Aliphatic Acid Anions in Oil-Field Waters--Implications for Origin of Natural Gas

Abstract

Concentrations of short-chain aliphatic acid anions (acetate, propionate, butyrate, and valerate) in 95 formation-water samples from 15 oil and gas fields in the San Joaquin Valley, California, and in the Houston and Corpus Christi areas, Texas, show three temperature regimes. The sandstone reservoir rocks range in age from Eocene through Miocene. The aliphatic acid anions of formation waters in zone 1 (subsurface temperatures lower than 80°C) are characterized by concentrations less than 60 mg/L and consist predominantly of propionate. The concentrations of aliphatic acid anions in zone 2 (temperatures 80 to 200°C) are much higher (up to 4,900 mg/L) than in zone 1, and decrease with increasing subsurface temperatures and age of their reservoir rocks; acetate forms more than 90% of the total anions. No aliphatic acid anions are believed present in zone 3, which is based on extrapolation of data in zone 2; the temperatures are higher than 200°C. Microbiologic degradation of acetate and dilution by mixing with meteoric water most probably explain the composition and concentration of aliphatic acid anions in zone 1. The trends in zone 2 and the absence of acid anions in zone 3 are explained by thermal decarboxylation of these acid anions as in the reaction: CH3COO- + H2O^rarrCH4 + HCO3-. Aliphatic acid anions generally contribute more than 50% and up to 100% of the measured alkalinity in the samples of zone 2. Their contribution to the alkalinity in zone 1 is small. Iodide concentrations generally increase with increasing concentrations of aliphatic acid anions, which supports the use of iodide as a good proximity indicator of petroleum. The aliphatic acid anions mainly result from the thermocatalytic degradation of kerogen. We believe that these anions, which are highly soluble, are produced and dissolved in the pore waters of the source rocks and are expelled to the reservoir rocks during dehydration of clays. Decarboxylation of these acid anions to the components of natural gas is believed to occur mainly in the reservoir rocks, thus resolving the difficult problem of explaining the primary migration of natural gas. Evidence for the formation of natural gas from decarboxylation of acid anions is provided by the ^dgrC13 values of total bicarbonate and CH4 and the good correlation between the proportions of these anions in formation waters (94% for acetate, 5% for propionate, and 2% for butyrate) and their decarboxylated gases in the natural gas produced (90% for methane, 5% for ethane, and 2% for propane). Calculations show that the amount of gas that can be generated from the decarboxylation of the reported acid anions is large and apparently adequate to produce the amounts of gas in these fields.