Quantitative assessment of free gas beneath gas hydrate stability zone from prestack seismic data and rock physics: a case of hole NGHP01-10A, Krishna-Godavari basin, India

Abstract

The distribution, saturation, and hydrate indicator of free gas beneath gas hydrate stability zone have been the difficulty in the research of gas hydrate. Amplitude versus offset (AVO) and rock physics can be used to quantitatively interpret free gas saturation. In this paper, seismic data, acquired at K-G basin, India, are processed to preserve true amplitude information. Based on the logging data and horizon calibration, free gas saturation of NGHP01-10A is assessed with effective medium theory and AVO modeling. It is found that free gas saturation is related to its distribution patterns. Assessments of free gas saturation in homogeneous and patchy distribution are 0.3%∼0.4% and 3%∼4%, respectively. The results show good agreement with the crossplot of Poisson's ratio which are calculated from the measurement at NGHP01-10D. Free gas is further deduced to be homogeneous distribution on the base of dry frame Poisson's ratio with the saturation of 0.3%∼0.4%.