Architecture of Reservoirs Confined by Early Diagenesis in Paleosols from Sequence Boundaries in Permian Eolianites

Richard P. Langford¹, David M. Tatum⁴, Pierre Depret³, Katy Duncan⁶, Krystal Pearson², and Luqman Adams^5
1Geological Sciences, University of Texas at El Paso, El Paso, TX
²Conoco-Phillips, Houston, TX
³Mid-Continent Business Unit, British Petroleum, Houston, TX
⁴ChevronUSA, New Orleans, LA
⁵Chemistry, Prairie View A & M University., Prairie View, TX
⁶El Paso Corporation E&P, Denver, CO

The Permian Cedar Mesa Sandstone of Southeastern Utah is divided into 20 to 40 m thick units by prominent erosion surfaces. These surfaces correlate with color changes from red to white within the unit and mark barriers to fluid flow. The white units were probably reduced in the Paleocene by hydrocarbon bearing fluids, and some may represent exhumed reservoirs. The erosion surfaces form permeability barriers within the sandstone that segregate reservoir and non-reservoir strata.

Some erosion surfaces represent dune seas that formed after the previous ergs were stabilized and partly eroded. Petrographic analyses show that two of these horizons contain fragmented marine fossils. The erosion surfaces are associated with large ponds, and are extensively vegetated; suggesting a high water table and correlation with transgressions associated with Permian sea level highstands. This correlation of eustacy with the erosion surfaces allows development of a sequence stratigraphic model for this and similar Permian eolianites, which are found on most continents.

The erosion surfaces mark highstands that flooded the sources of eolian sand. The intervening dune fields mark lowstands. Within the intervals, reservoirs are large transverse mounds, accumulated during erg aggradation. The barriers to flow are complex surfaces with discontinuous shales and diagenesis in soils that precipitated clay and hematite cements. Permeabilities, measured with an air permeameter drop from 10 to 10 of MilliDarcys to less than 1 md (the limits of our equipment across several meters at the boundaries). The basal 1 to 3 m of the overlying eolian sandstones are also cemented with early calcite cement. The origins of this cementation are uncertain, but it contributes to the ability to obstruct fluid migration.

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