Abstract
Direct measurements of gas composition by drilling at a few hundred meters below seafloor can be costly, and a remote sensing method may be preferable. The hydrate occurrence is seismically shown by a bottom-simulating reflection (BSR) which is generally indicative of the base of the hydrate stability zone. With a good temperature profile from the seafloor to the depth of the BSR, a near-correct hydrate phase diagram can be calculated, which can be directly related to the hydrate composition. However, in the areas with high topographic anomalies of seafloor, the temperature profile is usually poorly defined, with scattered data. Here we used a remote method to reduce such scattering. We derived gas composition of hydrate in stability zone and reduced the scattering by considering depth-dependent geothermal conductivity and topographic corrections. Using 3D seismic data at the Penghu canyon, offshore SW Taiwan, we corrected for topographic focusing through 3D numerical thermal modeling. A temperature profile was fitted with a depth-dependent geothermal gradient, considering the increasing thermal conductivity with depth. Using a pore-water salinity of 2%, we constructed a gas hydrate phase model composed of 99% methane and 1% ethane to derive a temperature depth profile consistent with the seafloor temperature from in-situ measurements, and geochemical analyses of the pore fluids. The high methane content suggests predominantly biogenic source. The derived regional geothermal gradient is 40°C km-1. This method can be applied to other comparable marine environment to better constrain the composition of gas hydrate from BSR in a seismic data, in absence of direct sampling.
Highlights
A bottom-simulating reflection (BSR), is commonly associated with the presence of gas hydrate under the seafloor
The temperature field is usually different from the temperature derived from depths of the BSR (TBSR_no_topo), which was forward-modeled based on a 99% methane hydrate phase diagram using water depths, subbottom depths of the BSR, and a regional geothermal gradient of 40°C km-1
2% salinity was used based on core samples from the Shenhu Area, northern South China Sea (Wu et al 2011), which are located in a similar tectonic setting to the west of our study area and the results of our geochemical modelling are consistent with previous studies (e.g., Feng and Chen 2015)
Summary
A bottom-simulating reflection (BSR), is commonly associated with the presence of gas hydrate under the seafloor. The depth of the BSR can be interpreted from 2D (e.g., Yamano et al 1982; Chi et al 1998; Chi and Reed 2008), and occasionally 3D seismic reflection datasets, dependent on the availability of data (e.g., Martin et al 2004; Hornbach et al 2012; Zander et al 2017). This interpreted BSR depth can be used to derive the temperature at depth of the BSR using a given hydrate phase boundary (e.g., Yamano et al 1982; Chi et al 1998; Chi and Reed 2008)
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