Print this pageAnalytical 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.