The U.S. Geological Survey (USGS) recently completed an assessment of in-place nahcolite (NaHCO₃) resources in the Piceance Basin, northwestern Colorado. Nahcolite is present in the oil shale deposits of the Parachute Creek Member of the Eocene Green River Formation. It occurs as disseminated aggregates, nodules, bedded units of disseminated brown crystals, and white crystalline beds associated with dawsonite (NaAl(OH)₂CO₃) and halite (NaCl). The nahcolite-bearing facies are divided into an unleached part containing the nahcolite and halite, which is estimated to be as much as 1,130 ft thick, and an upper leached part several hundred feet thick containing minor nahcolite aggregates and nodules. Locally, thick beds of halite and brown fine-grained nahcolite lie in the depocenter of the basin, but thin laterally away from the basin center and grade into beds of white, coarse-grained nahcolite. In the central part of the study area, the top of the nahcolite-bearing rocks range in depth from about 1,300 to 2,000 ft.

Dissolution of water-soluble minerals, mostly nahcolite and halite, in the upper part of the nahcolite-bearing facies has created a collapsed leached zone as much as 580 ft thick that consists of laterally continuous units of solution breccia and fractured oil shale containing solution cavities. The top of the leached zone is not yet defined in the basin, but it probably extends into the A groove in the upper part of the Parachute Creek Member.

First posted April 2, 2009