Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Depositional and Facies Controls on Infiltrated/Inherited Clay Coatings: Unayzah Sandstones; Saudi Arabia
(1) EXPEC Advanced Research Center, Saudi Aramco, Dhahran, Saudi Arabia.
(2) Consultant, Dallas, TX.
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 and their relationship is necessary to predict reservoir quality in the Unayzah sands.
Grain coatings are found in all depositional environments that have been investigated (Eolian, fluvial, lacustrine, glacial diamictite, and estuarine depositional settings). These coatings are especially abundant in sands associated with clayey paleosols. They are presently composed of illite and / or chlorite , but these may have had precursor clay minerals prior to burial diagenesis. 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.
Moreover, this study shows that the presence of these clay coatings is a grain-size dependent. For a given depositional setting there is a direct relation between the mean grain size of sandstones and the average percentage of coated grains. The finer the grains, the more clay coatings (~90%); and the coarser the grains, the fewer grain coatings (~ 50%) in the samples.
Chlorite is the dominant clay coating in eolian settings, especially associated with coarser eolian grains in dune and sand sheet sub-environments recognized on the upper part of 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 BKC), 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 Unayzah A and B units).
To summarize, we suggest that depositional environment, paleoclimate, and grain size are all factors in the genesis of clay coatings. Some clay coatings formed in-place by pedogenesis (soil-forming processes), and “inherited” clay coatings on grains transported by eolian (and fluvial) processes may have originally formed in pedogenic environments.