Modelling of the sub-Cretaceous unconformity across the Alberta basin is being undertaken to map the elevation of the unconformity surface, delineate the boundaries of stratigraphic units subcropping at the unconformity surface, and to provide a detailed reconstruction of the paleotopography of the sub-Cretaceous unconformity surface. This reconstruction is a representation of what the surface looked like before the deposition of Lower Mannville and equivalent strata, including highlands and drainage pathways.
Figure 1. Current state of sub-Cretaceous unconformity modelling.
Figure 2. 3D block model of the sub-Cretaceous unconformity and underlying strata in the Slave Lake and Peace River regions of Alberta.
Modelling of the sub-Cretaceous unconformity surface and zero-edge delineation of subcropping formations are an integral part of a larger project at the Alberta Geological Survey (AGS) to create a provincial-scale 3D geological framework model of Alberta’s subsurface.
Previous modelling of the sub-Cretaceous unconformity surface was completed using datasets from multiple sources, resulting in a variable quality due to inconsistent picking criteria between sources, co located data, and clustered data distribution. In order to increase our confidence in this important surface model, a revised dataset based on spatial distribution, log quality, and consistent picking criteria of unconformity picks data is required. Additional picks are based on newer log data, with new wells chosen to optimize the spatial distribution for modelling in areas of increased geological complexity. Unconformity picks are used to determine the extent of formation boundaries and complex erosional features along the unconformity surface.
Modelling of the sub-Cretaceous unconformity begins with mapping the present day elevation of the surface by making stratigraphic picks based on well logs. These picks provide the location (x and y coordinates) of the well and the elevation (z) at which the unconformity is identified in the well. These data are evaluated both visually and geostatistically to identify potential outliers. The data is then geostatistically modelled in ArcGIS using the ordinary kriging algorithm on a 500 metre grid cell size, accounting for regional data trends.
Figure 3. Elevation map of the sub-Cretaceous unconformity in the Slave Lake and Peace River regions of Alberta.
The paleotopography of the sub-Cretaceous unconformity is derived by subtracting the regional trend model from the modelled elevation. The residuals from this grid operation provide a representation of what the topography of the sub-Cretaceous unconformity would have looked like before the current structural setting and the onset of Lower Cretaceous Mannville and equivalent strata sedimentation.
Figure 4. Schematic illustration of paleotopography calculated from current subsurface elevation and regional trend of the sub-Cretaceous unconformity surface.
Figure 5. 3D oblique view of the sub-Cretaceous paleotopography in west-central Alberta.
Subcropping Stratigraphic Units
The stratigraphic units underlying the sub-Cretaceous unconformity are geostatistically modelled using the available pick data and integrated into a 3D geocellular model using Petrel 2013. The first step involves geostatistically modelling a 2.5D surface for each of the underlying stratigraphic units and projecting it through the modelled surface of the sub-Cretaceous unconformity. This ensures that there are no gaps within the 3D model along the complex topography of the sub-Cretaceous unconformity when the underlying stratigraphic surfaces are truncated by the unconformity. The 3D model is used to assess the form and geometry of the subcrop boundaries for each stratigraphic unit along the unconformity. To ensure that the modelled subcrop lines are consistent with the eroded formation picks, the model results are cross-validated with well control data, and adjustments are made to the subcrop lines where required.
Figure 6. Schematic diagram illustrating subcrop areas on the sub-Cretaceous unconformity surface resulting from the intersection of underlying stratigraphic units.
Figure 7. Plan view of study area showing the distribution of stratigraphic units subcropping the sub-Cretaceous unconformity in the Slave Lake and Peace River regions of Alberta.