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BAI, J., C. L. DECKER, and J. S. WATKINS, Department of Geology and Geophysics, Texas A&M University, College Station, TX

Abstract: Charge of Fault-Bound Reservoirs: Paths and Reserves as a Function of Fault Properties

Charge of a fault-bounded reservoir is primarily a function of hydrocarbon availability and the contrast between permeability and capillary pressure of the reservoir and the bounding faults. To determine migration paths of oil along faults and through a series of stacked permeable reservoirs separated by impermeable shale, and quantify potential reserves, we used reservoir simulation software to study the effect of varying fault and reservoir properties on hydrocarbon flow.

Since oil typically flows in the direction of increasing permeability and decreasing pressure, migration up high-permeability faults favors charge of upper reservoirs while migration up low-permeability faults favors charge of lower reservoirs.

Once the oil front moves across the reservoir, the reservoir charge continues, but a percentage of oil bypasses the reservoir and escapes. Both the time at which escape begins and the ratio of retained oil to escaped oil depend on bounding-fault properties. Low permeability faults result in longer time-to-escape and lower escaped/retained oil ratios.

An actively charging system can maintain anomalous column heights. Our models suggest that reservoirs adjacent to high-permeability faults may be fruitful targets if charge is ongoing.

If fault and reservoir capillary pressures are varied independently of permeability, charge paths are broadly the same as in the cases described above, but differ in detail, as does oil saturation distribution. Thus, while flow path depends on permeability, it is necessary to know capillary pressures to quantify oil reserves. 

AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana