Geochemical Effects of Light Hydrocarbon Diffusion Into Cap Rock Seals of Hydrocarbon Accumulations

D. Leythaeuser, B. M. Krooss

Diffusion of low molecular-weight hydrocarbons, especially methane through the water-saturated pore system of sedimentary rocks, is a common and ubiquitous process in the subsurface. Wherever concentration gradients develop, diffusion of mobile components should occur, especially in the contact region between gas-bearing reservoir rocks and overlying caprocks. If the cap rocks were originally devoid or lean in hydrocarbons, one should be able to recognize concentration gradients away from this contact and extending into the cap rock, or, ideally, diffusion haloes within the cap rock stratas.

Detailed light hydrocarbon analysis (C₂-C₈) of closely spaced samples by the hydrogen stripping technique revealed systematic depth trends for total and individual light hydrocarbon concentrations for a shale cap rock overlying a gas-condensate bearing sandstone in a producing field in North America. A regular relationship appears to exist between the recognized thickness of the enriched interval in the cap rock and the molecular size and type of the considered light hydrocarbon species.

A simple one-dimensional diffusion model (diffusion into a semi-infinite homogeneous medium) was used to reconstruct the concentration-depth trends observed within this cap rock. The reasonably good agreement between the experimental data and the fitted diffusion functions suggests that diffusion is indeed the predominant process of light hydrocarbon transport in this shale cap rock.

Two different approaches can quantify these effects. (1) Based on the geologic history of the reservoir/cap rock system, a time scale may be assessed for the diffusion process. This time scale can then be used to estimate the diffusion coefficients for the individual hydrocarbons in the cap rock. (2) Diffusion coefficients obtained from laboratory experiments with similar cap rock samples may be used to estimate the time scale of the diffusion processes within the cap rock sequence.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.