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Dissolved organic species are present in high concentrations in almost all subsurface waters, especially those associated with petroleum. The highest concentrations (as much as 10,000 mg/1) are present in formation waters from relatively young reservoir rocks with subsurface temperatures of 80 to 100° C. As a result of thermal decarboxylation, concentrations of dissolved organics decrease with increasing temperatures above 100° C. The concentrations of organic species in subsurface waters with temperatures lower than 80° C are generally less than about 100 mg/1 because of bacterial degradation. Short-chain (C2—C5) aliphatic acid anions (principally acetate and propionate) form as much as 90% of these dissolved organic species. Detailed chemical analysis using GC and GC/MS techniques has shown the presence of longer-chain aliphatic acid anions (C6—C9), dicarboxylic acid anions (C4—C10), phenols and other organic species in formation waters from High Island field, offshore Texas.

Dissolved organic species may play an important role in mineral diagenesis. This role is related to the following chemical properties. (1) They can be the dominant source or sink for the hydrogen ion (H+), and thus, directly or indirectly control the pH and buffer capacity of subsurface waters. (2) They behave as reducing agents controlling the oxidation state (Eh) of subsurface waters and the concentrations of multivalent elements (e.g., iron). (3) They can be decarboxylated, thermally or with the aid of bacteria, into carbon dioxide and hydrocarbon gases. (4) They form soluble complexes with metals and other inorganic species, but quantitatively these complexes are not as important as previously reported. Computations with a geochemical model show that when organic as well as inorganic species are considered, the diagenetic mineral sequences predicted by the model are in general agreement with those observed in the field.

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