‘Scale Is Everything’ the Permian Cedar Mesa Outcrop, a Qualitative Analog for the Permian Unayzah, ‘Wet’ Eolian Depositional System in Saudi Arabia

Christian J. Heine and John Melvin
Upstream Ventures (Joint Venture Exploration), Saudi Aramco, Dhahran, Saudi Arabia

One of the most important contributions outcrop analogs can provide to you as a geoscientist is a feeling for scale. Questions like; well spacing, reservoir compartments, model cell size, horizontal vs. vertical wells, and production anisotropies may at least be visualized and possibly understood at outcrop scale.

The Permian Cedar Mesa sandstone, a well documented ‘wet’ eolian deposit in southern Utah is an outstanding outcrop analog for the Permian Unayzah ‘A’ reservoir. Field observations from Permian Cedar Mesa outcrop along the Moqi Dugway road cut display the characteristic alternating ‘wet’ and ‘dry’ cycles of a ‘wet’ eolian depositional system. From core, image log studies and well-log cross sections, the eolian Unayzah reservoir was identified as a ‘wet’ eolian transverse dune depositional system. In well log cross-sections through the Unayzah ‘A’ reservoir, the ‘wet’ and ‘dry’ depositional cycles were recognized and incorporated into the geocellular model layering scheme as time lines.

The borehole image log was a critical tool for facies recognition in the Permian eolian reservoirs of Saudi Arabia. Four distinct depositional facies have been identified on image log and confirmed with detailed core description namely: dune, sand-sheet, paleosol and playa. A numerical proportion of each facies was determined from well data for each reservoir sequence and an object-based modeling technique was used to distribute the image log identified facies.

The resulting geocellular model was scaled to match the outcrop and visualized in 3-D. As the outcrop would suggest, the ‘wet’ and ‘dry’ cycles of eolian deposition were modeled as separate packages honoring the well facies proportions. In cross-section the model displayed the characteristic ‘wet’ and ‘dry’ cycles observed in the Cedar Mesa outcrop.

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