Combining impedance inversion and seismic similarity for robust gas hydrate concentration assessments – A case study from the Krishna–Godavari basin, East Coast of India

Gas hydrate saturations were calculated based on Archieʹs relation and rock-physics modeling utilizing log measurements of electrical resistivity and P-wave velocity through the gas hydrate stability zone (GHSZ) at two sites in the Krishna Godavari (KG) basin off the East Coast of India. Acoustic impedance inversion was then performed around the well sites for regional extrapolation of the borehole data. Well-log based gas hydrate concentration estimates and core data are in general agreement with the seismic impedance inversion results at the individual well sites. However, the correlation with seismic data and thus the confidence in the extrapolation decreases with distance from the well site. To address the general problem of unknown regional confidence limits in the extrapolation and aid in regional gas hydrate assessment analyses, a new approach is introduced by calculating the running-sum of the seismic similarity attribute across the gas hydrate stability zone. The running-sum of the similarity attribute can be used locally on a 2D seismic line or 3D seismic volume for defining the limit of well-data extrapolation around a given well site. By normalizing the running-sum of the similarity attribute from all available 2D seismic data in the KG basin, a regional map was generated yielding effective confidence limits for extrapolation of well-log data. Such maps of regional confidence limits can be used strategically in basin-wide gas hydrate assessments as they provide a measure of probability to find a given gas hydrate concentration, and may also offer a guide for defining a minimum regional spacing between well-sites to address the overall structural complexity of the basin (which is reflected in the similarity of the seismic data).