Himalayan Geology, Vol. 42 (1), 2021, pp. 69-84, Printed in India
A new concept for the appraisal of gas hydrates: An example from the Krishna-Godavari basin, in eastern Indian margin
VIVEKANAND PANDEY1*, KALACHAND SAIN2#, MRINAL K. SEN3
1CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad, 500007, India
2Wadia Institute of Himalayan Geology, 33 GMS Road, Dehradun, 248001, India
3The University of Texas at Austin, Austin, TX 78758, USA
Email: *mr.vivekanandpandey@gmail.com; #kalachandsain7@gmail.com
Abstract: Logging and coring during the first Indian National Gas Hydrates Program have confirmed Gas-hydrates in fractured shale of the Krishna-Godavari (KG) basin. Higher values of resistivity and sonic velocity logs at site 10 indicate gas-hydrates between 30 to 150 m below the sea floor, whereas the shallower and deeper sediments are water saturated. We propose a workflow to estimate the gas hydrates saturation from sonic or seismic velocity using velocity resistivity transformed data. Velocity resistivity transform is obtained from a cross plot between sonic velocity and normalized resistivity (formation factor) of gas-hydrate bearing sediments at the well location. Fractured reservoir is assumed to be made of two components, one is gas hydrates saturated fracture and other is water saturated sediment surrounding the fracture. Modelled Velocity and formation factor for the water saturated sediment surrounding the fracture has been used in velocity resistivity transform equation to get background formation-factor. A widely used relation between formation factor and water permeability and the Tokyo model relating relative permeability with gas hydrate saturation have been used to estimate the gas hydrate saturation and relative permeability. This approach has also been applied to sites 2, 3, 4, 5, 6, 7 and 11 in the KG basin, and further extended along seismic lines passing through the site 10. The estimated results show gas-hydrates saturation and relative permeability as varying both laterally and vertically up to 25% and 0.6 respectively along the seismic lines.
Keywords: Velocity-resistivity transform, gas-hydrates, Krishna-Godavari basin, Gas-hydrates Saturation.
