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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 952

Last Page: 952

Title: Capping Rock and Its Formation: ABSTRACT

Author(s): Kinji Magara

Article Type: Meeting abstract


Calcareous shales, called capping rocks, are found in the zone immediately above the deep undercompacted and geopressured intervals in the Gulf Coast district. The capping 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 leached grains and cement, could have been the prime source of such precipitating 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 CO2 gas associated with thermal maturation of organic matter and its solution in water, causing an acidic environment (low pH), are the prime causes for leached calcite and feldspar. Woody and herbaceous organic matter, which are commonly associated with deltaic sandstone deposition, seem to have produced more CO2 gas than algal organic matter, facilitating dissolution of calcite and feldspar grains and cement. The acidic solution, which contains these 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, temperature, and concentration of other ions in water between the deep and hot geopressured interval and the shallow and cool hydropressured interval may also be an additional cause for mineral precipitation at the shallow interval.

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