The Geology of Petroleum Reserve Formation
Petroleum does not form in reservoirs, but it does collect in them. For a reservoir (just a large accumulation of liquid crude) to form, three conditions must be present. First, there must be petroleum formed deep in the soil. Second, the rocks in which petroleum formation takes place must be porous so that the liquid can move around. Finally, there must be a solid surface above the reservoir that prevents it from being pushed to the surface. If all three of these elements are present, then a reservoir will form.
In the diagram below, the kerogen deposit is where all of the chemistry takes place and where all of the liquid petroleum is formed. This is where the heat and pressure work to convert organic matter into liquid hydrocarbon. The kerogen deposit itself is surrounded by porous rock, which is then surrounded by harder, non-porous rock. This is important because the hydrocarbon needs to be able to leak from the kerogen deposit, but not leak so much so as to reach the surface or be able to fall so deep that it cannot be recovered. So, the ideal scenario for the formation of an oil reserve is porous rock with non-porous rock above and below it.
Once the hydrocarbon hits solid rock, it will follow that rock until it reaches a depression. On the surface, this depression would form a lake or pond. Under the surface, the depression remains empty unless the geology is such that petroleum or some other liquid can reach it.
The surface of the reservoir is important. It must be solid. The reason it must be solid is that petroleum contains both liquid and gas. As the gas builds, pressure in the reservoir increases and without a solid surface, it would push the petroleum to the surface. In fact, this pressure is the reason that humans have been able to extract petroleum in the past. Only recently, as supplies dwindle, have we been forced to pump the liquid to the surface. Early wells had enough pressure to create the geyser-like explosions depicted in movies.
If a reservoir contains water, and they often do, the water will be found below the petroleum because it is denser. The heaviest petroleum molecules will float on the surface of the water and above them will be lighter molecules typically used in gasoline. At the very top of the reservoir will be natural gas.
If pressures are high in the reservoir, natural gas will be forced into solution. In other words, the natural gas will be dissolved in the liquid petroleum. Even then, however, it will make up the topmost fraction of the liquid. When pressures are low, such as when the well is drilled, the natural gas is the first to escape solution. It is the escape of natural gas that makes explosions a possibility when drilling for oil. Even the smallest spark can ignite methane.