Gypsum can form penecontemporaneously in carbonate sediments, and it may be altered to anhydrite during burial. Some workers have suggested that gypsum dehydration is complete at burial depths of 3500 ft (1067 m); however, gypsum has been documented at depths as great as 13,100 ft (3993 m).
Two examples of deeply buried gypsum illustrate contrasting origins. Gypsum in the Permian San Andres Formation in the Permian Basin of west Texas at a depth of 6000 ft (1829 m) occurs as anhydrite nodules with an outer rind of gypsum, suggesting that anhydrite was rehydrated to gypsum by interaction with interstitial brines. In contrast, gypsum in the carbonate beds of the Cretaceous Ferry Lake Anhydrite of the Mississippi Interior Salt Basin at 13,100 ft (3993 m) occurs as gypsum nodules with a rind of anhydrite, suggesting that the gypsum is primary and in the process of dehydration to anhydrite.
These contrasting origins are likely a function of evaporite and carbonate stratigraphy. In the Permian Basin, a thick section of carbonate rocks is overlain by a relatively thin section of evaporites. In the Mississippi Interior Salt Basin, carbonates are interbedded with multiple evaporite beds, which inhibit circulation of interstitial brines. The burial temperature, pressure, and salinity of these formations suggest that gypsum could be preserved as primary or secondary gypsum.
The presence of gypsum in reservoir rocks can affect porosity calculations, especially calculations using neutron porosity logs, which measure bound water of hydration in gypsum as porosity. Identification of gypsum at the depth of 13,100 ft (3993 m) indicates that log estimates of porosity in some deep carbonate reservoirs containing gypsum could have significant porosity and water saturation measurement errors.