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Abstract:

Clay coats, which may be continuous or discontinuous, originate from soils as cutans, from infiltration of clay in sand and sandstone, and authigenically as newly formed or regenerated clay minerals. Allogenic cutans and infiltration deposits have a laminar morphology, whereas authigenic-clay coats commonly have a radial morphology.

Thick, well-developed, continuous clay coats, regardless of origin, may retard quartz cementation by masking the surface of detrital-quartz grains and preventing the nucleation of quartz overgrowths. Chlorite is the most effective of the clay minerals in preserving intergranular porosity and appears to be important in very deep sandstone reservoirs. The most favorable amount of chlorite to preserve porosity is variable: 4 to 7 volume percent for the Berea Sandstone and 5 to 13 volume percent for the Tuscaloosa Sandstone, for example. Smaller amounts of chlorite permit quartz to nucleate and destroy porosity and greater amounts result in porosity reduction by infill of pores. Clay coats do not retard epitaxial cements (e.g., carbonates and sulfates), which may cover clay coats and occlude porosity. Clay coats may occur in highly lithic (e.g., >35% lithic material) sandstones, but are not important because physical compaction dominates diagenesis and destroys porosity.

Experimental growth of clay coats shows that clay flakes are flatly attached to detrital-sand grains and curl upward to form a radial-fibrous morphology. This attached root zone may explain why clay coats are effective at blocking nucleation of quartz cement. Experimental work also shows that mineralogy may provide an initial substrate control over the precipitation of clay coats by providing an in situ source of the cations needed to precipitate the clay. Later, the clay coats nucleate on other framework grains farther from the site of initial nucleation.

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