Hyeong Soo Kim

This study investigates the properties of natural hydrogen (H2) in two geothermal wells, PX1 and PX2, in the Pohang enhanced geothermal system (EGS) in the Korean Peninsula. Well PX1 has a steady H2 concentration level of ~30 ppm, whereas well PX2 has much higher levels, 6000 ppm at depths of 45 m and increasing at a rate of ~130 ppm/m with depth of 1–45 m. The H2 flux in PX2 is ~871 g/m2/day. Possible source rocks for the generation of H2 include gabbroic amphibolite xenoliths within granodiorite and ultramafic rocks. Thermodynamic modeling yields estimated maximum concentrations of H2 of ~120 ppm and ~900 ppm in the xenoliths and ultramafic rocks, respectively. The difference in H2 concentrations between PX1 and PX2 is ascribed to the different locations of the wells with respect to the position and geometry of the causative fault of the Pohang earthquake. PX1 is located in the hanging wall, whereas PX2 cuts the fault and penetrates the hanging wall and footwall. The impermeable gouge layer(s) and permeable damage zone of the fault are inferred to have sealed and facilitated the migration of H2, respectively. The H2 trap in the Pohang EGS site formed in response to activity along the causative fault and antithetic normal faults that developed during the evolution of the Pohang Basin. The occurrence of H2 reservoirs controlled by fault structures (i.e., structural traps), as observed at the EGS site, should be considered during exploration for economic accumulations of H2.