True, but thats after it can't go any lower. Stratigraphic traps and structural traps will determine how its stored underground, but all of the pore spaces in the rocks (or space amongst sand) are filled with a fluid, be it water, gas, or oil. Gas, being the lightest, moves to the top. Oil right beneath the gas, and water at the bottom.
Once the oil and gas reach an impenetrable layer, a layer that is very dense or non-permeable, the movement stops.
So there will be more forces acting upon it other than just gravity, being under pressure - But I was keeping it basic. I''ve drilled 100s of miles of horizontal wells, if not 1000s. SAGD, huff and puff - both for new wells, re entries, multi laterals, extended reach, you name it.