ABSTRACT: Numerical Simulations of How Cap-Rock Properties Can Control Differential Entrapment of Oil

ROSTRON, BEN J., University of Alberta, Edmonton, Alberta, Canada

The theory of differential entrapment of hydrocarbons, proposed by Gussow almost 40 years ago, has been used to explain the basinwide distribution of oil and gas in reef trends. However, examples of reef trends with hydrocarbon distributions different from the predicted distributions can be found. To date, there are no widely accepted explanations for these differences. In order to explain these discrepancies and better understand differential entrapment, a series of numerical simulations of oil migration in a reef trend were conducted.

Results show that the hydraulic properties of the system control oil migration and entrapment. If the cap-rocks are assumed to be impermeable (consistent with the original theory), then the spill point controls the height of oil column in the trap and the position of the trap within the trend controls the type of hydrocarbon present. If the cap-rocks have some permeability, then trapping is controlled by the interaction of the "sealing" properties (capillary-pressure curves and permeability distribution) and the "leaking" properties (fluid density and formation-fluid gradients) of the system. Simulations illustrate that with certain cap-rocks it is possible to have traps that cannot be filled to their spill points.

If the model of differential entrapment is expanded to include the cap-rock properties of each reef, then a better match could be obtained between observed and predicted distributions of hydrocarbons for the entire trend.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)