Vertical noble gas fractionation reveals non-equilibrium dual dynamics in subsurface multi-fluid systems

This study presents the first comprehensive noble gas study of a fully connected subsurface fluid system with gas, oil, and water, from a deep reservoir in Malaysia, located at depths of 3 to 5 km. Using noble gases as tracers, we examine fluid dynamics, mixing, and non-equilibrium processes across fluid phases. Wireline formation testers enabled precise, depth-resolved sampling, revealing noble gas gradients and compositional changes in hydrocarbon and water layers. Given the ongoing hydrocarbon charge, the system appears non-equilibrated. We supplemented our case study with a comparative noble gas database from other hydrocarbon, water, and CO2-rich systems, revealing consistent patterns of mass fractionation and dual phase equilibrium. The gas phase equilibrates one to two orders of magnitude faster than the water phase, producing distinct fractionation signatures within the same reservoir. Positive covariation of crustal (4He/40Ar)* and atmospheric (20Ne/36Ar) noble gas fractionation factors was commonly observed in gas systems. Despite their chemical inertness, noble gases are physically sensitive to temperature, density, and composition, leading to diffusion-, thermodiffusion-, and solubility-driven fractionation.The isotopic ratios 40Ar/36Ar and 38Ar/36Ar, being less influenced by the physical conditions in the reservoir than elemental ratios, demonstrated depth-dependent variation and showed strong sensitivity to fluid–fluid contact interfaces. A test using only gas and oil samples, excluding the water sample, successfully identified the oil–water contact depth by extrapolating local equilibrium signatures in hydrocarbon phases. These results demonstrate the diagnostic power of noble gas isotopic and elemental fractionation in identifying equilibrium zones and mapping fluid boundaries. Our findings advance the understanding of subsurface fluid behaviour in open, dynamic systems and provide valuable insights for CO2 storage, baseline aquifer characterisation, and more effective reservoir management.