Control of Reservoir Porosity and Permeability by Original Depositional Fabric: Emma San Andres Field, West Texas

Stephen C. Ruppel

The Emma San Andres field exhibits many of the classic signs of a depleted reservoir including declining annual production rates and high water cuts. By the end of 1986, oil production from the Emma reservoir, more than 19 million bbl, totaled nearly 100% of the estimated ultimate recovery. Recent studies, however, indicate that as much as 25 million bbl of recoverable mobile oil still remain in the reservoir. These studies also indicate that the observed poor recovery efficiency (32%, typical for San Andres/Grayburg reservoirs) is due to reservoir heterogeneity caused primarily by variations in original depositional fabric.

Two distinct porosity intervals are recognized in the Emma reservoir. The lower interval is composed of fusulinid packstone/wackestone that contains moldic and intercrystalline porosity and low (average 2 md) permeabilities. These deposits are continuous and relatively uniform throughout the area; net pay in this zone is controlled primarily by the field structure, a low-relief northwest-trending anticline. The upper interval comprises skeletal grainstone characterized by intergranular porosity and higher (up to 200 md) permeabilities. In contrast to lower interval rocks, these deposits exhibit marked variations in lateral extent and reservoir quality that are related to original depositional processes. Thick accumulations of porous skeletal grainstone are present well off the field s ructure.

Comparison of production data with faces mapping suggests that most oil production has been controlled by the distribution of skeletal grainstone and not by structure. Effective exploitation of this remaining mobile oil must include selective completion and injection programs based on variations in the distribution of skeletal grainstone. The absence of significant production from local grainstone thicks, for example, points to inefficient drainage in several parts of the field.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.