Modeling CO₂ generation, migration, and titration in sedimentary basins

Abstract

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.

He and Ne as tracers of natural CO2 migration up a fault from a deep reservoir

“…20 Ne is introduced into the subsurface as a component of air dissolved in water and, hence, is a useful tracer of groundwater interaction . Whilst there are means by which crustal CO 2 (CO 2 / 3 He > 10 10 ) could be added to an initial mantle rich CO 2 accumulation (Bradshaw et al, 2004;Cathles and Schoell, 2007), there is no plausible mechanism that would permit crustal CO 2 to be added whilst preserving the correlation between CO 2 / 3 He reduction and increases in noble gases derived from the groundwater. Therefore, changes in CO 2 / 3 He must be due to CO 2 loss in the subsurface by an amount directly related to the quantity of groundwater the CO 2 has contacted.…”

On the Processes that Produce Hydrocarbon and Mineral Resources in Sedimentary Basins

“…(B) Whether basin reservoirs can contain almost no, a few percent, or nearly 100% CO 2 depends on whether the sediments along the pathways between the CO 2 source at >330 C and the reservoir contain Ca aluminosilicates (almost no CO 2 at location A), just Fe or Mg aluminosilicates (a few percent CO 2 at location), or no aluminosilicates (~100% CO 2 at location C). From [68]. (C) shows how the titration process has been implemented in basin models.…”

Hydrogeochemical modelling of CO2 equilibria and mass transfer induced by organic–inorganic interactions in siliciclastic petroleum reservoirs

“…It has multiple origins and can be sourced from the Earth’s mantle (Holloway 1998). The breakdown of carbonates (siderite, magnesite) can generate high mole fraction CO 2 gas in parts of sedimentary basins that attain temperatures above approximately 330°C (603 K; Cathles & Schoell 2007). Contents above 15 wt% CO 2(gas) are relatively rare in sedimentary basins.…”

“…1999). Hydrocarbon reservoirs rich in CO 2 are of less economic value, and pose a significant risk to hydrocarbon exploration in some areas (Cathles & Schoell 2007). One option to increase their economic value is to separate this greenhouse gas and re‐inject it for long‐term storage (e.g.…”