AAs early as the 1920s, gold miners south of Johannesburg were surprised to encounter methane gas flowing out of assay boreholes drilled into ancient quartzites, with little explanation as to its origin. Termed “blower” wells, hundreds of these were drilled throughout the region over the next several decades as gold (and platinum) mining progressed deeper into the subsurface. Methane was recognized as a significant and near-ubiquitous hazard,  yet a convincing description of its origin puzzled geoscientists for decades. This is because most natural gas is derived from the thermal degradation of ancient organic matter, or kerogen, that lived millions to hundreds of millions of years ago. The rocks hosting these gold and platinum deposits are billions of years old, and lack the organic matter necessary to generate hydrocarbons.

It wasn’t until the early 2000s that a more credible explanation for the gas was discovered: a class of organisms called archaea are actively generating biogenic methane. Despite being a primitive life-form found in a variety of habitats, including the rumens of cattle, archaea were unknown to biology until the 1970s. In part this is because archaea tends to thrive in places where other organisms with more efficient metabolic pathways, such as bacteria, cannot exist. These “extremophiles” are found in environments where high temperatures, anoxia, salinities, and pH may preclude other life. Many archaea are methanogenic, creating methane as a metabolic product of partially-degraded organic material or by combining free hydrogen with carbon. In the case of the “blower wells”, geologic evidence supports the latter mechanism.

The mystery of the biogenic methane does not necessarily end with the recognition that archaea are involved. Their metabolism requires hydrogen, which is the most abundant element in the universe, and also one of the rarest to find in large concentrations as a pure gas. Hydrogen is not only the elemental building block for the universe, it is also a critical constituent for life—most directly for archaea. Free hydrogen in the biosphere is relatively rare, as organisms like archaea will convert it to methane if provided the opportunity. On earth, there are two processes that are currently believed to be responsible to create volumetrically-significant free hydrogen: radiolysis and hydration of iron silicates. Radiolysis is the process whereby radioactive decay splits water molecules into its constituent ions, liberating hydrogen. In the subsurface, the naturally-occurring radionuclides uranium, thorium, and potassium are responsible for most of this production. Hydration of iron silicates is another means of hydrogen production, where the alteration of minerals into more stable phases also liberates hydrogen. In South Africa, the widespread occurrence of basaltic dikes and intrusions across the Karoo are likely candidates for hydrogen production, as are more ancient lava flows (like the 2.7 billion-year-old Ventersdorp Group or the 2.3 billion-year-old Pretoria and Rooiberg groups of the Transvaal Supergroup). Notably, hydrogen has been found within the blower wells and free gas encountered in the deep mines in low concentrations as a mixture with methane.

As mentioned above, radioactivity may play an important, but indirect, role in sustaining a habitat for archaea. Naturally-occurring radioactive elements uranium and thorium also produce another important product as it decays: helium. The helium diffuses out of the mineralogic hosts of the uranium and thorium and gets dissolved into pore water that exists in small fractures and voids in the subsurface. The helium likely remains dissolved as an undersaturated species in subsurface fluids, but is “vapor stripped” or partitioned into the rising methane bubbles that are produced by archaea as it rises buoyantly through the water-filled fractures. Ultimately, mixtures of predominantly methane, along with helium and nitrogen, can be found in fractures and faults of the ancient “basement” rocks underlying the Free State of South Africa, or on occasion, in the sandstones and coal seams of the Permian Karoo Supergroup that overlies it.

Rhino Resources is exploring for these subsurface accumulations of helium and biogenic methane within South Africa.