Geochemical signatures of mud and core gases from the Milun Fault Scientific Drilling (MiDAS), Taiwan: Implications for fluid pathways and crustal degassing

We investigated the fluid processes within the Milun Fault Zone (MFZ), eastern Taiwan, by combining continuous monitoring of dissolved gases in drilling mud and gases extracted from cores with the compositional and isotopic analyzers during the Milun Fault Scientific Drilling (MiDAS) project. Although atmospheric inputs dominate the background signal, distinct multi-gas anomalies of CH₄, C₂H₆, CO₂, H₂, He, and Rn occur at the discrete depths (~440 m, 470–488 m, and ~ 520 m). These anomalies coincide with the fracture zones, organic carbon-rich horizons, and the fault gouge core, delineating structurally controlled fluid pathways. Core δ 13 C CO₂ values (−10 ‰ to −15 ‰) indicate organic degradation and methanogenesis as major sources of shallow CO₂, while clustered CH₄-C₂H₆ anomalies with He and Rn reflect episodic inflow of deep fluids along the fault conduits. Integration of the mud gas and core gas datasets demonstrated that the mud gas monitoring captured transient inflows and fracture controlled variability, whereas the core gases preserved in situ compositions and the isotopic fingerprints. Meanwhile, the data reveal the structural heterogeneity of the MFZ, providing geochemical confirmation of location near 520 m and the key horizons for long term hydrogeochemical monitoring. These findings highlight the potential of multi-parameter gas geochemistry to probe crustal fluid migration and its implications for fault activity and earthquake precursors. • Dual mud and core gas monitoring reveals fault-related degassing zones. • Multi-gas anomalies at ~440 m and ~ 520 m mark key Milun Fault Zone fluid conduits. • Isotopic and compositional data link CO₂-CH₄ enrichments to black layers.