ABSTRACT

Helium-enriched natural gas occurs in Upper Cambrian silicified siltstones and mudstones overlying a pronounced basement topographic high (the Aikens anomaly) nine miles north of Swift Current, Saskatchewan. Relief of 600 ft, relative to the regional basement elevation, can be attributed to the massive structure and stability to weathering of the undeformed and unmetamorphosed porphyritic rhyolite forming the topographic anomaly.

Delayed neutron-activation analysis of a rhyolite core sample from B. A. Wilhelm 1-9 indicates 4.9 ppm. U, only slightly above the western Canada basement average of 4.3 ppm. U. However, core samples from M.O.W.S. Cantuar 2-21 and M.O.W.S. Fosterton 15.3, 20 and 25 mi to the west respectively, contain 24.1 and 15.7 ppm. U. Compositionally both rocks are porphyritic granites. Textural features suggest epizonal emplacement.

The suggested origin of the helium accumulation is:

1. A Late Hudsonian volcanic-plutonic centre developed west of the present site of Swift Current.
2. Pre - Upper Cambrian erosion formed a series of hills above this complex.
3. Upper Cambrian transgression (Deadwood Formation) buried the hills. Authigenic silica tightly sealed parts of the siltstone and mudstone overlying the Aikens anomaly.
4. Circulating formation fluids carried helium from the subcrop of uranium-rich porphyritic granites along the Precambrian-Cambrian unconformity to the highest adjacent topographic feature.