A Multi‐Scale Geophysical–Geochemical Footprint of Persistent Methane Leakage at a Legacy Petroleum Well

Fugitive methane leakage from legacy oil and gas wells is assessed using surface flux and soil-gas measurements, yet these signals are modulated by meteorology and shallow processes, obscuring sustained leakage. Whether persistent leakage leaves a coherent imprint in the shallow subsurface remains unresolved. Here we present a multi-year field investigation at a decommissioned gas well, integrating repeated CH₄–CO₂ efflux surveys with geophysical and geochemical measurements. Surface flux mapping reveals a compact emission core despite strong interannual variability. In contrast, electromagnetic conductivity and geochemistry record broader, co-centered responses extending meters beyond the flux core. These observations define a nested, multi-scale geophysical–geochemical footprint anchored to a common source. The contrasting scales reflect temporal integration, with flux capturing instantaneous emissions and subsurface properties preserving longer-term alteration. Our results show that sustained methane leakage reorganizes shallow conditions, providing a transferable framework for delineating leakage at legacy wells and informing monitoring of subsurface energy systems.