised paleovalleys with high sedimentation rates, a more complete stratigraphic succession is preserved. The Lower Cretaceous McMurray Formation records a sedimentary history of repeated erosional and depositional cycles, resulting in significant unconformities at the base and top, and internal disconformities within the succession.

Former informal stratigraphic subdivisions for the McMurray Formation included the Lower, Middle and Upper McMurray (Carrigy, 1959). Other workers have subdivided the Wabiskaw/McMurray interval into 'basal McMurray' and 'Wabiskaw/McMurray'. During the present study detailed facies and biostratigraphic analyses of units formerly defined as Middle and Upper McMurray show no difference in age, and any facies variations appear to relate to paleogeographic controls. Additionally, units from what were originally called 'Middle' and 'Upper' appear to be a single, genetically-related and conformable succession with no regionally significant internal boundary or disconformity. By contrast units previously called 'Basal McMurray' or 'Lower McMurray' are distinct lithologically, have different biostratigraphic ages, and are overlain unconformably by younger units of the McMurray succession. Thus it is recommended here that the informal term 'Middle McMurray' be abandoned, and what was formerly mapped as Middle McMurray now be included as part of the upper McMurray. What was formerly designated as 'basal' or 'lower' McMurray is called Lower McMurray. In summary, it is proposed that the two mappable informal members of the McMurray Formation are the Lower McMurray and Upper McMurray, which are separated by a disconformity.

Units within the Lower McMurray, interpreted as part of a fluvial lowstand system, host rich bitumen and water-sand reservoirs that accumulated as incised paleovalley-fills cut within the karstic pre-Cretaceous landscape. Lower McMurray bitumen reservoirs include braided channel and bar sands and intraclast mudstone breccias. Localized to laterally extensive water sands occur in paleolows along the basal pre-Cretaceous unconformity. The upper McMurray concession, comprising estuarine and nearshore deposits of the overlying transgressive system, contains rich bitumen reservoirs, hosted within estuarine channel complexes that are often stacked above the Lower McMurray channel sands. Local water sands occur in the lower portions of the Upper McMurray succession, with somewhat isolated gas reservoirs in the upper parts of the succession, particularly alon trend with the Prairie Salt Scarp. The uppermost part of the succession records the final stages of estuarine sedimentation that includes abandonment filling of late stage channelling, crevasse splays and tidal mud and sand flats associated with coastal plain sediments.

Although there is a high degree of heterogeneity within the McMurray Formation, much of this apparent heterogeneity can be simplified when a unified facies classification, as developed in the present study, is used. This type of classification affords comparison between different areas on a regional basis. The lithofacies scheme developed here will allow future detailed descriptions of various reservoirs, including regional mapping and characterization of various the water-, gas-and bitumen-resources of the McMurray Formation.

Hein, F.J., Cotterill, D.K. and Berhane, H. (2000): An atlas of lithofacies of the McMurray Formation, Athabasca Oil Sands deposit, northeastern Alberta: surface and subsurface; Alberta Energy and Utilities Board, EUB/AGS Earth Sciences Report 2000-07, 217 p.