Tatapani is the most promising geothermal field located on Son-Narmada lineament in Central India. Two types of litho units comprising Proterozoic basement rocks viz. granite gneiss, biotite gneiss, phyllite, and Lower Gondwana group consisting of sandstone , shale are exposed in this area. Fractures in Proterozoic rocks control the permeability of the reservoir.

Reservoir temperature of 180-190 ?C is indicated by Na/K and 160?C by quartz geo thermometer, respectively. GSI has completed 5 production wells, upto depth of 350m, to exploit thermal water @ 1800 lpm of 100?C on the surface. Down hole well testing was completed by GSI-ONGC to decipher the sub -surface characters of shallow reservoir . The data from the well testing are discussed here.

The well testing observations suggest maximum temperature of 112.5 ?C in the borewell. The temperature profile shows gradual increase in temperature with depth but slight inversion in temperature is reported below the depth of 300m, indicating cold water incursion . The pressure ranges from 4-5 kg/cm2 at well head to 34 kg / cm2 at the depth of 350m. The pressure gradient is uniform and slightly more than the hydro static pressure. Permeable zones are interpreted around the depth of 110-150m, 175 to 275m and > 300m. Major thermal water feed zone is at 200m while the cold water incursion occurs at >300m and at shallow level. The interference tests show interconnection of borewell Tat/23 with Tat/6 and 24, though perceptible effect is not noticed on the discharge of these borewells on each other. The thermal profiles indicate convective flow at the depth of 175m and below. The P and T data indicate single phase liquid dominated reservoir. The well testing results are encouraging for further exploration work.

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