Analytical Models of Petroleum Migration In Homogeneous Strata: Implications for Migration Efficiency and Velocity
BROWN, ALTON A
The relationship between capillary properties, petroleum flux, and thickness of a petroleum-saturated pathway in a dipping, homogeneous carrier bed was analytically evaluated in a position behind the shock front. The transmissivity ratio (ratio between the total petroleum flux in the bed and the forces driving petroleum migration) can be related to the thickness of the oil-saturated zone for a rock with given capillary properties and porosity. The thickness of the zone and reservoir properties can then be used to estimate average velocity, saturation, and total oil-in-place in the migration pathway.
With maximum transmissivity ratios estimated from real petroleum basins, the maximum expected thickness of the saturation zone and average velocity can be quantified for different quality carrier beds. For good quality (>100 md) rocks, maximum thickness of migrating petroleum is measured in feet, whereas thickness in poor quality carrier beds (1-10 md) is measured in tens of feet. Velocity at reasonable transmissivity and dip is on the order of miles per million years, and increases with bedding dip. Average petroleum saturation is less than 20 percent, if critical saturation of 0 percent is assumed.
Model results indicate that secondary petroleum migration is usually fast (almost geologically instantaneous over short migration distances) and efficient (low saturation and thickness of oil saturated zone) in moderate to good quality, homogeneous reservoir rock. Because capillary pressure-saturation relationships result in selective charge to best quality accessible reservoir beds, petroleum will tend to migrate where the velocity is fastest and most efficient. Detecting migrating petroleum in good-quality carrier beds may be difficult during standard drilling due to thin saturated interval and low saturation. Reservoir heterogeneity decreases efficiency and average velocity.