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Title: Gas hydrate occurrence and Morpho-structures along Chilean margin
Authors: Vargas Cordero, Ivan De La Cruz
Keywords: BSRChilean margingeothermal gradientgas hydrate
Issue Date: 27-Apr-2009
Publisher: Università degli studi di Trieste
Abstract: During the last decades, the scientific community spent many efforts to study the gas hydrates in oceanic and permafrost environments. In fact, the gas hydrate occurrence has a global significance because of the potential energy resource represented by the large amount of hydrocarbon trapped in the hydrate phase. Moreover, it may play a role in global climate change, and it is also study because of the hazard that accumulations of gas hydrate may cause to drilling and seabed installations. In seismic data, the base of the gas hydrate presence is detected by a strong reflector, called BSR. Along the Chilean continental margin, in the last decades the BSR is well reported by several geophysical cruises. In particular, the BSR is recognized along the accretionary prism. An important aspect related to the gas hydrates is the estimate of gas concentration in the pore space by using seismic data. In fact, both compressional and shear wave velocities provide information about the presence of gas hydrate and free gas in marine sediments. A quantitative estimate of gas hydrate and free gas concentrations can be obtained by fitting the theoretical velocity to the experimental velocity. For this purpose, in this Thesis several seismic data are analyzed in order to detect, quantify and explain the gas hydrate presence in this region. Frontal and basal accretions were identified by interpreting six post-stack time migrated sections, which across the entire margin (continental shelf, continental slope, oceanic trench and oceanic crust). The trench infill southwards of Juan Fernandez Ridge is characterized by a succession of reflectors with high and low amplitude associated to turbidites. A thinner bed (0.3 s) was recognized in correspondence to the accretionary prism characterized by several morphological highs. These morphological highs were associated to different accretional stages. On the contrary, a thicker bed (0.8 s) was recognized in correspondence to an uplifted accretionary prism characterized by a smoother topography. Basal and frontal accretions can be related to the morpho-structures recognized in this part of the Chilean margin. Negative and positive flower structures can help to explain the deformational variability of the Chilean margin, because negative flowers structures are associated to transtensional domain, where the continental slope morphology is characterized by normal faults, submarine erosive channels and slump heads. Positive flower structures, instead, are associated to transpresional domain and could explain the presence of older re-activated thrusts, slightly deformed slope basins. Moreover a strike-slip component affecting the oceanic crust, can also involve the continental margin, in fact on the continental slope, positive and negative flower structures can be associated to strike-slip faults parallel to the coast or to Riedel shear. The BSR is an important indicator of gas hydrate and free gas presence and we performed a processing to enhance its presence. In all analysed sections, the BSR was recognized in correspondence to an ancient accretionary prism with different seismic characteristics along the margin. A strong and continuous BSR was recognized in the northern sector (offshore Itata) and southern sector (offshore Coyhaique), while a discontinuous and weak BSR was recognized in the central Chile (offshore Arauco and Valdivia). In order to quantify the gas-phase, an advanced processing was performed. Two portions of sections were selected of about 20 km length. The first one is located in the central part (offshore Arauco) and another one is located in the southernmost part (offshore Coyhaique). In the Coyhaique offshore, the seismic section evidences the presence of a structural high that acts as structural trap for the gas and the fluid upwards migrating. Here, the BSR depth varies from 250 mbsf (in the middle of the accretionary prism) to 130 mbsf (in the structural high), reaching its maximum (330 mbsf) in the fore-arc basin. This depth variability is partially due to the different water depth and partially to the variable geothermal gradient, which varies from 35 to 95° C/km, caused by fluid migration that modifies the gas hydrate stability field. In the Arauco offshore, the BSR is strong and continuous only in a limited area, where it is possible suppose that the fluid is accumulated below the gas hydrate layer and, somewhere, the fluid reaches the seafloor. In this area, the BSR depth reaches 500 mbsf. Here, the higher BSR depth with respect to offshore Coyhaique can be justified by the high water depth and the presence of a lower geothermal gradient (about 30° C/km). The results allowed us to recognize a high (2200 m/s) and low (1270 m/s) velocity layers associated to gas hydrate and free gas presence respectively. The highest gas hydrates and free gas concentrations were detected in the Coyhaique offshore (at 44.5 °S) with an average of 12% and 1% of total volume respectively. By using the instantaneous amplitude, in particular using the BSR/seafloor ratio, it is possible conclude that the section located northernmost in offshore Itata (close to 36 °S; RC2901-728 section), can be considered an interesting reservoir of gas hydrates and free gas, because of the high estimated values of the BSR/seafloor ratio (>0.5). This study suggests that the gas hydrate can play an important role in this part of the Chilean margin for two main reasons. The first one is related to the potentiality of the hydrate reservoir. In fact, the local high concentrations of both hydrate and free gas, as suggested by previous and our studies, could be considered as a future energy resources. The second one is related to the important geo-hazard related to the gas hydrate destabilization. For example, high amount of the free gas, presumably in overpressure condition (Coyhaique offshore), could be naturally released and trigger submarine slides, inducing hydrate instability. Moreover, a possible strong earthquake could generate anomalous sea waves, which could affect at vicinity coast, inducing the gas hydrate destabilization.
Description: 2007/2008
NBN: urn:nbn:it:units-7462
Appears in Collections:Scienze della terra

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