The sub-Cretaceous unconformity is a major component of the three-dimensional geological model of Alberta. This dataset is a two-and-a-half dimensional (2.5D) grid surface of the sub-Cretaceous unconformity paleotopography for the Lower Athabasca region of Alberta. The study area covers NTS map sheets 73M, 74D, 83P, and 84A.
To create this grid, stratigraphic picks of the top of the sub-Cretaceous unconformity were geostatistically analyzed and interpolated to model the unconformity surface elevation. A grid of the dominant regional trend was created and subtracted from the elevation grid, creating a residual grid that is a representation of the sub-Cretaceous unconformity paleotopography. The grid was then clipped by the study area boundary.
The vertical positional error in the paleotopography grid is a function of the unconformity pick set, including data density, the surface complexity (i.e., topographic variability), and the removal of points from the pick set identified as outliers.
Error in the formation pick elevation has two sources, error in the kelly bushing (KB) elevation, from which the pick elevation is derived, and actual errors in the making of the stratigraphic pick. Combined, these two sources of error are likely to have the largest impact on the grid’s vertical accuracy. The occurrence and magnitude of these errors are difficult to quantify. However, they were mitigated in the creation of this grid by geostatistical evaluation to identify and remove outliers.
Step 1: (Input) 25,082 picks of the sub-Cretaceous unconformity were created. 20,575 of the picks are located within the study area boundary, and an additional 4,507 picks are located outside the study area boundary to account for edge effects. These picks were geostatistically analyzed for the presence of outliers, defined as points with an elevation value that differs more than 50 m from the predicted value of the modelled surface. All outliers were reviewed, and adjustments were made where necessary to ensure all picks represented the sub-Cretaceous unconformity as accurately as possible.
Step 2: (Modelling the surface) The sub-Cretaceous unconformity picks were brought into the Schlumberger Petrel 2019 software. These data were used to model a grid of the structure top of the sub-Cretaceous unconformity. The Convergent interpolation method was used to model the sub-Cretaceous unconformity elevation surface. Convergent interpolation is a control-point orientated algorithm that converges on a solution iteratively and uses Taylor series projection as the basic gridder.
Step 3: (Modelling the trend surface) The paleotopographic surface of the sub-Cretaceous unconformity can only be evaluated once the predominant regional trends throughout the study area are subtracted from the structural elevation surface. A third order regional polynomial trend surface was determined to provide the optimal surface to capture the predominant regional trend. The third order trend was modelled in ArcGIS Pro using the original data point set in order to capture the regional trends of the study area.
Step 4: (Creating the paleotopography grid) The paleotopography of the sub-Cretaceous unconformity was produced by subtracting the regional trend surface from the structural elevation surface. The grid subtraction process was completed in Petrel and exported in ArcMap 10.8.1 using the Raster to ASCII tool as an ASCII grid file