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Bureau Seminar, December 5, 2008

Upper Permian Yates Formation Oil- and Gas-Reservoir Facies
Northwest and Northeast Central Basin Platform, Texas

H. S. Nance
Bureau of Economic Geology

The Upper Permian Yates Formation produces oil and gas from fine-grained sandstone facies along the northwest and northeast margins of the Central Basin Platform (CBP), West Texas. Yates reservoir-interval siliciclastics, carbonates, and evaporites represent repetitive inner- and middle-shelf successions that record relative sea-level cyclicity. Lowstand-transgressive phases are marked by siliciclastics that were deposited in coastal mudflats, wadi outwashes, and eolian dunes or sandsheets that were partly re-worked into tidal flats during transgressions. During later stages of transgression and highstands, marine-derived waters were subjected to net evaporation in regionally extensive brine pans, resulting in widespread precipitation of gypsum and halite. Brine incursion into pan-margin-margin sediments produced displacive gypsum in carbonate mud in down-dip locations and displacive halite in siliciclastics in up-dip locations. Over time, brine pans expanded both shelfward and basinward. During falling relative sea level, coastal eolian and alluvial siliciclastic environments occupied the region and fed siliciclastics to deep Delaware Basin.

Stratigraphic traps are composed of reservoir- and non-reservoir siliciclastics that are interbedded with low-permeability carbonates in down-dip areas and evaporites in up-dip areas. Up-dip concentration of evaporite cementation and structural closure also are components of trapping in Yates reservoirs. Post-depositional cementation with evaporites may record density-reflux from overlying brine pans or evaporative pumping from hypersaline water tables.

Although very well-sorted fine-grained eolian sandstones in the upper Yates inner-shelf originally possessed high primary porosity, permeability, and reservoir-potential, pore occlusion by halite has destroyed their reservoir potential but enhanced their potential as seals. In the absence of core-based calibration, well-log responses in this facies has been misinterpreted to indicate gas content.


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