The dataset includes subsurface stratigraphic picks for the top of the Foremost Formation (Belly River Group) in the Alberta Plains (Townships 1 to 52, Ranges 1W4 to 5W5) made from downhole wireline geophysical well logs. The top of the Foremost was picked at the base of a continuous sandstone or siltstone bed (low-gamma-ray) of variable thickness overlying the Taber coal zone.
Well data were screened to detect errors resulting from deviated wells, as well as incorrect ground and kelly bushing elevation data. We used statistical methods to identify local and regional statistical outliers, which were examined individually.
A stratigraphic pick in a well is a point defined in three dimensions (X, Y and Z).
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 latitude and longitude (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. All picks are ranked the same in quality.
As the dataset includes only vertical wells, all location data and well identifier data (UWI and UWI_MODIFIED) are unique. In non-vertical wells, surface and bottom-hole latitude and longitude should be different, and several wells might share a common surface location but have different bottom-hole locations. By choosing only vertical wells, this problem was avoided.
The data are from the Alberta Plains where deformation of the Cretaceous sedimentary succession is relatively minor. All points are east of the deformation front at a given latitude. Thus, rocks should not be thrusted or structurally duplicated. Therefore, the pick should only occur once in any given vertical well.
No data are missing.
Attribute values were checked to ensure reasonable values. For instance, the author plotted the well locations on a map and observed no obvious anomalous locations. A query checked for any deviations from vertical of the well surface location compared with the bottom-hole location. Wells for which well surface coordinates differed from bottom-hole coordinates were removed from the dataset. As all remaining wells should be vertical if the surface and bottom-hole coordinates are correct, measured depth and true vertical depth should be equal.
The horizontal datum for the well locations is the North American Datum of 1983 (NAD83).
The latitude and longitude coordinates for well surface and bottom-hole locations are 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. The bottom-hole latitude and longitude of each well location were compared with the surface latitude and longitude for each well to ensure they were the same. If either the surface or bottom-hole latitude and longitude are incorrect, some degree of vertical error may result. 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.
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.
Prior to making picks for a given surface, the author studied the published geological literature with emphasis on type and/or representative sections. Studies including both core and geophysical well logs were particularly valuable, as they provided a link between the rock and geophysical signatures.
Geophysical well logs (both digital and raster format) were examined using Petra® and Accumap® software and picks were recorded in a database. When sufficient well density and log availability permitted, we selected wells according to the following criteria:
1) vertical wells only;
2) wells with a spud date between 1975 and the present; and
3) wells with down-hole geophysical well log suites that include gamma-ray, density or sonic, and resistivity logs.
The author gave preference to wells where the bottom of surface casing shoe was at a depth of less than 50 metres below the kelly bushing. If sufficient well density was not available using the above criterion, the criterion was expanded to include wells with the bottom of surface casing in the 50-150 metre depth range. A minimum well density of one well per township was used, although, in most areas, well density greatly exceeded that number, especially where an anomalous structure was detected.
To correlate and check internal consistency, a series of north-south and east-west cross-sections, with a spacing of approximately 10 km (one township), was used to make picks. Therefore, a pick in a well was typically compared with three to four picks in nearby wells to ensure consistency. During picking, picks were gridded using the triangulation method using Petra® to identify and check outliers, which appear as "bull's eyes" on a structure contour map.
After making picks, and prior to modelling the surface, steps were taken to detect errors in:
- depth data (true vertical depth compared with measured depth data for non-deviated wells);
- kelly bushing (KB) elevation data for wells;
- ground elevation data for wells; and
- pick depth due to human error.
Picks and well header information, including kelly bushing elevation, ground elevation, well surface location (longitude and latitude in decimal format) and bottom-hole location (longitude and latitude in decimal format), were exported from Petra® (IHS) software into a comma-separated value file. The horizontal datum for the well location is the North American Datum of 1983 (NAD83) and the vertical datum is the Canadian Geodetic Vertical Datum of 1928 (CGVD28). The picks are in metres, given as measured depth relative to KB elevation. Pick elevations were calculated by subtracting measured depth from the KB elevation.
A query of the well surface location compared with the bottom-hole location was run to check for any deviations from vertical; we removed these wells from the dataset. If a well is deviated, its surface and bottom-hole coordinates should be different. As all remaining wells should be vertical if the surface and bottom-hole coordinates are correct, measured depth and true vertical depth should be equal.
Although incorrect KB elevation data can be difficult to detect, the author screened the data 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 of this range were excluded.
Data were then screened for both global and local outliers. Outliers are those values that are outside a specified normal range, compared with the entire dataset (global outliers), or within a local area (local outliers). If caused by errors, outliers can have several detrimental effects on the interpolated surface. One should either correct or remove outliers before creating a surface. Outliers may result from one or more of the following factors:
- incorrect ground elevation and/or KB elevation data not detected during the initial screening;
- incorrect location data for a well;
- deviated wells which are not marked as such and have either incorrect surface or bottom-hole location data;
- incorrect pick data due to human error; and
- geological structure.
The inverse-distance weighted interpolation method was useful in locating outlier data points, which appeared as bull's eyes on the resulting map. Outliers were flagged and the well data and geophysical logs were examined to determine whether the outlier was due to geological structure or incorrect data. In cases where no error could be identified, additional data were gathered to refine the definition of local structure. In these cases, if the data were a single point and no geological evidence corroborated the structure, then the data point was removed.
Once outliers were either removed or confirmed, we repeated the outlier screening process at least three times. This iterative process identified increasingly more subtle outliers. As each pick was made during this study and all statistical outliers were examined and some removed, the largest source of error and uncertainty in the elevation of the pick is likely to be related to the surveyed kelly bushing (and ground elevation) for a given well.