The link between ophiolitic chromitites, natural hydrogen and methane: Insights from 3D microtomography

Abstract

Ultramafic rocks in ophiolites are known as source rocks of abiotic hydrogen (H₂) and methane (CH₄), due to serpentinization and successive CO₂ hydrogenation. Ophiolites are therefore key targets in natural hydrogen exploration. While serpentinized peridotites are the main sources of H₂, chromitites host both hydrogen and large quantities of methane, as revealed by analyses of direct gas extraction from rocks and micro-Raman analyses. However, the fluid bearing properties of chromitites, as well as their mineralogical phases that are correlated to the gas genesis and evolution, are unclear. We conducted high-resolution X-ray computed micro-tomography (microCT) on chromitite samples from two ophiolites in Greece. The microCT analysis, using the X-ray attenuation coefficient (which is density-dependent), combined with 3D image analysis and pore-scale permeability simulations, revealed the geometry and distribution of pores and microfractures. This approach provided insights into their flow properties and spatial relationships with solid phases that could act as catalysts for CH₄ production (Platinum Group Elements - PGM), H₂ flow (altered PGM), and CO₂ hydrogenation (amorphous carbon). Microfractures appear as potential sites or microreactors for H₂-CO₂ conversion into CH₄, while also retaining residual, unreacted H₂. The microCT technique provides insights into the in-situ textural relationship between microfractures, gas pores and solid phases, unattainable through 2D traditional techniques, thus offering a valuable support for natural hydrogen exploration.