Hodgeman County Study, part 12 of 12
In the study area the aquifer is really two aquifers, the Cheyenne sandstone and the Dakota formation, which are separated by the Kiowa shale. The Cheyenne is saturated with salty water coming from the dissolution of underlying Permian salt beds while the Dakota formation is the unit of economic interest yielding fresh water. Throughout the area it is then important that the Cheyenne and the Dakota not be connected. At sites in group 23, a sandy Kiowa may result in an unavoidable leak in the sealing and one should be cautious with drilling and completions at sites in group 26 where the Cheyenne is thick.
Some of the group 23 anomalies may not be real, which would reduce the danger of leaks. There are grounds to believe that the gamma ray log scale reported in the logs may be incorrect for at least a few of those wells. Under-reported natural radiation results in units abnormally and incorrectly low in shale. The errors should not have a bearing upon the results of the classification of those wells outside group 23 and open another potential application of regionalized classification: detection of errors.
The third group to avoid is the most predominant group 81, where for some intriguing geological reason there has never been much deposition of sand. Thus the reasons to avoid drilling group 81 are economic rather than environmental.
Finally groups 36 and 65 should be primary targets for groundwater supply in the area. If one believes those geologists who postulate meandering river flood plains deposited the Dakota Formation, one may suggest that group 65 developed along a drainage system flowing to the northwest. The exceptionally favorable areas associated with group 36 tend to occur in the flanks of group 65.
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