GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.1 GNGTS 2023 A review of the geophysical methods to characterize the gas hydrate system: the Chilean margin case study G. Alessandrini 1 , I. Vargas-Cordero 2 , U. Tinivella 1 , M. Giustiniani 1 , L. Villar-Munoz 3 1 National Institute of Oceanography and Applied Geophysics – OGS, Trieste, Italy 2 Valparaiso, Chile 3 Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile Many studies related to gas hydrate occurrences worldwide have been reported mainly to better characterize reservoirs, their potential as energy resource and their role in global climate change. Concerning the former, large amounts of methane gas can be trapped in form of gas hydrate being considered as an important greenhouse gas. It is estimated that over a 20-year period, one ton of methane has a global warming potential 84–87 times greater than carbon dioxide and over a century, this warming potential is 28–36 times greater, according to the Intergovernmental Panel on Climate Change. For these reasons, in the last decades, several projects have been carried out to assess the global methane hydrate quantities. The estimation of the global methane reservoir has decreased, while the knowledge about hydrate reservoirs have increased. Several geophysical methods can be used to study the hydrate system, even if the most powerful method is the seismic method. The main indicator of gas hydrate occurrences in seismic sections is the bottom simulating reflector, the so-called BSR, which represents the boundary between the high seismic velocity gas hydrate-bearing sediments and the low seismic velocity underlying free gas. In fact, it is well known that the presence of gas hydrate and/or free gas in the pore space of marine sediments can be detected analysing P- and S-wave velocities. In addition, the base of the free gas layer is called BGR and it is not always detectable by seismic data. Knowing the relationship between hydrate and free gas saturation in the pore space of sediments and elastic velocity, it is possible to convert seismic velocity section into into gas-phase section. Several theoretical and empirical models are available in literature; here, we discussed and adopted the Tinivella’s method, based on the Biot-Gerstmann-Smith equation. In last decades, the Chilean margin has been extensively investigated to better characterize the complex geological setting through the acquisition of geophysical data and, in particular, seismic lines that allowed identifying the occurrence of gas hydrates and free gas and features related to them, such as the BSR. Moreover, their occurrence has also been confirmed by the presence of