PROGRESSIVE DEVOLITISATION OF NEW ZEALAND RESERVOIR ROCKS FROM SUB-CRITICAL TO SUPERCRITICAL CONDITIONS

Abstract

The source of volatiles such as Cl, S and CO2 in geothermal systems is an object of frequent debate. Early experiments concluded that they could be derived solely from water-rock interaction. Others infer that some component of magmatic input is necessary. We have conducted experiments to measure the volatiles released during progressive heating of the typical reservoir rocks of the Taupo Volcanic Zone: greywacke (Mountain et al. 2016) and rhyolitic pumice (this study). The experiments aim to simulate the chemical exchange occurring during recharge of the geothermal plume of a reservoir.
The pumice experiment was conducted over a period of 82 days using a high temperature and pressure fluid-rock interaction simulator. Temperature ranged from 20 to 400oC and a maximum pressure of 405 bar. The flow rate was maintained at 1 ml hr-1 throughout the experiment.
Up to a temperature of 250oC, only small amounts of Cl were released. Most Cl was released between 250 and 300oC. The integrated ?Cl flux was almost 1200 mg kg-1 rock. Sulfur released by the pumice was mostly SO4 (180 mg kg-1 rock) which occurred between 20 and 250oC while H2S release was minimal (6.5 mg kg-1 rock).
These preliminary results show that different geothermal reservoir rocks can influenced the composition of the fluids reacting with them. This can be a possible explanation for some of the chemical observations observed in between geothermal systems of the Taupo Volcanic Zone.