Natural Hydrogen at the Kempirsay Ophiolite Massif (Western Kazakhstan): Remote Sensing, Soil-Gas Validation, and Geochemical Investigation

Abstract

Natural hydrogen has been recognized as a promising low-carbon energy source, but efficient exploration methods are still scarce, especially in areas where surface expressions and subsurface processes have not been properly integrated. This thesis assesses natural hydrogen potential in western Kazakhstan (Aktobe region) where the Kempirsay ultramafic massif exists, focusing on natural hydrogen generation mainly through serpentinization of the chromitite-bearing ultramafic rocks. A multi-sensor remote sensing analysis was carried out in Google Earth Engine with Sentinel-2 (Level-2A), Landsat 8/9 thermal imagery (Collection 2 Level-2), and SRTM DEM. Spectral and thermal indices (NDVI, NDMI, NDSI, IOR, FMI, and land surface temperature) were combined with geomorphological information, and principal component analysis (PC1) was employed to capture main surface variability. Regional screening of the north-central Aktobe region identified areas where there is convergence of moisture response, oxidation response, salinity response, vegetation response, and thermal response. An in-depth examination of the Bogestay-Kenoy region revealed that areas with high NDMI/NDVI responses are associated with drainage corridors and valley junctions, while IOR/FMI and PC1 responses distinguish between oxidation-dominated valley margins/slope breaks and moisture-rich depressions. The validation of the results was carried out through GA5000 soil-gas surveys over mine areas and sub-circular depressions. The H2 concentration levels varied between background levels (0 ppm) and ~750 ppm over transects, with the highest levels recorded near the Bolashak mine site. The H2 anomalies of more than 50 ppm were recorded over sub-circular depressions, with the highest levels recorded at the depression margins rather than centers, which is consistent with boundary-controlled seepage reported for “fairy circles.” Two samples of underground gas, collected at about -480 m (bubbling pool and free gas), were analyzed for their composition by GC and IRMS. After correction for air, both samples show that H2 is dominant, comprising about 75.5% and up to 97%, while minor components of CH4, ranging from 0.36 to 1%, are also present. Isotopic composition shows that δD values for H2 range from about -725 to -736‰ V-SMOW, while δ¹³C-CH4 and δD-CH4 range from -13.5 to -10‰ VPDB and -343 to -277‰ V-SMOW, respectively, and show good agreement with abiotic origin and low-temperature equilibration at about 40-50°C, suggesting continental serpentinization involving meteoric waters. Petrography, SEM-EDS, XRD, and XRF studies verify the presence of serpentinized harzburgite/dunite with high amounts of serpentine (>70%) and chromite-bearing assemblages, which allows a conceptual model of a hydrogen system with ultramafic basement as a reactive hydrogen source.