This digital data release comprises regional lithostratigraphic picks and evaporite mineral interval data for the Elk Point Group in Alberta. The lithostratigraphic picks were made on wireline logs and represent the formation-, member-, unit-, and marker-bed-level stratigraphy of Alberta’s Elk Point Group, including: 1) the base of the Elk Point Group comprised by the eroded tops of the Cambrian and Ordovician systems in southern and central Alberta (sub-Devonian unconformity), as well as the top of the Granite Wash lithosome on and adjacent to the Peace River Arch in west-central Alberta, 2) the La Loche Formation, 3) the Lotsberg Formation, including the basal red beds, the lower Lotsberg halite, the middle red beds, and the upper Lotsberg halite units, and the L1, L2, and L3 marker beds from the lower and upper Lotsberg halite units, 4) the Ernestina Lake Formation, 5) the Cold Lake Formation, 6) the Contact Rapids and equivalent Chinchaga Formation, 7) the Keg River and equivalent Winnipegosis Formation, 8) the Prairie Evaporite and equivalent Muskeg and Eyot formations, including the Prairie laminites unit, the Whitkow and Telegraph members, the Shell Lake Member, the White Bear and Conklin marker beds, the Leofnard Member, the Black Creek Member, the Muskeg lower anhydrite unit, the Zama Member, and the remnant Prairie Evaporite collapse breccia, 9) the Sulphur Point Formation, 10) the Dawson Bay Formation, and 11) the Watt Mountain Formation. The dataset also includes picks of the sub-Cretaceous unconformity where it intersects strata of the Elk Point Group. Where any difference occurs, these stratigraphic picks should supersede those in equivalent wells in AER/AGS Digital Data 2017-0027.
The evaporite interval dataset was made using wireline logs and defines the top and base of evaporite minerals in the Lotsberg, Cold Lake, and Prairie Evaporite-Muskeg-Eyot formations, and includes: 1) halite in the lower Lotsberg unit, 2) halite in the upper Lotsberg unit, 3) halite in the Cold Lake Formation, 4) halite in the Whitkow and Telegraph members of the Prairie Evaporite and Muskeg formations, 5) halite in the upper Prairie Evaporite and Muskeg formations, 7) anhydrite in the Prairie Evaporite, Muskeg and Eyot formations, 8) undifferentiated halite in the Prairie Evaporite Formation within and east of the Prairie Evaporite halite dissolution scarp in northeastern Alberta, and 9) gypsum (gypsified anhydrite) within the zone of remaining sulphates in the Prairie Evaporite Formation predominantly east of the Prairie Evaporite halite dissolution scarp.
The accuracy of the pick depth, either in measured depth from the kelly bushing or with respect to sea level, is difficult to quantify and includes (but is not necessarily limited to) errors in
- well surface or bottom-hole Easting and Northing (X and Y);
- well ground elevation (Z);
- well kelly bushing elevation (Z);
- human error resulting from errors in picking the incorrect stratigraphic top (Z);
- data entry or data transfer (X, Y, and/or Z); and
- incorrect well log depth calibration (Z).
The data are tabular (point data with X, Y, and Z values).
The author made all of the stratigraphic picks and picks of evaporite mineral interval tops and bases. For the stratigraphic picks, some variation in quality exists due to older wireline log suites, and difficulty in making picks near the edge of unit extents.
As the dataset includes vertical and deviated wells, the location data and well identifier data (UWI) are not unique. In non-vertical wells, surface and bottom-hole latitude and longitude should be different. The X and Y location of picks from non-vertical wells are derived from the deviation survey within IHS Petra. The deviation survey uses the measurements of depth, inclination, and azimuth, and a minimum curvature method to interpolate the position of the stratigraphic pick along the non-vertical well path. The position of the stratigraphic pick on a non-vertical well path is therefore dependent upon the number of measurements of inclination and azimuth at depth during the time of drilling. In this dataset, the X, Y location for a stratigraphic pick in a non-vertical well is the X, Y location of that pick at depth along the deviated well path (i.e., not the surface hole X, Y location of the drilling rig).
The data are from the Alberta Plains where deformation of the Devonian sedimentary succession is relatively minor. All points are east of the deformation front of Paleozoic strata at a given latitude. Thus, rocks should not be thrusted or structurally duplicated. Therefore, the stratigraphic pick should only occur once in any given vertical well.
Two of the wells in the stratigraphic pick dataset are not oil and gas wells (100/SE-13-094-01W4/00 and 100/SE-24-094-01W4/00). These wells were originally drilled for mineral exploration purposes in northeastern Alberta and were cored through the Elk Point Group. These cores reside at the Mineral Core Research Centre (Alberta Energy Regulator) in Edmonton, Alberta. Synthetic unique well identifiers were given to the wells for the purposes of importing the stratigraphic tops data into IHS Markit's Petra geological software.
The horizontal datum for the well locations is the North American Datum of 1983 (NAD83).
The X and Y coordinates for stratigraphic pick locations are in 10TM (Forest) from IHS (Petra software). The horizontal positional accuracy is unknown.
The vertical datum for elevations in this data set is the Canadian Geodetic Vertical Datum of 1928 (CGVD28).
In vertical wells, the subsurface elevation of a pick in a well, measured with respect to sea level, is calculated by taking the elevation of the kelly bushing (on the drilling platform) and subtracting the measured depth of the pick on the geophysical well log.
Some uncertainty in the vertical depth of the pick will result if the borehole is not entirely vertical. In general, the amount of vertical depth due to deviations from the vertical in boreholes is deemed negligible with respect to other potential sources of vertical error in this study. In the case of non-vertical wells, the deviation survey from IHS Markit’s Petra was used to calculate the position of the stratigraphic pick along the well path. The deviation survey uses the measurements of depth, inclination, and azimuth, and a minimum curvature method to interpolate the position of the stratigraphic pick along the non-vertical well path. The position of the stratigraphic pick on a non-vertical well path is therefore dependent upon the number of measurements of inclination and azimuth at depth during the time of drilling.
Perhaps the greatest source of vertical uncertainty in this study is potential error in the elevation of the kelly bushing (KB). Any errors in surveying the ground elevation of the well site can result in vertical error. In addition, once the ground elevation is determined, the site is usually prepared for the drilling rig. If the original survey marker is disturbed or moved, this can result in potential vertical errors. The KB elevation is usually derived from adding the height of the drilling platform above the ground surface to the survey ground elevation. If this is not done correctly, it can introduce vertical error in the KB elevation, which is then propagated in the measured depth to the pick and the pick elevation.
Although incorrect KB elevation data can be difficult to detect, the data were screened by comparing the ground elevation and the KB elevation (derrick height) for each well. An acceptable range of derrick height (calculated by subtracting ground elevation from KB elevation) of zero to eight metres was used. Wells with derrick heights outside this range were excluded.
Vertical error in the pick elevation value can also result from human error resulting from uncertainty or incorrect placement of the pick on the well logs. The occurrence and magnitude of this error is difficult to identify, but comparison with existing published pick datasets and checks for internal consistency (such as identification of global and local outliers using statistical methods and gridding data while picking) minimized this source of error as much as possible.
Stratigraphic picks and evaporite mineral tops and bases were made within the IHS Markit’s Petra software application using well logging data from IHS.
Attribute values were checked to ensure reasonable values. For instance, the well locations were plotted on a map and no obvious anomalous locations were seen. A quality control process was used to test the quality of the deviation survey in non-vertical wells. Structural and isopach mapping were employed to check for anomalies in the stratigraphic picks in this dataset. Additionally, stratigraphic picks were modelled in the modelling software Petrel, and potential anomalies were reviewed.
A description of the methodology for picking the tops and bases of evaporite minerals can be found in AER/AGS Report 99. The tops and bases of the evaporite mineral intervals are in measured depth (MD) along the wellbore. In crooked or deviated wells these interval thicknesses will not represent true vertical thicknesses. True vertical thicknesses in non-vertical wells need to be calculated within a geological software capable of incorporating deviation surveys for the purposes of deriving true vertical thicknesses. For mapping of net-evaporite thicknesses within Petra, the program derives a true vertical thickness for the net-evaporite value; that is to say the net-evaporite isopach maps in AER/AGS Report 99 represent the true vertical thicknesses of the respective evaporite minerals.