Thermal state and concentration of gas hydrate and free gas of Coyhaique, Chilean Margin (44°30′ S)

In the last decades gas hydrate occurrence along the Chilean continental margin has been well documented. In order to better define the seismic character of the hydrate-bearing sediments, we performed a detailed velocity analysis by using the pre-stack depth migration on part of multichannel reflection seismic line RC2901-734 located offshore Coyhaique.The velocity model shows a hydrate bearing layer above the BSR, with high velocity (1700-2200ms^-1) and maximum thickness of 250m and a free gas bearing layer below the BSR, characterized by low velocity (1250-1400ms^-1). A weak reflector at about 70m below the BSR marks the base of the second layer.By knowing the BSR depth, the seafloor depth, and the sea bottom temperature, the geothermal gradient was estimated. The resulting gradient varies from 35 to 95°C km-1, with highest values at the structural high, and the lowest values located in the accretionary prism and in the fore arc basin.In order to quantify the amount of gas phase, the velocity model was converted into a gas-phase concentration model by using a theoretical approach. The results indicate that highest concentrations of gas hydrates, up to 23% of the total volume, are located in the fore-arc basin, and that highest concentration of free gas, up to 3% of the total volume, are located at the structural high, which may be considered as a natural trap for migrating fluids. Average concentrations are equal to 12% and 1% of total volume for gas hydrate and free gas, respectively.