An Attempt to Study Natural H2 Resources across an Oceanic Ridge Penetrating a Continent: The Asal–Ghoubbet Rift (Republic of Djibouti)

Abstract

Dihydrogen (H2) is generated by fluid–rock interactions along mid-ocean ridges (MORs) and was not, until recently, considered as a resource. However, in the context of worldwide efforts to decarbonize the energy mix, clean hydrogen is now highly sought after, and the production of natural H2 is considered to be a powerful alternative to electrolysis. The Afar Rift System has many geological features in common with MORs and offers potential in terms of natural H2 resources. Here, we present data acquired during initial exploration in this region. H2 contents in soil and within fumaroles were measured along a 200 km section across the Asal–Ghoubbet rift and the various intervening grabens, extending from Obock to Lake Abhe. These newly acquired data have been synthesized with existing data, including those from the geothermal prospect area of the Asal–Ghoubbet rift zone. Our results demonstrate that basalt alteration with oxidation of iron-rich facies and simultaneous reduction in water is the likely the source of the hydrogen, although H2S reduction cannot be ruled out. However, H2 volumes at the surface within fumaroles were found to be low, reaching only a few percent. These values are considerably lower than those found in MORs. This discrepancy may be attributed to bias introduced by surface sampling; for example, microorganisms may be preferentially consuming H2 near the surface in this environment. However, the low H2 generation rates found in the study area could also be due to a lack of reactants, such as fayalite (i.e., owing to the presence of low-olivine basalts with predominantly magnesian olivines), or to the limited volume and slow circulation of water. In future, access to additional subsurface data acquired through the ongoing geothermal drilling campaign will bring new insight to help answer these questions.

Natural hydrogen potential and basaltic alteration in the Asal–Ghoubbet rift, Republic of Djibouti

“…Gas analysis following the 1978 Ardoukoba eruption (in the volcanic core zone) also indicated 0.15% H 2 in the magma gas (Allard et al, 1979). At the surface, H 2 concentrations were found to increase towards the center of the rift (Pasquet et al, 2021).…”

“…Many H 2 emission zones have been discovered, yet the production of natural hydrogen (as in Mali; Prinzhofer et al, 2018) remains rare and natural dihydrogen exploration is still in its infancy. The geological and geothermal context of the active rift in the Afar Depression is favorable for natural H 2 production, with several sources proposed previously: alteration of iron minerals in basaltic material, pyritization, or volcanic degassing (Holloway and O'Day, 2000;Klein et al, 2020;Pasquet et al, 2021). Following surface gas surveys, analysis of cuttings from two wells is presented below and aims to improve our understanding of basalt alteration and Fe 2+ /Fe 3+ evolution at depth across the Asal-Ghoubbet rift in order to assess its potential H 2 resource.…”

“…Well data and a field campaign (2019) across the rift highlighted the presence of H 2 at depth in the area (Figure 2), in some geothermal wells along the southwestern margin (wells A3 and A6;D'Amore et al, 1997), and in the fumaroles along the southern inner margin near well A5 and the Fiale Caldera (Pasquet et al, 2021). The well data indicate H 2 percentages of ~0.25% (in dry gas, excluding air and water vapor); however, no H 2 sources were measured at the surface in the vicinity of the wells during the field campaign.…”

“…The well data indicate H 2 percentages of ~0.25% (in dry gas, excluding air and water vapor); however, no H 2 sources were measured at the surface in the vicinity of the wells during the field campaign. Along the inner margins and in the axial volcanic zone, although no gas data were acquired at depth, the intense hydrothermal activity present at the surface made it possible to measure gas contents: after correction for air, H 2 contents were found to be up to 3%, with an average value of 1% and a mean value of 0.77% (Pasquet et al, 2021). Gas analysis following the 1978 Ardoukoba eruption (in the volcanic core zone) also indicated 0.15% H 2 in the magma gas (Allard et al, 1979).…”

“…This natural dihydrogen has been observed in various geological contexts (Truche et al, 2020;Zgonnik, 2020), including mid-oceanic ridges (Charlou et al, 1998(Charlou et al, , 2002Kelley et al, 2001;Kelley, 2005); ophiolitic massifs such as the Sultanate of Oman, New Caledonia, or the Philippines (Neal and Stanger, 1983;Abrajano et al, 1988;Sano et al, 1993;Deville and Prinzhofer, 2016;Vacquand et al, 2018); and Archean and Proterozoic cratonic basins such as those in Namibia, Mali, Russia, Australia, and Brazil (Larin et al, 2015;Prinzhofer et al, 2018Prinzhofer et al, , 2019Boreham et al, 2021;Moretti et al, , 2022Malvoisin and Brunet, 2023). Steam at the hightemperature geothermal sites of Italy, Iceland, and Djibouti (Oskarsson, 1984;Ármannsson et al, 1989;Leila et al, 2021;Pasquet et al, 2021Pasquet et al, , 2022Combaudon et al, 2022) has also been shown to contain H 2 , possibly from multiple sources.…”