Depositional and Facies Controls from Infiltrated/Inherited Clay Coatings: Unayzah Sandstones, Saudi Arabia

Abstract

Clay coatings on detrital quartz grains inhibit precipitation of burial diagenetic quartz overgrowths and help preserve porosity and permeability in Unayzah sandstones. These clay coatings are physically emplaced, not neoformed (authigenic) clay coats such as fibrous illite or radial chlorite. Understanding the depositional and facies controls on these clay coatings is necessary to predict reservoir quality in the Unayzah sands. Petrographic and SEM analysis of sandstones from different depositional settings and stratigraphic units within the Unayzah were made to investigate the relationships between facies and the presence of grain coatings. Grain coatings are found in all investigated depositional environments--eolian, fluvial, lacustrine, glacial diamictite, and estuarine settings. These coatings are especially abundant in sandstones associated with clay-rich paleosols. They are presently composed of illite and/or chlorite, but they may have had precursor clay minerals prior to burial diagenesis (e.g., smectite, sepiolite, or palygorskite). The relative amounts of clay coatings depend not only on the type of depositional environment, but also on the stratigraphic unit within which the environment resides. This is interpreted to be a function of changing paleoclimates during deposition of the Unayzah. For example, in the fluvial setting, the percentage of clay coatings in the relatively warm fluvial systems in the upper part of the Unayzah A is much higher than in the cold lower fluvial systems of the Unayzah C. Moreover, this study shows that the presence of clay coatings is grain-size dependent. For a given depositional setting (e.g., fluvial environment and its sub environments), there is a direct relationship between the mean grain size of sandstones and the average percentage of coated grains in all samples of this facies. In finer-grained facies, as in a distal sheet flood, more clay coatings (~90%) occur. In coarser-grained facies, as in a fluvial channel, fewer grain coatings (~ 50%) occur. Chlorite is the dominant clay coating in eolian settings, especially associated with coarser eolian grains in dune and sand sheet sub-environments recognized in the upper part of the Unayzah (Unayzah A). Also, in this unit, grains deposited in fluvial settings may be coated with illite or chlorite. In estuarine, and fluvially dominated estuarine deposits (of the Basal Khuff Clastics), illite is the dominant clay coating. Both chlorites and illites are present (with different percentages) in the relatively finer grains deposited in floodplain/playa and interdune/distal sheet flood sub environments of the Unayzah A and B units.