Print this pageE.L. Rowan1, L.O. Anna2, P.G. Lillis2, S.M. Condon2, T.C. Hester2, J.L. Ridgley2
(1) U. S. Geological Survey, Menlo Park, CA
(2) U.S. Geological Survey, Lakewood, CO
ABSTRACT: Rates of Methane Migration by Diffusion: Implications for the Age of Gas Resources in North-Central Montana
Cretaceous sandstones and shales of north-central Montana host important resources of biogenic methane. Our study focuses on the Bowdoin dome region. Although poorly understood, the timing of methane generation is key to understanding its distribution today. Some previous studies have proposed generation beginning with shallow burial in the Cretaceous, whereas other work suggests generation as late as the Pleistocene. An important constraint on the timing of gas generation is the rate at which gas is lost by diffusion, moving as a dissolved species through groundwater. Strong capillary effects in the low permeability, shale-rich rocks make it unlikely that methane moved any significant distance as a free gas.
We modified the basin modeling program, Basin2, to simulate methane generation throughout the Cretaceous era and diffusive losses until the present day. We tested a range of diffusion coefficients from the literature. At the highest value tested (10 -5 cm2/s) nearly of all the methane escaped by the present day. However, a smaller diffusion coefficient appears to be more appropriate; using a value of 10 -7 cm2/s, most of the Cretaceous methane would still be present.
Deuterium analyses indicate isotopic equilibrium between groundwater and methane. In addition, oxygen and deuterium isotopic values plot distinctly below the meteoric water line, indicating an important connate (non-meteoric) component in groundwater. These analyses, therefore, do not support Pleistocene-Holocene generation of methane. If further work supports the lower diffusion coefficient, methane generated as early as the Cretaceous could still be present despite the continuous, slow loss by diffusion.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado