Molecular diffusion of light hydrocarbons in sedimentary rocks and its role in migration and dissipation of natural gas

The role of molecular diffusion during different stages of hydrocarbon migration bas been an issue of recurrent interest during the past 40 years. Controversial views on the importance of this transport mechanism can be partially attributed to inconsistent definitions and computational errors. Considerable amounts of light hydrocarbons can be released by diffusion, even from thick source rock sequences. At higher generation rates, primary migration of gas is, however, dominated by volume flow (Darcy flow). Molecular transport of C-5-C-9, hydrocarbons in source rocks deviates strongly from ideal diffusion behavior. Secondary migration is dominated by volume now, and diffusion plays only a subordinate role. Diffusion may be important in the dismigration of natural gas if seal leakage by volume flow does not occur over extended periods of time. Average diffusive fluxes for methane through seals reported in various studies range from 0.16 to 89 m(3)/km(3)/year. For comparison, the compressible volume flow of gas through a shale 50-450 m thick with a permeability of 1 nanodarcy (10(-21) m(2)) was calculated between 100 and 1000 m(3)/km(3)/year. Numerical simulation of diffusion in the context of integrated two-dimensional basin modeling improves the quantification of molecular transport of hydrocarbon gas in petroleum systems.