Permeability Distribution and Reservoir Continuity in Permian San Andres Shelf Carbonates, Guadalupe Mountains, New Mexico

Paul D. Hinrichs, F. Jerry Lucia, Robert L. Mathis

Outcrop exposures of Permian San Andres dolomites were examined in the Guadalupe Mountains of southeastern New Mexico to gain insight into the lateral continuity of permeable, reservoir-quality beds. The investigation focused on the continuity that can be expected between an injector and producer on 40-ac spacing or 1,320 ft. The exposed rocks closely resemble subsurface producing strata, both in their regional setting on a broad carbonate shelf and in their detailed petrographic and petrophysical properties. Weathering of the outcrop is not so severe as to invalidate the conclusions. Individual beds were sampled at intervals of 100 ft, 10 ft between selected 100-ft intervals, and 1 ft or less between selected 10-ft stations. More than 1,000 permeability plugs were analyz d.

Permeable beds are found in the middle of the San Andres section starting about 350 ft below the top of the San Andres Formation. The middle and upper sections of the San Andres are well bedded, and abrupt lateral facies changes were not found. Beds with "pay"-type permeability are interbedded with laterally continuous impermeable mudstone that should act as effective barriers to crossflow. All permeable beds would have to be perforated to drain or flood this type of reservoir effectively. Permeability within a given reservoir bed is highly variable, with the range of values averaging three orders of magnitude and the standard deviation averaging one order of magnitude. This range is present irrespective of the sampling interval. The pay-quality rock appears continuous on the 40-ac sc le within any of the reservoir beds. The permeability path is tortuous, however, suggesting that oil could be trapped or bypassed in low-permeability pockets or reentrants.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.