Lithological properties of natural gas hydrate-bearing sediments in pressure-cores recovered from the Krishna-Godavari Basin

We investigated the sediment properties and gas hydrate saturations in pressure-cores recovered during the National Gas Hydrate Program Expedition 02 (NGHP-02) in Areas B, C, and E of the Krishna-Godavari Basin (K-G Basin) off the eastern margin of India. These investigations included the analyses of grain size, mineralogy, and grain density and employed scanning electron microscopy (SEM). Sediments with median grain sizes below 90 mu m were generally observed within most of the pressure-cores, whereas sediments with median grain sizes exceeding 250 mu m were only observed in a few pressure-cores. Higher concentrations of gas hydrate were found in sediment layers with median grain sizes of 10-90 mu m; the occurrence of gas hydrates was lower in sediment layers with median grain sizes below 10 mu m. Three types of gas hydrate-bearing sediments containing above 50 vol.% gas hydrate saturations were observed: (1) sediments with silt grain size ratios above 70 vol.%, (2) sediments with sand grain size ratios above 35 vol.%, and (3) coarser sandy sediments with gravel. Sandy sediments with high gas hydrate saturations (above 50 vol.%) were very minor constituents in the pressure-core sediments. The gas hydrate saturations were lower in sediments with a clay fraction above 30 vol.%. SEM images revealed microfossil debris, fragments of sedimentary rock, and silt/sand-sized-agglomerated grains throughout the pressure-cores, which indicate that the sediments were deposited at and/or transported to the foot of the continental slope through reworking of upper-slope deposits. The geological setting and SEM observations indicate that the pressure-core sediments are mainly a terrigenous source, with a small component from pelagic or hemipelagic sedimentation. The results of the pressure-core analyses confirm that natural gas hydrates can occur in silt-rich sediments. The occurrence of gas hydrates in the pressure-core sediments recovered during the NGHP-02 is controlled by the lithology of the sediments, especially their clay content.