An integrated experimental/modelling investigation was conducted on the potential of newly discovered structure-H (sH) hydrates formation in petroleum reservoir fluids. A thermodynamic model was validated against a series of experimental data on a natural gas with various quantities of methylcyclohexane (MCH). Three mixtures were tested where structure-it (sII), sII/sH and sH were the stable structures. The validated thermodynamic model was employed to predict the stable hydrate structure for a large number of real reservoir fluids, (i.e., black oils, volatile oils and gas condensates). The results showed that sII is the stable hydrate structure for all systems investigated in this work. However, sH might co-exist with sII in some reservoir fluids where transportation of hydrates as slurry is being considered.