--> Abstract: Computer Modeling of Vertical Migration of Light Hydrocarbons, by R. W. Klusman; #90937 (1998).

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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