Abstract
The presence of a gas hydrate reservoir and free gas layer along the South Shetland margin (offshore Antarctic Peninsula) has been well documented in recent years. In order to better characterize gas hydrate reservoirs, with a particular focus on the quantification of gas hydrate and free gas and the petrophysical properties of the subsurface, we performed travel time inversion of ocean-bottom seismometer data in order to obtain detailed P- and S-wave velocity estimates of the sediments. The P-wave velocity field is determined by the inversion of P-wave refractions and reflections, while the S-wave velocity field is obtained from converted-wave reflections received on the horizontal components of ocean-bottom seismometer data. The resulting velocity fields are used to estimate gas hydrate and free gas concentrations using a modified Biot-Geertsma-Smit theory. The results show that hydrate concentration ranges from 10% to 15% of total volume and free gas concentration is approximately 0.3% to 0.8% of total volume. The comparison of Poisson’s ratio with previous studies in this area indicates that the gas hydrate reservoir shows no significant regional variations.
Highlights
Gas hydrates are ice-like crystalline solids composed of water and low-molecular-weight gases, which form under conditions of high pressure, low temperature, and sufficient gas concentration [1]
1.68 to 1.73 km/s at the seafloor to 2.0 to 2.1 km/s at the bottom simulating reflection (BSR). This high P-wave velocity layer with a thickness varying between 150 and300 m just above the BSR can be associated with the presence of gas hydrates
The analysis of ocean-bottom seismometer (OBS) data has enabled the obtaining of the information of both P- and S-wave velocity and of it provides into the distribution and quantification of gas hydrate and OBSinsights data has allowed us to characterize the gas hydrate reservoir in free gas in marine sediments in the
Summary
Gas hydrates are ice-like crystalline solids composed of water and low-molecular-weight gases (mostly methane), which form under conditions of high pressure, low temperature, and sufficient gas concentration [1]. Hydrates are widespread in the shallow marine sediments along continental margins and in permafrost areas [2]. Gas hydrates in the marine sediments have commonly been inferred on the basis of seismic reflection profiles from the presence of a so-called bottom simulating reflection (BSR) that marks the base of the gas hydrate stability zone [3]. A BSR is generated due to the strong impedance contrast between hydrate-bearing sediments above and underlying free gas-bearing sediments. In the majority of situations, where no direct measurements are available, the analysis of seismic velocity provides an efficient way to identify and characterize the distribution of gas hydrates and Energies 2018, 11, 3290; doi:10.3390/en11123290 www.mdpi.com/journal/energies During the last few decades, much effort has been expended on the study of gas hydrates because of their economic potential as a future energy source [4,5] and their potential role in geohazards [6,7,8] and global climate change [9,10,11,12,13,14,15].
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