The Relative Roles of Channel Types and Facies for Reservoir Characterization in Fluvial Tight-Gas Sands, Upper Williams Fork Formation, Piceance Basin, Colorado

McDowell, Bryan; Plink-Bjorklund, Piret

Fluvial sandstones in the Upper Williams Fork Fm show a variety of geometries, sedimentary facies, and amalgamation styles important for hydrocarbon production in the Piceance Basin. This study is based on measuring stratigraphic sections, acquiring outcrop gamma ray curves, and measuring lateral extent using GPS data in Rifle Gap and Coal Gulch, CO. Outcrop gamma ray logs are used for comparison with subsurface logs. Paleoflow data help correct for apparent vs true channel width, and determine local drainage patterns. This high-resolution dataset will be integrated with a regional sequence stratigraphic and fracture dataset for a comprehensive understanding of the Upper Williams Fork Fm and the eastern Piceance Basin. Four sandbody types have been recognized: (1) single-story isolated channels, (2) laterally-amalgamating channels, (3) multi-story channels, and (4) small single-story channels/crevasse splays. Sandbody types 1-3 are potential reservoirs, whereas type 4 is evaluated to be too small for economic production. Type 1 sandbodies (60% of sandbodies) are medium to very fine grained, structureless to cross-stratified sandstones with little-to-no muds. Type 2 (10%) consists of similar sandstones, but 0.5-1.5 m mud layers dissect the sand-rich intervals. Type 3 (20%) facies are similar to type 2. Type 4 (10%) is fine to very fine grained, structureless to cross-stratified and thinly-laminated sandstone, with little-to-no muds. Sandstone facies variability is relatively low within the identified sandbody types, and thus less significant, compared to the channel dimensions and amalgamation style. In contrast, differences in mud distribution and architecture play a substantial role in pressure communication between sand-rich intervals within individual wellbores and adjacent wells. Stratigraphically, type 3 sandbodies are volumetrically more significant in the lower part of the section. Type 1 occurs throughout the section, but most commonly in the Middle and Upper Williams Fork. Types 2 and 4 do not have a clear relationship to stratigraphic position at the studied localities. Geographically, type 3 is more common in western part of the basin, whereas type 1 in the Rifle Gap area. This new understanding of geometry, architecture, and occurrence of sandbody types aims to help guide future drilling within Piceance fields, as well as similar fluvial tight-gas sand plays.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013