Porosity, permeability, and grain size of sediment cores from gas-hydrate-bearing sites and their implication for overpressure in shallow argillaceous formations: Results from the national gas hydrate program expedition 02, Krishna-Godavari Basin, India

The existence of overpressure in shallow sediments, which is often constrained by hydraulic properties, influences the gas-hydrate formation process and gas production. Porosity, permeability, and grain size measurements in laboratory experiments were conducted on core samples from gas-hydrate-bearing regions offshore from the Krishna-Godavari Basin, eastern India. Porosity was found to decrease with increasing effective stress; this is explained by the exponential decay curve along which porosity decreases from 65 % at 0 MPa to 40 % at 10 MPa. Permeability and the corresponding hydraulic diffusivity decrease from 10⁻¹⁷ to less than 10⁻¹⁸ m² and from 10⁻⁷ to 10⁻⁸ m²/s at 0.5 and 5 MPa, respectively. Grain sizes were larger and the sand fraction more scattered in channel-filled sediment sites compared to slope sediment sites. The preconsolidation stresses evaluated from consolidation curves indicate the absence of overpressure at shallow depths. In contrast, a comparison of ship-board measurements and standard compaction curves suggested that measured porosity was higher than the predicted porosity at greater depths. These porosity anomalies are interpreted as a sign of overpressure that approaches near lithostatic values at greater depths in slope sediment sites. A one-dimensional sedimentation model recreated overpressure profiles similar to those predicted by porosity gaps, under the assumption of large basial fluid influx or lower permeability than that derived from laboratory data. The modeling results suggest that near-hydrostatic pressure at shallow depths and significant overpressure at greater depths proposed by the porosity gap method is explained by the non-linearity of transport properties. The relatively small overpressure generation in channel-filled sites compared with slope sites can be explained by the higher permeability due to coarser grain size and larger sand fraction and by the smaller basal influx. On the contrary, considerably large basal influx associated with clay mineral dehydration and methane gas supply from deep sediments was expected to promote overpressure at slope sites, which is confirmed by Cl⁻ concentration-depth profiles.