The Dakota sandstones of the Moxa Arch and surrounding Green River Basin remain attractive targets for explorationists in the Rockies. This regional stratigraphic study encompasses an 18,000 square-mile area includes Sublette, Lincoln, Uinta, Sweetwater, and portions of Teton and Fremont counties in Wyoming; portions or all of Summit, Duchesne, Daggett, and Uintah counties in Utah; and Moffatt County, Colorado. The project, representing more than seven man-years of geologic research, used 1318 wells to evaluate the sequence stratigraphic framework of the Dakota and Muddy formations in the Greater Green River Basin. Along the trend of the Moxa Arch, every well that penetrated the Dakota section was included in the study database. In the surrounding area, at least one well per section, where available, was selected for regional correlations.

      Stratigraphic picks, used to subdivide these formations, were correlated along laterally continuous marker horizons that represent transgressive disconformities or their equivalent flooding surfaces. This produced a time-stratigraphic framework for evaluating the distribution of sandstones and reservoirs. The thickness of gross and porous sandstone (based on reservoir-derived porosity) was determined for each stratigraphic interval. Five regional cross sections document the stratigraphic framework for the Dakota and Muddy formations. In addition, regional maps illustrate gross and net sandstone distribution, the occurrence of reservoir quality, and reservoir

development for the most significant stratigraphic intervals.

     Twelve outcrops and cores from 129 wells of the Dakota and Muddy formations in the Greater Green River Basin were described for this study. Lateral and vertical variations in lithologies as well as primary and biogenic sedimentary structures from this database were used to interpret the depositional sequences. This rock database illustrates the relationships among lithology, log response, porosity and permeability, and other log-derived measures of reservoir quality.

     The petrographic study of 546 thinsections from 50 cores provided data for interpreting textural and diagenetic characteristics of Dakota and Muddy reservoir lithologies. Specific emphasis of the petrographic study was to determine the compositions of the sandstones, define the diagenetic sequence, and to relate cementation to variations in porosity and permeability.

     Two field areas on the Moxa Arch were selected for the characterization of reservoirs from shallow-marine (Swan/Blue Forest area) and fluvial (Lucky Ditch/Henry area) depositional environments. These field studies provide a detailed analysis of reservoir quality and reservoir development. Data in these areas relate core-derived measures to log-derived parameters. Concepts derived in these field areas were applied to the regional evaluation of reservoir potential of the Dakota and Muddy formations in the Green River Basin.