Porosity mapping of shallow subsurface sediments: a case study in Mahanadi basin, eastern margin of India

We have used 2D seismic reflection data along a profile AC with available well logs of the Expedition-01 of the Indian National Gas Hydrates Program (NGHP) for the evaluation of lateral variation of porosity – a petrophysical property. The porosity is estimated using two different approaches based on acoustic impedance (AI) transformation and direct seismic inversion. The AI transformed porosity section illustrates erroneous porosity values within the target zone, irrespective of a strong coefficient of determination (R2) of 0.82 and 0.85 between AI and porosity at sites NGHP-01-19B and NGHP-01-9A, respectively. The accuracy of the porosity results obtained from the direct inversion approach depends on the best linear regression fit established between acoustic and porosity reflectivity- based models with a coefficient of determination as 0.91 at Site NGHP-01-19B and 0.94 at Site NGHP-01-9A. The porosity distributions are also corroborated with the density-porosity log values. The porosity results at the two sites signify the overall good performance of the direct seismic inversion method and also indicate that the probability of free-gas occurrences in shallow sediments is higher compared to that of gas-hydrates.

Comparison of porosity mapped using direct inversion method and AI transformation method with porosity log at well sites NGHP-01-9A and NGHP-01-19B

Citation: Laxmi Pandey & Kalachand Sain (2021): Porosity mapping of shallow subsurface sediments: a case study in Mahanadi basin, eastern margin of India, Exploration Geophysics, DOI: 10.1080/08123985.2021.1994352