Why does mineral oil occur below natural gas? It was asked in our science book in the Geology section and I didn't get a clear explanation from the teacher.
I'm expanding upon a couple of the answers because none of them used the term "buoyancy" explicitly. Also, don't use the term "mineral oil", just call it oil or petroleum. Mineral oil is a distillation by-product of petroleum and is not found in subsurface reservoirs.
Let's consider a simple example of a thick reservoir quality sandstone with constant lithology throughout the whole reservoir. Then there are two forces acting on fluids in the sandstone pore space in an undisturbed reservoir. The pore space is the space between the rock grains in the sandstone. By undisturbed, I mean a reservoir which has not been drilled.
The two forces are the buoyancy pressure and the capillary force. The buoyancy pressure is due to the difference of specific gravities between fluids in the pore space. The specific gravity is a unit-less proxy for density. More formally, the specific gravity is simply the ratio of the density of the substance to the density of water at 4 degrees Celsius.
The capillary pressure is due to fluids becoming trapped in narrow pore throats, and because of this pressure there is always an irreducible water saturation in the reservoir. Put simply, even in the gas or oil zone, water always occupies some fraction of the pore space. In other words, for most rocks water is sticky and oil is not. As a result, a bit of water is left behind even when the rock is full of oil or gas. This is the " irreducible water saturation".
See these links:
Once an exploration well has been drilled, figuring out the water and hydrocarbon saturations in the reservoir are critically important because many subsequent decisions will be based upon these numbers.
In a reservoir containing gas, oil and water. The fluids will occupy the pore spaces of the formation from shallowest to deepest-- gas first, then oil and finally water.
In a reservoir, one may have only gas or oil, but there is always water because water always exists as the major constituent of the pore space of underground formations unless it has been replaced by gas or oil.
In addition, due to the capillary pressure, the saturation of the reservoir fluids changes inside each gas/oil/water zone with depth and proximity to the nearest fluid contact. For example, near the oil-water contact, the water saturation is highest and decreases upwards to a constant minimum as it nears the gas-oil contact. Around that point, the oil saturation is at a maximum. The oil saturation then decreases from a maximum as the gas saturation increases, however, the water saturation is still a constant and minimum value.
More detail can be found here - http://wiki.aapg.org/Fluid_contacts .
I'm a geoscientist who works with petroleum engineers-- hope this helps!
it is all about the density.gass have a lower density than oil and oil have a lower density than water.
oil will float on water and gass will float on oil.
some gasses are lighter than air like hydrogen gass so this will float on air because hydrogen have lower density than air.
your question is about schoolwork so i asume this Q&A will be closed soon.
The obvious answers have been posted, but I believe the question really should be why do they occur together?
The answer to such a question is because their source is the same - organic remains subjected to specific conditions: no oxygen, pressure, temperature, and a lot of time.
LOTS of time for density differences to cause migration miles through relatively solid rock. And beyond the level of the question to point out there is always water and often CO2 which make for some complicated ( for me) physical chemistry.