Stratigraphic measured section of the Upper Cretaceous Mancos Shale (upper part) and Mesaverde Group (lower part), Moffat County, Colorado

Abstract

Twelve sections of the uppermost part of the Mancos Shale and the lower part of the Mesaverde Group (lies Formation) of Late Cretaceous age have been measured in a linear distance of about 50 mi (83 km) in northwestern Colorado as part of a study of depositional environments of impermeable (tight, <0.1 md) gas-bearing reservoirs in the Sand Wash basin of the Greater Green River Basin* This study focused on interpretation of facies based on lateral and vertical associations of rock types, textures, sedimentary structures, contact relations, and faunas. The depositional model derived from this study (Kiteley, 1979) indicates that most reservoirs shallow shelf sand bars, barrier beaches, delta margin sands, distributary and fluvial channels -are discontinuous but nevertheless systematic and predictable, and that reservoir quality is related to depositional environment. Most gas production in the Sand Wash basin has been from stratigraphic traps on small anticlines at shallow depths. This production is only marginally economic because of low permeability and lenticularity of reservoirs. Gas production from these reservoirs has been improved somewhat by hydraulic -fracturing in the report area. Nuclear fracturing in the nearby Piceance Creek basin has been generally unsuccessful. The present study shows that quality and distribution of the producing sand bodies can be predicted so that exploration can be better focused. Moreover, reservoir characteristics such as geometry, distribution of pore space, and barriers influencing fluid flow, which are needed in enhanced recovery programs, can be delineated in the subsurface based on the stratigraphic data presented herein (Kiteley, 1979).