Deep drilling in the interior salt basin of Mississippi has revealed geopressured oil, gas, and water with high-pressure gradients. These geopressures are mainly in the Jurassic Smackover and Norphlet Formations, but also may be in overlying formations. The geopressures rise stratigraphically in a basinward direction and increase their gradients with depth. The highest documented Smackover pressure gradient in Mississippi is 1.06 psi/ft recorded in saltwater flows from a 23,455-ft wildcat. The highest Smackover gas gradient is 0.99 psi/ft at 22,250 ft. Pressure-gradient reversals are recorded in some parts of the basin.

Deep Smackover geopressures differ from relatively "leaky" geopressures in the Gulf Coast Tertiary in that they underlie nonshale crystalline seals with no transition zone. Deep Smackover geopressures cannot be predicted from compaction trends because cores reveal that geopressured Smackover sandstones are compacted severely, whereas their vuggy porosity is secondary in origin and is not a result of "undercompaction" related to geopressures as in the Gulf Coast Tertiary.

Geopressured gas mixes range up to 100 percent carbon dioxide and 78 percent hydrogen sulfide. The nature and distribution of these gases suggest they are late thermal migrants and late thermal metamorphic alterations of former oil reservoirs. The geopressures they have generated are young pressures in this "old" basin and are termed inflated and phase pressures, respectively. Associated geopressured acidic fluids appear to have dissolved available soluble minerals, thereby creating late secondary porosity in compacted sandstones which are now the deep gas reservoirs.

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