Limits of Conventional Well Logs for Differentiating Hydrogen and Methane in Subsurface Gas Reservoirs

Hydrogen (H2) is increasingly recognised as a potential subsurface energy resource, both as a naturally occurring gas and as a candidate for underground storage. A critical challenge for hydrogen exploration and monitoring is whether existing wireline logging technologies—developed primarily for hydrocarbon systems—can reliably discriminate hydrogen from methane (CH4) in gas-bearing formations. This study quantitatively evaluates the diagnostic sensitivity of conventional open-hole well logs to gas composition under realistic reservoir conditions. The analysis is based on high-quality wireline data from multiple confirmed methane-saturated gas intervals in sandstone reservoirs of the Northwest Shelf, Western Australia. Using depth-dependent pressure–temperature modelling, real-gas property estimation, hydrogen-index scaling, and forward log-response calculations, synthetic hydrogen-saturated log responses were generated and directly compared with measured methane-saturated density (RHOB), neutron porosity (NPHI), and sonic transit-time (DT) logs for the same depth intervals. This approach isolates the effect of gas substitution from lithology, porosity, and depth-related trends. Results show that complete substitution of methane by hydrogen produces only marginal changes in conventional log responses. Across all wells and depth intervals, the log differences remain small, with ΔRHOB ranging from −0.04 to 0.15 g/cm3 (mean ≈ 0.06 g/cm3), ΔNPHI from −0.05 to 0.06 porosity units (mean ≈ 0.005), and ΔDT from −18 to 18 µs/ft (mean ≈ 2.8 µs/ft). These variations are small, depth-dependent, and dominated by rock-frame effects rather than fluid type. In all cases, computed contrasts fall well below typical tool resolution and repeatability limits. Other conventional logs (gamma ray, resistivity, PEF and NMR) were assessed qualitatively from measurement physics, which indicates they are not expected to be sensitive to gas molecular identity. These findings demonstrate that conventional petrophysical well logs can reliably indicate the presence of gas but lack the intrinsic sensitivity required to discriminate hydrogen from methane in subsurface reservoirs. The inability to distinguish these gases is shown to arise from fundamental tool-physics constraints rather than data quality or modelling assumptions. The results highlight the need for advanced downhole fluid-characterisation technologies to support natural hydrogen exploration and underground hydrogen storage applications.