Now that the oil is in the sand, it is free to move about or “migrate.” As the sediments and organisms were being deposited, salt water was also deposited and trapped in the sediments.
Flows up hill?
Since oil is lighter than water and will not stay mixed, the two fluids will separate and the oil will be on top of the water. In separating, the oil will appear to flow up hill, but is is merely seeking it’s way to the top of the salt water.
Since oil is lighter than water, the oil droplets will move (float) upward and be above the water. The oil and water cannot get out of the sand because the shale acts as a seal above and below the sand. The oil will continue to move upward within the sand until it reaches a barrier and can move no farther.