AN ENHANCED UNDERSTANDING OF THE DENVER BASIN AQUIFER CONFIGURATIONS

The Denver Basin includes four principal aquifers, the Dawson, Denver, Arapahoe, and Laramie-Fox Hills, and an overlying alluvial aquifer. The Dawson and Arapahoe aquifers are further classified by the State into upper, lower, and undivided regions for regulatory purposes. An understanding of the geometry of these aquifers is important to assist policy makers in regulating the future of these aquifers. It has been nearly twenty years since detailed analysis of the geometry of the Denver Basin was conducted. Much new data has arisen since that time, and under the South Platte Decision Support System project, being developed by the State’s Department of Natural Resources, more recent geophysical and drilling logs were collected and analyzed to help further define the Denver Basin geometry.

Data used to update the understanding of the aquifer configurations were collected from USGS reports, the SEO geophysical log database, scientific reports, and from recent field data. Picks for the top, bottom, and net sand thickness for each aquifer were compiled from the sources and formatted into a single database. Using GIS it was possible to assign Dawson and Arapahoe picks to their appropriate regions of either upper, lower, or undivided regions. Once a database had been built then the picks were queried to contour the top, bottom, and net sand thickness surface for each bedrock aquifer using an automated contouring package. To define the base of the alluvial aquifer, structural contour maps were digitized from a series of reports since raw geophysical and drilling log data were not readily available to define the alluvial system.

After the data for each surface had been compiled, they were imported into the modeling preprocessor package GMS, and a solids model representing the geometry of the Denver Basin was created. The model was sliced to produce two north south cross sections and five east west cross sections of the Denver Basin. The individual surfaces and cross-sections have helped to demonstrate the spatial variability in net sand thickness and the asymmetrical anticline structure that exists in the aquifers.