Identification of Gas-hydrates and Free Gas Zones using Acoustic Impedance Inversion Integrated with AVA Analysis

ABSTRACT The methane hydrates are often recognized in seismic data by means of identifying a distinct seismic reflection called the Bottom Simulating Reflector (BSR). However, their presence is established affirmatively by employing the acoustic impedance inversion schemes using seismic and well log data. The present study aims to delineate the BSR and the free gas zones in the Krishna-Godavari (KG) basin, located in the eastern coast offshore basin, India, using the acoustic impedance inversion followed by Amplitude Versus Angle (AVA) analysis. The pre-stack and post-stack seismic data are used along with the log data from the study area for carrying out impedance inversion, and the obtained results are then analysed and compared the final models. The pre-stack inversion model clearly delineates the BSR at ~1680 ms as a laterally continuous reflector separating high-impedance (~2930 (m/s) * (g/cc) with cyan color) gas-hydrate-bearing sediments from underlying low-impedance (~2400 (m/s) * (g/cc) with yellow to green color) gas sands. Post stack impedance final model shows the BSR at ~1690 ms with impedance of ~2600 ms (m/s) * (g/cc). Subsequently, the pre-stack data were also analyzed using the AVA analysis, and the corresponding reflection coefficients (RC) were plotted. AVA analysis was carried out using angle gathers in the range of 5°-40°. A Class IV response is observed in the third layer, indicating the BSR, while the fourth and fifth layers represent gas zones showing class-II variations indicating lateral variation from northwest to southeast along the profile. The pre-stack inversion results have shown a clearer BSR, compared to the post-stack inversion. AVA analysis strongly supports the presence of both BSR and gas sands in the basin.