[MIS17-P15] Anomalously high porosity and the implication of overpressure in shallow gas-hydrate-bearing sites in eastern India
Keywords:overpressure, gas-hydrate, porosity, permeability, National Gas Hydrate Program Expedition 02, Krishna-Godavari Basin
Consolidation tests of core samples showed overconsolidation of core samples at shallower depths, which can be explained simply by the effect of past maximum stress. However, the curves were consistent with normal consolidation curves at greater depths. Permeability of mud in four sites was quite low, which decreases from 10-17 to less than 10-18 m2 from 0.5 to 5 MPa, respectively. Therefore, continuous overburden loading in low permeable sediments is the primary cause of overpressure.
Comparing the channel-filled site with c, the former is more consolidated and has less generated excess pore pressure than those normal slope site. This variation is probably induced by differences in grain size, where coarser-grained sediments are more abundant in channel sites than in slope sites. This is because coarser-grained particles are easier to compact owing to their smaller specific surface area and higher permeability.
The porosity–decay curves are well fitted by modified exponential decay curves, in contrast to the traditional Athy’s law; therefore, we recommend this modified decay curve for predicting porosity–depth curves and pore pressures, especially in shallow argillaceous sediments.