Seismic characterization of hydrates in faulted, fine-grained sediments of Krishna-Godavari Basin: Full waveform inversion

Abstract

In fine-grained, faulted sediments, both stratigraphic and fault-induced structural variations can simultaneously determine the gas hydrate distribution. Insights into hydrate distribution can be obtained from P wave velocity (V sub(P)) and attenuation (Q sub(P) sup(-1)) character of the gas hydrate stability zone (GHSZ). In this paper, we apply frequency domain full-waveform inversion (FWI) to surface-towed 2D multichannel seismic data from the Krishna-Godavari (KG) Basin, India, to image the fine-scale (100×30m) V sub(P) and Q sub(P) sup(-1) variations within the GHSZ. We validate the inverted VP model by reconciling it with a sonic log from a nearby (apporx. 250 m) well. The V sub(P) model shows a patchy distribution of hydrate. Away from the faults-dominated parts of the profile, hydrates demonstrate stratigraphic control which appears to be permeability driven. The Q sub(P) sup(-1) model suggests that attenuation is relatively suppressed in hydrates-bearing sediments. Elevated attenuation in non-hydrate-bearing sediments could be driven by the apparent pore fluid immiscibility at seismic wavelengths. The V sub(P) and the Q sub(P) sup(-1) models also suggest that fault zones within the GHSZ can be hydrate- or free-gas-rich depending on the relative supply of free gas and water from below the GHSZ.