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

From early 1962 through early 1966, fractured Precambrian rocks at a depth of 12,000 ft were used as a disposal reservoir for toxic effluent produced at the Rocky Mountain Arsenal near Denver. The disposal well is now shut in pending investigation of the possible relationship of waste injection to Denver-area earthquakes, which increased in frequency and magnitude during the injection period.

Data on the Precambrian of the Denver basin are sparse because only a few test wells penetrated it in search for oil and gas. Two borings on the Apishapa uplift indicate 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 sandstones in the Cretaceous Dakota Group and the "Hygiene zone" of the Pierre Shale. The Lyons, Dockum, and Dakota are best suited for waste disposal in the southern part of the basin. Exploration for oil and gas in this part of the basin has been very slow 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 limited to the southeast part of the basin. The Jelm-Entrada and Hygiene-zone sandstones are potential disposal 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 over large areas. Beneath the populous strip along the Front Range, the shale is covered locally by as much as 2,000 ft of Upper Cretaceous and Tertiary transitional to continental strata.