CO2 and hydrocarbon gases are generated in large quantities in deeply buried organic rich sedimentary sequences. Gas composition at any given depth is to some extent predetermined by the elemental composition of the buried organic material, and the subsequent thermal exposure, but it is also strongly influenced by the detrital mineralogy, and the interactions between organic and inorganic components that occurs during sediment burial. We present a 1D (depth; aqueous phase concentration) reaction-transport model (advection, diffusion) involving several sources and sinks for CO2.
CO2 and CH4 generation is controlled by total organic carbon (TOC), genetic potentials and kinetic parameters. Further generation of CO2 by the reaction between hydrocarbons and water based on thermodynamic constraints given by Barker and Takach (1992) is also implemented. At moderately large depths (< 4-5 km) thermodynamic equilibration between CO2 and hydrocarbon gases is too slow to be of significance. Carbon isotope data from natural gases show that CO2 and CH4 are approaching equilibrium at depths between 4 and 5 km. This equilibration will in most cases (dependent on thermal gradient and rock composition) result in CO2 generation at the expense of CH4 at depths between 5 and 10 km.
Loss of CO2 by mineralization is controlled by the availability of bivalent metals derived from non-carbonate minerals. The impact of pressure and temperature on both CO2 and carbonate solubility is incorporated.
Below aqueous solubility levels CO2 and hydrocarbon gases may be buried to great depths, whereas the excess gas above aqueous solubility will migrate upwards in the gas phase. CH4 aqueous solubility is also strongly dependent on both temperature and pressure, but unlike CO2, this gas is not removed by mineralization.
The shape of the concentration profile of CO2 with depth, will thus in addition to the sources and sinks described above, be influenced by phase separation and porosity decrease.
Various scenarios for the consequences of upward migration of CO2 and CH4 with oil and/or gas phases, solubility of CO2 and CH4 in oil, cracking of organic acids and higher hydrocarbons, the importance of TOC for CH4 survival, and the more detailed effects of mineralogical variation, will be presented.
The resulting profiles of CO2 concentration with depth has several interesting applications: They constrain the environment of carbonate cementation and formation of secondary porosity from mineral dissolution, and they also represent a tool for the prediction of gas quantity and composition in sedimentary basins, which is vital for the evaluation of deep and/or hot hydrocarbon prospects. Finally, the model also has some impact on the quantification of gases in the deep crust, and consequently contributes to improved insight into the global carbon cycle.
Barker, C. & Takach, N.E., AAPG Bull. 76, 1859-1873 (1992).