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Predicting Deep Porosity Preservation in Eolian Reservoirs from Early Grain-Coat Formation Processes: A Modern Desert Sand Study

Previous HitJoannaTop Ajdukiewicz1, William Esch2, Pete Rumelhart2, Stephen Franks3, Clemens Van Dijk3, Bill Carrigan3, and Richard Larese4
1Exxonmobil Exploration Company, Houston, TX
2Exxonmobil Upstream Research Company, Houston, TX
3Saudi Aramco, Dharan, Saudi Arabia
4Consultant, Durango, CO

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 composition 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. This paper reports results from an arid setting in modern Saudi Arabia. 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 sand-sheet sediments after fluvial or sheet flooding, or illuviation of airborne dust during early soil formation. Coated grains blown from wadis or soils into active dunes lose their coats by abrasion during eolian transport, driving a multicyclic process of coat formation and destruction. In arid climates, dunes remain active, and infiltrated/illuviated coats are abraded more quickly than they can form. Consequently, more continuous coats are found in environments prone to periodic flooding, soil formation, or rising water table.


AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas