Untapped resource, or threat of greenhouse gases, discovered under the axis of division of the Okinawa coastline

By analyzing the reflections of seismic pressure waves by sub-soil geology in southwest Japan, researchers from Kyushu University discovered the first evidence of a massive gas reservoir in which the Earth's crust is being separated. Depending on its nature, the trapped gas could be an untapped natural resource or a source of greenhouse gases waiting to escape, which would reinforce the need to publicize similar reservoirs around the world.

While the ocean may appear calm at the surface, ocean depths may undergo intense thermal activity as warm magma infiltrates from locations where the upper layers of the Earth are separated, process called rifting. In these areas, high levels of carbon dioxide and methane may be present in the water, possibly escaping magma or produced by microbial organisms or by the interaction of rich sediments. in organic matter with hot water.

In a new study published in Geophysical Research Letters, researchers at the Kyushu University International Institute for Research on Carbon Neutral Energies (I²CNER) now indicate that some of these gases may actually be trapped in the ground, which would result in the existence of a huge gas reservoir below the axis along which there is a crack in the pit. Okinawa.

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

"Seismic pressure waves generally propagate more slowly in gases than in solids," says co-author Andri Hendriyana. "Thus, by estimating the velocity of seismic pressure waves across the ground, we can identify underground gas reservoirs and even obtain information on their saturation in the middle of the Okinawa Trough, indicating areas filled with gas. "

At this stage, researchers are still unsure whether the tanks are mainly filled with carbon dioxide or methane. If methane gas could be a potential natural resource. However, carbon dioxide and methane both contribute to the greenhouse effect, so the rapid and uncontrolled release of one or the other gas from such a large reservoir could have a significant impact on the environment.

"While many people focus on man-made greenhouse gases, a wide variety of natural sources also exist," says author correspondent Takeshi Tsuji. "Large gas reservoirs located along one axis of separation could be another source of greenhouse gases that we need to keep an eye on and could also prove to be an important natural resource. "

With respect to gas trapping, it is possible that layers of impermeable sediment, such as clay, prevent the gas from escaping porous layers of underlying materials, such as stone. pumice. Based on the heat flow around the study area, the researchers believe that another possibility is that a cape of methane hydrate with low permeability, an ice containing methane , plays the role of cover.

"Areas such as the one we have explored are not uncommon along the faults, so I suppose that similar reservoirs could exist elsewhere in the Okinawa Trench as well as in other basins. Continental background covered with sediment in the world, "says Tsuji.

Provided by
Kyushu University