Abstract: Stratigraphic Variation in Porosity Types: Frontier Formation at Depths Greater than 10,000 Ft., Uinta and Sweetwater Counties, Wyoming, USA

VINOPAL ROBERT J.

Matrix porosity in the Frontier Formation consists of subequal amounts of microporosity, intergranular porosity, and dissolution porosity. Principal diagenetic parameters affecting porosity in Frontier sandstones include degree of silica cementation, pressure solution compaction, ductile grain compaction, presence-absence of clay coatings, and grain dissolution. Facies related porosity parameters are grain size, sorting and sandstone composition. Sandstone facies and resultant diagenesis show relationships that create a stratigraphically related variability in porosity types. Reduced intergranular porosity and microporosity occur in fine-medium grained marine and fluvial sandstones that contain illite grain coatings which inhibit silica cementation. Dissolution porosity, formed through leaching of rock fragments, is fairly well connected to the intergranular pore system. Fine to medium grained sandstones typically show clay blockage of pore throats or pervasive silica cement. Coarse grained fluvial sandstones that contain high percentages of contacting chert grains show good intergranular porosity and constitute the best potential reservoirs. Silica overgrowths are absent on the chert grains, but occur on monocrystalline quartz grains. Clays are minor in coarse grained sandstones that lack volcanic and argillaceous rock fragments. Pinch outs of coarse grained channel facies into finer grained facies that are silica cemented or pinch outs of clay coated sandstones into non-coated silica cemented sandstones are stratigraphic variables that influence Frontier trap geometry.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado