Abstract: The Rate of Generation of Natural Gas Based on Diffusional Loss

Jon S. Nelson, E. C. Simmons

No reservoir cap rock is a perfect barrier to natural gas migration. All cap rocks leak. The generally accepted slowest loss mechanism is molecular diffusion. To maintain reserves for geologic periods of time, there must be an influx to the reservoir to balance any loss. Therefore, the minimum rate of influx necessary to maintain the reserves may be estimated using a Fick's First Law of Steady State Diffusion calculation. The minimum rate of influx is the minimum rate that natural gas must be generated and/or migrated to the reservoir. For reserves to increase, generation and migration must exceed the molecular diffusion loss rate.

A modified Fick's First Law equation using the Stokes-Einstein Relation and the Archie Equation was used along with solubility data and reserves estimates to obtain conservative estimates of the fluxes and turnover times of methane and ethane through the cap rocks of seventeen Pennsylvanian Morrow Formation fields in SE Colorado. For a 5% porosity, salt water saturated shale cap rock, the turnover times for methane were 0.231-4.15 million years, for ethane, 0.505-10.0 my.

Unless these fields have undergone a precipitous decline in reserves in the last hundred thousand years, there must be active generation and migration of natural gas to the reservoirs to balance diffusional loss. For methane the minimum generation/migration rates must meet or exceed 0.115-33.2 Mcf/yr; for ethane 0.43-22.3 Ccf/yr.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994