Lithostratigraphic analysis of sand and silt facies from NGHP 01 gas hydrate accumulations in the Krishna-Godavari Basin

In 2006, an international effort led by the Indian National Gas Hydrate Program (NGHP) and the U.S. Geological Survey conducted the first large-scale exploration of gas hydrate accumulations. Seven sites were drilled within the Krishna-Godavari (KG) basin, a large syn-tectonic rift basin off the eastern shore of India, with the deepest hole penetrating ~300 mbsf. The sedimentary section in the KG basin includes up to 7 kilometers of Late Carboniferous to Holocene sediments from which commercial oil and natural gas production has been established. Detailed lithologic descriptions and physical properties measurements obtained from cores were combined with electrical log data to characterize the sedimentology and stratigraphy at each site. Our analyses indicate that sediments within the Gas Hydrate stability zone (GHSZ) in the KG basin record a Quaternary (<2MYA) history of deposition dominated by dark grey to black colored nannofossil bearing to rich clay and silty clay sourced from the nearby Krishna and Godavari Rivers. Relatively minor amounts of silt to fine sand beds and lamina (1-5 cm thick) were also observed as well as visible terrestrial organic material. The most pervasive mode of gas hydrate occurrence observed during NGHP-01 in the KG basin sites were disseminated hydrates in low-permeability silt-clay facies. Secondary gas hydrate accumulations were recovered in fracture fill, nodular and lens-like occurrences, or as pore-filling cement in the more permeable sand-silt beds and lamina. Thin sand beds and lamina were recovered at 6 of the 7 sites in the KG basin. ~330 sand beds were reported for all 6 sites with a typical bed thickness of ~3 cm. Net sand to gross sediment ratios ranged from 0.026 to 0.405. No major sand beds (>1 m thick) were recovered at any of the sites. Underlying the GHSZ in the KG basin are Pliocene and Pleistocene age sediments deposited during low-stand conditions. However, rising sea-level from the late Pleistocene to the present resulted in a gradual decrease in the volume of coarse grained material transported across the shelf, and slope related deposition during the Holocene has largely been controlled by episodic failure of shelf-edge deltaic deposits via turbidity flows in slope channels. At two KG sites, significantly greater net sand to gross sediment ratios were observed, perhaps indicating a closer proximity of those sites to turbidite channel-levee systems. While the channels may contain sand facies, given the slope dominant location of the NGHP well locations, it is likely that transported sand bypassed the well sites and was deposited in base-of-slope fans in deeper water to the east. Therefore, exploration for gas hydrate-bearing sands in the KG region should focus on the identification and location of the slope levee sand deposits or toe of slope sand rich fans within the GHSZ.

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