Stratigraphic traps in the Upper Jurassic Frisco City sandstone surround isolated and buried Appalachian basement highs or inselbergs near the ump margin of the Gulf Coast Basin in southwest Alabama. Reservoir sands and gravels, representing arid coastal plain environments, onlap the basement highs at depths of 9,000-12,000 ft. Traps with four-way closure have formed where the sands and basement highs are overlain by top sealing marine shales. The largest of these fields discovered to date (North Frisco City) is expected to produce over 24 MMBO from 16 wells.

Ephemeral stream channel and sheet flood processes deposited much of the Frisco City sand. Aprons of coarse sand and metamorphic clasts accumulated around the inselbergs as rockfall and debris-flow deposits. The basinward margin of the clastic wedge was reworked by eolian and marine shoreface processes.

Frisco City sands sharply overlie the Smackover Formation, Buckner anhydrite, or Paleozoic basement, marking a basinward shift of facies and sequence boundary (141.5 Ma) at the contact. Basal Frisco City deposits consist of coarse alluvium, but these pass upward into coastal eolian and shoreface sands, and black marine shales of the middle Haynesville Formation. This succession indicates that deposition occurred during an overall base-level rise. However, some facies stacking patterns suggest that the overall rise was punctuated by several high-frequency base-level transit cycles or local tectonic relaxations during the final stages of extention in the Jurassic Gulf basin.

High-resolution 3-D seismic lines display stratal patterns indicative of retrogradational shoreface or alluvial lobe sand bodies onlapping basement highs. Differing oil/water contacts between these sand bodies may indicate compartmentalized reservoirs.

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