How is oil naturally pushed to the top of reservoirs?

[Charles123]

I am not talking about drive mechanisms used in oil extraction, like water or gas injection. I am referring simply to the fact that oil migrates to the top of reservoirs until it reaches an impermeable top layer that traps it.
"A trap forms when the buoyancy forces driving the upward migration of hydrocarbons through a permeable rock cannot overcome the capillary forces of a sealing medium." - http://en.wikipedia.org/wiki/Petroleum_reservoir
From what I understand oil is usually found in a layer of porous rock. Sometimes there is water at the bottom, but let's assume that is not the case. What causes this buoyancy?
Thank you
Regards

 

[CWatters]

Don't think of the ground as "solid". Think or it as a very viscous liquid. The oil simply trys to float up through the denser rock.

 

[Charles123]

Thank you for your answer.
But, and thinking about the most common oil reservoirs, made of sandstone. It's not easy to image it as a viscous liquid. If it was only made up of small loose grains of sand then I would understand it, but it is not the case, it is more of a porous solid…
Regards

 

[Charles123]

No more thoughts on this?

 

[Charles123]

I just wanted to share this:
I asked this question to a petroleum engineer, and apparently the answer is that water is always present so when hydrocarbons are formed they interact with water (not chemically) and being lighter than water are pushed up-dip due to gravity segregation.
Regards