--> ABSTRACT: Mineralogy and Geochemistry of the Parachute Creek Member of the Green River Formation, Piceance Basin, Colorado, U.S.A

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Mineralogy and Geochemistry of the Parachute Creek Member of the Green River Formation, Piceance Basin, Colorado, U.S.A

Malicse, Ariel 1; Inan-Villegas, Esra 2
(1) Shell Exploration & Production Co., Houston, TX. (2) Shell Exploration & Production, Houston, TX.

The study describes the Colorado oil shale using core samples in the Piceance Basin, Colorado, U.S.A. The Parachute Creek Member consists of mineral zones that reflect the chemical conditions prevailing in the ancient lake. The mineral zones identified: 1) calcite-analcime zone, 2) illite zone, and 3) dawsonite-saline zone.

The calcite-analcime zone is a high TOC interval characterized by high amounts of calcite and analcime but low amounts of quartz and potassium feldspar. Albite increases with depth in this zone. The underlying Illite zone is a relatively lean, low TOC interval. This zone is characterized by interbedded brecciated and non-brecciated beds. The mineralogy is characterized by high amounts of illite, potassium feldspar, albite, and pyrite; analcime and dawsonite are absent. The illite zone contains anomalously high amounts of Fe, Cr, Zr, and several chalcophile trace elements.

The Illite zone marks a major stratigraphic change from the underlying dawsonite-saline zone. Lake conditions were marked by long periods of increased stream runoffs followed by periods of relatively high salinity (up to gypsum saturation). Periods of increased runoff are characterized by lean dolomicrite with detrital illite, whereas, periods of higher salinity are characterized by more organic-rich strata with gypsum pseudomorphs. The abundance of pyrite and metals can be correlated to the mobility of metal-bearing particulates in relation to the redox conditions in the lake column.

The dawsonite-saline zone is a high TOC interval distinguished by the abundance of dawsonite, nahcolite and quartz. Potassium feldspar and albite are common; the abundance of albite and dawsonite are inversely correlated. Vugs lined by pyrite are common.

The kerogen content, mineralogy, and elemental abundances in each mineral zone were strongly influenced by the Eh-pH, salinity, and PCO2 in the ancient lake. For example, minerals in the calcite-analcime zone reflect stratified meromictic lake with low PCO2, which precluded the precipitation of nahcolite and dawsonite. The buildup of high PCO2 was prevented by the periodic overturning of the water column.

 

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.