Fluvial deposits of the Upper Williams Fork Formation contain a variety of geometries and facies that change both spatially and stratigraphically throughout the Piceance Basin. This study identifies that the geometrical variability of these fluvial sandstones is greater than the lithofacies variability within the sandbodies. The latter, and the lack of lateral continuity of the fluvial sandbodies, makes it difficult to identify sandbody types in subsurface. For this study, individual sandbodies were measured for thickness and apparent lateral extent in Rifle Gap, Colorado using a combination of GPS measurements, satellite photography, and stratigraphic sections. This dataset will later be integrated with a regional sequence stratigraphic framework and fracture characterization to better understand the controls on reservoir occurrence and quality in the eastern Piceance Basin. Outcrops in the eastern part of the Piceance basin are only miles away from producing wells, and thus highly interesting study location.
Four sandbody types have been recognized:
single-story, isolated channels;
multi-story channels; and
crevasse channels and splays. Types I through III represent potential hydrocarbon reservoirs; whereas Type IV appears insignificant for economic production. Lithofacies are dependent on sandbody type, but of relatively low variability, except for sandbody type IV. Type I sandbodies form ca. 57% of sandbodies present and contain little-to-no muds. Type II (5%) occurs when single-story channels amalgamate at the tips, forming a single, laterally-continuous and vertically-isolated sandbody with a thin highly-rippled sandstone/mudstone separating individual channels. Type III deposits (25%) are composed of laterally- and vertically-stacked channels, and form the majority of potential reservoir volume. These multi-story channels contain a significant mudstone component between individual channels which may act as baffle to fluid/gas flow and communication between sandstone compartments. Type I sandbodies are typically 6.3 m thick and 158 m wide; Type II = 10.8 m thick, 496 m wide; Type III = 21.3 m thick, 463 m wide; and Type IV = 1.9 m thick, 38 m wide.
The established relationships aid future exploration in the Piceance Basin by providing:
geometric relationships between thickness and sandbody type,
lithofacies percentages present within sandbodies, and (3) interpreted flow baffles found within potential reservoirs.