Abstract
Bottom-simulating reflectors (BSRs) were observed beneath the seafloor in the northern continental margin of the South China Sea (SCS). Acoustic impedance profile was derived by Constrained Sparse Spike Inversion (CSSI) method to provide information on rock properties and to estimate gas hydrate or free gas saturations in the sediments where BSRs are present. In general, gas hydrate-bearing sediments have positive impedance anomalies and free gas-bearing sediments have negative impedance anomalies. Based on well log data and Archie's equation, gas hydrate saturation can be estimated. But in regions where well log data is not available, a quantitative estimate of gas hydrate or free gas saturation is inferred by fitting the theoretical acoustic impedance to sediment impedance obtained by CSSI. Our study suggests that gas hydrate saturation in the Taixinan Basin is about 10 - 20% of the pore space, with the highest value of 50%, and free gas saturation below BSR is about 2 - 3% of the pore space, that can rise to 8 - 10% at a topographic high. The free gas is non-continuous and has low content in the southeastern slope of the Dongsha Islands. Moreover, BSR in the northern continental margin of the SCS is related to the presence of free gas. BSR is strong where free gas occurs.
Highlights
The hydrate-bearing sediment has an anomalously high compressional velocity, and gas hydrate saturation can be estimated when a reference velocity curve is known (Tinivella 1999)
Acoustic impedance profiles can be obtained by Constrained Sparse Spike Inversion (CSSI), which provides a quantitative approach to determine the presence of gas hydrate and free gas and to estimate their saturations
Gas hydrate saturation is about 10 - 20% of the pore space; in general, it increases to about 30% of the pore space near Bottom-simulating reflectors (BSRs)
Summary
The hydrate-bearing sediment has an anomalously high compressional velocity, and gas hydrate saturation can be estimated when a reference velocity curve is known (Tinivella 1999). The free gas-bearing sediment has low elastic impedance, low P-impedance, non-anomalous background S-impedance, low λρ and low Poisson’s ratio. The tectonic regime in the northern SCS is mainly controlled by faults trending in NE and NEE directions and BSRs are observed to the south of Taiwan on reflection seismic records and at the Xisha Trough (Chi et al 1998, 2006; McDonnell et al 2000; Wu et al 2005; Liu et al 2006). Acoustic impedance profiles can be obtained by Constrained Sparse Spike Inversion (CSSI), which provides a quantitative approach to determine the presence of gas hydrate and free gas and to estimate their saturations. The gas hydrate saturations in the Taixinan Basin and in the Pearl River Mouth basin west of the Dongsha Islands are inferred by using CSSI on two seismic profiles 0102 and 97301, respectively (Fig. 1)
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