ABSTRACT

A petrophysical evaluation was made of the Apalachicola embayment Jurassic rocks from all available wells, using ores, cuttings, and electric logs. From this information, I propose that the embayment was formed by basement faulting, which resulted in a grabenlike structure. Two basins were formed on either side and within the embayment, together with a structurally high area at the entrance of the bay. These structures strongly influenced the patterns of water circulation and sediment deposition throughout the Jurassic. The Louann Salt was laid down throughout the embayment but was later eroded, with some of the Eagle Mills Formation, by the Norphlet fluvial sands. These Norphlet sands were deposited in a very arid climate by braided streams and then moved into eolian dunes. The initial marine transgression of the Smackover eroded the top of most of these sand deposits and laid down, in place, a fine-grained dense carbonate throughout the embayment in a tidal-flat environment. During the late Smackover marine regression, additional sediment was deposited, which was subsequently subaerially exposed to freshwater percolation, resulting in dolomitization. Lower Haynesville Buckner anhydrite was laid over the Smackover limestone, but much of the anhydrite was eroded by the continental sediments of the Haynesville. The Haynesville sediments show that there were marine fluctuations into the embayment throughout this time. The Cotton Valley sandstones were deposited throughout the area including the embayment. As hydrocarbon potential exists in the Norphlet and Smackover, a petrologic examination was concentrated on these formations. The fluvial Norphlet sands are conglomeratic, fine grained with interbedded red shales in the updip position, but were white to red, fine grained, and quartzitic in the center and at the mouth of the embayment. The Smackover limestones and dolomites are shallow, tidal-flat marine deposits and are composed of algal-coated grains, fecal pellets, Foraminifera, echinoid spines, and mollusk shells. Porosity and permeability reduction is due to large calcite crystal growth and closely packed dolomite rhombs in the pore spaces and pore throats. From a model application of a modern analog of a carbonate tidal flat, the most favorable reservoir rock would be expected in intertidal channels and sediment buildups at the embayment mouth.

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