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abstract

Carbon dioxide produced by decarboxylation of organic matter is not a dominant factor in secondary porosity development. In typical sedimentary basins it may account for a regional average of a few volume percent secondary porosity, but it is insufficient to account for all of the secondary porosity in Texas Gulf Coast sandstones.

Petrographic data indicate dissolution of a minimum of 5 volume percent feldspar from Paleogene sandstones of the Texas Gulf Coast. The amount of acid required for feldspar dissolution depends on its mineralogy and the fate of aluminum. Feldspar dissolution in Texas Gulf Coast sandstones probably requires at least 1 mole of protons for every mole of feldspar dissolved.

Carbonate dissolution, a process of debatable importance in sandstones, was probably of major significance in many Gulf Coast shales. This is indicated by the strontium isotopic composition of carbonate cements in sandstones, and the decreasing weight percent calcite in shales in some wells. Carbonate cementation in sandstones probably occurred in response to lowered CO2 solubility in upward-migrating formation water.

Considerations of the solubility of carbon dioxide and the distribution of organic acids with respect to temperature suggest that these species will be of great importance in porosity enhancement only if generated locally. Other potential sources of acid for dissolution reactions are reverse weathering reactions in shales and hydrous pyrolysis reactions between organic carbon and oxygen in H2O (including synthesis of organic acids from kerogen).

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