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Lacustrine Deposits Examined as a Sratigraphic Control on Migration and Compartmentalization in Eolian Reservoirs

K. A. Duncan, The University of Texas at El Paso, 500 University Avenue, Department of Geologic Sciences, El Paso, TX 79968, phone: (915) 731-6338, [email protected] and R. P. Langford, University of Texas at El Paso, 500 University Avenue, Department of Geologic Sciences, El Paso, TX.

The Cedar Mesa Sandstone was deposited in an ancient sand dune sea and is an exhumed stratum that has been a host for petroleum migration and accumulation. The Sandstone consists of interbedded and intertonguing eolian, fluvial, pedogenic and lacustrine strata. The focus of the research conducted is the small scale lacustrine, and associated pedogenic, lithologies that formed when the water table was high or when floods filled the interdune areas of the ancient desert. These lacustrine and pedogenic zones are one example of barriers that will affect the compartmentalization and migration of petroleum in a dune deposited reservoir type. The geometric distribution of groups of ponds stacked within sandstones is predictable using outcrop observations and wind direction to reconstruct the dune topography. Lacustrine deposits surveyed run parallel to the long axis of dunes in the interdune areas and are found between beds where petroleum has migrated and where it has not. Two end member types of lacustrine deposits were recognized. Deposits containing dolomite, limestone and dolomitized sandstone are interpreted as deposits of freshwater ponds fed by groundwater. Deposits containing numerous shale beds were repeatedly inundated from stream floodwaters. Many lacustrine deposits contain both shale and limestone and are interpreted to have formed in episodically flooded groundwater fed ponds. The Cedar Mesa's interbedded lacustrine lithologies, their thicknesses, and associated dune interaction zones are compared to better understand how these depositional features may inhibit or enhance production of eolian type reservoirs once they are buried and matured.