Gas hydrates saturation using geostatistical inversion in a fractured reservoir in the Krishna–Godavari basin, offshore eastern India

Abstract

A reservoir of gas hydrates filling fractures was discovered in the Krishna–Godavari (KG) Basin during the Indian National Gas Hydrate Program (NGHP) Expedition 01 at site NGHP01-10. The existing methods for estimating gas hydrate saturation from seismic data rely on establishing an empirical relation between acoustic impedance and density porosity from well logs assuming isotropic pore-filling gas hydrate. This method, however, yields a misleading saturation for fractured clay-dominated sediments. Here we present a methodology to estimate gas hydrate saturation from seismic data based on geostatistical inversion. It integrates the verticals detail of well log data with the lateral details from seismic data to produce highly detailed estimates of gas hydrate saturation. First, gas hydrate saturation is calculated from P-wave velocities assuming anisotropic distribution at the well site. Then, probability density functions (PDFs) between acoustic impedance and the calculated gas hydrate saturation at the well site are analyzed. A Markov Chain Monte Carlo method is employed to integrate well logs with the seismic data to produce acoustic impedance. Finally, crossplots and histograms at the well site are used to estimate gas hydrate saturations along the seismic line from inverted acoustic impedance. The spatial distribution of gas hydrate varies both laterally and vertically along the line with an average saturation of 22.5%. The estimate matches reasonably with the value at the wells.