Numerical investigation of the effect of volatilization and the supercritical state of pore water on maturation of organic matter in the vicinity of igneous intrusions

Abstract

This study presents a numerical investigation of the effect of volatilization and the supercritical state of pore water on maturation of organic matter in host rocks based on the heat flow models assuming the instantaneous and finite-time intrusion mechanisms of magma. A 15 m thick, well-dated basic sill in the DSDP 41-368 hole near Cape Verde Rise, eastern Atlantic is selected as an example due to the sufficient thermophysical parameters of rocks and the definite burial and thermal history of the shale host rocks. Results indicate: (1) The effect of the temperature-dependent thermal properties of pore water at a hydrostatic pressure of 414 bar on the predicted vitrinite reflectance ( R r) is less than 0.1% no matter which intrusion mechanism of magma is assumed and can hence be ignored reasonably; (2) The consideration of volatilization of pore water can reduce the predicted R r of host rocks significantly. In case of the instantaneous intrusion mechanism, the maximum deviation of the predicted R r caused by pore-water volatilization reaches 1.3% at the location of half the sill thickness away from the contact (i.e. X / D = 0.5), and the deviation above 0.5% can occur in the region from 0.3 to 1.0 in the form of X / D . In case of the finite-time intrusion mechanism, the maximum deviation of the predicted R r due to pore-water volatilization attains 1.15% at X / D = 0.25, and the region where the deviation is larger than 0.5% lies between 0.15 and 0.6 in the form of X / D ; (3) If hydrothermal convection in the host rocks is allowed for, the predicted R r of the overlying host rocks is less than that of the underlying host rocks at the same X / D in the inner region of the contact aureole of igneous intrusions, whereas the phenomenon is converse in the outer region. In contrast, the measured R r profile shows that at the same X / D , R r of the overlying host rocks is totally higher than that of the underlying host rocks. Thus, it is not the hydrothermal convection in the overlying host rocks that resulted in the asymmetry of the current R r profiles below and above the sill; (4) The predicted R r based on the heat conduction model assuming the finite-time intrusion mechanism and pore-water volatilization matches well with the measured one out of the region where the R r geothermometer is unreliable due to the effect of volatilization of pore water. This demonstrates that the finite-time intrusion mechanism of magma, together with pore-water volatilization, possibly represents natural conditions. ► The temperature dependence of thermal properties of pore water can be ignored. ► Volatilization of pore water can reduce the predicted R r significantly. ► Hydrothermal convection in host rocks can not result in the present R r profile. ► Finite-time intrusion and pore-water volatilization represent natural conditions.