ABSTRACT

There is considerable interest at the present time in the origin and distribution of dissolved volatile fatty acids (VFAs) in oil-field brines. This interest has developed as a result of the recognition of the role these compounds may play: 1) in subsurface acid attack and porosity enhancement, 2) as precursors of natural gas, and 3) as possible proximity indicators of hydrocarbon accumulations. The detailed study of the distribution of dissolved VFAs on a field scale provides much useful information on processes of generation, transport, and degradation of these compounds.

At the Iberia oil field, one of several fields we have studied in the Tertiary section of South-Central Louisiana, spatial variations in pore water compositions and temperatures indicate the presence of an on-going, dynamic, subsurface circulation system. Deep brines with VFA levels in excess of 150 mg/1 are migrating up the south flank of the Iberia salt dome a vertical distance of at least 2 km (6000 ft). The VFAs in these waters are dominated by acetate and propionate. As these waters ascend, they mix with an ambient mass of water having total VFA concentrations of 20 mg/1 or less and dominated by n-butyrate. Preferential decarboxylations of acetate and propionate relative to iso- and n-butyrate and iso- and n-valerate are occurring in this system. The byproducts of these decarboxylation reactions should be methane, ethane, and bicarbonate. Temperatures are cool enough (<80°C) in the shallower parts of the sequence to permit the existence of bacteria that coud break down acetate through fermentation.

The spatial distribution of individual dissolved VFA's is complex but systematic, and must ultimately be related to the rates of advective transport, dispersive mixing, and chemical reaction. We believe that a potential new application of the study of these dissolved organic compounds lies in helping to unravel the dynamics of some types of subsurface flow systems.