About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

Earth Science Bulletin (WGA)


Earth Science Bulletin
Vol. 17 (1984), No. 1. (Annual), Page 104a

Abstract: Paleotectonic, Stratigraphic, and Diagenetic History of the Weber Sandstone in the Rangely Area

Mark Koelmel1

Rangely Field is situated in Rio Blanco County, Colorado, on a doubly plunging anticline of Laramide age. The Rangely structure is asymmetrical with the steepest flank to the southwest. The Permo-Pennsylvanian Weber Sandstone is the primary producing formation with cumulative production exceeding 670,000,000 bbls. The Weber is a subarkosic arenite deposited in an eolian regime. It interfingers with the alluvial Maroon Formation in the southern and southeastern portions of Rangely Field. Isopach maps of the Pennsylvanian-aged Formations suggest a paleotectonic platform in the Rangely area and a Permo-Pennsylvanian north-south trending arch west of the Laramide-age Douglas Creek Arch. Hydrocarbons migrated into the Rangely area prior to the Laramide Orogeny and were stratigraphically trapped at the Weber-Maroon transition zone. Subsequent Laramide structure localized the hydrocarbon accumulation.

Diagenetic history of the Weber Sandstone differs between the Uinta and Piceance basins. Weber diagenesis in the Uinta basin is dominated by silica precipitation and porosity appears to be residual primary. Weber diagenesis in the Piceance basin includes dissolution of detrital material and precipitation of a complex sequence of carbonate cements. Weber porosity in the Piceance basin appears to be both residual primary and secondary. The boundary between these two diagenetic regimes appears to coincide with a Pennsylvanian paleoarch.

A diagenetic model is proposed for the Rangely area. The model assumes a paleotectonic basin geometry consisting of a gently dipping western limb and a steeply dipping eastern limb. Silica precipitation commenced after Weber deposition throughout the Rangely area. Pre-Laramide salt tectonics may have caused sufficient faulting to permit fluid communication between the Eagle Valley Evaporites and the Weber Sandstone. Saline solutions from the Eagle Valley Evaporites migrated into the Weber Sandstone in the Piceance basin halting such precipitation and initiating precipitation of carbonate cements. Precipitation of silica continued in the Uinta basin. Development of secondary porosity in the Piceance basin occurred prior to, or simultaneously with oil migration.

Acknowledgments and Associated Footnotes

1 Mark Koelmel: Chevron, Inc.

© Wyoming Geological Association, 2015