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The concentrations of short-chain aliphatic acid anions (acetate, propionate, butyrate, and valerate) in 120 formation-water samples from 25 oil and gas fields in Alaska, California, Louisiana, and Texas were determined to study the formation of natural gas from decarboxylation of these anions. The reservoir rocks consist of sandstones ranging in age from Triassic through Miocene.
The samples from Tertiary rocks depict three temperature zones. The aliphatic acid anions of formation waters in zone 1 (subsurface temperatures < 80°C) are characterized by concentrations less than 60 mg/L and consist predominantly of propionate. The concentrations of 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 acid anions are present in zone 3 (temperatures < 200°C) or in formation waters from Triassic rocks. Microbiologic degradation of acetate and dilution by mixing with meteoric water most likely explains the composition and concentration of acid anions in zone 1. The rend in zone 2 and the absence of acid anions in zone 3 and Triassic rocks are explained by thermal decarboxylation of these anions as in the reaction: CH3COO- + H2O ^rarr CH4 + HCO3-.
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 anions to the components of natural gas in the reservoir rocks provides a mechanism that does not require the primary migration of natural gas.
Evidence for the formation of natural gas from decarboxylation of these acid anions is provided by ^dgrC13 values of total bicarbonate and CH4 and the correlation between the proportions of these anions in formation waters and their decarboxylated gases in the natural gas produced. Calculations show that most of the gas in these fields may have been generated from these anions.
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