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. This study aims to investigate the prospect of gas hydrate accumulation in this area by integrating well-log analysis and seismic methods with rock-physics modelling. First, gas hydrate saturation is estimated at these two holes from the observed impedance using the three-phase Biot-type equation. Then, a linear relationship is estimated 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, a 2D gas hydrate-distribution image is produced 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 present method indicates that it varies from 10% to 20% and 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 no presence of gas hydrate is seen throughout the seismic section above the BSR.
Fig.: Comparison of the various structures above the BSR in the (a) seismic and (b) corresponding impedance con- trast/difference section. Seafloor, R1, R2, R3, R4, R5, and BSR are the observed horizons. The different layers between the horizons are marked by 1, 2, 3, 4, and 5.
Very bright BSR observed in some parts of the seismic section, is most probably due to the interference between the BSR and the reflection just above it. A small amount or no gas hydrate present a few meters above the BSR between CDPs 1200 and 3200 is mostly due to the closed structures obstructing the free-gas movements upslope.
Maheswar Ojha1 and Ranjana Ghosh1
Assessment of gas hydrate using prestack seismic inversion in the Mahanadi Basin, offshore eastern India