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Petrology and Diagenesis of Pocono-Price Big Injun Sandstone, Granny's Creek Field, West Virginia

David L. Swales

Two cores of the Lower Mississippian Pocono-Price Big Injun sandstone were studied megascopically and petrographically to determine the effects of mineral content and diagenesis on porosity.

Most of the samples studied are fine-grained sublitharenites. Quartz, polycrystalline quartz, and argillaceous rock fragments comprise up to 95% of the framework grains. Many samples also are characterized by abundant pseudomatrix resulting from the squeezing and degradation of mica flakes, argillaceous rock fragments, and feldspars.

Initial porosity probably was very high because of the well-sorted and angular nature of the sand grains, although reservoir porosities generally range only up to 15%. Mechanical compaction, diagenetic cements, and degradation of sand grains have significantly reduced initial porosity. The general sequence of postdepositional events includes: (1) mechanical compaction, (2) illite and chlorite grain coatings, (3) quartz and barite pore filling and replacement, (4) calcite pore filling and replacement, (5) dolomite, siderite, and pyrite replacement, and (6) dissolution of feldspar grains. Chlorite coatings are abundant in some samples and, where present, help to preserve primary porosity by inhibiting the growth of quartz cement. Minor secondary porosity has resulted from dissolution of feldspar.

Clean, high quartz sandstones with well-developed chlorite coatings form the best reservoirs in this area.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.