Early grain-coat formation in modern eolian sands: Implications for prediction of deep porosity

Category Sedimentology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Ajdukiewicz, Joanna۱; Esch, William۲; Rumelhart, Peter۲
Holding Date 23 September 2008

Early-formed grain coats preserve favorable reservoir quality in deeply buried sandstones by inhibiting formation of high-temperature quartz cement during later burial. Deep eolian reservoirs with grain-coat-preserved porosity include the Norphlet, USA; Rotliegendes, Germany; and Unayzah, Middle East. Petrographic observations and quartz-cement modeling indicate that coat effectiveness in preserving porosity increases with greater coat continuity. Infiltrated/illuviated clays, diagenetic clays, and microcrystalline quartz have all been shown to form effective grain coats. Consequently, reliable deep-porosity prediction requires accurate prediction of early grain-coat presence, continuity, and composition.
A joint Saudi Aramco-ExxonMobil study was undertaken to document the distribution and genesis of grain coats in modern eolian settings. Sand and water samples were collected from a range of depositional environments in arid and semi-arid settings (Saudi Arabia and New Mexico, respectively). Results indicate that continuous clay grain coats in eolian environments form by infiltration of muddy water into wadi and nearby eolian sediments, illuviation of airborne dust into sands during soil formation, or diagenesis at/below the water table. Grains blown from coat-forming environments into active dunes lose their coats by abrasion during eolian transport, in a multicyclic process of coat formation and abrasion. Climate and water-table level are strong controls on coat formation and preservation. In arid climates, dunes remain active, and grain coats are abraded more quickly than they can form. Eolian sands in semi-arid settings, however, are stabilized and more frequently water-saturated during wetter climatic periods, enhancing coat-forming processes.