A basin of biological gas
In the vast, intermontane Zaisan Basin of East Kazakhstan, a unique geological process is underway. The ground is releasing significant quantities of methane, the primary component of natural gas. This is however not ancient gas trapped for millions of years. Instead, it is being actively generated by immense underground communities of microorganisms. The basin's Cenozoic coal seams, relatively young geological formations, are a fuel source; they are the substrate for a living methane factory.
The process is known as microbial methanogenesis. Groundwater flowing through the porous coal seams introduces communities of microbes. A consortium of different bacteria and archaea works to break down the complex organic molecules of the coal. First, bacteria ferment the coal, producing simpler compounds like acetate, hydrogen (H₂), and carbon dioxide (CO₂). Then, a group of archaea called methanogens consumes these byproducts to produce methane. The dominant pathway in many coal beds is hydrogenotrophic methanogenesis, where archaea use hydrogen as an energy source and carbon dioxide as a carbon source, releasing methane as waste. This ongoing biological activity makes the Zaisan Basin a site of secondary biogenic gas formation.
The isotopic fingerprint
Scientists can prove the methane's modern, biological origin through stable isotope analysis. Methane gas is composed of carbon and hydrogen atoms, which exist in slightly different forms, or isotopes. The lighter isotope, Carbon-12 (¹²C), is much more common than the heavier Carbon-13 (¹³C). Microbes preferentially use the lighter ¹²C isotope during their metabolic processes. As a result, the methane they produce is significantly depleted in ¹³C compared to the surrounding environment.
This creates a distinct isotopic signature. Methane produced by geothermal heat and pressure, known as thermogenic gas, has a δ¹³C value typically heavier than -50‰ (parts per thousand). By contrast, biogenic methane is much lighter, often with δ¹³C values less than -60‰. The gas seeping from the Zaisan Basin falls squarely into this biogenic range. Analysis of both carbon and hydrogen isotopes (δ²H) confirms that the methane is not ancient gas that has been trapped, but is the result of contemporary microbial activity breaking down the coal. This living process is dependent on the local geology, where tectonic activity created faults that allow surface water and microbes to infiltrate the coal beds.