Imaging the root of the Bulqizë ophiolite and it associated H2 system through magnetotellurics

The Bulqizë Ophiolite in Albania is a site of active hydrogen degassing (84 vol% H₂, ~200 tons/year) observed in a deep chromite mine (Truche et al., 2024). This intense degassing may result from serpentinization at depth, a process where ultramafic rocks react with water, producing H₂ as a byproduct. The depth, shape, and major structures of this ophiolite are currently poorly constrained. Gaining insights into these key parameters is essential to understand the conditions that promote H₂ generation, migration and eventual trapping whithin these ultramafic rocks. To fill this gap, we conducted a magnetotelluric (MT) survey across a east-west transect of seven measurement points crosscutting the ophiolite and encompassing the adjacent sedimentary units. Data processing and analysis, including a 2D inversion executed using Geotools software, unveiled the subsurface structural configuration of the area.

Our results confirmed the existence of a bowl-shaped ophiolite structure at approximately 6 km depth, surrounded by sedimentary rocks and consistent with existing geological cross-sectional data. The structure displays an asymmetric elongation towards the East, with a thicker profile towards the West. This left-leaning bowl shape aligns with the geological characteristics of the Supra-Subduction Zone region. Additionally, our analyses identified that the ophiolite rock consists of an upper layer of fresh Peridotite transitioning to serpentinised Peridotite at greater depths. This transition zone, marked by significant chromite deposits – Bater, aligns with the observations that the chromite deposits tend to be accumulated in Ophiolite transition zones.

This geophysical characterization enhances our understanding of the subsurface configurations crucial for targeting potential H₂ reservoirs. While this study primarily focuses on mapping and characterizing the subsurface structures, the implications for H₂ exploration are significant. Our findings lay the foundation of a workflow for H₂ exploration in ophiolite and ultramafic mantle bodies emplaced at shallow depth in the crust.