GNGTS 2017 - 36° Convegno Nazionale

594 GNGTS 2017 S essione 3.1 southern sector, where the highest values are located in vicinity of the accretionary prism (2 to 10 km; about 30% of total volume; Fig. 3). On the other hand, the highest free gas concentrations (0.35% of volume) in the northern sector are detected upwards (between 7 and 12 km). Finally, in the northern sector we obtained an average of gas hydrate and free gas concentrations equal to 12% and 0.2% respectively, while in the southern sector the average concentrations of gas hydrate reaching 18% of volume. Discussion and conclusion. The main points found are: a) the seismic velocity analysis indicates strong lateral variations in both analysed seismic lines; b) in the northern part the BSR depth variability is due partially to the different water depth and partially to the variable geothermal gradient, from 35 to 87°C/km, while the higher BSR depth (compared to the southern sector) can be explained by the high water depth and by the presence of a constant and lower geothermal gradient (about 35°C/km); c) the highest velocities can be related to two main factors: - gas hydrate presence and - change in petro–physical properties; d) the high velocity above the BSR means high gas hydrate concentrations (until 30% of volume in the northern sector and 22% of volume in the southern sector) and the low velocity below the BSR means high free gas concentration (0.35% in the northern sector). Here a lateral migration upwards can be deduced. This study suggests that gas hydrate can play an important role in this part of the Chilean margin. In fact, the high local concentrations of both hydrate and free gas, as suggested by several studies, could be considered as a potential future energy resource also geological risk due to eventual gas hydrate dissociations. Acknowledgments  The authors are very grateful to Joyce Alsop and Volkmar Damm for seismic data provided by the Lamont Doherty Earth Laboratory (LDEO), USA and the Federal Institute for Geosciences and Natural Resources (BGR), Germany respectively. We are grateful to CONICYT (Fondecyt de Iniciación N°11140216), which supported this work. References Bangs, N.L. and Brown, K.M.; 1995: Regional heat flow in the vicinity of the Chile Triple Junction constrained by the depth of the bottom simulating reflection . In: Lewis S.D., Behrmann J.H., Musgrave R.J. and Cande, S.C. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, 141, 253–258. Bangs N.L., Sawyer D.S. and Golovchenko X.; 1993: Free gas at the base of the gas hydrate zone in the vicinity of the Chile triple Junction . Geology, 21 , 905–908. Brown, K.M., Bangs, N.L., Froelich, P.N. and Kvenvolden K.A.; 1996: The nature, distribution, and origin of gas hydrate in the Chile Triple Junction region . Earth and Planetary Sciences Letters, 139 , 471–483. Diaz–Naveas, J.; 1999: Sediment subduction and accretion at the Chilean convergent margin between 35° and 40°S. Dissertation, University of Kiel, Germany, 130pp. Grevemeyer I., Diaz–Naveaz J.L., Raner C.R., Villenger, H.W. and Ocean Drilling Program Scientific Party; 2003: Heat Flow over the decensing Nazca plate in Central Chile, 32°S to 41°S: observations from ODP Leg 202 and the occurrence of natural gas hydrates . Earth and Planetary Sciences Letters, 213 , 285–298. Grevemeyer I. and Villinger H.; 2001: Gas hydrate stability and the assessment of heat flow through continental margins. Geophysical Journal International, 145 , 647-660. Morales E.; 2003: Methane hydrates in the Chilean continental margin. Electronic Journal of Biotechnology. Available via DIALOG. http://ejb .ucv.cl/content/vol6/issue2/issues/1/ Rodrigo C., Gonzalez–Fernández A. andVera, E.; 2009: Variability of the bottom–simulating reflector (BSR) and its association with tectonic structures in the Chilean margin between Arauco Gulf (37°S) and Valdivia (40°S). Marine Geophysical Researches, 30 , 1–19. Tinivella U., Loreto M.F. and Accaino F.; 2009: Regional versus detailed velocity analysis to quantify hydrate and free gas in marine sediments: The south Shetland margin target study. Geological Society of London Special Publications, 319 , 103-119. Vargas-Cordero I.C., Tinivella U., Accaino F., Loreto M.F., Fanucci F. and Reichert, C.; 2009: Analyses of bottom simulating reflections offshore Arauco and Coyhaique (Chile). Geo Marine Letters, 30 , 271–281, DOI 10.1007/ s00367-009-0171-5.