The mid-point of the sampled interval is calculated value and its accuracy is dependent on the accuracy of the measured depths and elevations provided in the well header. For the oil and gas wells, the well elevations and locations are surveyed in, so accuracy will be to the same significant digits provided by the source, even though there may be considerable uncertainty as to where the sample actually came from.

The location of the sampled interval in oil and gas wells may be accurate to within millimetres in accordance with surveying standards at the time of drilling.

Process steps for selection of formation water analyses from oil and gas wells:

1. Records containing tested interval depth information were selected;

2. The tested interval was constrained to less than or equal to 50 m;

3. The acceptable charge balance error was constrained to ±10%;

4. Samples labelled as “incomplete analysis" were excluded;

5. Records were allocated to the Wabiskaw HSU based on the depth information of the tested interval;

6. Records with tested intervals straddling the top or bottom of the Wabiskaw HSU were excluded, unless they were from wells in data-poor areas and it was verified that less than 30% of the tested interval extended into the formation directly above or below the HSU, but not into any other formation;

7. In case of collocated data, the most representative sample was selected, but in some instances averaging was allowed;

8. The concentration of total dissolved solids (TDS) was determined either by summing the concentration of major constituents (if available), or by using the TDS values as reported in the source dataset (if major constituents were not available).

9. The remaining formation water analyses were subjected to additional screening to identify potential contamination of the formation water by drilling fluids such as acid water, corrosion inhibitors, mud filtrates, and alcohols.

The final dataset includes only those records that were used in the production of the final TDS surface.

Details about the screening process can be found in the following reference: Jensen, G.K.S., Rostron, B., Palombi, D. and Melnik, A. (2013): Saskatchewan Phanerozoic Fluids and Petroleum Systems project: hydrogeological mapping framework; in Summary of investigations 2013, v. 1, Saskatchewan Geological Survey, Saskatchewan Ministry of the Economy, Miscellaneous Report 2013-4.1, Paper A5, 10 p. URL http://publications.gov.sk.ca/details.cfm?p=80100 [October 2014].