Predicting Carbonate Mineral Precipitation/Dissolution Events during Progressive Diagenesis of Clastic Rocks

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There is an observable, regular progression of early and late carbonate cements that is separated by carbonate mineral dissolution in many sandstones during progressive burial and diagenesis. The distribution of early cements is a function of incipient hydration of framework grains, sulfate reduction, and bacterial methanogenesis. These early cements typically precipitate from the sediment water interace to burial depths corresponding to about 80C. The distribution of late carbonate cements is a function of the relationship of organic acid anions, aluminosilicate reactions and CO{sub 2} in formation waters. Elevated PCO{sub 2} in a fluid where the pH is buffered by organic acid anions or aluminosilicates results in precipitation of a late carbonate cement (typically ferroan); these late carbonate cements generally form over the temperature interval of 100-130C. The late and early carbonate cement events are separated in time by a period of carbonate mineral dissolution or nonprecipitation. This dissolution is related to the increase in concentration of carboxylic acid anions resulting from the thermocatalytic cracking of oxygen-bearing functional groups from kerogen and/or redox reactions involving kerogen. Examples of the importance of early carbonate cementation/decementation to hydrocarbon reservoirs include the Campos basin of Brazil and the U.S. Gulf Coast. An example ofmore » late carbonate cementation/decementation includes the Norphlet Formation. This observed sequence of cementation and decementation can be modeled, and the modeling results can be used to predict enhanced porosity in the subsurface.« less