Depret, Pierre-Andre¹, Richard P. Langford¹
(1) University of Texas at El Paso, El Paso, TX

ABSTRACT: Eolian Facies in Southeast Utah and Northwest Colorado, and Reservoir Quality

Widespread eolian sediments were deposited during the Late Pennsylvanian and early Permian in Utah and Colorado and formed hydrocarbon reservoirs. Some are exhumed, and some are productive. The exhumed and productive reservoirs are similar in facies, and reservoir quality, composed of ergs, deposited between marine, fluvial, and playa environments. Depositional environment strongly influenced early diagenesis, which, in turn controlled development of reservoir porosity and permeability. Both formations are characterized by three facies: (1) dune, (2) wet and dry interdune, and (3) extradune. Dune deposits are well-sorted subarkoses and are main reservoir facies. Interdune shales and laminated siltstones and extradunal playa deposits and sand sheets are low porosity and permeability intervals sandwiched between thick eolian units. Samples from outcrops and illustrate the importance of stratigraphic boundaries on diagenesis. Samples were collected in profiles illustrating diagenesis associated with: (1) wet and dry interdunes, (2) extradunes, and (3) eolian exposure surfaces (super surfaces) Within both reservoir systems, the highest porosities are associated with the lower portions of the thick eolian sandstones. These were least affected by the early cementation related to stratigraphic boundaries. Within the exhumed sandstones an additional setting with high porosities is associated with eolian sands that intertongue with playa deposits, as grain dissolution was higher in this setting. Early porosity was lost through calcite cementation and compaction associated with pressure solution. An important leaching episode resulted in much secondary porosity after calcite, dolomite, and feldspar dissolution. This dissolution resulted in the migration of producible hydrocarbons in both settings.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.