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abstract

Advective diagenesis predicts that large-scale mineral migration will occur when saturated pore fluids move across isotherms (or isobars). A general picture of the change in a given porosity field can be calculated if the fluid flow and temperature (or pressure) fields are known. For more detailed analysis it is necessary to know the details of the chemical equilibria involved, their temperature and pressure dependence, the host rock mineralogy and, in particular, the thermal mass transfer coefficients for the phases involved. In this paper thermal mass transfer coefficients for quartz and calcite have been calculated in detail over the temperature range 0- 200°C. Simple approximations (involving only equilibrium constants) have been obtained for the concentrations of the aqueous species as well as for the solid mass precipitated or dissolved.

Quartz solubility is prograde over the entire temperature range of interest, and the mass transfer coefficients for quartz range from 0.1 ppm/C° at 0°C to about 4 ppm/C° at 200°C.

In a closed system calculation shows that calcite solubility is prograde up to 125°C, and then becomes retrograde, while in open systems calcite solubility is retrograde over this temperature range. The mass transfer coefficients for calcite range from +0.1 ppm/ °C in a closed system at 0°C to greater than −60 ppm/°C at 0°C under 62 atm of CO2 pressure. At higher temperatures these coefficients seem to trend toward a common asymptotic value of 0(?) ppm/°C at temperatures greater than 200°C.

These calculations suggest that low temperatures and high CO2 pressures are most effective in mobilizing calcite but that high temperatures are more effective in mobilizing quartz. The calculations support recent suggestions that decarboxylation of organic matter is involved in the development of secondary porosity.

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