Comparative petrology of Tertiary sandstones of southern Piceance Creek Basin, Colorado
— Allan M. Ochs and Rex D. Cole

Comparative petrographic analyses of channel-form and tabular sandstone bodies in the upper Wasatch Formation (Paleocene- Eocene) and lower Green River Formation (Eocene) in the southern Piceance Creek basin show that compositional and textural variability reflects the environment of deposition.

Sandstones from the sampled intervals are generally similar in texture and composition but have varying concentrations of (1) angular to well-rounded monocrystalline quartz grains, some with abraded overgrowths; (2) fresh and slightly altered potassic and sodic feldspars; and (3) volcanic-lithic fragments, mostly andesite. Wasatch sandstones generally contain slightly more lithic fragments than those of the Green River, which are more quartzose. The difference is attributed to the fluvial mode of deposition recognized in the Wasatch Formation which contrasts with the marginal-lacustrine nature of the sampled portion of the Green River Formation. Lacustrine sandstone also commonly contains accessory analcime and pyrite.

Paleocurrent data suggest a south, southwest, and southeasterly source for the lower Tertiary sediment. Petrographic evidence further suggests that the source terranes were compositionally consistent during the development of the lake and that the sediment was primarily derived from Mesozoic and Paleozoic sedimentary rocks and early Cenozoic silicic volcanics and intrusives.

This paper describes lacustrine and fluvial sandstones of the Wasatch and Green River Formations as they outcrop at several localities along the southern margin of the Piceance Creek basin. The description combined with petrographic analyses of thin- sectioned samples provide a basis for determining the provenance of the sand, as well as a crude comparison of compositional and tex-tural features that resulted from differences in depositional pro-cesses.

Sandstones in marginal lacustrine settings in the Wasatch and Green River Formations are proven reservoirs for significant hydro-carbon reserves in the Uinta and Piceance Creek basins. Alta-mont-Bluebell field in the northern part of the Uinta Basin is a major field discovery with an estimated ultimate recovery of 250 million barrels (40 billion liters) (Lucas and Drexler, 1975). The trapping mechanism is stratigraphic in nature, comprising stacked lacustrine and fluvial sandstone bodies that are sealed by an updip prograding sequence of alluvial facies composed of red siltstones, shales, and sandstones. Similar traps are found in the Piceance Creek basin, although not as large as Altamont-Bluebell. Although the producing sandstones are fractured, production is greater from lacustrine sandstones than from fluvial sandstones. This sugests that lacustrine and fluvial sandstone porosity and permeability may be related to compositional and textural modifications of the sediment during deposition.