Reservoir rock commonly refers to the porous rock that contains oil or the non-porous rock that holds water. In the oil-based usage, a reservoir rock is formed when a non-porous layer shears a porous rock that contains oil. The oil is trapped against the new layer of rock and pools under the non-porous layer. When referring to water, a reservoir rock traps water and prevents it from seeping into the ground. This non-porous layer acts as a bowl, holding water above the normal water table.
Oil is found in porous sedimentary rocks such as limestone and sandstone. As these layers of rock move up and down, oil is squeezed out towards the surface. While any oil-baring rock is technically a reservoir rock, the term is generally used to describe systems where the oil is blocked from any additional upwards movement.
This upwards motion is usually stopped by some kind of non-porous rock. When a crack forms and the rock layers become mismatched, the oil can’t continue through the non-porous material. Even though it can get anywhere, the oil continues to move upwards and creates an area that is very oil -rich right under the crack.
It is possible to locate these oil-rich areas by examining the oil-filled layer where it comes to the surface. The oil that made it to the other side of the crack before it happened continued to move up the layer, and this oil may be found near the surface. Once oil-filled rock is located, the rock layer simply needs to be tested for oil-rich reservoirs.
Water-based reservoir rock works in the opposite way. Water attempts to sink down into the ground until it reaches a natural equilibrium. This equilibrium forms the water table; above the table, there is very little groundwater, while below, there is much more.
Reservoir rock arrests water’s sinking by creating a barrier that it can’t move through. This layer catches the water and holds it at an unnatural level. The water is still trying to move downwards, but the non-porous rock layer prevents it.
This process can be both good and bad. On the good side, this creates mountain lakes and water reserves that are outside of normal for the area. This water access is vital for some plants and animals, allowing life to thrive in areas where it wouldn’t be able to otherwise. On the downside, this water is often unable to filter properly and may taste foul or contain harmful bacteria. These contaminates would normally be filtered by the water’s downward trip.