Print this pageStratigraphic Traps in Base-Level Rise Deposits of Braided Alluvial and Arid Coastal-Plain Sandstones (Frisco City Sand, Jurassic Haynesville Fm.), Southwest Alabama
HANDFORD, C. ROBERTSON, and LAWRENCE R. BARIA
Stratigraphic traps in the Upper Jurassic Frisco City sandstone surround isolated and buried Appalachian basement highs or inselbergs near the updip 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 Fields with four-way closure have formed where the sands and basement highs are overlain by topsealing marine shales. The largest of these fields (North Frisco City) will produce >30 MMBO from 16 wells.
Ephemeral stream-channel and sheetflood 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 either the Smackover Fm., 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 Haynesville Fm. 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, perhaps driven by local tectonics during the extension of the basin.
High-resolution 3-d seismic lines display stratal patterns interpreted as retrogradational shoreface or alluvial-lobe sand bodies onlapping basement highs. Differing oil/water contacts between these sand bodies may indicate compartmentalized reservoirs.