Reservoir-Scale Analysis of Sandstone-Body Dimensions in a Braided Fluvial System, Upper Williams Fork Formation, Main Canyon, Piceance Basin, Colorado

Sommer, Nicholas K.¹, Matthew J. Pranter², Rex D. Cole³ (1) University of Colorado at Boulder, Boulder, CO (2) University of Colorado, Boulder, CO (3) Mesa State College, Grand Junction, CO

This study presents the results of an outcrop analysis of the Upper Williams Fork Formation (Late Cretaceous) of the Mesaverde Group located in Main Canyon, western Colorado (U.S.A.). The exposures within Main Canyon are stratigraphically analogous to the upper-middle portion of the undifferentiated high net-to-gross ratio interval of the Upper Williams Fork Formation that forms petroleum reservoirs within the Piceance Basin. The sandstone-body characteristics and architecture were evaluated using high-resolution aerial lidar, digital orthophotographs, photomosaics, and field measurements.

The Upper Williams Fork Formation in Main Canyon is characterized by sheet-like sandstone bodies and associated mudrocks that include planar-to-trough cross-stratified sandstone, convolute-to-wavy-laminated siltstone, and calcareous, fissile, silt-dominated shale. These sandstones bodies are interpreted to have been deposited in a low-sinuosity sand-dominated, braided alluvial plain setting.

Fluvial sandstone bodies include single channels to multiple-story-channel complexes with dimensions ranging from less than one meter to tens of meters in thickness (or composite thickness) and tens to hundreds of meters in apparent width. The sheet-like sandstone bodies typically have apparent width-to-thickness ratios (W:T) of greater than 30:1. Between-well correlation of individual sandstone bodies at a reservoir scale is difficult. Amalgamation of sandstone bodies is common in this high net-to-gross system; however, thin laterally extensive interbedded mudrocks exist that could compartmentalize sandstone bodies at the reservoir scale.

The sandstone-body dimensional and descriptive data of the Williams Fork Formation are useful to constrain reservoir models and aid in predicting the width and lateral continuity of sandstone bodies between wells when only thickness data are known.