Geothermal modeling of the gas hydrate stability zone along the Krishna Godavari Basin
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ORIGINAL RESEARCH PAPER
Geothermal modeling of the gas hydrate stability zone along the Krishna Godavari Basin Uma Shankar • Michael Riedel • A. V. Sathe
Received: 21 October 2009 / Accepted: 24 May 2010 / Published online: 12 June 2010 Ó Springer Science+Business Media B.V. 2010
Abstract A wide-spread bottom simulating reflector (BSR), interpreted to mark the thermally controlled base of the gas hydrate stability zone, is observed over a close grid of multichannel seismic profiles in the Krishna Godavari Basin of the eastern continental margin of India. The seismic data reveal that gas hydrate occurs in the Krishna Godavari Basin at places where water depths exceed 850 m. The thickness of the gas hydrate stability zone inferred from the BSR ranges up to 250 m. A conductive model was used to determine geothermal gradients and heat flow. Ground truth for the assessment and constraints on the model were provided by downhole measurements obtained during the National Gas Hydrate Program Expedition 01 of India at various sites in the Krishna Godavari Basin. Measured downhole temperature gradients and seafloor-temperatures, sediment thermal conductivities, and seismic velocity are utilized to generate regression functions for these parameters as function of overall water depth. In the first approach the base of gas hydrate stability is predicted from seafloor bathymetry using these regression functions and heat flow and geothermal gradient are calculated. In a second approach the observed BSR depth U. Shankar (&) National Geophysical Research Institute (Council of Scientific and Industrial Research, New Delhi), Hyderabad 500 606, India e-mail: [email protected] M. Riedel Natural Resources Canada, Pacific Geoscience Center, Geological Survey of Canada, 9860 W. Saanich Rd., Sidney, BC V8L 4B2, Canada e-mail: [email protected] A. V. Sathe Oil and Natural Gas Corporation Ltd., KDM Inst. Of Petroleum Exploration, 9 Kaulagarh Road, Dehradun 248195, India
from the seismic profiles (measured in two-way travel time) is converted into heat flow and geothermal gradient using the same ground-truth data. The geothermal gradient estimated from the BSR varies from 27 to 67°C/km. Corresponding heat flow values range from 24 to 60 mW/m2. The geothermal modeling shows a close match of the predicted base of the gas hydrate stability zone with the observed BSR depths. Keywords Gas hydrates Bottom simulating reflector Krishna Godavari Basin Base of hydrate stability zone Heat flow Thermal modeling
Introduction Gas-hydrates are ice-like solids belonging to the class of clathrate compounds in which gas molecules (mainly methane) are trapped within cages of water molecules. Gas-hydrates are considered as potential future energy resource for India (Collett et al. 2008; Sain and Gupta 2008). Gas hydrates also plays a role in global climate change and as submarine hazards (e.g. Kvenvolden 1988). High pressures and low temperatures are required for the formation of gas hydrates. These requirements restrict natural gas hydrate
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