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 elevation for the Lower Athabasca region of Alberta. The study area boundary covers NTS map sheets 73M, 74D, 83P and 83A.
To create this grid, stratigraphic picks of the sub-Cretaceous unconformity were geostatistically analyzed and interpolated to model the unconformity surface elevation.
The vertical positional error in the sub-Cretaceous unconformity elevation grid is a function of the quality of the formation top pick set, including data density, the surface complexity (i.e., topographic variability), and the removal of points from the pick set that are 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 through geostatistical evaluation to identify and remove outliers.
Step 1: (Input) 25,802 stratigraphic picks of the sub-Cretaceous unconformity were made. 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. The points were geostatistically analyzed for the presence of outliers, defined as points with an elevation value that differed more than 50 m from the predicted value of the modelled surface. These data points were reviewed and adjustments were made where necessary to ensure all of the 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 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.
The following table summarize the parameters used to model the sub-Cretaceous unconformity surface:
Algorithm parameters for sub-Cretaceous unconformity elevation surfaces
Topic (Petrel process tabs) Parameters Elevation Surface statistics
Boundary Same boundary polygon for all interpolated surfaces not applicable
Geometry Mesh type Structured; 4 nodes to each cell
Grid increment, X 100 m
Grid increment, Y 100 m
Grid rotation 0 degrees
Grid position, X min 539,819
Grid position, Y min 6,081,787
Grid position, X max 828,420
Grid position, Y max 6,344,087
Pre-processing line data pre-processing not applicable
input trend surface none
Algorithm extrapolation method normal
residual tolerance none
min value none
max value none
initial coarsening factor none
number of nodes to snap 16
order of projection linear
do all nodes get a value? Yes
number of smoothing iterations none forced
fixed sampling interval do not ensure
Post-processing pre-clipping smoothing none
clipping none
post-clipping smoothing none