Assessment of Tatapani Geothermal Field, India, Based on Hydrothermal Mineral Assemblages

Abstract

Tatapani is the most promising geothermal field located on Son-Narmada lineament in Central India. Two types of lithounits comprising Proterozoic basement rocks viz. granite gneiss, biotite gneiss, phyllite, and Lower Gondwana group consisting of sandstone, shale are exposed in this area. The contact between Proterozoic and Gondwana rocks is marked by Tatapani fault trending ENE-WSW. Hydrothermal alteration in form of silica sinter is observed in this area. Fractures in Proterozoic rocks control the permeability of the reservoir. Five production wells have been drilled at Tatapani Geothermal Field, for establishing a binary cycle pilot power plant. The core samples recovered from these bore holes have revealed a variety of hydrothermal alteration minerals in veins and cavities. The prominent hydrothermal minerals observed are stilbite, quartz, laumontite, smectite, albite, with subordinate illite, and calcite. The assemblage stilbite - smectite suggests alkaline conditions and temperature range of 120 -150?C. Illite - smectite association is reported around 200m depth, indicating that the temperatures around 180?C prevailed in this zone. Platy calcite observed in the boreholes suggests presence of zone of loss of CO2 and boiling. The possibility of two phase conditions prevailing in the reservoir is supported by the occurrence of platy calcite in the bore hole cores. The temperatures suggested by hydrothermal mineral assemblages are comparable with the reservoir temperatures suggested by the aqueous geothermometers while the maximum temperature recorded in the borewells is 112.5?C. The difference between temperature indicated by hydrothermal alteration and temperatures recorded in the borewells may be attributed to the dilution or the heat loss by conduction, without attaining corresponding equilibrium in the chemical composition.