About This Item
Share This Item
Rangely field is located in Rio Blanco County, Colorado, on a Laramide doubly plunging anticline asymmetrical to the southwest. The Pennsylvanian Weber Sandstone is the primary producing horizon, with cumulative production exceeding 650,000,000 bbl. The Weber is a sub-arkosic arenite deposited in an eolian regime. It interfingers with the alluvial Maroon Formation in the southern and southeastern portions of Rangely field. Lower Pennsylvanian stratigraphy suggests a paleotectonic high in the Rangely area. Hydrocarbon migration into the Rangely area probably occurred prior to the Laramide with stratigraphic entrapment at the Weber-Maroon transition. The subsequent Laramide structure localized the hydrocarbon accumulation.
Diagenetic history of the Weber Formation differs between the Uinta and Piceance basins. Weber diagenesis in the Uinta basin is dominated by silica precipitation; porosity appears to be residual primary. Weber diagenesis in the Piceance basin includes dissolution of matrix material and precipitation of a complex sequence of carbonate cements. Weber porosity in the Piceance basin appears to be secondary. The boundary between these two diagenetic regimes seemingly coincides with the present Douglas Creek arch.
A diagenetic model is proposed for the Rangely area. The model assumes a paleotectonic high in the Rangely area or early expression of the Douglass Creek arch. Silica precipitation commenced after Weber deposition throughout the Rangely area. Pre-Laramide expression of the Grand Hogback may have caused sufficient faulting to permit fluid communication between the Eagle Valley Evaporites and the Weber Formation. Saline solutions from the Eagle Valley Evaporites had sufficient head or diffusion drive to replace the Weber Formation fluid in the Piceance basin. The saline solutions halted silica precipitation in the Piceance basin and initiated precipitation of the carbonate cements. Precipitation of silica continued in the Uinta basin. Development of secondary porosity in the Piceance ba in occurred prior to or simultaneously with oil migration.
End_of_Article - Last_Page 1346------------