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Abstract

The Anadarko basin is one of the most prolific hydrocarbon provinces in North America. Examination of more than 50 cores from the Pennsylvanian Morrow sandstones reveals a complex diagenetic history. Although quartz is the major framework constituent of sandstones, shell fragments, glauconite, and clayey matrix occur in considerable amounts throughout the section. Diagenetic complexity is a function of depositional environment and burial and thermal history of the basin.

Most porosity in the Morrow sandstones throughout the Anadarko basin is secondary in origin. Such porosity results from the dissolution of clayey matrix, carbonate fragments and cement, glauconite, and quartz grains and their overgrowth.

Evolution of secondary porosity is related to the hydrogen (H+) ions produced directly from the organic material maturation processes of Morrow shales. Carbon dioxide (C02) gas, with concentrations ranging from 0.3 to 4.7% by volume, was detected in more than 150 natural gas wells monitored in the basin. Based on geothermal and geopressure gradients and on experimental investigations of the solubility potential of CO2 in formation fluids under elevated temperatures and pressures, a good estimate of solubility of CO2 in the Morrow formation water may be attained. Because the concentration of CO2 appears to increase with depth in the basin, secondary porosity should not be restricted to a particular zone or to particular depths but should persist with depth. Organic acids at shallow depths and H2S in deeper zones may be important in secondary porosity enhancement.

Amounts of porosity and the geometry of pore space are directly related to the original lithology. A better understanding of lithofacies is critical in evaluating reservoir quality.

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