Modeling CO 2 generation, migration, and titration in sedimentary basins

High mole fraction CO 2 gases pose a significant risk to hydrocarbon exploration in some areas. The generation and movement of CO 2 are also of scientific interest, particularly because CO 2 is an important greenhouse gas. We have developed a model of CO 2 generation, migration, and titration in basins in which a high mole fraction CO 2 gas is generated by the breakdown of siderite (FeCO 3 ) and magnesite (MgCO 3 ) where parts of the basin are being heated above approximately 330°C. The CO 2 reacts with Fe‐, Mg‐, and Ca‐silicates as it migrates upward and away from the generation zone (CO 2 ‐kitchen). Near the kitchen, where the Fe‐, Mg‐, and Ca‐silicates have been titrated and destroyed by previous packets of migrating CO 2 , gas moves upward without lowering its CO 2 mole fraction. Further on, where Fe‐ and Mg‐silicates are still present but Ca‐silicates are absent in the sediments, the partial pressure of CO 2 is constrained to 0.1–30 bars and reservoirs contain a few mole percent CO 2 as described by Smith & Ehrenberg (1989) . Still further from the source, where Ca‐silicates have not been titrated, partial pressure of CO 2 in migrating methane gas are orders of magnitude lower. A 2D numerical model of CO 2 generation, migration, and titration quantifies these buffer relations and makes predictions of CO 2 risk in the South China Sea that are compatible with exploration experience. Reactive CO 2 transport models of the kind described could prove useful in determining how gases migrate in faulted sedimentary basins.