Devonian Bedrock Mapping

Devonian Bedrock Mapping

Devonian Bedrock Mapping in the Area Subject to the Lower Athabasca Regional Plan

       



Figure 1: Location of the study area in northeastern Alberta overlain on a hill-shaded digital elevation model (SRTM). The model boundary is in black. The administrative boundary of the Lower Athabasca Regional Plan (LARP) is in yellow.

What

Regional-scale lithostratigraphic mapping of the entire Paleozoic succession in northeastern Alberta was undertaken to support the production of a high-resolution 500 × 500 m grid-cell 3-dimensional model. The stratigraphic dataset comprises formation- and member-level picks from 1307 wells covering 874 townships in an area encompassing the area subject to the Lower Athabasca Regional Plan (LARP). The study area is extensive, covering an area of 81 550 km2, which is roughly 12% of the area of Alberta.

Why

To date, there does not exist a comprehensive regional-scale lithostratigraphic model for the geology of northeastern Alberta—an area of critical importance due to intense hydrocarbon production. Whereas publicly available local-scale mapping and modelling have been produced for relatively small portions of the bitumen-bearing lower Mannville Group strata, no such efforts have been focused on the underlying Paleozoic strata. A comprehensive mapping and modelling program aims to construct an integrated and geologically consistent model using substantive geostatistical methods for the Paleozoic succession. This model will augment efforts to regulate oil and gas activities and provide a stratigraphic framework for hydrogeological assessments aiming to characterize water resources in the area.

The Prairie Evaporite Dissolution Scarp
Detailed mapping of the Prairie Evaporite Formation was undertaken to redefine the location and extent of the Prairie Evaporite salt scarp, which is a well-known intrastratal dissolution feature that runs south-southeast through the study area. The dissolution of greater than 200 m of halite from within the Prairie Evaporite Formation resulted in structural compensation and collapse of the overlying Devonian Beaverhill Lake Group. Redefining the location and extent of the Prairie Evaporite dissolution scarp was accomplished through detailed mapping of internal member and marker bed stratigraphy, combined with net-pay mapping of halite and anhydrite using modern spectral density log suites.



Figure 2: 3-dimensional oblique view of the structural elevation of the Prairie Evaporite Formation (50× vertically exaggerated) with the location of the dissolution scarp (red lines).

The Sub-Cretaceous Unconformity Surface
The top of the Paleozoic succession in the study area is the sub-Cretaceous unconformity surface—an angular unconformity. This surface forms the interface between Paleozoic and Mesozoic strata, and its proper reconstruction is integral in understanding depositional trends in the latter, as well as basin hydrodynamics in northeastern Alberta. The deep regional well control used for stratigraphic mapping was employed to produce a detailed model of the location of Devonian subcrop belts at the sub-Cretaceous unconformity surface.



Figure 3: Paleotopographic reconstruction (relative height in metres) of the sub-Cretaceous unconformity surface, with Devonian subcropping formations and the Prairie Evaporite dissolution scarp.