CARBONATE CEMENT ZONATION PATTERNS IN TWO LOWER PENNSYLVANIAN SANDSTONES, CENTRAL APPALACHIAN BASIN: STRATIGRAPHIC CONTROL AND RESERVOIR QUALITY IMPLICATIONS

Two lower Pennsylvanian sandstones, the Upper and Lower Raleigh, from the central Appalachian basin contain anomalous carbonate cement zones of varying mineralogy. Each zone contributes significantly to the whole rock volume and occurs near stratigraphically significant intervals (e.g., along disconformities). Control on the distribution of early cements is related to changing depositional conditions during sediment aggradation. Spacing between zones ranges from 4 to 10 meters.

Carbonate mineralogy and abundance has been characterized using electron probe microanalysis and standard petrographic point-counting methods. All samples were collected from core. The Lower Raleigh (lithic arenite) contains an interval of ferrous dolomite, near the middle of the sand body, that accounts for nearly 50% of bulk rock volume. An erosional contact between the Upper (quartz arenite) and Lower Raleigh sandstones is marked by small (25-50 microns) subhedral siderite crystals (18% by volume) surrounding quartz grains. Additional zones within the Upper Raleigh include a dolomite-rich (16% by volume) interval located 10 meters above the sandstone base and “wheat seed” siderite (6% by volume) occurring 15 meters above the base.

Carbonate zones found in sandstones have far-reaching reservoir quality implications. Compartmentalization, caused by the presence of such zones, could affect reservoir geometry by inhibiting fluid migration. In addition, subsequent dissolution of once pore occluding carbonate minerals could enhance porosity.