Early Grain-Coat Formation in Modern Eolian Sands: Implications for Prediction of Deep Porosity
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 model-ing indicate that coat effectiveness in cement inhibition increases with greater coat continu-ity. Infiltrated/illuviated clays, diagenetic clays, and microcrystalline quartz all have been shown to form effective grain coats. Consequently, reliable deep-porosity prediction re-quires accurate models for the presence, continuity, and composition of early grain coats. A joint Saudi Aramco-ExxonMobil study was undertaken to document the distribution and genesis of grain coats in modern eolian settings. Water and sand samples were collected from a range of depositional environments in arid and semi-arid settings (Saudi Arabia and New Mexico, respectively). Coat characteristics were evaluated using petrographic analyses and laboratory experiments. Early results indicate that most continuous infiltrated coats in eolian environments are clay, formed either by percolation of muddy water into wadi sedi-ments, or illuviation of airborne dust into dunes and sand-sheets during early soil formation. Coated grains blown from wadis or soils into active dunes lose their coats by abrasion dur-ing eolian transport. Climate appears to be a control on coat formation and preservation. In arid climates, dunes remain active, and illuviated coats are abraded more quickly than they can form. Dunes and sand-sheets in semi-arid settings, however, are stabilized during wet-ter climatic periods. This stabilization allows longer-term illuviation and drives a multi-cyclic process of coat formation and abrasion.
AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain