Evolution of Secondary Porosity in Pennsylvanian Morrow Sandstones, Anadarko Basin, Oklahoma
Published:January 01, 1986
Zuhair Al-Shaieb, Patty Walker, 1986. "Evolution of Secondary Porosity in Pennsylvanian Morrow Sandstones, Anadarko Basin, Oklahoma", Geology of Tight Gas Reservoirs, Charles W. Spencer, Richard E. Mast
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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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Geology of Tight Gas Reservoirs
Tight gas reservoirs occur in low-permeability, gas-bearing formations that are present to some extent in all gas-producing basins worldwide. This is the first volume to bring together data on tight reservoirs for a variety of basins and different geologic settings. The papers in this volume discuss characteristics of some of the most significant tight gas areas in the United States; however, these data are equally applicable to many other recognized and unrecognized tight gas provinces in other nations. In general, tight reservoirs in the United States are grouped into tight gas sandstones and eastern Devonian shales. The Devonian shale sequences are dominantly marine shale but include some siltstone and sandstone. Tight gas sandstone formations of other than Devonian age are present throughout the United States and consist primarily of fluvial and marine sandstones and siltstones. In addition, gas also occurs in low-permeability marine carbonate reservoirs. The 14 papers in this volume cover such topics as: coal-bed methane and tight gas sands interrelationships; gas-bearing shales in the Appalachian basin; exploration and development of hydrocarbons from low-permeability chalks; and geologic characterization of low-permeability gas reservoirs.