We study the processes by which petroleum originates in source rock and generates a transport path enabling some of it to leave. We show that diffusion through the source rock is too slow to account for the migration of petroleum. However when kerogen converts into petroleum within pores, it expands, and this expansion is sufficient to fracture the rock around the pores. Thus the transport of petroleum depends on whether these fractures connect up to form a macroscopic transport path. We develop a simulation tool that lets us study pressurized fluid in disk-shaped domains which expand and fracture the surrounding material. Examining pairs of pressurized pores, we obtain a lower limit for critical porosity in shale rock, ϕcrit.=0.15. When kerogen saturation exceeds this value, long-range transport paths become possible. This critical porosity is comparable to the porosities observed in immature shales.