3:45 PM - 4:00 PM
[MIS26-07] Vp anomalies of gas hydrate-bearing sediments inferred from BSR and BGHS
Keywords:gas hydrates, BSR, velocity anomaly, free gas
High amplitude, bottom simulating reflectors with reversed polarity (BSR) have been attributed to the base of gas hydrate stability (BGHS). TWT of the BSR of hydrate bearing sediments in Nankai trough were observed to be about 0.32 sec. below seafloor at about 945m of water depth (Tesuji et al., 2004), whereas they were 0.18 to 0.20 sec. at 895m to 1000m along the eastern margin of the Sea of Japan. Assuming that the average Vp of shallow subsurface marine sediments of 1.50km/sec, the actual depth of BSRs are estimated to be 240mbsf and 135 to 150mbsf respectively, which are assumed to be the depth of BGHS, an important factor of hydrate exploration. On the other hand, the depth of BGHS is identified by coring and logging penetrating through BSR profiles. In Nankai trough, Main Hole and Post Survey Wells have clearly recognized the BGHS at 263mbsf (Tsuji et al, 2004; Matsumoto et al., 2004), implying that the Vp of hydrate-bearing sediments of Nankai trough is not 1.50km/sec but 1.64m/sec for the interval between the sea-floor to the BGHS due to high Vp (~3.8km/sec) hydrates.
In the Sea of Japan, methane hydrates occur as massive and disseminated aggregates in gas chimney structure (Matsumoto et al., 2017). The stability of methane hydrate in seawater is determined by the pressure and temperature, and the depth of BGHS is uniquely determined by the temperature of bottom water, thermal gradient, and water depth. Thermal gradients on the Umitaka Spur at about 900m (W.D.) off Joetsu are approximately 10.0 +/- 1.0 degreeC, and the temperature of bottom water of 0.20 degreeC (Tomaru et al., 2019; Matsumoto et al., 2017). Therefore, the equilibrium depth of the BGHS is calculated to be 100 +/- 10 mbsf. On the other hand, a number of coring and logging holes on the Umitaka Spur identified the BGHS at the depth of 100 to 120mbsf, which are consistent with the equilibrium depth of BGHS.
BSR on the Umitaka Spur and Joetsu Knoll off Joetsu area appears at around 0.20sec TWT, whereas it occurs at 0.12sec to 0.14secTWT within hydrate-concentrated gas chimney structures. Vp of the host sediments off gas chimneys is calculated to be 1.0km/sec while it is 1.40 to 1.70km/sec for hydrate-bearing sediments in gas chimneys (Ohkawa et al., 2017). Vp off gas chimney is much lower than the velocity of water, anomalously low for marine sediments. This anomaly strongly suggest that free gas (methane) bubbles developed over the hydrate-bearing Spur and Knoll sediments. Vp of the gas chimney sediments (1.40 to 1.70km/sec) looks similar or a bit higher than “normal” marine sediments, however, the Vp is also unexpectedly low, considering that massive gas hydrates accumulates in gas chimneys by 25 to 80% by volume (Matsumoto et al., 2017). Based on the field observation, exploration and theoretical consideration of the occurrence of BSR and BGHS, we came to the hypothesis that development of free gases in shallow subsurface marine sediments is a critical factor to accumulate massive gas hydrates within gas chimneys, and the existence of free gas is to be identified by anomalously low Vp inferred from BSR and BGHS.
In the Sea of Japan, methane hydrates occur as massive and disseminated aggregates in gas chimney structure (Matsumoto et al., 2017). The stability of methane hydrate in seawater is determined by the pressure and temperature, and the depth of BGHS is uniquely determined by the temperature of bottom water, thermal gradient, and water depth. Thermal gradients on the Umitaka Spur at about 900m (W.D.) off Joetsu are approximately 10.0 +/- 1.0 degreeC, and the temperature of bottom water of 0.20 degreeC (Tomaru et al., 2019; Matsumoto et al., 2017). Therefore, the equilibrium depth of the BGHS is calculated to be 100 +/- 10 mbsf. On the other hand, a number of coring and logging holes on the Umitaka Spur identified the BGHS at the depth of 100 to 120mbsf, which are consistent with the equilibrium depth of BGHS.
BSR on the Umitaka Spur and Joetsu Knoll off Joetsu area appears at around 0.20sec TWT, whereas it occurs at 0.12sec to 0.14secTWT within hydrate-concentrated gas chimney structures. Vp of the host sediments off gas chimneys is calculated to be 1.0km/sec while it is 1.40 to 1.70km/sec for hydrate-bearing sediments in gas chimneys (Ohkawa et al., 2017). Vp off gas chimney is much lower than the velocity of water, anomalously low for marine sediments. This anomaly strongly suggest that free gas (methane) bubbles developed over the hydrate-bearing Spur and Knoll sediments. Vp of the gas chimney sediments (1.40 to 1.70km/sec) looks similar or a bit higher than “normal” marine sediments, however, the Vp is also unexpectedly low, considering that massive gas hydrates accumulates in gas chimneys by 25 to 80% by volume (Matsumoto et al., 2017). Based on the field observation, exploration and theoretical consideration of the occurrence of BSR and BGHS, we came to the hypothesis that development of free gases in shallow subsurface marine sediments is a critical factor to accumulate massive gas hydrates within gas chimneys, and the existence of free gas is to be identified by anomalously low Vp inferred from BSR and BGHS.