Natural hydrogen potential and basaltic alteration in the Asal–Ghoubbet rift, Republic of Djibouti
Gabriel Pasquet, Amin Mohamed Idriss, Lou Ronjon-Magand, Magali Ranchou‐Peyruse, Marion Guignard, Mathieu Duttine, Anthony Ranchou‐Peyruse, Isabelle Moretti
Abstract
The Asal–Ghoubbet active rift in the Republic of Djibouti is a site of interest for geothermal energy and natural hydrogen, and previous studies have indicated that dihydrogen (H 2 ) emanates from this rift. However, the well-known serpentinization reaction does not appear to be the main mechanism generating H 2 at this site. Rather, the H 2 is generated as follows: (1) by alteration of basaltic lava at depth via reaction with seawater flowing from Ghoubbet Bay towards Lake Asal; (2) by simple degassing of the volcanic chamber located a few kilometers below the Fiale Caldera in the rift axis; or (3) as a result of pyritization processes via the oxidation of H 2 S. Study of microorganisms did not indicate any production or consumption of H 2 , CO 2 , or CH 4 ; therefore, it is unlikely that microorganisms affected H 2 gas contents measured at the surface. However, air contamination at fumaroles is typically considerable and may limit interpretation of such processes. Drill cuttings from the Fiale 1 (F1) and Gale le Goma 1 (Glc1) wells (located on the inner and outer rift margins, respectively) were analyzed to determine where H 2 is generated. Total rock analyses indicated distinct zones at depths of 464 m and 280 m for F1 and Glc1, respectively, representing the boundary between the Asal and Stratoïd Basalts. 57 Fe Mössbauer analyses show a decrease in the percentage of Fe 3+ at depth, indicating that Fe 2+ -rich minerals, particularly in the Stratoïd Basalts, may be a source of H 2 . Based on well data from the rift center and the outer rift margin, it is evident that H 2 is present at the surface in the rift axis and that this area offers good remnant potential because of the presence of Fe-rich chlorite. Conversely, few H 2 emissions were measured at the surface on the outer rift margins, although well data showed some H 2 (∼0.25%) at depth. The presence of a cap rock in the rift axis has not yet been proven; however, the high loss on ignition and the mineralogy in well Glc1 may indicate that the rocks are sufficiently altered into clays to offer potential as a H 2 seal. If so, the rift margins would offer greater exploration potential than the rift center.