LEVEY, RAYMOND A., and LIANGQING XUE, Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

ABSTRACT: Distribution of the Largest Gas Reservoirs in Third-Order Depositional Sequences and Systems Tracts of the Frio Stratigraphic Unit in the Gulf Coast Basin

The gas-prone Oligocene Frio stratigraphic unit (FSU) is the dominant hydrocarbon-bearing lithostratigraphic unit of the seven major siliciclastic Paleogene depositional episodes in the northwest Gulf of Mexico Basin. Cumulative production from gas reservoirs in the FSU exceeds 66 Tcf of the greater than 300 Tcf produced in Texas. Analysis of an extensive onshore well log network of depositional-dip- and depositional-strike-oriented cross sections led to the identification of unconformity-bounded units, correlative conformity surfaces, and flooding surfaces in the FSU. Regionally correlatable surfaces define two third-order depositional sequences in the FSU from the Rio Grande Embayment of South Texas and Mexico to the Houston Embayment of the Texas and Louisiana border.

More than 65 reservoirs from 47 Frio gas fields that produced greater than 100 Bcf account for more than 21 Tcf cumulative gas production in 12 gas plays. Regional cross sections and type logs from the largest gas fields illustrate the spatial relationships of these major gas-bearing reservoirs to depositional sequences and systems tracts in the FSU. The most prolific gas reservoirs are primarily associated with transgressive (>10 Tcf, 50 percent) and highstand (9 Tcf, 41 percent) deposits. In contrast to previously published concepts of exploration applications of sequence stratigraphy and reservoir distribution, reservoirs in lowstand deposits are relatively minor, accounting for only 10 percent (2 Tcf) of the gas produced in the FSU. Infield reexploration in mature fields may be optimized by scaling down to understand the distribution of gas resources remaining in smaller reservoirs that have been bypassed or incompletely developed. Understanding the distribution of gas reservoirs can help to identify and rank other opportunities.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.