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

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