Hydrogeochemical Evaluation of the Uinta Formation and Green River Formation, Piceance Creek Basin, Northwestern Colorado, USA

摘要

The Piceance Creek Basin in northwestern Colorado contains extensive oil shale deposits that produce natural gas and which could potentially yield ~1.5 trillion barrels of shale oil. However, much of the oil shale lies at depths too great for traditional mining practices and various innovative approaches for in situ conversion of kerogen to oil have been proposed. A firm understanding of the existing hydrogeochemistry is needed as resulting mineralogical changes or rock-fluid reactions may affect rock porosity and permeability. Using an existing database complied by the USGS, the water chemistry of 267 surface and groundwater samples in the Piceance Creek primary drainage basin have been evaluated by mapping major ion concentrations and mineral saturation indicies with respect to hydrostratigraphic units and geologic structures. Controlling processes have been further assessed using statistical correlation and factor analysis.Results indicate that shallow waters in recharge zones are dominated by mixed cations (Na, Ca, Mg) and bicarbonate anions but with increased depth, groundwater transition to nearly 100% sodium bicarbonate type water. The chemistry of lower aquifer waters are principally controlled by nahcolite dissolution, but evidence of sulfate reduction and cation exchange aid in maintaining a sodium-bicarbonate water type. Ion evolution in surface and upper aquifer waters are influenced by an increase in sulfate concentration which is necessary to evolve water to an intermediate stage with sulfate-dominant anions. The source of sulfate is speculative, but likely due in part to the oxidation of sulfide-enriched groundwater and possible dissolution of sulfate-bearing carbonates. Surface and upper aquifer water chemistry in the northern portion of the basin is the result of discharge of deeper groundwater which is controlled to some degree by preferential pathways created by faults. Lower aquifer water migrates upward and mixes with the less-concentrated near-surface water, resulting in sodium bicarbonate type water in all hydrologic units.