How is oil formed? Part 4 – Oil migrates

Did you know that oil migrates? (Updated)

Birds migrate…so does oil.

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.

Migration of Oil Upward

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.


Here is a great explanation of the migration and trapping of oil and gas:

Why do some wells flow naturally?

Very often, as the oil is being formed, gas is also formed in the process. Sometimes the gas will stay mixed with the oil and sometimes it will separate, depending on how much gas is present. The gas that is mixed in with the oil is what causes the pressure enabling some wells to flow naturally. If there is no gas present in the reservoir, a well must be pumped.

The oil and gas move through the sand trying to get above the water until it reaches a barrier and can go no father. When it reaches this barrier, it is said to be trapped. These “traps” are what we look for in our search for oil.

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