A reconnaissance subsurface geologic study has shown that three types of reservoirs are available for liquid waste disposal in the Denver basin. These are fractured Precambrian rocks, porous sandstone reservoirs, and thick shale suitable for disposal by the hydraulic-fracturing technique.

From 1962-1965, fractured Precambrian rocks at a depth of 12,000 ft. were being used as a disposal reservoir for toxic effluent produced at the Rocky Mountain Arsenal near Denver. The disposal well presently is shut in pending investigation of the possible relation between waste injection and Denver-area earthquakes which increased in frequency and magnitude during the injection period. Data points on the Precambrian of the Denver basin are sparse because only a few test wells reached it in search for oil and gas. Two borings on the Apishapa uplift indicate the presence of a good fractured Precambrian reservoir.

Porous sandstone reservoirs considered most favorable for waste disposal are the Permian Lyons Sandstone, the Triassic Dockum Sandstone, the Triassic-Jurassic Jelm-Entrada Sandstone, and sandstone bodies of the Cretaceous Dakota Group and Hygiene zone. The Lyons, Dockum, and Dakota are best suited for waste disposal in the southern part of the basin. Exploratory activity for oil and gas in this part of the basin has been very low in recent years, whereas activity is moderate to brisk in the central part of the basin where the Lyons and Dakota are prime drilling objectives. The Dockum Sandstone, potentially the best disposal reservoir volumetrically, is restricted to the southeast part of the basin. The Jelm-Entrada Sandstone and sandstone beds in the Hygiene zone offer potential dispo al reservoirs along the heavily populated strip between Denver and Cheyenne.

Cretaceous marine black shale suitable for disposal by the hydraulic-fracturing technique is present everywhere in the basin. The shale crops out in large areas. Beneath the populous strip along the Front Range, the shale locally is covered by as much as 2,000 ft. of Late Cretaceous and Tertiary transitional and continental rocks.

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