Stratigraphic architecture and connectivity of a low net-to-gross fluvial system: Combining outcrop analogs and multiple-point geostatistical modeling, lower Williams Fork Formation, Piceance Basin, Colorado

Fluvial deposits of the lower Williams Fork Formation (Mesaverde Group; Upper Cretaceous) form the main reservoirs at Grand Valley Field in the Piceance Basin of Colorado. In the eight-section study area, the lower Williams Fork Formation consists of approximately 1,500 ft (457 m) of mudstone with numerous isolated-to-amalgamated, lenticular-to-channel-form sandstone deposited by meandering river systems within a coastal-plain setting. Given the uncertainty in the geometry and distribution of the fluvial reservoirs, this study addresses the field-scale architecture and distribution of the fluvial deposits by combining outcrop-analog data with 3-D reservoir modeling using multipoint statistical simulation (MPS).;Log data from 328 wells and core data from 10 wells were described to determine key facies (N=12), facies associations, and architectural elements (point bar, crevasse splay, coal, floodplain). These observations and interpretations were compared to existing outcrop data and observations to evaluate stratigraphic variability within the area.;Core-to-log comparison was used to develop criteria to calculate lithology logs and interpret architectural elements from conventional well logs. Both point bars and crevasse splays form reservoir-quality sandstones. The range in thickness for point bars is 2-30 ft (0.6-9.0 m) with an average of 5.4 ft (1.6 m). Crevasse splays range in thickness from 0.5-20 ft (<0.2-6.1 m) with an average of 1.7 ft (0.5 m). For comparison, dimensional data from outcrops located approximately 20 mi (32 km) to the southwest show that point bars (N=116) range in thickness from 3.9-29.9 ft (1.2-9.1 m) and from 44.1-1699.8 ft (13.4-518.1 m) in apparent width, and crevasse splays (N=279) range in thickness from 0.5-15.0 ft (0.2-4.6 m) and from 40.1-843.3 ft (12.2-257.0 m) in apparent width.;Three-dimensional reservoir modeling was used to investigate three different scenarios, representing different spatial distributions of the fluvial deposits. A nested modeling approach was used to model sinuous channels that represent a "string-of-beads" pattern for comparison to individual, and partially disconnected point bars and crevasse splays that form isolated-to-amalgamated sandstones. Within modeled meander belts, MPS was used to model the detailed distribution of fluvial deposits to capture the inferred spatial patterns of the fluvial reservoirs. The differences between the scenarios were explored in terms of static and dynamic connectivity and dynamic fluid-flow simulation to address the potential impact on reservoir performance.