The activation of methanogenic Archaea in the context of subsurface hydrogen storage may lead to permanent hydrogen conversion to methane. The objective of this proof-of-concept study is to experimentally investigate these activities, and it is focused on reservoir analogues from the Cretaceous and Triassic periods. These analogues have been selected based on their varying porosities, which range from 8% to 24%. Methanothermococcus thermolithotrophicus was used as the model organism due to its relatively high activity and growth rate. The microbial activities in various water-saturated reservoir rocks with either similar bulk or pore volumes, as well as inoculated media containing sand particles and rock fragments, were experimentally studied and compared to values obtained in bulk solutions. Measured activities in the water-saturated rock specimens with identical bulk volumes varied between 0.17 and 1.22 mM H2 /h largely correlating with the pore volume. Furthermore, the results indicated that activities in the water-filled pore space of the respective rocks were higher by a factor of 8-10 compared to activities in bulk solutions. This observation, in conjunction with the measured activities in inoculated media containing sand particles and rock fragments, as well as in rocks with similar pore volume, supported the notion that the surface area available for microbial colonization is another factor in controlling activity when the amount of substance is held constant. Additionally, the study suggests that methanogenic activities used to quantify hydrogen conversion in reservoirs must potentially be revisited because they are typically measured on bulk solution rather than within intact rocks.