FACIES AND DIAGENESIS OF THE CEDAR MESA SANDSTONE

The Cedar Mesa Sandstone on the Colorado Plateau is an outcrop deposited as a sand dune sea between a marine environment on the West, a fluvial environment to the Northeast, and a playa (sabkha) environment to the Southeast. Aqueous environments had an important impact on diagenesis. Consequently, diagenetic cements change laterally or vertically due to superposition of wet or dry depositional environments. Diagenetic fabric vary in association with three features: (1) depth below eolian sequence boundaries (super surfaces), (2) depth below surfaces of playa and fluvial floods into the dune sea, and (3) lateral changes away from intradunal ponds. Samples from outcrops have been analyzed in thin sections to characterize authigenic mineralogy, and effects of cementation and compaction on reservoir quality. Sand Dune Sea forms the best reservoir quality through the Cedar Mesa Sandstone. Even if those dunes have a good porosity, they are characterized by intertonguing red and white zones inside sandstones. Red sandstones show a higher cementation, rich in hematite and calcite. Hematite occurs as rims around sand grains. Reducing rich hydrocarbon fluids probably leached hematite and calcite cements, and as a consequence, white zones with good porosity have been created in large dune sands.