A mechanism for hydrocarbon expulsion from low temperature (T = < 20 - 150 degrees C), low maturity (R(o)=< 0.6), overpressured sediments (clays, shales and enclosed sands) with active hydrocarbon concentration and/or generation is outlined. Low temperature polymerisation of light hydrocarbons (e.g. biogenic methane) is considered to be a potential source for some oils found in association with hydrates (resulting from fluidisation discharges from overpressured zones), and some oils found in shales displaying suppression of vitrinite reflectance. It is observed that low temperature polymerisation will increase the potential pressure load retained within an overpressured zone and increase the overall volume of gas/fluids discharged on pressure release. Field observations, including measured recharge volumes and the fluid discharge volumes through a chimney from an overpressured zone, have been used to produce a triple porosity, poroelastic fluidisation expulsion model which links the discharge volume to pressure loading. The model predicts that expulsion from an active pressure mound will be cyclic and episodic. Published geochemical results from seismic chimneys in the Lower Congo Basin have been reinterpreted using the model to demonstrate that expulsion through a chimney is episodic, and to identify overpressured zones where the dominant fluid is oil and others where the overpressured zone contains both oil and gas. It is suggested that some of the oil in these overpressured zones, currently interpreted as thermogenic, may be derived from the polymerisation of biogenic gas.