2.4.1. CO2 Generation and Titration

Some reservoirs trap almost pure CO2, and it is not uncommon for gas in reservoirs to contain a few percent CO2. In the latter case, the partial pressure of CO2 depends systematically on temperature, which indicates chemical equilibrium with siderite or magnesite [67], as shown in Figure 12A. Most reservoirs contain almost no CO2. These observations can be understood and modeled as indicated in Figure 12.

**Figure 12.** (**A**) Log partial pressure of CO2 in equilibrium with aluminosilicates and various carbonate minerals. From [67]. (**B**) Whether basin reservoirs can contain almost no, a few percent, or nearly 100% CO2 depends on whether the sediments along the pathways between the CO2 source at >330 C and the reservoir contain Ca aluminosilicates (almost no CO2 at location A), just Fe or Mg aluminosilicates (a few percent CO2 at location), or no aluminosilicates (~100% CO2 at location C). From [68]. (**C**) shows how the titration process has been implemented in basin models. From [68].

At 350 ◦C the partial pressure of water vapor and CO2 exceeds the fluid pressure in over-pressured basins, and a separate gas phase is formed. But dissolved CO2 is highly reactive, and as it migrates upward it will react with any silicate minerals that contain Ca, Mg, or Fe to form carbonates. The CO2 in the rising gas will be consumed. So long as the stratigraphy contains calcium aluminosilicates, the partial pressure in the gas and in reservoirs will be very low. If the Ca aluminosilicates have been fully reacted but Mg or Fe aluminosilicates remain, the partial pressure of CO2 in the reservoirs will depend on temperature and lie on the Smith and Ehrenberg (S&E) [67] buffer trend shown in Figure 12A. When the Mg or Fe aluminosilicates are titrated, there is no buffer control, and the reservoirs can be filled with pure CO2. The titration is illustrated in Figure 12B. Reservoirs at location C can contain 100% CO2, reservoirs at location B a few percent CO2, and reservoirs at location A will contain no CO2. Figure 12C shows how this titration process has been implemented in basin models (see [68]). Documentation of titrated migration pathways could reveal a lot about how gas migrates over substantial intervals of time in basins.
