| Abstract: |
In addition to relatively shallow geothermal resources associated with the structurally controlled deep circulation of groundwater, significant geothermal resources exist in Colorado’s sedimentary basins. One basin previously unexplored for geothermal resources is the Piceance Basin, a Laramide-age structural basin, which includes a sedimentary rock foundation of Mississippian and older platform deposits and basin deposits of the broader Maroon trough that preceded the Piceance Basin. The modern Piceance drainage basin comprises four smaller drainage basins and is much smaller than the structural basin. A few thermal springs occur on the eastern margin of the structural basin, and high heat-flow values have been reported near the southeastern salient of the basin, but no significant geothermal manifestations occur in the basin. Bottom-hole temperature (BHT) data have been compiled from 10,372 hydrocarbon wells in the Piceance basin with an average depth of 2103 ± 685 (± standard deviation) m. The data were combined in 0.4 by 0.4 degree blocks by their geographic coordinates and average geothermal gradients calculated for each block. These gradients ranged from 22.7 to 41.8°C/km. A correction for the effect of the circulation of drilling fluid on the BHTs was estimated from BHTs from second cement bond logs and fluid temperatures from drill stem tests. Block gradients calculated from corrected BHTs ranged from 27.3 to 51.5°C. km. A general increase in gradients from the north to the south of the basin was observed. No significant correlation was found between block geothermal gradient and well depth or well collar elevation, tentatively suggesting that neither basin-wide thinning of formations toward the basin margins nor basin-wide thermal convection by groundwater flow were the primary cause of the regional distribution of geothermal gradients. Increased gradients to the south correlate with Quaternary faulting in the Uncompahgre uplift to the southwest and Tertiary volcanism to the southeast. Uncorrected BHTs indicate geothermal resources at temperatures of 100 to 250°C in the depth range of 2.5 to 5 km. Corrected BHTs reduce this range to 1.7 to 4.2 km. General permeability at these depths is likely to be low. The Leadville Limestone, a Mississippian karst-forming limestone is likely to underlie most of the basin, shallowing on the southwest margin of the basin. Observations of this limestone at other locations indicate that it is a very permeable aquifer. Production from similar fractured karst limestone aquifers in Germany has generated ?3.0 MWe from single wells. Alternatively, impermeable strata could be hydrofractured to produce an enhanced/engineered geothermal system (EGS). An analogous sedimentary basin EGS project is currently in the early stages of development in the Raton basin of southern Colorado. |