Shallow gas occurrence in the eastern Lago Fagnano (Tierra del Fuego)

BARADELLO, L; DARBO, A.; LODOLO E.; CAFFAU, M; TASSONE A.; LIPPAI, H.; GROSSI, M.; DE ZORZI; G.

Torino

Congreso; VIII Forum Italiano di Scienze della Terra; 2011

Federazione Italiana di Scienze della Terra

Lago Fagnano, the southernmost ice-free reservoir of water in the world, occupies a deep tectonic depression developed along the E-W-trending Magallanes-Fagnano transform fault in Tierra del Fuego. It is 105-km-long, 7-km-wide in average, and has a maximum depth of 206 m. In the frame of an Italian-Argentinean scientific project funded by the Italian Ministry of Foreign Affairs, and jointly coordinated by the Istituto Nazionale di Oceanologia e di Geofisica Sperimentale (OGS) of Trieste and the Instituto de Geofísica “D. Valencio” of the Universidad de Buenos Aires, a series of high-resolution single-channel seismic reflection profiles were acquired on November 2009 and March 2010. These seismic data, collected by a Boomer source (150 kJoule) and a 10-hydrophones solid-state, single-channel streamer, complement and complete the seismic surveys carried out in the past in the Lago Fagnano. Ten gravity piston cores (with recovered sediment samples lenghts variable from 80 cm to 170 cm) have been also collected to analyze the stratigraphy of the most recent (Middle to Late Holocene) sedimentary cover in the eastern sector of the basin. Along some high-resolution seismic profiles the actual presence of shallow gas layers has been inferred because the typical very strong reflection and multiple reflections and acoustic blanking characterizing the seismic signal. The gas-related features observed on the seismic profiles include an amplitude phase reversal on top that creates multiple reflections. The gas abundance is well detected in seismic section because the sediment/gassy sediment interface (SGI) has a characteristic reflector with strong amplitude and reversed phase with respect to the bottom. The gassy sediments exhibit high attenuation (blanking) that hide geological sub-surface structures. The lake-floor morphology does not reveal any evidence of clear gas escape from the floor. The top of the acoustically high-amplitude layer is located between 0.3 and 1.7 m below the lake-floor. Seismic characteristics and velocity data seem to suggest a low concentration of gas, most likely less than 1%. We assume that the main origin of gas could to some extent be linked to the presence of a shallow, thin peat-rich layer of Middle-Late Holocene age. In fact, the mapped gassy zone occurs in correspondence of the outlet of the Rio Turbio, the principal input of eastern Lago Fagnano, which discarges the waters coming from a relatively small sag pond located immediately to the east of the eastern shore of the lake, one of the numerous peatlands that characterize most of the Tierra del Fuego lowlands.