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Transitions from carbonate to terrigenous clastic sedimentary deposits are commonplace in the Mesozoic-Cenozoic section of the northwestern Florida shelf. On a regional scale, these transitions are responsible for a large seismic velocity variation between the areas of the Destin dome and the Middle Ground arch. In the Destin region, clayey shales and sands are more prevalent, interspersed with carbonates and evaporites, with the result that seismic transmission velocities are relatively low. Toward the south on the Middle Ground arch, the increased carbonate-evaporite content of the section results in much higher velocities. An example of this variation is that a reflection two-way travel time of 2 sec corresponds to a depth from 2.5 to 2.8 km (1.5 to 1.7 mi) in the Dest n area while this same reflection time corresponds to a depth of 4 km (2.5 mi) in the vicinity of the Texaco 2516 well on the Middle Ground arch. Analyses of stacking velocities indicate that the transition is a gradual one to the north and west of Middle Ground arch.
On a local scale, transitions or terminations related to facies changes, erosion, or sediment body geometries are a potentially important factor in prediction of reservoir rock on the as yet uncondemned, 12 km (7.4 mi) broad, deep structural culmination west of the Destin tests and on the untested, 9 km (5.5 mi) broad, deep structure 20 km (12.4 mi) south of the Destin dome. The deep Exxon test on the Destin dome encountered 20 m (66 ft) of Norphlet quartz sand with porosity ranging from 20 to 30% and permeability of 1 darcy. This potentially excellent reservoir bed at a depth of 5,224 m (17,138 ft) is more than 150 m (492 ft) below the deep structural crest on the Destin dome. The Sun test, 25 km (15.5 mi) east of the Exxon well, penetrated 6 m (19.6 ft) of Smackover oomoldic dolomit with porosities of 13 to 15% and failed to find any Norphlet sand as it bottomed in Louann salt immediately below the Smackover. A study of a combination of velocity analyses, density and velocity logs, and synthetic seismograms allows speculations that the deep Destin dome and the structure on its south flank are still viable exploration prospects.
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