Abstract: Computer Modeling of Vertical Migration
of Light
Hydrocarbons
KLUSMAN, RONALD W., Dept. of Chemistry and Geochemistry, Colorado School of Mines
Vertical migration
of methane and light hydrocarbons is the
basis for all of the geochemical methods used in surface
prospecting. The buoyancy of microbubbles” originally proposed
by MacElvain (1969) is the currently favored
migration
mechanism
for the formation of surface anomalies. This mechanism has been
incorporated into a computer program which models the buoyancy
mechanism using a finite difference solution of the differential
equations. Above the water table, a diffusion mechanism is
employed. The program can incorporate different physical and
chemical properties for up to ten strata in the saturated section
and ten strata/soil horizons in the unsaturated sedimentary
section. Microbial degradation and isotopic fractionation can be
incorporated into the modeling of gas transport through the
unsaturated section.
The results of the simulations support the observation that
methane and light alkanes leaking at low rates from a reservoir can
be detected at the surface. Anomalies at the surface will be apical
in nature for a reservoir at steady-state, or for an undiscovered
reservoir. The gas potential, and to a lesser extent, the degree of
gas saturation will change in a relatively short time in response
to pressure changes in the reservoir. These data are supportive of
a transition from an apical to a halo-type anomaly during
production. The simulations also support a fractionation process
for light hydrocarbons during migration
. The degree and rate of
change in the overlying hydrocarbon distribution are particularly
sensitive to the depth of the reservoir, the thickness,
permeability, and capillary entry pressure of the caprock.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah