Fluvial Architecture of the Lower Williams Fork Formation (Middle Mesaverde Group), Douglas Creek Arch, Colorado

Ericka Harper, Rex Cole, and Matthew Pranter

The main coal-bearing and associated intervals of the upper Cretaceous middle Mesaverde Group in the Douglas Creek Arch, northwestern Colorado, are representative of marginal-marine to lower coastal-plain depositional environments. These strata serve as outcrop analogs to laterally equivalent deposits that form petroleum reservoirs in the adjacent Piceance and Uinta basins. The study interval has a relatively low net-to-gross ratio (<50% sandstone) and includes approximately 540 feet (164.6 meters) of sandstone and mudstone. Sandstones within the study area are mainly very fine to fine grained, and commonly exhibit ripple lamination and cross-stratification. Eleven facies were defined based on lithology and sedimentary structures. Paleocurrent data obtained from outcrops (N=172, vector mean=86.5°) indicate an overall easterly direction of sediment transport. Architectural elements include: 1) single-story channel body, 2) multi-story channel body, 3) crevasse splay, and 4) lacustrine. Gamma-ray data for each facies suggest no clear correlation based solely on sedimentary structure variability. Architectural-element gamma-ray analysis indicates that sandstones predominantly show lower gamma-ray values than siltstones; however, overlap of gamma-ray values between architectural elements may pose problems for interpretation based on well-log data alone. Five detailed measured sections were used to correlate sandstone bodies within the 0.5 square mile (1.3 square kilometer) study area in efforts to understand stratigraphic variability. Apparent widths, thicknesses, and geometries aided in identifying architectural elements. Channel bodies are generally more laterally continuous and thicker near the middle portion of the study interval, with multi-story channel bodies most commonly occurring above and below a potential sequence boundary. Crevasse splays commonly occur in the lower net-to-gross ratio portions of the interval. Comparison to existing data from Coal Canyon, near Palisade Colorado, shows that depositional environments and sandstone-body types are very similar, but dimensional data were less consistent. Architectural-element geometries, dimensions, orientations, and spatial variability can be used to constrain reservoir models for basins with similar depositional styles and reservoir types.

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