AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Stratigraphic Characterization and Modeling of Fluvial Deposits of the Lower Williams Fork Formation Combining Outcrop Analogs and Multipoint Statistical Simulation (MPS), Grand Valley Field, Piceance Basin, Colorado
(1) Geological Sciences, University of Colorado at Boulder, Boulder, CO.
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 8-section study area, the lower Williams Fork Formation consists of approximately 1500 ft (457 m) of mudstones with numerous isolated-to-amalgamated, lenticular-to-channel-form sandstones 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 330 wells and 10 cores were described to determine key facies (eleven), facies associations, and architectural elements (point bar, crevasse splay, coal). 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 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 two “end-member” scenarios, representing different spatial distributions of the deposits. A nested modeling approach was used to model sinuous channels that represent a “sting of beads” pattern for comparison to individual 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 deposits. The differences between the scenarios are explored in terms of static and dynamic connectivity and address the potential impact on reservoir performance.