A promising source of clean and renewable energy

Natural hydrogen gas is known on Earth since the 1920’s. However, its potential interest as an exploitable source of energy has been growing in the past ten years. Early discoveries were either forgotten and neglected (Australia, Kansas, USA, Brazil, Mali) or located in remote areas where little if no economic interests can be devised (Mid-oceanic ridges, mountain chains).

Hydrogen exploration has recently generated a renewed R&D interest for Earth scientists. It is well known that H₂ can be found in mid ocean ridges (MOR), in ophiolites (slices of oceanic crust thrusted onto mountain chains), in continental cratons (very old continental rocks) and in hydrothermal fluids. So far, industrial research projects and potential added value of this resource has been developing mainly in continental cratons. It is indeed in this kind of habitus that natural hydrogen production seems to be more effective with low-cost technologies, and with high H₂ concentrations and fluxes. This document presents two cases, where exploration and business plans are beginning: a hydrogen field in Mali, and an area with active hydrogen systems in Brazil.

The Malian case study

In 1987, a water well drilled 110 meters deep on the outskirts of the Bourakébougou village (Kati circle) encountered a pocket of gas that subsequently ignited at the wellhead. The well was cemented and abandoned until the Hydroma company re-opened it in 2011, and all the while acquiring block 25 in the vicinity for exploration, with a surface area of 43.174 km². The objective was to carry out exploration operations of hydrogen in the area. “Fairy circles” were observed in the eastern part of Block 25 (similar as the one on Figure 2 located in Brazil), and hydrogen emissions were measured in those superficial structures (Prinzhofer et al., 2018).

Emitted gas in the pioneer well (called Bougou-1) is composed of 98 % hydrogen and the remaining 2 % consists of methane and nitrogen with traces of heavier gaseous hydrocarbons and helium. Shortly thereafter, it enabled the company Hydroma to install a pilot gas exploitation unit, in order to supply the Bourakébougou village with electricity. Since then, much geological, geophysical and geochemical studies have been performed. A seismic reflection survey was carried out in the Bourakébougou area, enabling a mapping of the dolerite that extend in the area and which acts like a seal and could accumulate hydrogen in reservoirs.

Finally, twenty-four additional wells were drilled, devoted to hydrogen exploration. All wells are placed within a circle of 20 km diameter all around the Bourakébougou village. Hydrogen was encountered in all the wells at various depths. The first accumulation is confirmed to occurs at a depth of about 100 meters, and four other deeper reservoirs were discovered. A borehole crossing the sedimentary series to reach the basement showed that it is also impregnated with hydrogen (Prinzhofer et al., 2018).

The sedimentary series, about 1,400 meters thick, is intersected by numerous doleritic sills of Triassic age. This sedimentary series represents the oldest part of the Taoudéni basin (Tamboura sub-basin, essentially neo-Proterozoic), which extends over a large part of West Africa. The Paleozoic formations which surmount it are particularly developed in Algeria and present well-known petroleum systems. In contrast, Proterozoic rocks are very poor in organic matter, and these are today considered hyper-mature, with no petroleum potential. This virtual absence of organic carbon probably explains the good conservation of the natural hydrogen produced. Otherwise, the latter would be expected to quickly react with carbon by the Sabatier reaction to form essentially methane.

… Read more in the latest H2-International e-Journal, Feb. 2021

Author: Alain Prinzhofer, GEO4U, Rio de Janeiro, Brazil