Preliminary Facies Analysis, Regional Sequence Stratigraphy and Distribution of Stratigraphically controlled Mechanical Units of the middle and upper Williams Fork Formation, Piceance Basin, CO

Michele Wiechman and Jennifer Aschoff

The Piceance Basin, northwest Colorado, is home to one of the most important basin-centered tight-gas accumulations in North American. 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. Outcrop-to-subsurface stratigraphic correlation and detailed analysis of facies can elucidate potential stratigraphic controls on geographic and stratigraphic zones with better production. The objectives of this project are to: (1) characterize the specific types of fluvial and marine facies in the middle and upper Williams Fork Formation, (2) delineate the regional distribution of, and transitions between these depositional facies, (3) determine provenance of key facies (4) assign potential mechanical properties to these facies based on fracture types, spacing and abundance within each facies, and (5) disentangle potential relationship between depositional and mechanical properties of the facies that may control gas production. Here, we present the first phase of the project including preliminary regional sequence-stratigraphic framework for the middle and upper Williams Fork Formation. The database focuses on outcrop data and consists of 12 new detailed (10cm scale) measured sections, 7 published sections, 9 outcrop gamma-ray profiles, detailed facies and channel geometry descriptions, 3D channel dimension analysis, paleocurrent data and well-logs that were used to build regional cross-sections that identify regional stratigraphy. The Gazzi-Dickinson point-counting method was used on 15 thin sections from core to determine provenance. Thirty-one lithofacies were distinguished and grouped into six assemblages: (1) high-sinuosity, meandering fluvial, (2) isolated, low-sinuosity anastomosed fluvial, (3) tidally influenced fluvial, (4) estuarine, (5) regressive marine shoreline, and (6) transgressive marine shoreline barrier systems. The fluvial facies tend to be laterally extensive with variations in channel type throughout the basin while marine and tidal facies show lateral discontinuity in the northwestern sections of the Basin. Initial observations suggest that the anastomosed fluvial and marine shoreline facies have the most fractures with varying intensity in the facies. Future work will build on the array of detailed facies, and sequence-stratigraphic context to understand relationships between facies, fractures, and production from tight-gas sandstones.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012