Highlighting Natural H2 Generation Potential of the Eastern Mediterranean Ophiolites by Implementing 232Th-238U-40K Concentration Measurements and Thermal Modeling

Ophiolitic succession of the Eastern Mediterranean region includes one of the most famous natural H₂ leakage spot, globally known as “Chimera Gas Seepage”, noted since ancient times. Geochemical analysis on the seepage revealed that the origin of the gas is abiotic and along CH₄, %10-12 of H₂ is associated with the seepage due to the serpentinisation process which is widely accepted as one of the main mechanisms for the natural H₂ generation.

Radiolysis, considered as another natural H₂ generation process, is defined as the decomposition of H₂O by decay of ²³²Th-²³⁸U-⁴⁰K causing an increase in radioactivity levels. Therefore, increasing radioactivity levels can be detected to identify potential natural H₂ generating zones by calculating the radiogenic heat generation. This study aims to test this hypothesis by implementing the usually neglected or overlooked ²³²Th-²³⁸U-⁴⁰K concentration measurements, also known as SGR logs. A-1 well drilled in the onshore portion of the Antalya Bay, SW Turkey, includes ²³²Th-²³⁸U-⁴⁰K concentration measurements covering an allochthonous ophiolitic section. Penetration into the ophiolites by a well, proximity of well location to the Cirali gas seepage (60 km NE of the seepage) and 2D seismic sections acquired in the region make the study area a perfect spot to test the applicability of integrated methods for natural H₂ exploration.

The most significant finding along the ophiolitic section of the A-1 well is the presence of a peak in radiogenic heat generation that might indicate a potential natural H₂ generation zone. On the other hand, thermal models derived from the interval velocities of 2D seismic survey nearby indicate that vast majority of generated H₂ by serpentinisation process must have migrated from the deepest sections of the ophiolites as temperatures are generally quite low in the area. Apart from that, thermal models also demonstrate the presence of temperature anomalies exhibiting themselves as rapid lateral increases in temperatures that can be associated with the fluids in the sedimentary succession.

As a conclusion, this study provides a unique workflow to reveal potential natural H₂ generating zones that can be applied all along the wells if ²³²Th-²³⁸U-⁴⁰K concentration measurements cover zone of interest not only in the Eastern Mediterranean but for any region. In terms of play fairway, 2 play types have been identified. Naturally generated H₂ can accumulate both in the serpentinites as it is already proven by Chimera gas seepage, or it can migrate into Plio-Miocene aged reservoirs in the area. In terms of expulsion mechanism, heavy deformation and compressional tectonic phase controlled by ongoing convergence of African and Anatolian plates create faults and fracture zones that might allow migration of natural H₂ from the deeper sections into the shallower structures. However, detailed geomechanical analysis should be performed to understand and prevent potential seal breach risks. The methodologies provided by this study might unlock the path to a potential natural H₂ discovery that can turn the Eastern Mediterranean region into a unique natural H₂ exploration theatre.