Petrophysical variations within the basaltic lava flows from Tural-Rajawadi hot springs, Western India and their bearing on the viability of low-enthalpy geothermal systems

Abstract

The Tural-Rajawadi hot springs (Temp. 30-62 °C) are a part of the West Coast Geothermal Province of India, located at the foothills of the Western Ghats in Maharashtra state. The hot springs manifest as several small pools/puddles along lineaments within the valley that exposes five (5) surficial lava flows belonging to the Cretaceous Deccan Traps. In addition, six (6) distinct lava flows were identified based on their internal structure, textural variations, vesiculation and presence of glassy rinds in the 203m deep exploratory borehole drilled at Rajawadi. These lavas are of pahoehoe sheet flow and rubbly pahoehoe morphologies. Temperature of water increased from 51 at the top to62 °C at the bottom. XRD studies of vesicle and joint fillings within the lava flows reveal peculiar assemblage of calcite, nontronite, clinopitolite, stilbite, apophyllite, cristobalite, etc. indicating mineralisation related to circulating hot geothermal fluids. The detailed petrophysical properties like textural diversity, crystal size distribution, specific gravity (2.766 to 2.999), resistivity (295.5 to 1790Ωm at 50Hz) and thermal conductivity were measured and their variations within two (2) thick subsurface lava flows is presented. These variations can be attributed to the crystallisation, cooling history, post emplacement hydrothermal alteration and mineralisation the lava flows have undergone. Understanding these variations is essential for assessing viability and commercial feasibility in using and exploiting low-enthalpy geothermal systems.