Permian Salt in the Northern Denver Basin: Controls on Occurrence and Relationship to Oil and Gas Production from Cretaceous Reservoirs

Abstract Subsurface study of Permian evaporites and related strata in the Denver Basin reveals that the distribution of upper Wolfcampian, Leonardian, and Guadalupian salt is related to: 1) the configuration of evaporite basins during precipitation; 2) truncation or near-surface dissolution below a pre-late Jurassic unconformity; and 3) subsurface dissolution at various times since the Jurassic. Correlations across the basin have identified 13 stratigraphic intervals that, in places, are salt-bearing. Precipitation of Leonardian and upper Wolfcampian salts was influenced by a northeast-trending positive element related to the Transcontinental Arch. During late Leonardian and Guadalupian time this structural feature had less influence on the configuration of the evaporite basin; instead the present eastern limits of these uppermost salts are controlled largely by truncation or near-surface dissolution below a pre-Upper Jurassic unconformity. Subsurface dissolution has partially removed salts from all stratigraphic intervals at various times since the Jurassic. Collapse of overlying strata in response to subsurface salt dissolution created additional accommodation space, causing localized thickening of post-salt formations, and allowing for the dating of dissolution events. Timing of salt dissolution has influenced the distribution and trapping mechanism of oil and gas fields on the eastern flank of the Denver Basin. In the western part of the D and J Sandstone Fairway, dissolution took place largely during Jurassic and Early Cretaceous time, resulting in compensatory thickening of the Jurassic Morrison and Lower Cretaceous Cheyenne formations. Because dissolution and collapse occurred prior to deposition of the D and J sandstones, structure is relatively simple at the level of these important reservoirs, and stratigraphic traps predominate. In the eastern part of the D and J Sandstone Fairway and along the shallow Niobrara gas trend, where dissolution occurred after deposition of the Cretaceous reservoirs, perhaps due to introduction of fluids along Laramide-activated basement faults, structure is more complex and structural or structural-stratigraphic traps predominate. Discovery of oil in the subsalt Paleozoic section in 1980 sparked an exploration effort in the northern Denver Basin that has greatly increased the amount of subsurface control with which to examine the stratigraphy of Permian salt-bearing units. A subsurface nomenclature, which primarily uses Midcontinent terminology for Leonardian lithostratigraphic units and terminology from the Powder River Basin and surrounding areas for Guadalupian formations, is recommended for the Denver Basin.