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     Joanna Ajdukiewicz -- Geologist
ExxonMobil Exploration Company, Houston, TX, USA

 Favorable reservoir quality in deeply buried sandstones is preserved by early formed grain coats that inhibit growth of high-temperature quartz cement during later burial. Deep eolian reservoirs with grain-coat-preserved porosity include the Norphlet, USA; Rotliegendes, Europe, Nugget/Navaho, USA, and Unayzah, Middle East. Porosity- preserving coats in these reservoirs are mainly composed of infiltrated and diagenetic clays. Coat effectiveness increases with greater coat continuity. Reliable deep porosity prediction requires accurate models for the presence, continuity, and distribution of early grain coats.

 

A  collaborative Saudi Aramco-ExxonMobil study was undertaken to document the distribution and genesis of grain coats in modern eolian settings. Studies of coat formation in eolian settings with different climatic conditions were undertaken in Saudi Arabia, New Mexico, and Namibia. Coat characteristics were evaluated using petrographic analyses and laboratory experiments. Results indicate that coat continuity and effectiveness vary by EOD as well as climate. Infiltrated clay coats form either by percolation of muddy water into wadi and nearby eolian sands after periodic flooding, or illuviation of airborne dust during early soil formation. Continuous diagenetic coats form early in sediments buried to or below the water table. Coated grains blown from these environments into active dunes lose their coats by abrasion during eolian transport, most likely in a multicyclic process of coat formation and destruction. In arid climates, dunes remain active, and grain coats are abraded more quickly than they can form. Wetter climates favor sediment stabilization and coat formation. Consequently, more continuous coats and highest porosity preservation occur in dune sands prone to periodic wetter climate conditions, overbank and sheetflooding, and/or rising water tables.