Soil gas analysis, western Pyrenees

Native hydrogen (H2) may represent a new carbon free energy resource, but to date there is no specific exploration guide to target H2-fertile geological settings. Here, we present the first soil gas survey specifically designed to explore H2 migration in a region where no surface seepage has been documented so far. We choose the Pyrenean orogenic belt and its northern foreland basin (Aquitaine, France) as a playground to test our strategy. The presence of a mantle body at shallow depth (<10 km) under the Mauléon Basin connected to the surface by major faults is considered as a preliminary pathfinder for H2 generation and drainage. On this basis, more than 1,100 in situ soil gas analysis (H2, CO, CO2, CH4, H2S, and 222Rn) were performed at ~1 m depth at the regional scale along a 10 × 10 km grid spanning over 7,500 km2 25 . The analysis campaign reveals several hot spots to the north of the Mauléon Basin where H2, CO2 and 222Rn concentrations exceed 1000 ppmv, 10 vol% and 50 kBq m-3 , respectively. Most of these hot spots are located along the North Pyrenean Frontal Thrust and other related faults rooted in the mantle body. These results, together with evidence of fluid migration at depth, suggest that H2 may be sourced from mantle rocks serpentinization and carried to the surface along major thrusting faults. Hydrogen traps remain unidentified up to now but the presence of salt-related structures (diapirs) near these hot spots could play this role.

原生氢（Native hydrogen, H₂）或可成为新型无碳能源资源，但迄今为止尚无针对富氢地质背景的专属勘探指南。本研究首次开展专为探测氢运移而设计的土壤气调查（soil gas survey），研究区域此前尚无地表渗氢的相关记录。我们选取比利牛斯造山带（Pyrenean orogenic belt）及其北部前陆盆地（northern foreland basin，法国阿基坦Aquitaine地区）作为研究试验区，以验证我们的勘探策略。莫利翁盆地（Mauléon Basin）下方存在浅部（<10 km）地幔体（mantle body），且该岩体通过大型断裂与地表连通，这被视为氢生成与运移的初步找矿标志。基于此前提，我们在覆盖7500 km²的区域内，沿10×10 km的网格在约1 m深度处完成了超1100组原位土壤气分析（in situ soil gas analysis），检测组分包括H₂、CO、CO₂、CH₄、H₂S及²²²Rn[25]。本次分析工作在莫利翁盆地北部发现多处异常高值区，其中H₂、CO₂及²²²Rn的浓度分别超过1000 ppmv、10%体积分数及50 kBq·m⁻³。这些高值区大多分布于北比利牛斯逆冲断裂带（North Pyrenean Frontal Thrust）及其他与浅部地幔体相连的相关断裂沿线。结合深部流体运移的相关证据，上述结果表明氢可能源自地幔岩石的蛇纹石化（serpentinization）作用，并沿大型逆冲断裂运移至地表。目前尚未发现氢的圈闭构造，但这些高值区附近的盐相关构造（salt-related structures，即底辟构造diapirs）或可充当此类圈闭。