Assessment of gas hydrate using prestack seismic inversion in the Mahanadi Basin, offshore eastern India

Abstract

The Indian National Gas Hydrate Program Expedition-01 in 2006 has discovered gas hydrate in the Mahanadi offshore basin along the eastern Indian margin. However, well-log analysis, pressure core measurements, and infrared anomalies reveal that gas hydrates are distributed as disseminated within the fine-grained sediment, unlike massive gas hydrate deposits in the Krishna-Godavari Basin. The 2D multichannel seismic section, which crosses holes NGHP-01-9A and 19B located at approximately 24 km apart, indicates a continuous bottom-simulating reflector (BSR) along it. We aim to investigate the prospect of gas hydrate accumulation in this area by integrating well-log analysis and seismic methods with rock-physics modeling. First, we estimate gas hydrate saturation at these two holes from the observed impedance using the three-phase Biot-type equation. Then, we establish a linear relationship between the gas hydrate saturation and the impedance contrast with respect to the water-saturated sediment. Using this established relation and impedance obtained from prestack inversion of seismic data, we produce a 2D gas hydrate-distribution image over the entire seismic section. The gas hydrate saturation estimated from resistivity and sonic data at well locations varies within 0%–15%, which agrees well with the available pressure core measurements at hole 19. However, the 2D map of gas hydrate distribution obtained from our method indicates that the maximum gas hydrate saturation is approximately 40% just above the BSR between the common-depth points of 1450 and 2850. The presence of gas-charged sediments below the BSR is one of the reasons for the strong BSR observed in the seismic section, which is depicted as low impedance in the inverted impedance section. Closed sedimentary structures above the BSR are probably obstructing the movements of free-gas upslope, for which we do not see the presence of gas hydrate throughout the seismic section above the BSR.