But is it natural gas or carbon dioxide or perhaps helium or sulfur dioxide?

Kyushu University researchers used an automated method to create a high-resolution map of the seismic velocity below the seafloor where they found a large-scale gas reservoir in an area where the Earth’s upper layers are being separated. This reservoir, the first of its kind, and the potential for others like it could have implications from a natural resource or environmental standpoint depending on whether the trapped gas is methane, carbon dioxide or others and whether it remains trapped.

By analyzing reflections of seismic pressure waves by the subseafloor geology off southwestern Japan the researchers at Kyushu University have found the first evidence of a massive gas reservoir where the Earth’s crust is being separated.

While the ocean can seem calm on the surface, the ocean depths can experience intense thermal activity as hot magma seeps from locations where the Earth’s upper layers are being pulled apart – a process called rifting. In such areas, elevated levels of methane and carbon dioxide gases can be present in the water, possibly escaping from magma or being produced by microbial organisms or the interaction of organic-rich sediment with hot water.

In a new study published in Geophysical Research Letters, researchers from Kyushu University’s International Institute for Carbon-Neutral Energy Research (I2CNER) now report that some of these gases may actually get trapped underground, leading to the existence of a massive gas reservoir beneath the axis along which rifting is occurring in the Okinawa Trough.

To find the reservoir, the researchers analyzed measurements of how geological structures reflect seismic pressure waves generated by an acoustic source carried by a boat to the study area. Applying an automated calculation technique to this seismic data, they were able to create a two-dimensional map of the velocities at which the pressure waves travel through the ground with a much higher resolution than previous manual techniques.

Study co-author Andri Hendriyana explained, “Seismic pressure waves generally travel more slowly through gases than through solids. Thus, by estimating the velocity of seismic pressure waves through the ground, we can identify underground gas reservoirs and even get information on how saturated they are. In this case, we found low-velocity pockets along the rifting axis near Iheya North Knoll in the middle of the Okinawa Trough, indicating areas filled with gas.”

At this stage, the researchers are still not sure if the reservoirs are mainly filled with carbon dioxide or methane. If its methane, the gas could be a potential natural resource. However, both carbon dioxide and methane contribute to the greenhouse effect, so the rapid, uncontrolled release of either gas from such a large reservoir could have significant environmental implications.

Corresponding author Takeshi Tsuji said, “While many people focus on greenhouse gases made by humans, a huge variety of natural sources also exist. Large-scale gas reservoirs along a rifting axis may represent another source of greenhouse gases that we need to keep our eyes on. Or, they could turn out to be a significant natural resource.”

As for how the gas is trapped, one possibility is that layers of impermeable sediment such as clay could prevent the gas from escaping porous underlying layers of materials such as pumice. Based on the flow of heat around the study area, the researchers think another possibility is that a low-permeability cap of methane hydrate – a methane-containing ice – acts as the cover.

Tsuji explained, “Zones like the one we investigated are not uncommon along rifts, so I expect that similar reservoirs may exist elsewhere in the Okinawa Trough as well as other sediment-covered continental back-arc basins around the world.”

One can be sure this will get explored. East Asia, Japan in particular, need a much more local supply of fuel. Your humble writer hopes they choose a capable oil and gas firm and they find natural gas soon. The worst outcome would be something like sulfur dioxide.


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