Geochemistry of the Green River Formation, Piceance Creek Basin, Colorado

Jeremy Boak and Jufang Feng
Colorado School of Mines

The Green River Formation of Colorado, Utah and Wyoming constitutes one of the richest and thickest petroleum source rock deposits in the world. Although Lakes Uinta and Gosiute, where these sedimentary rocks were deposited, were unusual, even extreme, in their chemistry, the rocks provide clues to the conditions required to produce and preserve such source rocks. We have analyzed major and trace element chemical data from current work (82 samples) and previous work by the U. S. Geological Survey (172 samples), integrating them with stratigraphic and mineralogical studies conducted at Colorado School of Mines. A rise in Na2O and Na/Al occurs in the basin center during the Transitional Lake Stage, coincident with sharp reduction in clay mineral content and rise in dawsonite and later nahcolite content. This change reflects a transition from mesosaline to hypersaline conditions. Na2O drops near the top of the Rapidly Fluctuating Lake Stage reflecting a dissolution zone in which nahcolite was removed across a stratigraphically transgressive surface. Na2O and Na/Al remain elevated because of the presence of authigenic albite. Dawsonite and quartz abundance decrease at the top of the R5 zone, reflecting increase in silica activity with continued hypersaline conditions. A wide variety of metal ratios and other chemical measures of redox potential devised mainly for marine systems support the long-held conclusion that the Colorado portion of Lake Uinta was stratified, with a deep zone (basin center samples) that was persistently at least dysoxic, and commonly anoxic, and a shallow zone (basin margin samples) that was less persistently dysoxic. Whereas a moderately strong correlation of Fe to Al indicates an origin in clastic constituents (clay and oxide/hydroxide minerals), calculated values of Ca/[Ca+Mg+Fe*] (where Fe* represents Fe not in pyrite) for basin center samples match carbonate mineralogy, and the Mg/[Mg+Fe*] indicate ferroan dolomite with XFe of ~0.2 as the dominant reservoir for Fe. Metal/Al ratios show enrichment with respect to the same ratio in average shale, in the following general order: Mo>Cu>U>V>Co. These data, along with thick organic rich zones and the presence of buddingtonite support the presence of chemical stratification, brackish-mesosaline-hypersaline conditions and dysoxia/anoxia from Garden Gulch Member onward. Potentially fresh water conditions found in part of the section in the basin margin may have been relatively local.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013