William R. Bryant, 2003. "Permeability of Clays, Silty-Clays and Clayey-Silts", Siltstones, Mudstones and Shales: Depositional Processes and Characteristics, Erik D. Scott, Arnold H. Bouma, William R. Bryant
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Permeability is the property or capacity of a porous rock, sediment, or soil for transmitting a fluid; it is a measure of the relative ease of fluid flow under unequal pressure (Bates and Jackson, 1980). The most significant characteristic of mudstones, siltstones and shales are their extremely low permeability. Fine-grained sediments have some of the lowest permeability of any natural occurring mineral, rock or sediment. The porosity of a fine-grained sediment is the major factor that controls the permeability of sediment. The relationship between porosity and permeability of a fine-grained sediment is one of nature’s largest contrasts, one that covers 13 to 15 orders of magnitude. Determining the permeability of fine-grained sediments, that are virtually impermeable, is a difficult accomplishment and can only be achieved accurately by certain geotechnical measurements, such as the consolidation test. Consolidation tests performed on a multitude of fine-grained sediments from the Gulf of Mexico resulted in the determination of the relationships between porosity and permeability of a fine-grained sediment.
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Siltstones, mudstones and shales have been studied mainly with regard to clay mineralogy and general transport-deposition. Recent studies on deepwater deposits from cores and outcrops have shown that fluid flow properties of deepwater reservoirs are greatly affected by the presence of finer-grained deposits in the reservoir. Initial analysis indicates that the majority of these finer grained deposits have a large silt component and are closer to siltstones rather than mudstones, commonly called shales To date, little attention has been given to their characteristics resulting from different depositional processes.