Sandstone-Body Connectivity in a Meandering-Fluvial System: from Example from the Williams Fork Formation, Piceance Basin, Colorado

Nicholas K. Sommer¹, Matthew J. Pranter², and Rex D. Cole^3
1EnCana Oil & Gas (USA) Inc., Denver, CO
²Department of Geological Sciences, University of Colorado at Boulder, Boulder, CO
³Department of Physical and Environmental Sciences, Mesa State College, Grand Junction, CO

This study explores the static connectivity of fluvial deposits of the lower Williams Fork Formation (Late Cretaceous) of the Mesaverde Group in western Colorado (U.S.A.). The lower Williams Fork Formation is a relatively low net-to-gross ratio (< 50%) succession of approximately 700 feet of fluvial channel-fill sandstone bodies, crevasse splays, floodplain mudstones, and subordinate coal interpreted to have been deposited in a highly sinuous meandering river system in a coastal-plain setting. These deposits, as exposed within Coal Canyon, serve as an excellent reservoir analog; the strata dip gently eastward into the Piceance Basin where they produce natural gas. Static connectivity of these high-sinuosity meandering-fluvial sandstones is most sensitive to sandstone-body width; therefore width statistics are a critical input parameter to constrain 3-D numerical models of these reservoirs. For the lower Williams Fork Formation, the shape and orientation of the sandstone bodies have a less pronounced effect on static connectivity.

Three-dimensional architectural-element models constrained to outcrop-derived sandstone-body dimensional statistics show that connectivity is controlled largely by sandstone percent (net-to-gross ratio). Connectivity analyses reveal an S-curve relationship between net-to-gross ratio and connectivity at 160-acre well spacing. As well density increases, the S-curve relationship diminishes and connectivity increases linearly with increasing net-to-gross ratio. At dense well spacings, there are enough wells to intersect a high proportion of sandstones, regardless of sand content. Even at 10% net-to-gross, 10-acre spacing connectivity is typically above 60% and can be as high as 85%. Infill drilling scenarios increasingly intersect a higher percentage of crevasse splays.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado