Gas Hydrate and Free Gas Concentrations in Two Sites inside the Chilean Margin (Itata and Valdivia Offshores)

Vargas-Cordero Iván, Tinivella Umberta, Villar-Muñoz Lucía
2017 Energies  
Two sectors, Itata and Valdivia, which are located in the Chilean margin were analysed by using seismic data with the main purpose to characterize the gas hydrate concentration. Strong lateral velocity variations are recognised, showing a maximum value in Valdivia offshore (2380 ms −1 above the BSR) and a minimum value in the Itata offshore (1380 m·s −1 below the BSR). In both of the sectors, the maximum hydrate concentration reaches 17% of total volume, while the maximum free gas concentration
more » ... is located Valdivia offshore (0.6% of total volume) in correspondence of an uplift sector. In the Itata offshore, the geothermal gradient that is estimated is variable and ranges from 32 • C·km −1 to 87 • C·km −1 , while in Valdivia offshore it is uniform and about 35 • C·km −1 . When considering both sites, the highest hydrate concentration is located in the accretionary prism (Valdivia offshore) and highest free gas concentration is distributed upwards, which may be considered as a natural pathway for lateral fluid migration. The results that are presented here contribute to the global knowledge of the relationship between hydrate/free gas presence and tectonic features, such as faults and folds, and furnishes a piece of the regional hydrate potentiality Chile offshore. 2 of 11 concentrations of 18% and 1% of volume of gas hydrate and free gas concentrations, respectively [17] . Nowadays, in the literature, a limited number of studies are performed to quantify gas hydrate in marine sediments along the Chilean margin [17] [18] [19] [20] [21] [22] . However, a regional estimation is needed in order to map the gas-phase distribution along this margin. 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-wave and S-wave velocity [23, 24] . In fact, knowing the relationship between hydrate saturation in the pore space of sediments and elastic velocity, it is possible to convert seismic velocity section into gas-phase section. Several theoretical and empirical models are available in literature; here, we briefly overview the ones that are most commonly used. The empirical model of Ref. [25] is based on a weighted equation model. The first theory that includes the concept of cementation caused by the hydrate presence in proximity of the grains was proposed by [26] ; as observed by Ref. [27] this theory predicts much higher seismic velocities than those that are normally observed in nature. The theory proposed by Ref. [28] considered the hydrate as a part of the skeleton; they verify the effective medium theory by using the ODP data (Leg 164, site 995, Blake Ridge area). The most used models were compared by Ref. [29] ; they concluded that the main difference among resulting velocities versus gas hydrate concentration is not related to the empirical or theoretical approach adopted, but to the petrophysical properties hypothesized. In this study, the method proposed by [30, 31] was adopted; it is based on the Biot-Gerstmann-Smith equation and was successfully verified by using direct measurements (i.e., Ocean Drilling Program-ODP, Leg 146 and 164; [23, 32] ) that gives results in agreement with the three-phase Biot theory [33] . The study area is located in Itata and Valdivia offshores along the Chilean continental slope between 36 • S and 40 • S (Figure 1 ), which is characterized by the subduction of the oceanic Nazca Plate below the South American continental plate along the Peru-Chile trench. The rate of subduction is estimated by Ref. [34] [35] [36] , and is of about 66 km·Ma −1 .
doi:10.3390/en10122154 fatcat:x6uwyztdp5h4tey3dosl7qlfa4