Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Stratigraphic Variability of Coastal-Plain and Marginal-Marine Deposits of the Middle Mesaverde Group, Douglas Creek Arch, Colorado
(1) Department of Geological Sciences, University of Colorado at Boulder, Boulder, CO.
(2) Physical and Environmental Sciences, Mesa State College, Grand Junction, CO.
The middle Mesaverde Group of the Douglas Creek Arch, northwestern Colorado, is represented by a succession of fluvial and marginal-marine strata that serve as outcrop analogs to laterally equivalent reservoirs in the Piceance and Uinta basins. The relatively low net-to-gross (<50% sandstone) interval includes ~770 ft (~230 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).
Sandstone bodies are very fine- to medium-grained, mature, and commonly exhibit ripples and cross stratification. Based on 656 paleocurrent values from sedimentary structures, the vector-mean azimuth is ~130°. Architectural elements include: 1) point bar/channel fill; 2) crevasse-splay; 3) discrete flood body; 4) tidal barform; 5) foreshore; 6) lacustrine delta; and 7) middle shoreface. The base of the interval is approximately 205 ft (63 m) above the top of the upper Sego Sandstone.
The interval can be divided into three major units based on architectural element stacking patterns. Unit one (0-200 ft [61 m]) is composed of isolated to amalgamated coastal-plain fluvial-channel deposits, and represents an overall lowstand systems tract based on identified sequence boundaries. Unit two (200-360 [61-109.8 m]) is composed of estuarine and marine-embayment deposits, and represents an overall transgressive systems tract based on an overall retrogradational stacking pattern and an identified maximum flooding surface. Unit three (360-770 ft [109.8-230 m]) is composed of open-marine to coastal-plain deposits, and represents and overall highstand systems tract based on an overall progradational stacking pattern.
Sequence-stratigraphic framework and detailed architectural-element characteristics are useful for subsurface correlation and mapping in the Piceance/Uinta basins (producing fields as close as 15 mi [24 km]) and in other basins with similar deposits and reservoirs. Major sequence-stratigraphic surfaces (sequence boundaries, maximum flooding surfaces) can be used to identify subsurface units synonymous with the units identified in the field. These data and relationships can be used to constrain reservoir models in terms of observed architectural element spatial variability, dimensions, geometries, and orientations.