Sequence-stratigraphic controls on reservoir-scale architecture of the middle Mesaverde Group, Douglas Creek Arch, Colorado.

摘要

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 relatively low net-to-gross (N:G) (50% sandstone) 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).;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) foreshores; (7) tidal barforms; (8) middle shorefaces; and (9) washover fans. Based on 480 paleocurrent values from sedimentary structures, the vector-mean azimuth is approximately 130°. The stratigraphic study interval reveals two depositional sequences, which record a retrogradation followed by a progradation. Based on 38 sandstone-body measurements, channel bodies have an apparent width (W) of 287.7 ft (87.7 m), and thickness (T) of 4.1 ft (1.3 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=2.0 ft [0.6 m]). Facies, facies associations, and architectural elements are more diverse in the study interval (Kmvl-lower Kmvc) as compared to previous studies completed in Coal Canyon, Colorado. Sandstone bodies are larger in Coal Canyon by almost 50%. Based on thin section analysis, the relative reservoir qualities of foreshore and middle shoreface architectural elements are good to excellent. Net-to-gross ratios (N:G) in the stratigraphic study interval show direct ties to the sequence-stratigraphic framework and provide a predictive tool for subsurface reservoir characterization. High N:G ratios lie above sequence boundaries within the early lowstand systems tract and fine upward. Low N:G ratios are present within the late lowstand systems tract. Moderate N:G ratios are present within the transgressive and early highstand systems tracts.