Abstract

Keystone field, Winkler County, Texas, is located in the northwestern part of the Central Basin Platform of the Permian basin, approximately 10 mi from the platform margin. The principal trapping mechanism is a northwest-southeast-trending anticline. The Keystone (San Andres) reservoir, which is approximately 400 ft thick, produces solution-gas-driven oil from a depth of 4,000 ft. Evaluation of logs and drill cuttings and comparison with cores from an analog reservoir provide the basis for geologic description.

The reservoir is composed of an upward-shoaling sequence of subtidal grainstone/packstone that is thoroughly dolomitized and partially cemented with sulfates. The upward-shoaling sequence is capped by tidal flat deposits, principally finely crystalline mudstone. Locally, post depositional leaching has increased porosity and permeability and some anhydrite has been altered to gypsum. The combined effects of depositional environment and diagenetic leaching and alteration control rock properties used to define five reservoir flow units.

This study divides the Keystone (San Andres) reservoir into five flow units based on depositional environments and distribution of gypsum. Oil production is principally from porous, leached subtidal grainstone/packstone and leached subtidal packstone/wackstone (units I and II) in the updip, western part of the study area, and principally from porous, leached subtidal packtone/wackstone (unit IV) in the downdip, eastern part, of the study area. Several opportunities for recompletion in units I and II are present in an area of more than 4 mi² surrounding the crest of the structure, where relatively few wells are in communication with these proven productive reservoir zones.