Reservoir Geometry of the Regressive Fox Hills Sandstone: Control on Aquifer Quality

Marieke Dechesne and Robert G. Raynolds. Denver Museum of Nature & Science, 2001 Colorado Boulevard, Denver, CO 80205, phone: 303-370-6047, [email protected]

In the Denver Basin the littoral to near-shore Fox Hills Sandstone was deposited during the early phase of the Laramide orogeny as the Cretaceous Interior Seaway retreated to the northeast. This approximately 68 MY regressive sandstone is composed of a series of shingles reflecting episodic seaway retreat. We illustrate the detailed geometry of the sandstone elements that make up the Fox Hills Sandstone. This unit is an important aquifer in the Denver Basin and the distribution of porosity, permeability, and facies tracts is important in predicting the performance of water wells.

A data base of thousands of oil and gas wells and thousands of water wells is available for study. A subset of these wells has been analyzed to create a three-dimensional model using ArcGIS software. Seven shingles are mapped in detail. Our three dimensional model permits the effective and efficient communication of critical reservoir parameters to resource planners. Comparisons are made to other well-studied regressive systems in the San Juan and Greater Green River basins. We propose that non-uniform subsidence has influenced the stacking pattern of this regressive succession, controlling both the distribution of net sandstone and of coal-bearing strata. These parameters are critical to predicting reservoir performance and quality. The patterns we document are relevant to any effort to extract fluids from rocks of this facies association.