Discovery of a new natural gas formation process in black shale

While historically there have been three main types of natural gas, scientists have discovered a fourth source of production. This gas, called radiolytic, is formed by the interaction between organic matter and radio elements present in unconventional deposits.

Originally, the European project SECURe set itself very specific research goals: assessing the risks and improving the monitoring of the potential impact on the environment when using the subsoil for gas extraction from shale or for CO2 storage. This work, started in 2018, was successfully completed and finished last year. But it also led to an unexpected scientific discovery. Indeed, researchers from the BRGM (Bureau of Geological and Mining Research) and Canadian and Swiss experts have highlighted a new process of formation of natural gas in black shale, called radiolysis. Your discovery has just been published in the magazine PNAS.

Historically, there are three main types of natural gas, explains Wolfram Kloppmann, researcher at the BRGM and specialist in the application of isotope techniques. The first two come from organic material and are typically those found in black shale. Organic matter, when in a low-temperature anaerobic medium, serves as food for microorganisms, which then produce a microbial gas. When organic material is buried at great depths and exposed to very high temperatures, it decomposes and forms a gas called thermogenic. A final typology arises from the modification of certain rocks in contact with water, leading to complex chemical reactions involving natural hydrogen, leading to the production of what is known as abiotic gas (1). »

By observing atypical levels in the isotopic and chemical signature of certain gases, the researchers are highlighting a fourth source of natural gas. This is composed of various elements, mainly methane and to a lesser extent higher alkanes such as ethane, propane, butane and hexane. If a gas is isotopically poor in carbon-13, the scientific evidence accumulated to date has suggested a microbial origin fairly quickly. Chemically this was correct as it contained a large proportion of methane at the expense of higher alkanes. The fact that microorganisms produce methane by feeding on organic matter explains this composition.

Natural radiation can last up to 500 million years

By analyzing a large number of gases around the world, scientists have discovered the formation of a gas that is both very low in carbon 13 and rich in higher alkanes. Experiments conducted by Canadian researchers exposing organic material to a source of radiation led to the conclusion that this gas is formed thanks to radio elements interacting with organic material. “These radio elements are, for example, uranium, thorium or potassium 40says the researcher. Their occurrence in organic material is very common, but in very low concentrations. Because they exist for very long periods of time, sometimes up to 500 million years, their association with organic matter causes significant natural radiation and therefore creates a small amount of gas that accumulates in impermeable rock. »

This discovery will make it possible to improve the scientific understanding of the formation of gases in unconventional reservoirs. Experts can thus include this ongoing production of low-dose radiolysis gas in the black shales to better quantify the available resource estimate.

It should also make it possible to avoid the interpretations that have so far been carried out quite quickly when monitoring shale gas production, especially in the United States and Canada. ” The whole difficulty of our work is to know whether the leaks observed are from the well or not. Up to now, the isotopic signature has been assumed to mean that a gas with a low carbon-13 content is necessarily a sign that it is of microbial origin. In some cases, we could then rule out the hypothesis that the leak was related to the drilling and conclude that it originated in the environment, such as swamps, which can also produce gas from the fresh organics they contain. Now we know that a gas can both have a microbial signature at first glance and be produced by the radiolysis of organic material in black shale.” concludes Wolfram Kloppmann.

(1) Abiotic: Environment in which life is impossible

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