Basin-scale Sequence Stratigraphy and Distribution of Depositional and Mechanical units in the Middle and upper Williams Fork Formation, Piceance Basin, Colorado

Michele Wiechman and Jennifer Aschoff
722 S Beech St, Lakewood, CO

The Piceance Basin, northwest Colorado, is home to one of the most important basin-centered tight-gas accumulations in North America. A wide range of geologic controls are responsible for the variation in gas production from the heterogeneous, low-permeability reservoirs, but these are not well understood. This study focuses on potential stratigraphic and mechanical controls on tight-gas sandstone reservoirs in the middle and upper Williams Fork Formation by building a regional sequence-stratigraphic framework integrating outcrop and subsurface data. The database includes detailed stratigraphic profiles (12), outcrop gamma-ray profiles (10), two cores, detrital mineral compositional changes, detailed facies (27) and facies associations (8) descriptions and interpretations, paleocurrent data and well-logs. Fluvial facies are laterally extensive with channel type variations throughout the basin while marine and tidal facies show lateral discontinuity in the northwestern sections of the Basin. Five facies associations have been identified as having the best reservoir potential based on their internal heterogeneity and lateral extent, (1) high-sinuosity, meandering fluvial, (2) isolated, low-sinuosity anastomosed fluvial, (3)tidally influenced fluvial channels, (4) regressive marine shoreline, and (5) transgressive marine shoreline barrier system. Natural fractures, important components of production from fluvial sandstones within the Williams Fork, were indentified in six facies. These associations were (1) high-sinuosity, meandering fluvial, (2) isolated, low-sinuosity anastomosed fluvial, (3) undifferentiated floodplain, (4) tidally influenced fluvial, (5) estuarine systems, and (6) transgressive marine shorelines. Fractures seem to be controlled by facies composition and bedding character. Fracture and fracture swarm spacing is relatively proportional to bedding thickness, with thicker beds showing higher fracture spacing then thinner beds. Sandstones with high fractures concentrations are also found to have cementation trends that increase the sandstones brittleness.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013