Petrophysical and fluid transport properties of Deccan trap basalts from analysis of geophysical well logs in a 3 km scientific borehole at Koyna, India

Abstract The knowledge of in situ petro‐physical properties of flood basalt is key to understanding the groundwater potential, deep (>1 km) hydrological networks, and the nature of the subsurface aquifers (confined or unconfined). Porous flow tops and permeable columnar joints/natural fractures can store and transport groundwater throughout the thickness of the basaltic pile, respectively. Additionally, bulk formation porosity (isolated or effective/interconnected) and fracture network provide further insights towards exploring the potential of Deccan basalt for CO 2 sequestration. Thus, information on in situ properties of basalts is a pre‐requisite to take up such exploration programs. In the present study, in situ petrophysical properties of a ∼750 m long, continuous vertical section of Deccan basalt (depth 500–1247 m) are determined from analyses of high‐resolution geophysical well log data acquired in a 3 km deep scientific borehole, KFD1, in the Koyna region, Western India. KFD1 passed through 1247 m thick Deccan traps and continued 1767 m into the underlying granitic basement. Well log data, including natural gamma, caliper, electrical resistivity, self‐potential (SP), formation density, neutron porosity, sonic velocities, temperature, nuclear magnetic resonance and wellbore image data, were acquired using standard Schlumberger logging tools. Salient results from the study are as follows. (i) The massive and non‐massive (comprising vesicular/amygdaloidal, flow top breccia and red bole horizons) parts of individual lava flows are delineated by contrasting physical properties. (ii) Empirical relationships are proposed for density, electrical resistivity, P and S wave velocities as a function of porosity for Deccan basalt formation. (iii) The geophysical well log datasets provide evidence for deep percolation of groundwater. Although the non‐massive parts of lava flows facilitate storage of groundwater, the presence of water‐saturated, permeable fractures in massive basalt sections favours deep hydrological networks in Deccan basalt.