Review Article

Natural H₂ exploration: tools and workflows to characterize a play

¹ Institut de physique du globe de Paris (IPGP), CNRS, Université Paris Cité, 1 rue jussieu, 75005, Paris, France
² Laboratoire des Fluides Complexes et leurs Réservoirs (LFCR), E2S UPPA, Université de Pau et des Pays de l’Adour, Avenue de l’université, 64000 Pau, France
³ Institut des Sciences de la Terre de Paris (ISTeP), Université Pierre et Marie Curie, 4 place Jussieu, 75005 Paris, France
⁴ IFP Energies Nouvelles (IFPEN), 1-4 Av. du Bois Préau, 92852 Rueil-Malmaison, France
⁵ CVA-Engineering, 2 Rue Johannes Kepler, 64000 Pau, France

^* Corresponding author: dlevy@ipgp.fr

Received: 15 June 2023
Accepted: 1 August 2023

Abstract

Natural dihydrogen (H₂) exploration is now active in various countries, but tools and workflows that help to characterize prospective zones are still poorly defined. This review paper is dedicated to share our experience in characterizing H₂ plays based on exploration efforts carried out in many countries in Europe, North and South America, Africa, and Oceania between 2017 and 2023. We decided to focus on onshore exploration where three main reactions are generating H₂: (i) redox reactions between Fe²⁺ and H₂O, (ii) radiolysis of water and, (iii) organic late maturation where H₂ comes from hydrocarbons. This leads to classify the H₂ generating rocks (H₂_GR) into four types that seem us the more likely to be of economic interest: basic and ultrabasic rocks of oceanic/mantellic affinity (H₂_GR1), iron-rich bearing sedimentary and intrusive rocks, (H₂_GR2), radioactive continental rocks (H₂_GR3) and organic matter-rich rocks (H₂_GR4). For the pre-fieldwork, the workflow aims to target new promising areas for H₂ exploration. Cross-referencing the presence of H₂_GR in the basement, classical geological-hydrodynamic features (fault, water source), and already-known H₂ occurrences at the surface remain essential but should be accompanied by remote sensing analyses to detect possible H₂ occurrences. For the fieldwork, the focus is made on gas and rocks. A discussion is led concerning the importance of punctual measurements and long-term monitoring of gas seepages, that allow to conclude on dynamics of H₂ leakage from depth through space and time. For the post-fieldwork, we present the most useful analytical tools to characterize H₂ gas seepages and the suspected H₂_GR. The critical parameters to estimate the H₂ potential of a rock are the content in Fe²⁺/Fe_tot (H₂_GR1 and H₂_GR2), the content of radioactive elements U, Th, K (H₂_GR3), and the total organic content (H₂_GR4). The hydrogen exploration is in its infancy and all the profession is attempting to define an automated and fast workflow. We are still far away from it due to a lack of data, yet this review presents a practical guide based on the current knowledge.