Natural H2 exploration: tools and workflows to characterize a play

Abstract

Natural dihydrogen (H 2 ) 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 H 2 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 H 2 : (i) redox reactions between Fe 2+ and H 2 O, (ii) radiolysis of water and, (iii) organic late maturation where H 2 comes from hydrocarbons. This leads to classify the H 2 generating rocks (H 2 _GR) into four types that seem us the more likely to be of economic interest: basic and ultrabasic rocks of oceanic/mantellic affinity (H 2 _GR1), iron-rich bearing sedimentary and intrusive rocks, (H 2 _GR2), radioactive continental rocks (H 2 _GR3) and organic matter-rich rocks (H 2 _GR4). For the pre-fieldwork, the workflow aims to target new promising areas for H 2 exploration. Cross-referencing the presence of H 2 _GR in the basement, classical geological-hydrodynamic features (fault, water source), and already-known H 2 occurrences at the surface remain essential but should be accompanied by remote sensing analyses to detect possible H 2 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 H 2 leakage from depth through space and time. For the post-fieldwork, we present the most useful analytical tools to characterize H 2 gas seepages and the suspected H 2 _GR. The critical parameters to estimate the H 2 potential of a rock are the content in Fe 2+ /Fe tot (H 2 _GR1 and H 2 _GR2), the content of radioactive elements U, Th, K (H 2 _GR3), and the total organic content (H 2 _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.