--> --> Abstract: From Outcrop-to-Subsurface: The Regional Sequence Stratigraphic Framework and Facies Distribution Within the Lower Williams Fork Formation, by Sarah Edwards and Jennifer L. Aschoff; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

From Outcrop-to-Subsurface: The Regional Sequence Stratigraphic Framework and Facies Distribution Within the Lower Williams Fork Formation

Sarah Edwards1; Jennifer L. Aschoff1

(1) Geology Dept., Colorado School of Mines, Golden, CO.

Exploration and production of tight-gas in the Piceance Basin, CO are plagued by low reservoir permeability and high reservoir heterogeneity. Much of the basin produces at 10-20 acre spacing with wells requiring hydraulic fracturing. Identifying zones with higher than average permeability and less heterogeneity is crucial to the success of these gas fields. Regional stratigraphic analysis provides a predictive framework to place depositional facies that can control tight-gas reservoir characteristics. We present a new outcrop-to-subsurface dataset and regional correlation through the Rollins Sandstone Member of the Iles Formation to lower Williams Fork Formation. The dataset consists of 16 stratigraphic profiles, >100 well logs and 9 outcrop gamma-ray curves. Depositional environments in this interval are recorded by 10 lithofacies and reveal facies diversity contributing to reservoir heterogeneitiy in the basin. Lithofacies are interpreted as coastal plain-paludal, fluvial- distal flood plain, marine shoreface, marine foreshore-upper shoreface, estuarine, tidally influenced fluvial, fluvial (proximal flood plain- upper point bar), fluvial (shallow, high sediment supply meandering channel), meandering fluvial (main channel-belt), and anastomosed fluvial channels. Facies interpretations are consistent with previous work, and suggest an overall stratigraphic trend from marine shoreface to coastal plain/estuarine deposits in the lower part of the succession to fluvial/flood plain depositional environments in the upper part. Potentially higher reservoir quality was identified in 3 lithofacies in (1)marine shoreface, (2)tidally influenced deposits, and(3)anastomosed fluvial channels. Shoreface deposits are typical of RSM, found primarily in eastern basin, thinning westward. Tidal deposits are found above the RSM within a coastal/paludal zone, associated with carbonaceous shale, tidal bars and channels in the lower WFF and in stratigraphically higher tidal-fluvial facies.Anastomosed fluvial deposits are dominantly in the upper zone of the lower WFF. 2 regionally extensive flooding surfaces were identified and correlated utilizing extensive open marine and tidal facies that rest above fluvial facies. Regional correlations of the RSM through lower WFF show a strong geographic, as well as stratigraphic partitioning of facies within a single stratigraphic unit. These regional facies distributions may be related to subsidence patterns in the Piceance basin.