Sequence-stratigraphic controls on reservoir-scale architecture of the middle Mesaverde Group, Douglas Creek Arch, Colorado
Kimberly Hlava, Matthew Pranter, and Rex Cole
The middle Mesaverde Group of the Douglas Creek Arch, northwestern Colorado, is represented by a complex succession of fluvial to marine strata that serve as outcrop analogs to laterally equivalent natural gas reservoirs in the Piceance and Uinta basins. The interval includes ~380 ft (~115.9 m) of mudrock, coal, and sandstone within the lower (Kmvl) to main coal-bearing (Kmvc) intervals of the Mesaverde Group (equivalent to the upper Iles and lower Williams Fork formations of the Piceance Basin). Based on 2,488 ft (758.5 m) of measured section, facies associations include: (1) coastal plain; (2) estuarine; (3) lagoon; and (4) shallow marine. Nine architectural elements are identified and include: (1) channel bodies; (2) crevasse splays; (3) discrete-flood bodies; (4) a bayhead delta; (5) an estuarine assemblage; (6) lagoonal foreshores; (7) tidal barforms; (8) middle shorefaces; and (9) washover fans. The study interval records an overall transgression from coastal-plain to shallow-marine facies associations followed by a regression from shallow-marine to coastal-plain facies associations and fit within the larger sequence-stratigraphic framework derived by previous studies in the Piceance, Uinta, and Sand Wash basins. Paleocurrents vary between depositional environments; however, the vector-mean orientation is 177 deg (N=485). Thirty-eight coastal plain sandstone bodies were evaluated in detail. Channel bodies have a mean apparent width (W) of 287.7 ft (87.7 m), and a mean thickness (T) of 4.9 ft (1.5 m) and are larger than crevasse splays (W=90.5 ft [28.0 m]; T=1.8 ft [0.5 m]) and discrete-flood bodies (W=61.5 ft [18.8 m]; T=1.6 ft [0.5 m]). The stratigraphic variability of architectural elements and net-to-gross ratio (N:G) is directly associated with the sequence-stratigraphic framework. High N:G intervals characterized by amalgamated channel bodies lie above sequence boundaries and fine upward into low N:G intervals characterized by isolated coastal-plain architectural elements. Late transgressive to early regressive strata create moderate to low N:G characterized by estuarine, lagoon, and shallow-marine architectural elements. The lowest N:G interval contains the maximum flooding surface.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012