Abstract
In this study one seismic section offshore Chiloe Island was analyzed to better define the seismic character of the hydrate-bearing sediments. The velocity analysis was used to estimate the gas-phase concentration and relate it to the geological features. The velocity model allowed us to recognize two important layers that characterize hydrate- and free gas-bearing sediments above and below the BSR respectively: one located above the BSR, characterized by high velocity (1,800-2,200 m/s) and a second one, below the BSR, characterized by low velocity (1,600-1,700 m/s). A weak reflector at about 100 m below the BSR marks the base of the second layer. AVO analysis and offset stack sections confirming that the reflector interpreted as BGR is related to free gas presence in the pore space. The velocity field is affected by lateral variation, showing maximum (above the BSR) and minimum (below the BSR) values in the sector. Here, the highest gas hydrate and free gas concentrations were calculated, obtaining 9.5% and 0.5% of total volume respectively. A variable BSR depth (from 300 to 600 mbsf) can be justified supposing a variable geothermal gradient (from 25 to 45 °C/km).
Highlights
Gas hydrate occurrence in the natural environment has a global significance because of the potential energy resource represented by the large amount of hydrocarbon trapped in the hydrate phase (Milkov, 2004)
AVO analysis and offset stack sections confirming that the reflector interpreted as Base of free Gas Reflector (BGR) is related to free gas presence in the pore space
The Bottom Simulating Reflector (BSR) presence allows defining the boundary between the high seismic velocity and the low seismic velocity because of the free gas presence (e.g., Shipley et al, 1979; Hyndman and Spence 1992; Berndt et al, 2004)
Summary
Gas hydrate occurrence in the natural environment has a global significance because of the potential energy resource represented by the large amount of hydrocarbon trapped in the hydrate phase (Milkov, 2004). Geophysical studies along the Chilean margin through seismic ref lection data analysis have given us the opportunity to recognize the Bottom Simulating Reflector (BSR) and to map its presence (Bangs et al, 1993; Brown et al, 1996; Díaz-Naveas, 1999; Morales, 2003; Vargas-Cordero et al, 2011; VillarMuñoz et al, 2014). This ref lector, representing the acoustic limit between a gas hydrate layer above the BSR and free gas layer below the BSR, is extended along the Chilean margin between 33° to 47° S. The results of these studies have allowed estimating the gas hydrate and free gas amounts in limited areas of Valdivia, Itata and Coyhaique offshore
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