Helium and hydrogen were the first elements of the universe, formed by nucleosynthesis three (3) minutes after the original explosion (big bang). A quarter of the universe mass consist of helium, hydrogen representing the other three quarters. The rest of the heavy elements account only for 2% of the universe mass. Although abundant in the cosmos, helium remains one of the rare elements on earth. It comes from the disintegration of radioactive elements uranium and thorium, which result into lead and alpha particles. Radiogenic alpha decay emits stable helium atoms. However, the earth’s gravitational field is powerless to prevent helium from escaping from our atmosphere. Thus, the atmospheric concentration of helium represents a dynamic balance between the diffusion of helium from the earth’s crust and the loss of helium into deep space.
The concentration of helium in the dry air of the atmosphere has the following value:
5.24 + or – 0.05 parts per million (ppm) or 0.0005%.
The low concentration of helium in the atmosphere is an economical obstacle for the helium extraction from the atmosphere. Helium could likewise be extracted from the following four sources:
1. Monoazite ore, containing around 10% thorium oxide, the rest is rare earth oxides.
2. Tritium, a heavy isotope of hydrogen used to enhance the yield of nuclear weapons. Helium-3 is a decay product of tritium. Helium-3 is harvested from dismantled or refurbished nuclear weapons.
3. Hot plumes from underwater hydrothermal vents. The helium content of the hot plumes ranges from 1% up to 2% of helium.
4. The hot gases from thermal springs. The helium content varies from 1% up to 10% of helium.
5. The untreated natural hydrocarbon gases from some natural gas fields. The helium content can range from traces up to 8% by volume.
The most interesting source is the untreated hydrocarbon gases from natural gas fields. Therefore it is the most used source for the extraction of helium. For this source, helium comes from deep crystalline rocks with a high content of uranium and thorium and after migration is trapped in sedimentary rocks, along with other gases. Natural gas analyzes have shown that the concentration of helium increases generally with the geological age of the reservoirs, up to a ratio of 1 to 2. Sedimentary rocks from erosion of volcanic rocks can likewise be a source of helium due to their uranium and thorium content. Thus, the analysis of the uranium and thorium contents of metamorphic and volcanic rocks, underlying the sedimentary rocks would be useful to identify the sources and explain the distribution of helium in the natural gases.