Quantification of carbon dioxide sourced by mineral reactions in ultradeep sedimentary basins

The accumulation of CO2 in some natural gas fields represents a risk for exploration, and calls for a way of integrating its occurrence in basin modelling. Among several possible sources we investigate mineral reactions able to take place within bottom sediments of particularly deep and hot sedimentary basins, i.e. at conditions of low-grade metamorphism (up to 500 °C and 2500 bar). These reactions involve lithologies where carbonate and alumino-silicate minerals are combined to form a « mixed composition ». For appraising decarbonation quantitatively we propose a method founded on thermodynamic calculations, that follows a modelling approach commonly used in metamorphic petrology. At given bulk composition and temperature-pressure (T-P) conditions, the composition and proportions of solid and fluid phases that coexist at equilibrium are computed using a Gibbs Energy Minimization (GEM) algorithm. On the basis of R.G. Berman's database we incorporate thermodynamic parameters for a variety of solid solutions including clay minerals, and for a CH4-CO2-H2O fluid mixture represented by the CPA-Electrolyte equation of state. A large range of decarbonation reactions is explored through a systematics involving 85 model compositions, and a unique T-P path typical of the targeted basins. We confirm that pure carbonate lithologies do not behave as CO2 source in the conditions explored. In contrast, we demonstrate that mixed compositions are likely to yield CO2 from successive mineral reactions, in which clay minerals play a prominent role. We also show that the temperature of incipient decarbonation, staggered over 325-425 °C, depends on the Ca/Mg/Fe composition ratio of the bulk sediment. CO2-prone reactions occurring from a selection of natural sediments are then presented. The CO2-prolific character of particular facies associated to the rift phase of a basin evolution is illustrated, notably in the case of lacustrine oil shales. Finally, we indicate a protocol that integrates the decarbonation scheme derived from thermodynamics into a CO2-generation formalism compatible with the usual formalism of a basin model.