ABSTRACT: Evolution of Porosity in "Deep" Sandstones of the Permian Upper Part of the Minnelusa Formation, Powder River Basin, Wyoming

C. J. Schenk, R. M. Pollastro, J. W. Schmoker

Analysis of intergranular volume in deep (>15,000 ft) sandstones of the Permian upper part of the Minnelusa Formation has demonstrated that porosity loss in these sandstones is largely a function of cementation rather than mechanical compaction or pressure solution. Intergranular volume, defined as the sum of porosity and cements, averages 42% of the rock volume, which approximates the initial (depositional) porosity of eolian and related sands. Quartz cement averages 49% of the intergranular volume, dolomite averages 35%, and porosity averages 13%. Quartz and dolomite cementation are the main cause of porosity loss. The few relatively low values of intergranular volume indicate that compaction occurred locally, possibly owing to a lack of early quartz cement.

Porosity averages 5% of the whole-rock volume and ranges up to 23%. Secondary porosity is present locally, formed by the dissolution of anhydrite cement and potassium feldspar. Sandstones with porosities greater than 8% have a component of porosity formed by anhydrite or feldspar dissolution. Sandstones with porosities less than 8% are those with high percentages of quartz cement. The porosity is widely spaced and isolated by extensive quartz overgrowths.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990