Organic ligand distribution and speciation in sedimentary basin brines, diagenetic fluids and related ore solutions

Abstract

Abstract The nature, distribution, and interactions of dissolved organic ligands in the shallower zones of sedimentary basins are highly variable and poorly understood. Better understood is the chemistry of dissolved aqueous carboxylic acid species in the deeper regions of sedimentary basins. In most oil-field brines, acetate is generally the dominant organic ligand and is followed in level of concentration by longer chained aliphatic monocarboxylic acid anions. The total concentration of dicarboxylic acid anions is probably lower than previously reported and likely less than 500 mg l −1 with succinate and glutarate being the dominant species. The highest concentrations of organic acid anions are present in formation waters at 80–120° C. Based on the available field and laboratory evidence, the concentrations of organic anions are controlled mainly by the rates of their generation from kerogen and their destruction thermally or by bacteria. An extensive compilation of experimentally determined stability constants and other thermodynamic data for a large number of non-humic organic ligands is available for temperatures near 25° C. However, corresponding high-temperature data are available for only a few species of interest; most notably, acetate complexes of some rock- and ore-forming metals and protonated species of several carboxylate ligands. Simulations of speciation in oil-field brines show that significant amounts of Ca, Mg, Fe, and Al can be complexed by carboxylate ligands, in the pH range 4–6, if ligand concentrations are on the order of 10–100 mg l −1 . Calculated speciation in model ore fluids for Mississippi Valley-type deposits and red-bed related base metal deposits show that optimum conditions for Pb and Zn transport by organic ligand complexation are oxidized ore fluids with total reduced inorganic sulphide concentration less than 10 −9 molal.