Abstract: An Integrated Study of the Grayburg/San Andres Reservoir, Foster and South Cowden Fields, Ector County, Texas

Hoxie W. Smith, James J. Reeves, Robert C. Trentham, David A. Rowland

A cooperative study of the Grayburg/San Andres reservoir is being conducted in response to the United States Department of Energy's Class II Oil Program. The purpose of the study is to preserve access to existing well bores by identifying additional reserves. Production problems associated with a shallow-shelf carbonate reservoir will be evaluated by a technical team integrating subsurface geological and engineering data with 3D seismic data. The proposed 3D seismic survey and study will cover five leases in a 1.5 square-mile area of the Foster and South Cowden fields. Results of the study will be used to design a 3D reservoir simulation model and subsequent waterflood.

The spatial distribution of flow parameters important in reservoir characterization are controlled by the sequences and parasequences forming the reservoir. Within the project area the Grayburg can be divided into at least two and possibly three shallowing upward sequences (each 100-150 feet thick). Each sequence consists of a series of stacked, shallowing upward parasequences (5-20 feet thick) that make up the flow units.

The planned 3D seismic survey has been designed to resolve changes in the subsurface on the megascopic scale, allowing the project reservoir engineer to correlate petrophysical properties and stratigraphic information between boreholes. Pre-planning for the survey has also included: seismic modeling, use of aerial photos to optimize fold in an area with cultural obstructions and acquisition design for bin fractionization of the data.

Several 3D seismic surveys have been acquired over Grayburg/San Andres fields by major oil companies (Chevron-McElroy field, Amoco-North Cowden field, Shell-Wasson field). However, this type of survey has not been undertaken by independent oil operators due to the perceived high cost of imaging shallow reservoirs. This project specifically addresses the application of 3D seismic technology by independents who operate smaller leaseholds of Grayburg/San Andres production.

It is expected that this study will demonstrate a methodology for reservoir characterization and subsequent development of the Grayburg/San Andres reservoir that is feasible for independent operators. Foremost, it will stress the importance of utilizing an integrated multidisciplinary approach for reservoir development. This method of reservoir evaluation should be relevant to shallow-shelf carbonate reservoirs throughout the United States.

AAPG Search and Discovery Article #90960©1995 AAPG Southwest Section Meeting, Dallas, Texas