HYDROGEOCHEMISTRY OF THE UINTA AND GREEN RIVER FORMATIONS, PICEANCE CREEK BASIN, NORTHWESTERN COLORADO, USA

The Piceance Creek Basin in northwestern Colorado hosts large quantities of natural gas and one of the world’s thickest and richest oil shale deposits. Responsible development of this resource will require a firm understanding of the existing hydrogeochemistry of the basin. Using a database compiled by the USGS, the water chemistries of 267 surface and groundwater samples in the Piceance Creek primary drainage basin were evaluated by mapping major ion concentrations and mineral saturation indices relative to hydrostratigraphic units and geologic structures. Controlling processes were assessed using statistical correlation, cluster and factor analyses. Cluster analysis suggests five distinct hydrogeochemical groups. These groups are largely differentiated by their water type (surface water versus groundwater), Na concentrations and alkalinity, Ca/Mg ratios, and sulfate concentrations. These differences in aqueous concentrations also reflect equilibration with different minerals. Data from two of these groups were used to estimate the solubility product and mole fraction of Fe(II) in ankerite (CaMg_1-XFe_X(CO₃)₂), a mineral commonly reported in the Piceance basin that was not included in any of the thermodynamic databases we considered. Group 2 comprises mostly samples from the lower of two recognized aquifers and yields an estimated log solubility product and mole fraction of Fe(II) of -16.25 ±0.34 and 0.48 ±0.13 while Group 4 comprises mostly upper aquifer samples which yield a log solubility product and mole fraction of Fe(II) of -16.51 ±0.44 and 0.34 ±0.15. Mineral saturation maps for minerals commonly found in the basin (calcite, dolomite, dawsonite, ankerite, analcime, albite, K-feldspar, quartz, chalcedony) were created for the upper aquifer, lower aquifer, and surface samples. Halite and nahcolite were not included since none of the samples in the database were equilibrated with these phases. The mineral saturation maps are generally consistent with mineral distributions reported in the literature. The geochemical evolution of groundwaters in the basin are discussed in consideration of these results.