Inherited Grain-Rimming Clays in Sandstones from Eolian and Shelf Environments: Their Origin and Control on Reservoir Properties
Michael D. Wilson, 1992. "Inherited Grain-Rimming Clays in Sandstones from Eolian and Shelf Environments: Their Origin and Control on Reservoir Properties", Origin, Diagenesis, and Petrophysics of Clay Minerals in Sandstones, David W. Houseknecht, Edward D. Pittman
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Inherited grain-rimming clays are common in sandstones from eolian dune and marine-shelf environments. Inherited clay rims are defined as clay coats that form on framework grains prior to their deposition. Development of such clays requires that these clays become attached to framework grains at some other location and that they then be recycled with the host grain to form the present deposit. Such clays have been recognized in a number of major eolian-reservoir units in North America and the North Sea. Inherited clay rims also are extensive in major shelf-sandstone reservoirs. The relatively high levels of reservoir quality of many of these dune and shelf sandstones are at least partially attributable to the presence of inherited clay rims.
The distribution and composition of inherited clay rims allow them to be distinguished from other types of detrital clays and from neoformed clays. Characteristics most useful in identifying inherited clay rims include: (1) presence at points of contact between framework grains; (2) widely varying rim thickness; (3) increased thickness in embayments; (4) absence on the surfaces of diagenetic components; (5) clay mineralogy similar to that of clay interbeds; (6) enhanced development in finer grained sandstones where clay-filled depressions are more abundant; and (7) tendency to be developed and preserved only in selected environments. Recognition can be difficult where significant regeneration has occurred, where rims are extremely thin, or where rims are masked by detrital matrix or by authigenic cements.
In eolian settings, clay rims initially form in sabkha and interdune environments. The coatings are interpreted to form by infiltration of clay-charged waters or by adhesion to wetted sand-grain surfaces in eolian-soil and sabkha environments. Mild reworking allows for preservation of complete clay rims, whereas extended transport results in the removal of rims from projections on grain surfaces. Inherited clay rims are absent in water-washed environments or in coastal dunes derived directly from beach sands.
In shelf environments, inherited clay rims are generated when sand grains passing through the digestive tracts of organisms, or disturbed by burrowing, become coated with clay. Later reworking may reduce the thickness of clay rims or remove them altogether. Clay rims are destroyed in high-energy nearshore environments.
Because the presence of inherited clay rims can severely limit quartz-overgrowth development, high levels of intergranular porosity may be preserved to depths where associated clay-free sandstones exhibit extremely low porosities. Inherited clay rims are not entirely beneficial. The presence of clays at grain contacts may significantly accelerate the process of pressure-solution suturing. Commonly, clays in the rims are regenerated at depth to forms having a much higher surface area and containing a large amount of microporosity.
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Origin, Diagenesis, and Petrophysics of Clay Minerals in Sandstone–This volume grew out of a symposium held at the 27th Annual Meeting of the Clay Minerals Society in Columbia, Missouri on October, 1990. The symposium was designed to present a current synthesis of research devoted to the origin, diagenesis and petrophysics of clay minerals in sandstones. International authors demonstrate a multidisciplinary approach to interpreting the origin and diagenesis of clay minerals in sandstones and to evaluating their influence on reservoir quality. This volume bridges a gap between petrographic and geochemical researchers and reservoir geologists and engineers.