Genesis of the saline waters of the Green River Formation, Piceance Basin, northwestern Colorado

Abstract

The northern Piceance Basin of northwestern Colorado is noted for vast reserves of kerogenaceous dolomite and highly saline sodium bicarbonate ground water. The local formations are the Uinta Formation, a silty sandstone, the Green River Formation, containing oil shale and a sequence of saline minerals, mainly nahcolite and halite, in its Parachute Creek Member, and the Wasatch Formation. The Uinta Formation is a poor aquifer, the Parachute Creek Member is a variable aquifer which is most permeable in areas of fracturing and/or zones of leached saline minerals. The ground-water flow pattern is from recharge areas in the southern highlands to discharge areas on the northern border of the basin. The hydrologic system consists of two aquifers separated by the Mahogany Zone confining layer of the Parachute Creek Member. The upper aquifer contains two relatively fresh water quality zones, one in the Uinta Formation and one in the upper part of the Parachute Creek Member. The lower aquifer contains fresh and saline water high in trace constituents, notably fluoride and boron. The lower aquifer is the source of high concentration (up to 50,000 mg /1 dissolved solids) brines found in the northern part of the basin. Past analyses of water quality assumed that the lower aquifer contained a sodium bicarbonate brine over most of Piceance Basin. Much of the information for this view came from wells left open to zones of soluble saline minerals. Careful sampling of drilled wells reveals a much more restricted area of brine occurrence. A large fracture zone on the northern rim of the basin is believed responsible for the present areas of brine discharge. The fractures act as vertical "planar wells" allowing present day continuous dissolution of saline minerals.