Permeability Characteristics of Continental Slope and Deep-Water Carbonates from a Microfabric Perspective
Permeability, the rate at which fluids move through a porous medium, affects the consolidation and reduction in porosity of a sediment with time and overburden pressure. Most marine clays consist of smectites and illites that are fine-grained and platey. In contrast, carbonate sediments are composed of multishaped, multisized components. In general, clay-rich sediments consolidate to lower porosities and are less permeable than carbonates at given overburden pressures. This difference is related to microfabric.
Numerous samples recovered from the continental slope to midocean depths were consolidated in the laboratory, and permeabilities were determined directly by using falling head permeameters and indirectly by consolidation theory. In the marine clays, porosity values ranged between 75 and 40%, and permeability values ranged between 1 × 10−6 cm/s and 1 × 10−10 cm/s. In contrast, carbonate sediments with porosities between 60 and 40% had permeabilities that ranged between 1 × 10−4 and 1 × 10−7 cm/s. Sediments with varying percentages of carbonate content, but with a matrix of clay particles, fall within the same range as the clays. At given porosities, permeabilities within the carbonate sediments are determined by microfabric. For example, grain-supported samples have a higher permeability (1 × 10−4 cm/s) than matrix-supported samples (1 × 10−5 and 1 × 10−6 cm/s). In general, matrix-supported carbonate sediments composed of aragonite needles have higher permeabilities (1 ×10−5 cm/s) than matrix-supported sediments of low-magnesian calcite composed predominantly of coccoliths (1 × 10−6 cm/s). Analysis of scanning and transmission electron micrographs of carbonate and clayey sediments confirm the more open microstructure of carbonate sediments as opposed to the more evenly distributed but smaller sized clay micropores.
KeywordsContinental Slope Carbonate Sediment Pore Throat Initial Permeability Ocean Drill Program Site
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