Grayburg Formation (Permian, Lower Guadalupian) Uplift-Related Porosity-Permeability Enhancement, Permian Basin, USA
Lindsay, Robert F.
The Grayburg Formation (Permian, Lower Guadalupian) was deposited on a distally steepened, mixed carbonate-siliciclastic ramp that was dolomitized and hosts the second largest suite of producing reservoirs in the Permian Basin of west Texas and southeastern New Mexico, USA. Following burial, regional uplift during Eocene to Early Miocene created a large recharge area west of the Permian Basin, referred to as the Southern Rocky Mountain Epeirogene and Rio Grande Rift. Uplift was due to emplacement of Trans-Pecos magmatic province intrusives and extrusives. This uplifted terrane recharged hot meteoric water (113° C to 224.5° C) into the subsurface, with a minimum drop in elevation of 1676 m (5500 ft) over 160 Km (100 miles). Recharging meteoric water swept through porous-permeable ramp margin strata, created an elevated geothermal gradient into the west side of the basin, formed residual oil intervals, and solution-enhanced porosity in grain-rich strata. This included up-dip solution enhancement of stratigraphic trap lithofacies, which repositioned stratigraphic traps farther up-dip. Dissolution created highly-etched dolomite crystals that turned the "classic" West Texas-southeastern New Mexico tight dolomitized carbonate reservoirs into laterally well connected, porous and permeable strata that responded to secondary (waterflood) and tertiary (CO2 assisted waterflood) recovery operations, with secondary recovery matching primary recovery. In some oil fields meteoric water rehydrated anhydrite beds/nodules to gypsum and hydraulically fractured individual evaporite beds/nodules. Some fractures were solution-enlarged, while others were cemented with gypsum.
In the Middle to Late Miocene, extension of the Rio Grande Rift tore apart the large east-dipping limb recharge area and developed a series of horsts and grabens. From the center of the rift at the Rio Grande River eastward these horsts and grabens are the: Franklin Mountains, Hueco Balson, Hueco and Sierra Diablo mountains, and Salt Flat graben, with the Sacramento, Guadalupe, Apache, and Glass mountains remaining attached to the west and south sides of the Permian Basin. At this time high volumes of highly pressured, hot meteoric water ceased to recharge into the basin and was replaced by low volumes of low pressured, cool meteoric water that was not capable of sweeping through reservoirs. The Permian Basin is now a cool basin with Grayburg reservoirs containing bottom-hole temperatures of 37° C.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013