A New Approach for Quantification of Gas-hydrates in Fracture Shale in KG basin, India from MCS Data

The coring of NGHP has recovered gas-hydrates in fracture shale in KG basin, India. Higher resistivity and sonic logs at site 10 indicate gas-hydrates between 30 to 150 mbsf. The simple Archie's formula to resistivity log overestimates gas-hydrates when compared to pressure core measurements. This is due to its isotropic assumption, as fractures cause anisotropy. Considering gas-hydrates as part of matrix and brine in pores, we establish a relation between normalized resistivity (NR) and velocity of sediments with and without gas-hydrates respectively from log data, and estimate gas-hydrates by modified Archie's formula to NR with the saturation exponent (n) that matches with pressure core measurement by incorporating effect of anisotropy. We derive seismic velocities from MCS data along 6 in-lines and 6 cross-lines (each of 8 km long) around the drilling site in KG basin covering an area of ~14.7 sq km. The modified Archie's formula to velocity-resistivity transformed data show gas-hydrates between 5 to 50% along different lines. Average volume saturation is calculated as 12.28% using 60% average porosity observed in density-porosity log. Total volume of gas within 168 m average thickness (indicated by high velocity anomaly) of hydrate-bearing sediments is calculated as 51.5 BCM in the study area.