Laura C. Zahm¹, Scott W. Tinker¹
(1) Marathon Oil Company, Littleton, CO

Abstract: 3-D reservoir characterization in the absence of seismic; Olson Field, San Andres/Grayburg, West Texas

The Olson reservoir characterization project applied an iterative approach for constructing a 3-D reservoir model by integrating core, log, production data, and a depositional model within a reasonable sequence stratigraphic framework. The primary goal was to represent the geologic system accurately in 3-D, including its barriers to flow and distribution of good reservoir quality rocks, without 3-D seismic data. Olson field is located on the Ozona Arch. Olson field is now in secondary recovery with a total of 130 wells penetrating the San Andres/Grayburg Formations.

This project integrates all available data into a 3-D reservoir model. Two critical steps in this non-seismic interpretation were petrophysical work, and sequence-stratigraphic interpretation using a depositional model. A primary challenge was the calibration of porosity log measurements and other unscaled log values into an acceptable field-wide range. A customized end-point scaling technique applied to total gamma ray, raw neutron counts, sonic, and density measurements. From cores (14 wells, 1,470 feet total), six key facies were defined. The two primary reservoir facies are oolitic grainstones and fusulinid grain-dominated packstones. Facies were predicted in wells without core using discriminant analysis. A depositional model was developed from cores to help interpret the sequence stratigraphy of the San Andres/Grayburg reservoir interval. This interval was divided into eight high-frequency sequences, which prograde in a basinal (NNE) direction. Three-dimensional facies and predicted facies distributions helped constrain reservoir distribution and diagenetic products in the 3-D model. Perforated intervals, initial production rates, and oil-type were also distributed in the 3-D model. Since its completion in 1998, the model has been utilized to examine OOIP, EOR, injection efficiency by well pattern, and in selection of infill well locations.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana