RESERVOIR CHARACTERIZATION OF THE MID-CRETACEOUS CEDAR MOUNTAIN AND DAKOTA FORMATIONS, EASTERN UTAH

In the southern Uinta basin of eastern Utah, fluvial channel sandstones of the Neocomian-Cenomanian Cedar Mountain and Dakota (CMD) formations are economic gas reservoirs. Recovery of gas from these formations, however, has proven challenging due to the complex stratigraphic relationships between these formations, as well as local variability in reservoir-sandstone thickness, distribution, and quality. In order to evaluate the physical characteristics of potential reservoir sandstones, we constructed six photo-mosaics of individual CMD fluvial channel complexes, measured 13 vertical sections with accompanying outcrop gamma-ray logs, and recorded >1,100 paleocurrent orientations. Outcrop data were compared with subsurface geophysical/petrophysical and production data from the southern Uinta basin to evaluate possible ties between channel sandstone attributes and hydrocarbon occurrence. CMD channel deposits consist of upward-fining conglomerates and coarse- to fine-grained sandstones containing trough- and planar -cross stratification, horizontal stratification and ripple-cross lamination. Paleocurrent orientations indicate an overall NE direction of paleo-flow. The geometry of internal bounding surfaces indicate deposition by in-channel dunes and both lateral- and downstream-accreting marcroforms. In the Cedar Mountain Formation, channel sandstone complexes are 100-1300 m wide and are up to 15 m thick. Width/Thickness ratios range between 10-100. In the Dakota Formation, individual channel sandstones are 200-1000 m wide and 8-15 m thick. Individual channel forms, however, form amalgamated sheet-like bodies that are up to 1.6 km wide and 30 m thick. These amalgamated sand bodies have W/T ratios of 100-125. Based on our evaluation of both outcrop and subsurface data, the primary controls of economic gas accumulations that occur within the CMD stratigraphic interval are depth, stratigraphic position, and reservoir architecture. Secondary controls such as structure, fracturing, and diagenesis also contribute to the occurrence of economic gas accumulations. The lowest risk intervals to pursue in the study area are the amalgamated sheet sandstones of the middle Cedar Mountain Formation and the basal Dakota Formation.