Upper Permian (Guadalupian) Coastal Tidal Flat and Shelf Lagoon Deposits: Outcrop Model and Subsurface Examples of Stratigraphic Traps, Permian Basin
Paul M. Harris, Robert F. Ward
Over half a century of exploration and development drilling has shown that hydrocarbons reservoired in Upper Permian (Guadalupian) deposits of west Texas and southeastern New Mexico have accumulated at the contact between shelf-lagoon dolomites or siltstones and their updip coastal evaporite equivalents. Facies relations responsible for such stratigraphic traps can be examined in outcrops found in the Guadalupe Mountains area. Outcrop exposures reveal diverse sediments that spanned a reef-rimmed carbonate-siliciclastic shelf of late Guadalupian age. These same shelf stratigraphic units (Seven Rivers, Yates, and Tansill Formations) produce in the subsurface surrounding the Delaware basin, and a direct comparison can be made between outcrop and subsurface facies relations. ower Guadalupian shelf equivalents (San Andres, Grayburg, and Queen Formations) are even more prolific hydrocarbon producers throughout the Permian basin, and can be equally well understood with the same outcrop model.
Production from any of the Guadalupian shelf units similarly occurs from stacked reservoirs of dolomites or siltstones. Dolomites comprise shoaling cycles of deposition: intercrystalline and moldic porosities typify basal dolomudstones and dolowackestones as well as overlying dolopackstones, whereas capping dolomudstones may contain fenestral porosity but usually are tight and interlayered with anhydrite. Interparticle porosity occurs in siltstones that are interbedded with the dolomites. Reservoir development is more a problem of updip seal than porosity, as sediments other than lagoonal dolomites and siltstones are porous. Porous carbonate sands accumulated in a backreef position and the shelf margin reef and associated slope debris apron have developed porosity secondarily through olution, fracturing, and minor dolomitization. Hydrocarbons migrated from presumed basinal source rocks through the margin and backreef, and continued updip into shelf lagoon deposits that pinch out into tight anhydrite-cemented equivalents as well as interbedded evaporites of coastal tidal-flat origin.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.