Natural H2 exploration: tools and workflows to characterize a play

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+/Fetot (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.