A direct observation of a hydrogen-rich pressurized reservoir within an ophiolite (Tișovița, Romania)

Abstract

Subsurface natural hydrogen accumulations that can be economically and safely recovered are a major target of the present roadmap towards the use of hydrogen as an alternative and C-free energy resource. The existence of H2-rich reservoirs in ophiolites, where H2 is produced via serpentinization, was theoretically predicted based on the intensity of gas seeps in Turkey and Albania. Here, we examine the geological bases supporting the potential existence of H2 reservoirs within ophiolites in general, also considering available data on ophiolitic petroleum reservoirs, and we document a direct observation of a H2-rich accumulation within an ophiolite. We studied the case of a borehole, drilled in the 1970s for metal mining exploration purposes within the Tișovița–Iuți ophiolite, in Romania, which accidentally encountered pressurized gas with ∼29 vol% H2 at a depth of about 800 m. We reconstructed the geometry of the ophiolite sequence in correspondence with the well and suggested that the reservoir is within cataclastic dunite near the tectonic contact between layered dunite and the harzburgite-dunite peridotites; the upper, non deformed portion of serpentinized layered dunite, likely with the contribution of tonalite veins, acted as a fluid trap. Although plugged, we detected H2 leakage from this well and from a second well in the same area, with H2 concentrations exceeding 100 ppmv in the air and 0.1 mg H2/L in the water within the wellhead open casing. We propose a broad conceptual model in which deformed, fractured, and non-deformed, impermeable sections of serpentinized peridotites can determine ophiolitic H2-rich reservoirs and traps, respectively, as factors that shall be considered in the guidelines for H2 reservoir exploration.