Stratigraphic relationships between the Webber Sandstone and the Maroon Formation, Northwestern Colorado, US

The study of interbedded eolian and fluvial deposits, their inherent heterogeneities, and the impact on reservoir quality are the main subject of this research. The analysis of more than 2000 feet of core from four wells along with petrophysical data from 50 wells in the Rangely oil field, northwestern Colorado, allowed the characterization of textural and diagenetic parameters which ultimately control the reservoir quality. Compositional and compactional parameters were determined for the reservoir lithologies of both facies. Dune deposits lithology include subarkosic sandstones, arkose and lithic arkose sandstones. They have an architectural assemblage of F70M1C17P12 and major grain ratios of Q77F16R7. Fluvial channel deposits show an assemblage of F79M3C14P4 with a major grain ratio of Q64F23R13. Textural and compositional parameters clearly show that eolian dune sandstones are more mature texturally than their fluvial counterparts. These textural differences influenced the path of porosity loss processes and controlled the performance and quality of the reservoir. Quantification of compactional parameters as Intergranular Volume (IGV), Compactional Porosity Loss (COPL), Cementational Porosity Loss (CEPL) and Index of Compaction (Icomp) enable the determination of the best reservoir facies. IGV values for eolian dune deposits average 26.7%, with COPL and CEPL averaging 24.8%, 11.1% and 0.69, respectively. Sand sheet sandstones show average values of 25.4%, 26% and 14% with an Index of Compaction (Icomp) in the order of 0.6 Average values for fluvial deposits regarding the same porosity loss and compactional parameters result in an IGV=16.8%, COPL=33.3%, CEPL=9.4 and Icomp=0.8 The vertical pattern and relative proportions of eolian and fluvial facies allowed the subdivision in two intervals designated informally as Upper and Lower Weber. At a reservoir scale the thickness and spatial distribution of contrasting eolian and fluvial textures as well as their locally relative proportions ultimately determined the zones of better quality in the area of Rangely Field. A steady thickening trend of eolian deposits is observed in the informal Upper Weber. Conversely, a thinning and regular thinning trend is apparent in fluvial facies. Lack of data in the north and northwestern zones prevented a determination of spatial trends for the Lower Weber.