Boiling and Thermal-Pressure Recorded in Calcite-Hosted Fluid Inclusions of the Ngawha Geothermal Field, New Zealand

Abstract

Calcite-hosted fluid inclusions separated from cuttings and cores of Ngawha wells were studied to understand boiling-mixing processes affecting reservoir fluids and the origins of calcite. Evidence of boiling is represented by co-existing liquid-rich and vapour-rich inclusions at most depths and the common occurrence of platy calcite. Over 1000 fluid inclusions have Th values that range from 160 to 306 "C and Tm values that range from -0.4 to -2.5 "C. Crushing experiments and Tm data suggest that inclusion fluids contain up to 3.7 wt % CO2, approximately twice as much as in the modem fluid. It thus appears that calcite and the fluid inclusions formed under a wide range of pressures, some of which approached lithostatic. The build up of high partial pressures of CO2 is likely due to the impermeable nature of the shallow sedimentary host rocks.