Mapping Formation-Top Offsets in the Athabasca Oil Sands Area, Alberta: Methodology and Results

This report presents the results of subsurface bedrock structure mapping in the Athabasca Oil Sands Area, Alberta, using well log data and geostatistical analysis. The project study area extends from Townships 87 to 99 and from Ranges 1 to 13, west of the Fourth Meridian.

Two horizons, the sub-Cretaceous unconformity and the top of the Lower Cretaceous Wabiskaw Member (Clearwater Formation), were selected for mapping formation-top offsets. The sub-Cretaceous unconformity represents approximately 220 Ma of non-deposition, erosion, and karstification, which resulted in active and relict sinkholes, enlarged joints, and faults related to the dissolution of halite in the Devonian Prairie Evaporite Formation. The top of the Wabiskaw Member represents the maximum flooding surface for the Mannville Group in this area and can be considered to have been originally smooth and therefore ideal for structure mapping. Numerous local offset structures were recognized and highlighted using residual maps of both the sub-Cretaceous unconformity and the top of the Wabiskaw Member.

A formation-top offset is herein defined as any vertical displacement of a formation top that can be detected using well log data and geostatistical analysis. The offset features identified on the two surfaces were compared to each other and to an isopach map of the McMurray Formation and Wabiskaw Member combined. This methodology provides a way to track the structural features and identify of the following types of offset features:

- features that offset the sub-Cretaceous unconformity surface only and became inactive prior to the initial deposition of the McMurray Formation,

- features that offset the sub-Cretaceous unconformity surface only, but remained active during the deposition of the McMurray Formation or the Wabiskaw Member and became inactive after the deposition of the Wabiskaw Member,

- features that offset both the top surface of the Wabiskaw Member and the sub-Cretaceous unconformity surface and remained active during and after the deposition of the Wabiskaw Member, and

- features that are not evident on the sub-Cretaceous unconformity, but offset the top surfaces of the McMurray Formation and the Wabiskaw Member at earliest after the deposition of the Wabiskaw Member.

The map of bedrock surface offsets generated in this report can be used to identify potential faults or salt dissolution collapse features. The methodology used to map formation-top offsets using well log data and geostatistical analysis is also presented.