PETROGRAPHIC AND PRIMARY POROSITY ANALYSIS OF SELECTED CORE SAMPLES FROM THE DUNCANNON MEMBER OF THE CATSKILL FORMATION, LUZERNE COUNTY, PENNSYLVANIA

NQ-diameter core was recovered from a 500 ft deep monitoring well installed in conjunction with the first deep Marcellus gas well drilled in Luzerne County, Pennsylvania. The continuous core (>95% recovery) extends from near the top of bedrock at 39 ft below the surface to the bottom of the bore hole at 500 ft. The entire core length occurs within the Duncannon Member of the Devonian Catskill Formation and consists of interbedded gray and red sandstones, red siltstones and red silty claystones arranged in repetitive, fining-upward cycles ranging up to 50 ft. Six core samples were selected from the basal sandstone zones of fining-upward cycles distributed throughout the core length. A petrographic analysis was performed on each sample with the goal of assessing primary intergranular porosity and testing the hypothesis that intergranular porosity is negligible and that groundwater flow paths occur largely along planar zones of secondary porosity, mainly bedding planes and fractures. Thin sections were impregnated with a blue dye that allowed the determination of intergranular porosity under plane-polarized light. Collectively, sample textures and grain compositions varied narrowly. Samples were classified as very fine-grained to fine-grained, mature quartz or feldspathic wackes having moderate to moderately-well sorting, moderate sphericity and low degree of roundness. Detrital grain compositions in order of generally decreasing modal abundances were quartz (60-85%), feldspar (alkali and plagioclase, 2-8%), muscovite (1-5%), lithic fragments (1-5%), and opaques (1-5%). The matrix component ranged between 5 and 15% and consisted of variable amounts of chlorite, epidote, quartz, calcite and silica cement. Intergranular calcite is present and interpreted to occur at the expense of intergranular pore space which ranges up to 2%. The petrographic results indicate that the original primary porosity was reduced by secondary deposition of calcite in addition to the effects of Paleozoic deformational episodes.