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An Unusual Deep Occurrence of Gypsum in the Ferry Lake Previous HitAnhydriteNext Hit, South Mississippi Salt Basin

F. O. Amadi1, R. P. Major1, and Lawrence R. Baria2
1Geology and Geological Engineering, The University of Mississippi, University, MS
2Jura-Search, Inc., Jackson, MS

The anhydritization of primary microcrystalline gypsum is well established in previous literature to be initiated in the shallow subsurface by high temperature, reaction to high-salinity brines, and continued burial diagenesis. The depth of gypsum to Previous HitanhydriteNext Hit transition varies from region to region. Many workers have suggested that this transition will occur at a depth greater than 6,000 ft (1,800 m), thus leading to the generalization that gypsum is not expected to exist below this depth. A study of the Lower Cretaceous Ferry Lake Previous HitAnhydriteNext Hit in the southern Mississippi Salt Basin at depths greater than 13,000 ft tests the validity of this generalization.

The Ferry Lake Previous HitAnhydriteNext Hit unit within the south Mississippi Salt Basin is composed of approximately 250 ft (76 m) of alternating carbonate and evarporite beds that have been interpreted as being deposited within a stratopycline free-flow exchange basin. The carbonate beds contain various sizes of evaporite nodules ranging from 2-10 mm. A petrographic study of samples from the carbonate beds at depths of 13,320 to 13,450 ft (4,060 - 4,099 m) indicates approximately 20 % gypsum in the evaporite nodules. We observe partial to complete diagenetic anhydritization of microgranular gypsum, with Previous HitanhydriteTop laths oriented preferentially along the cleavage planes and enclosing the gypsum crystals.

The presence of gypsum in reservoir rocks can effect porosity calculations, especially using neutron porosity logs, which measures bound water of hydration in gypsum as porosity. Identifying gypsum at the depth of 13,320 ft (4,060 m) indicates that log estimates of some deep carbonate reservoirs that contain evaporite crystals could possibly have significant porosity and water saturation measurement errors as a consequence to this generalization, and as such should be evaluated petrographically whenever possible.


AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas