Clay Filtration and Sandstone Reservoir Quality

David W. Houseknecht, Keith S. Matlack

Clay grain coatings are recognized as the earliest diagenetic feature in many sandstones. Such coatings significantly affect reservoir quality directly by modifying porosity and permeability, and indirectly by influencing subsequent diagenesis. Nevertheless, the distribution and origin of clay grain coatings remain unexplained in most studies of sandstone diagenesis.

Laboratory experiments and SEM examination of various Holocene facies demonstrate that filtration of muddy water through sand can result in the formation of pervasive clay grain coatings within centimeters of the depositional surface. Additionally, filtration commonly forms annular ridges of clay around grain contacts; these ridges render the sand semiconsolidated and are so distinctive that they can be used as a criterion for recognizing filtrated clays in certain ancient sandstones. The formation of such coats and ridges is distinctly facies-dependent; environments with high concentrations of suspended sediment, fluctuating water levels, and limited sediment reworking (e.g., rivers) are most favorable, whereas other environments (e.g., beaches) are much less favorable. Clay grain co tings and annular ridges also form by filtration during dewatering of turbidites.

Filtration provides a possible explanation for the consistently early occurrence of clay grain coatings in paragenetic sequences of ancient sandstones. Moreover, facies-dependent variation in sandstone reservoir quality is, in many cases, the result of the presence or absence of filtrated clays and their effects on subsequent diagenesis. However, the degree to which filtrated clays are modified by burial diagenesis and the establishment of definitive criteria for recognizing filtrated clays in ancient sandstones are currently unresolved research problems.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.