Modeling CO2 generation,migration, and titration in sedimentary basins

High mole fraction CO₂ gases pose a significant risk to hydrocarbon exploration in some areas. The generation and movement of CO₂ are also of scientific interest, particularly because CO₂ is an important greenhouse gas. We have developed a model of CO₂ generation, migration, and titration in basins in which a high mole fraction CO₂ gas is generated by the breakdown of siderite (FeCO₃) and magnesite (MgCO₃) where parts of the basin are being heated above approximately 330°C. The CO₂ reacts with Fe‐, Mg‐, and Ca‐silicates as it migrates upward and away from the generation zone (CO₂‐kitchen). Near the kitchen, where the Fe‐, Mg‐, and Ca‐silicates have been titrated and destroyed by previous packets of migrating CO₂, gas moves upward without lowering its CO₂ mole fraction. Further on, where Fe‐ and Mg‐silicates are still present but Ca‐silicates are absent in the sediments, the partial pressure of CO₂ is constrained to 0.1–30 bars and reservoirs contain a few mole percent CO₂ as described by Smith & Ehrenberg (1989) . Still further from the source, where Ca‐silicates have not been titrated, partial pressure of CO₂ in migrating methane gas are orders of magnitude lower. A 2D numerical model of CO₂ generation, migration, and titration quantifies these buffer relations and makes predictions of CO₂ risk in the South China Sea that are compatible with exploration experience. Reactive CO₂ transport models of the kind described could prove useful in determining how gases migrate in faulted sedimentary basins.