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