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Abstract

Petrographic and geochemical studies of cap-rock core of two salt domes, Gyp Hill in South Texas and Oakwood in East Texas, reveal significantly different diagenetic histories for each dome. Cap rock on Gyp Hill is now forming within shallow meteoric aquifers. In contrast, cap rock on Oakwood Dome formed principally during the geologic past within deeper saline aquifers in the East Texas Basin.

Gyp Hill cap rock, which is 271 m (890 ft) thick, is composed of 149 m (490 ft) of anhydrite overlain by 122 m (400 ft) of gypsum. An uncemented anhydrite sand marks the salt/cap-rock interface. From 4 m (13 ft) above the interface to the top of the anhydrite, porosity is occluded by poikilotopic gypsum cement. Occurrence of gypsum cement indicates low-temperature and low-salinity conditions during cap-rock formation, that is, dome dissolution in a shallow meteoric aquifer. The overlying gypsum results from hydration of anhydrite by meteoric ground water.

Oakwood cap rock, which is 137 m (450 ft) thick, is composed of 78 m (256 ft) of anhydrite overlain by 59 m (194 ft) of calcite. In contrast to Gyp Hill anhydrite, the Oakwood anhydrite is entirely devoid of gypsum cements except that the interface between anhydrite and calcite. The anhydrite has been deformed and recrystallized into a moderately well-developed granoblastic texture that is indicative of high-temperature and high-pressure conditions. The anhydrite section is thought to have formed by salt dissolution at deep, high-temperature, saline conditions. Petrographic, geochemical, and isotopic evidence from the dark calcite indicates that it is the product of calcium sulfate reduction by hydrocarbons in a saline, deep-basinal fluid. Another deep-basinal fluid more enriched in Sr, Ba, Mg, and Mn, dissolved dark calcite, which reprecipitated as coarsely-crystalline light calcite. The only effect of meteoric water on the Oakwood cap rock is the presence of gypsum in the calcite/anhydrite transition zone.

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