Influence of the chemical composition of the fluid on its Rayleigh number. These computations were made with a fixed permeability K_{p, m} = 10⁻¹² m², P = 20 bar, T = 90 K, H = 1 km, and a thermal gradient of 0.6 K km⁻¹. The influence of ethane content (blue curve) has been explored by forcing the mole fraction of nitrogen to follow that of methane: . The role of nitrogen abundance (red curve) has been studied by keeping ethane mole fraction constant: .