Short-chain aliphatic acid anions in deep subsurface brines: A review of their origin, occurrence, properties, and importance and new data on their distribution and geochemical implications in the Palo Duro Basin, Texas

Abstract

Short-chain aliphatic acid anions, which are produced during the thermal maturation of sedimentary organic matter, are common in oil-field brines and other deep formation fluids. Their abundance and distribution are controlled primarily by their rates of formation, thermal and bacterial destruction, and reservoir flushing. This first part of this manuscript reviews literature on their abundance and distribution in deep basin brines, their geochemical and biogeochemical properties, and their geochemical importance, which far outweighs the number of analytical measurements that have been published. The second part reports new data from the Palo Duro Basin in the Texas Panhandle. Deep brines from the Palo Basin contain short-chain aliphatic acid anions in concentrations ranging up to 440 mg/l. The observed order of abundance is acetate ≫ propionate > butyrate > valerate. Aliphatic acid anion contents vary randomly with subsurface temperature, depth, and geologic age, but correlate well with groundwater residence time, as inferred from δ 18O measurements. Groundwaters having the longest residence times contain the highest organic acid anion contents. Organic content, I −, and Br − in these deep brines appear to have a common source, and all of the aliphatic acid anions, most of the I − and some portion of the Br − are interpreted to have been derived from lipid-rich sedimentary organic matter. This observation complicates the use of Br − as a conservative marker constituent when inferring groundwater origin and evolution using Cl −/Br − relationships. Also, simplified calculations indicate that these organic acid anions are not important in complexing metals. Their presence in relatively high concentrations in this basin with its low petroleum potential indicates that they cannot be used alone as petroleum proximity indicators.