AMBROSE, WILLIAM A., and RAYMOND A. LEVEY, Bureau of Economic Geology, University of Texas at Austin, Austin, TX, JOSE M. VIDAL, ResTech, Inc., Houston, TX, MARK A. SIPPEL, Research and Engineering Consultants, Inc., Englewood, CO, JAMES R. BALLARD, Envirocorp Services and Technology, Houston, TX, DAVID M. COOVER, JR., Pintas Creek Oil Company, Corpus Christi, TX, and WALTER E. BLOXSOM, Coastal Texas Oil and Gas, Houston, TX

ABSTRACT: Secondary Natural Gas Recovery in Mature Fluvial Sandstone Reservoirs, Frio Formation, Agua Dulce Field, South Texas

An approach that integrates detailed geologic, engineering, and petrophysical analyses combined with improved well-log analytical techniques can be used by independent oil and gas companies for successful infield exploration in mature Gulf Coast fields that larger companies may consider uneconomic. In a secondary gas recovery project conducted by the Bureau of Economic Geology

and funded by the Gas Research Institute and the U.S. Department of Energy, a potential incremental natural gas resource of 7.7 bcf, of which 4.0 bcf may be technically recoverable, was identified in a 490-ac lease in Agua Dulce field. Five wells in this lease had previously produced 13.7 bcf from Frio reservoirs at depths of 4600-6200 ft. The pay zones occur in heterogeneous fluvial sandstones offset by faults associated with the Vicksburg fault zone. Poorly drained or isolated reservoir compartments occur where lenticular channel-fill and crevasse-splay sandstones are separated by these faults. The compartments may each contain up to 1.0 bcf of gas resources, with estimates based on previous completions and the recent infield drilling experience of Pintas Creek Oil Company, an indep ndent exploration and production company in Corpus Christi, Texas.

Uncontacted gas resources occur in thin (typically less than 10 ft) bypassed zones that can be identified through a computed log evaluation that integrates 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 mmcf/d. The expected ultimate recovery is 0.4 bcf.

Methodologies developed in the evaluation of Agua Dulce field can be successfully applied to other mature gas fields in the south Texas Gulf Coast. For example, Stratton and McFaddin are two fields in which the secondary gas recovery project has demonstrated the existence of thin, potentially bypassed zones that can yield significant incremental gas resources, extending the economic life of these fields.

AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.