To maintain natural gas reserves for even short periods of geologic time, there must be an influx of gas to the reservoir to balance any loss. Because molecular diffusion through the reservoir cap rock is generally regarded as the slowest loss mechanism, the minimum influx rate necessary to maintain the reserves is the same as the diffusive loss rate. Fick's first law of diffusion, modified with the Stokes-Einstein relation and the Archie equation, provides a conservative estimate for the rate of diffusion through a porous medium, such as the reservoir cap rock.

As an example, the diffusive flux of methane and ethane through the cap rock of the McClave field in southeastern Colorado was calculated. Assuming a shale cap rock porosity of only 5%, the entire volume of methane would have to be replaced in less than 2.3 m.y., and 5.3 m.y. for ethane, to maintain the reserves. Using 10% porosity, the replacement time falls to less than 500,000 yr for methane and 1.2 m.y. for ethane.

Unless natural gas fields are short-lived ephemeral phenomena, there must be ongoing generation and migration of gas to the reservoir to at least balance the diffusional loss. This balance requires that catagenesis be an ongoing and recent phenomenon.