ABSTRACT: Estimation of Reservoir Quality in Low-Permeability Sandstones from Production Data, with Application to the Lower Vicksburg Gas Trend of South Texas

LEHMAN, JAY A., ARCO Oil and Gas Co., Houston, TX

The Lower Vicksburg gas trend of south Texas is characterized by prolific production from thick intervals of low-permeability sandstones. These reservoirs are difficult to evaluate with standard log analysis owing to rapidly changing stratigraphy, thinly laminated reservoirs, and low-resistivity pay. To circumvent these log interpretation problems, a method is proposed that utilizes production information and structural mapping to evaluate the reservoir quality of producing formations.

The foundation for this analysis is the direct relationship between water saturation and structural position based on the principles of capillary pressure behavior in low-permeability rocks. In these reservoirs, the higher a well is on structure, the lower the water saturation and, hence, the higher the gas production per unit of reservoir rock. Deviations from the linear relationship between gas production and structural position are attributed to mappable variations in reservoir quality.

The key ingredients of this approach are developed in a study of East Jeffress field, a 100-bcf Lower Vicksburg accumulation in South Texas. The 14 producing wells in the field are plotted on a graph of structural position versus gas production. Deviations from a linear trend on the graph guide the interpreter to mappable reservoir-quality

units. At East Jeffress field, this analysis identified a northwest-southeast-trending fairway of excellent reservoir quality that quickly diminished to the northeast and southwest. This field example demonstrates the power of this method to evaluate and map reservoir-quality variations in the face of difficult log analysis, poor-quality logs, or even no logs at all.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)