Distribution of dissolved volatile fatty acids in some Louisiana oil field brines

Abstract

Interest in the distribution of dissolved volatile fatty acids, VFAs, in subsurface brines has been stimulated by the role these substances may play as proximity indicators to hydrocarbons, as potential precursors of some natural gas, as aqueous complexing agents, and as diagenetically-reactive acids. We have studied the distribution of individual VFA species in subsurface brines from three oil fields in the hydropressured portion of the Tertiary section of South Louisiana. Total VFA concentrations vary from 2 to 166 mg/l over a sampled depth range of from 660 to 2900 m and a temperature range of from 40 to 90°C. The VFAs in part have apparently been generated at greater depth and transported upward where they have undergone progressive degradation. In some areas, acetate appears to have been preferentially degraded over propionate, and propionate preferentially degraded over iso- and n-butyrate and iso- and n-valerate. VFAs contribute from 5 to 35% of the total titration alkalinity of most of the waters analyzed. The spatial distribution of individual dissolved VFA species at each of the three fields studied is unique, complex, but internally systematic, and must ultimately be controlled by local variations in the rates of production, degradation, and transport of these compounds. The VFAs thus have the potential for helping to delineate the dynamics of on-going subsurface mass transport. As such, further understanding of controls on their distribution will aid in the exploration for hydrocarbons.