Joint Meeting: University of Wyoming Department of Geology and Geophysics Wyoming Geological Association Geological Survey of Wyoming: April 2-4, 1982 Laramie, Wyoming: Subsurface Practices in Geology and Geophysics Abstracts of Papers - Compiled by James R. Steidtmann

Reservoir quality in a cored interval of the Frontier Formation from the Big Horn Basin is controlled principally by sandstone composition and clay authigenesis. Optimum porosity and permeability occur in tidal channel sandstones (11.3% and 0.24 md) and in foreshore and uppermost shoreface sandstones at the tops of progradational shoreline sequences (10.7% and 0.87 md). Both porosity and permeability systematically decrease with depth in each progradational sequence. Decreased grain size, sorting, and grain sphericity in lower and middle-shoreface sandstones are accompanied by an increase in the ratio of ductile, lithic to quartzofeldspathic components and by the introduction of detrital clay. Ductile grain deformation is the major agent of porosity destruction in lithic-rich sandstones.

Interangular porosity in foreshore and tidal-channel sandstones has been preserved by the precipitation of chlorite pore linings which have inhibited the formation of quartz overgrowths. Significant secondary porosity has also been generated by dissolution of detrital plagioclase. In addition to ductile grain deformation, intergranular porosity has been destroyed by the precipitation of illite and mixed-layer illite/smectite, chlorite, calcite, potassium feldspar overgrowths, zeolites, microcrystalline quartz, and quartz overgrowths. The distribution and abundance of these phases and of secondary porosity is, in part, related to detrital composition and to the timing or rate of the particular diagenetic event relative to that of ductile grain deformation. Triangular pore-classification diagrams and semilog cross-plots suggest formation permeability to be principally a function of total porosity, pore type, and type and abundance of authigenic clay.