Application of constrained AVO inversion: 2-D modelling of gas hydrates and free gas in Mahanadi basin, India

Abstract

We present an application of Constrained Amplitude versus Offset (AVO) inversion for 2-D modelling of gas hydrates and free gas saturation in the Mahanadi offshore basin. In order to reliably model gas hydrate distribution, it is essential to derive both the P- and S-wave velocities (VP and VS). Hence, we use the method of Constrained AVO inversion, in which we obtain both the VP and VS using a combination of travel time and amplitude inversion. We employ this exercise on the 2-D seismic reflection data in the Mahanadi offshore basin and have derived 2-D sections for both VP and VS. Information about porosity has been derived from the model-based inversion by utilizing a porosity wavelet. The 2-D sections of VP, VS and porosity are then utilized in rock physics modelling for the saturation estimation of gas hydrates and free gas respectively. We have estimated gas hydrates saturation using Three-Phase Weighted equation (WE) and free gas saturation using Gassmann equations for fluid substitution. Further, we have used the Effective Medium Modelling (EMM) for gas hydrates saturation and understanding their distribution by considering two cases: (a) gas hydrates cementing the grain contact and (b) gas hydrates occurring away from the grain contact. We obtain an average gas hydrates saturation as 7% and 10% from EMM and WE modelling respectively, and free-gas saturation in between 1 and 3%. The base of gas hydrate stability zone is found gently dipping down from the Common Depth Point (CDP) 2200 to CDP 2900 at the depth range of 1840–1970 m. Cross plot of velocity versus saturation and validation with the well log saturation suggests that the latter model or uncemented nature of gas hydrate distribution holds good in the study area.