Locating, Investigating, and Defining the Upper Laramie Aquifer, Northeastern Colorado

The Cheyenne Basin in northeastern Colorado has been explored and exploited for oil and gas reserves, resulting in a wealth of self-potential (SP) and resistivity borehole geophysical logs which have facilitated aquifer characterization in the basin. The Upper Laramie aquifer in the Cheyenne Basin was initially identified through geophysical logs and has since been further characterized through 3-D seismic data, test holes, production wells, cores, and outcrop data. The depth and thickness of the lower confining unit separating the Upper Laramie aquifer from the underlying Laramie-Fox Hills aquifer has been largely delineated using geophysical logs. Additionally, in areas where shallow geophysical data is sparse, the geometry of the bottom of the Upper Laramie aquifer and the underlying confining unit has been inferred by mirroring the geometry of the underlying Upper Pierre Shale, which is more commonly captured in deep geophysical logs for oil and gas wells. The Upper Pierre Shale exhibits a characteristic geophysical signature and is conformable with the Fox Hills Formation which is, in turn, conformable with the Laramie Formation.

Geophysical logging has thus played a crucial role in assessing the Upper Laramie’s potential connectivity to other aquifers and to surface water. A large portion of the aquifer which has been explored has been determined to be nontributary – that is, only negligibly connected to surface water - allowing water rights or regulatory determinations to be obtained which permit well owners to withdraw large quantities of groundwater from the aquifer without the need for augmentation. Future investigation will continue to delineate the geometry of the aquifer and its nontributary extent.

This talk will focus on where groundwater is located in the Upper Laramie aquifer, how an aquifer is determined to be nontributary, and the use of borehole geophysics in exploring and managing Colorado’s groundwater resources.