Effective medium modeling of gas hydrate-filled fractures using the sonic log in the Krishna-Godavari basin, offshore eastern India

Abstract

[1] Indian National Gas Hydrate Program Expedition 01 has established that clay-rich marine sediment from the Krishna-Godavari (KG) basin in the eastern Indian margin hosts one of the richest gas hydrate deposits in the world. Resistivity at-bit images and pressure cores reveal that the gas hydrate morphology in clay-rich sediment varies from complex vein structures (grain displacing) to invisible pore filling. Existing rock physics models, which relate acoustic data to in situ gas hydrate concentrations, generally assume isotropic pore-filling gas hydrate, which yields misleading concentration estimates for fractured fine-grained sediments. The anisotropic KG basin sediment presents additional complications. Here we apply differential effective medium theory to incorporate grain-displacing morphologies by which gas hydrate is included as vertical ellipsoids with aspect ratios ranging from those of thin veins up to those of nodules in an elastic anisotropic background. We have estimated gas hydrate concentrations from sonic velocities at hole 10D in the KG basin considering three basic gas hydrate morphologies: (i) pore filling, (ii) grain displacing, and (iii) a combination of grain displacing and pore filling. Average gas hydrate saturations for these three cases are 35–42%, 27–30%, and 33–41% of the total porosity, respectively, in the depth range 60–140 m below seafloor (mbsf). Saturation is highest at ∼67 mbsf for any morphology but the values differ between morphologies. For the pore-filling morphology, the maximum gas hydrate saturation of 56% is 18–22% higher than the grain-displacing morphology and 2–9% higher than the combined morphology. Estimates differ by ±6% of the sediment volume with rotations of gas hydrate veins from vertical to horizontal.