LOWER LIMITS OF HYDROTHERMAL CIRCULATION IN THE TIWI GEOTHERMAL FIELD, LUZON

摘要

In high-temperature geothermal systems, the lower limit of hydrothermal circulation is controlled by a decrease in permeability. This limit is a fundamental aspect of the circulation volume and the heat budget, and, as a consequence, it will influence the ultimate heat recovery from geothermal systems and the procedures that are applicable for heat mining below the permeability interface. In this paper, we consider the apparent termination of permeability with depth as documented in corehole Matalibong-25 in the Tiwi geothermal field. The bottom of convective circulation at Tiwi cannot be related to either the transition from brittle to ductile behavior which occurs at a temperature of about 400 deg C, or to changes in rock lithology. We suggest that the reduction in permeability is due to a stress reorientation with depth. The termination of permeability with depth, whatever the cause, requires that heat be transferred to the convective portion of the system by conduction. Temperatures within the overlying convective zone are buffered by the boiling point to depth curve. We use time-temperature constraints imposed by (40)~Ar/~39Ar age spectra on adularia, hydrothermal alteration mineralogy, and fluid-inclusion geothermometry to constrain the intrusive history of the system and the transfer of heat across an impermeable barrier. In the Tiwi system, three intrusive episodes are required to match the observed time-temperature relationships.