Abstract In situ oil sands are dense uncemented fine-grained sands that contain substantial amount of methane and carbon dioxide gases in pore water and heavy oil. Gas evolves when the pore pressure drops to the bubble point pressure (liquid–gas saturation pressure) due to a decrease in confining pressure or fluid production. The volume and pore pressure changes in live oil-filled sand specimens due to decrease in confining pressure under undrained condition were examined in laboratory. A mechanistic model based on kinetics of gas bubble growth due to solute diffusion in supersaturated oil liquid was formulated and presented to interpret the observed time-dependent non-thermodynamic equilibrium behaviour of pore pressure and volume changes. It was found that the bubble sizes could be estimated indirectly by matching the pore pressure response of the live-oil filled system.