Richard Langford¹, Peirre Andre Depret¹

(1) University of Texas at El Paso, El Paso, TX

ABSTRACT: Stratigraphic controls on diagenesis and porosity in the Cedar Mesa Sandstone, southeastern Utah

The Cedar Mesa Sandstone on the Colorado Plateau, is an outcrop analog of the Rotliegende hydrocarbon reservoirs in the North Sea. The Cedar Mesa was 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. Results illustrate the importance of stratigraphic boundaries on diagenesis. Samples were collected in profiles illustrating diagenesis associated with: (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 were analyzed in thin section to characterize primary and secondary porosity, authigenic mineralogy, and effects of cementation and compaction. Reservoir-quality sands form as thin sabkha-margin dune sands and within the erg, well below bounding surfaces. Up to 30% porosity is preserved where early calcite cements were dissolved prior to compaction and post-compaction grain dissolution has enhanced intergranular porosity. The highest porosities are associated with white eolian sands that intertongue with playa muds, as grain dissolution was higher in this setting. Early porosity was lost through calcite cementation and compaction associated with pressure solution. Clay cements are associated with the tops of some erg-margin dune sands. Roots and soil fabrics extend several meters below super bounding surfaces. The surfaces are associated with increased early calcite cement and are tight. Aqueous environments had an important impact on early diagenesis. Cementation increases and porosity decreases below ponds, and rooted horizons.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado