Abstract

The Grayburg Formation (Permian, Lower Guadalupian) was deposited on a distally steepened, mixed carbonate-siliciclastic ramp that forms the second largest producing suite of dolostone reservoirs in the Permian Basin, in west Texas and southeast New Mexico, USA. Regional uplift during the Late Eocene to Early Miocene, referred to as the Southern Rocky Mountain Epeirogen and Rio Grande Rift, created a large recharge area west of the Permian Basin. Uplift was due to emplacement of Trans-Pecos magmatic province plutons. 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 and permeable ramp margin strata, created an elevated geothermal gradient within the west side of the basin, formed residual oil intervals, and solution-enhanced porosity in grain-rich strata. This included solution enhancement of stratigraphic trap lithofacies, which repositioned the trap farther up-dip. Dissolution created highly-etched dolomite crystals that turned west Texas-southeastern New Mexico tight dolostone reservoirs into laterally well-connected, porous and permeable strata. These dissolution-enhanced reservoirs responded to secondary (waterflood) and tertiary (CO₂ assisted waterflood) recovery operations, with secondary recovery matching primary recovery. In one oil field meteoric water rehydrated anhydrite beds and nodules to gypsum and hydraulically fractured individual evaporite beds and 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 this 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 Bolson, 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 with bottom-hole well log temperatures of 37° C.