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Calcareous shales, called cap rocks or capping rocks, are present just above the deep undercompacted and geopressured intervals in the Gulf Coast district of the United States. The cap rocks are believed to have been formed by precipitation of minerals carried in aqueous solution by compaction water moving vertically upward and/or horizontally from the geopressured intervals.
Deep sandstones, in which significant secondary porosity has been developed by leaching of grains and cement, could have been the prime source of such precipitated minerals. Differences of physical and chemical environments (especially pressure, temperature, water salinity, and pH) between the geopressured and normally pressured zones may have been the principal cause of mineral leaching and precipitation.
It has been suggested that the generation of carbon dioxide gas during maturation of organic matter, and solution of the gas in water to form an acidic (low pH) environment, are the prime causes for leached calcite and feldspar. Woody and herbaceous organic matter, commonly associated with deltaic sandstone deposition, seems to have produced more of the gas than algal organic matter, facilitating dissolution of calcite, feldspar grains, and cement. The acidic solution containing the mineral ions would have moved to shallower intervals and mixed with more normal brines, thus increasing pH. The minerals would be precipitated there.
Differences of pressure and temperature between the deep and hot geopressured interval and the shallow and cool hydropressured interval may be an additional cause for mineral precipitation in the shallow interval. Relatively high concentration of other ions in the lower part of the hydropressured zone may also have facilitated precipitation of the minerals.
Volumetric and cumulative estimates of compaction water squeezed from sedimentary rocks suggest that the water volume throughput is excessively large in the zone just above the geopressured interval, where cap rocks are normally formed. Thus, the compaction water is an important agent for transporting minerals from deep to shallow subsurface and for precipitating them in the zone just above the geopressured interval.
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