ABSTRACT

Integration of detailed geologic, engineering, and petrophysical analyses, combined with improved well-log analytical techniques, may be used by independent oil and gas companies for successful infield exploration in mature fields in the Gulf Coast that larger companies may consider uneconomic. The secondary gas recovery (SGR) project, conducted by the Bureau of Economic Geology and funded by the Gas Research Institute, the U. S. Department of Energy, and the State of Texas, identified a potential additional natural gas resource of 7.6 billion cubic feet (BCF), of which 3.1 BCF may be recoverable in a 490-acre lease in Agua Dulce field. Five wells in this lease had previously produced 13.7 BCF from Frio reservoirs at depths of 4,600 to 6,200 ft. Pay zones occur in heterogeneous fluvial reservoirs that are offset by faults associated with the Vicksburg Fault Zone. Poorly drained or isolated reservoir compartments occur where channel-fill and crevasse-splay reservoirs are partitioned by these faults. Some compartments may contain <=0.5 BCF of gas resources, based on previous completions and recent infield drilling by Pintas Creek Oil Company, an independent exploration and production company.

Uncontacted gas resources at Agua Dulce field occur in thin (typically <10-ft), bypassed zones that can be identified through computed log evaluation integrating open-hole logs, wireline pressure tests, fluid samples, and cores. At Agua Dulce field, such analysis identified a 4-ft bypassed zone uphole from previously produced reservoirs. This reservoir contained original reservoir pressure and flowed at rates exceeding 1 million cubic feet per day. The expected ultimate recovery is 0.4 BCF.

Procedures developed in evaluating Agua Dulce field can be successfully applied to other mature gas fields in the South Texas Gulf Coast. For example, Stratton and North McFaddin are two other fields in which the SGR project has demonstrated the existence of thin, potentially bypassed zones that may yield large additional gas resources, extending the economic life of these fields.