Rangely Field is situated in Rio Blanco County, Colorado, on a doubly plunging anticline of Laramide age. The Rangely structure is asymmetric with the steepest flank to the southwest. The Pennsylvanian-Permian Weber Sandstone is the primary producing formation, with cumulative production exceeding 670 million bbl. The Weber is a subarkosic arenite deposited in an eolian regime. It interfingers with the alluvial Maroon Formation in the southern and southeastern part of Rangely Field. Isopach maps of the Pennsylvanian formations suggest a paleotectonic platform in the Rangely area and a Pennsylvanian-Permian north-south-trending arch west of the Laramide Douglas Creek arch. Hydrocarbons migrated into the Rangely Field area prior to the Laramide orogeny and were stratigraphi ally 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 involved dissolution of detrital material and precipitation of a complex sequence of carbonate cements. Weber porosity in the Piceance basin is both residual primary and secondary. The boundary between these two diagenetic regimes coincides with a Pennsylvanian paleoarch. The diagenetic model proposed for the Rangely area 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 tect nics 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 in the Piceance basin halting silica 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.