Diagenesis in Sandstone Reservoirs of Appalachian Basin: ABSTRACT

Abstract

Diagenetic processes have had an important effect on porosity in the sandstones of the Appalachian basin. The Mt. Simon and Rose Run sandstones (Upper Cambrian and Lower Ordovician respectively) in the deep parts of the basin are generally of low porosity mainly because of quartz and carbonate cementation rather than compaction attending pressure solution or crushing from deformation. Locally, small voids have developed as a result of dolomitization of original calcite. These sandstones, at moderate depths in southeastern Ohio, are porous particularly in the highly feldspathic parts where cementation is incomplete and solution of feldspar is appreciable. Locally, argillaceous coatings inhibited cementation but promoted pressure solution. The Tuscarora Sandstone (Lower Silurian) is generally well-cemented with quartz except in some of the very coarsest lenses. Small uncemented patches are present where argillaceous coatings or possible gas pockets prevented growth of secondary quartz. The lower part of the Williamsport (Newburg) Sandstone (Upper Silurian) has low porosity mainly because of dolomite filling pores. The upper part of the formation is quartzose with good porosity. Quartz cementation was retarded by argillaceous coatings on grains but locally anhydrite filled pores. Porosity is developed best in the quartzose phases of the Oriskany Sandstone (Lower Devonian), especially in the western part of the Appalachian basin where cementation is less complete. Primary porosity decreases eastward as the amount of cement increases. Quartz and calcite cements were available both before and after folding. Widespread leaching of carbonate is not indicated. Secondary fracture porosity is important in deformed areas particularly in the well-cemented quartz sandstones which were more brittle. The Benson sand (Upper Devonian) is commonly of silt size and has very low porosity because of argillaceous material and secondary carbonate. Porosity is good in some of the coarser fractions (very fine-grained sand) where quartz cementation is incomplete and carbonate content is low. Locally, porosity was increased by partial solution of feldspar. In the Berea Sandstone (Lower Mississippian) sericite and illite promoted pressure solution and led to considerable reduction in pore space in many areas. Porosity is relatively high, however, in the Cabin Creek and Gay-Fink trends where chlorite coatings were important in restricting quartz cementation. Solution of feldspar increased porosity in some places. End_of_Article - Last_Page 1893------------