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Most clayey marine sediments being deposited in today's oceans will eventually make excellent seals for hydrocarbons. Extensive consolidation testing of marine sediments indicates that most fine-grained marine deposits of the oceans are underconsolidated indicating the presence of excess pore water pressures. This condition is attributed to the low permeability of the sediments and indicates the potential of these sediments as seals for hydrocarbons.
Analyses of the consolidation, permeability, and microstructure characteristics of clayey marine sediments from most of the world's oceans show that the permeability decreases under an imposed load 7 to 8 order of magnitude faster than porosity. This decrease in permeability is controlled by the amounts and type of clay present as well as its microstructure. Clay microstructure refers to two important properties of a sediment--the fabric and physicochemistry. The fabric, a geotechnical property of the sediment is defined as the orientation and arrangement of the solid particles and the particle to particle relation. The relation between depth of burial, porosity, permeability, and fabric of marine clays will be discussed in relation to seals for hydrocarbons.
The consolidation tests have shown that the compressibility and permeability of the three clay types, kaolinite, illite, and bentonite, can be represented by power law functions of the porosity (n). They are ^sgr = AnB (psi) for the vertical stress supported by the mineral grains and K = CnD (md) for the absolute permeability measured in the vertical direction with seawater. For these different clay types the following table lists the constants A, B, C, and D.
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