Tank rock holds oil or water, with a non-porous layer blocking upward oil movement and preventing water from sinking. Oil is found in porous rocks and trapped against the non-porous layer, while water is held above the natural water table. This process creates unique water supplies but can also lead to contamination.
Tank rock commonly refers to porous rock that holds oil or non-porous rock that holds water. In oil-based use, a reservoir rock forms when a non-porous layer cuts through 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 like a bowl, holding water above the normal water table.
The oil is found in porous sedimentary rocks such as limestone and sandstone. As these rock layers move up and down, oil is squeezed towards the surface. While any rock that bares oil is technically a reservoir rock, the term is generally used to describe systems where the oil is blocked from any further upward movement.
This upward movement is usually stopped by some type of non-porous rock. When a crack forms and the rock layers mismatch, the oil cannot continue through the non-porous material. Even though it can get anywhere, the oil keeps moving up and creates a very oil-rich area right under the crack.
You can locate these oil-rich areas by examining the oil-filled layer where it comes to the surface. The oil that got to the other side of the fissure before this happened has continued to move up the layer and this oil can be found near the surface. Once the oil-filled rock is located, the bedrock simply needs to be tested for oil-rich deposits.
Water-based reservoir rock works the opposite way. The water tries to sink into the ground until it reaches a natural equilibrium. This equilibrium forms the water table; above the table is very little aquifer, while below is much more.
The rock in the tank stops the water from sinking by creating a barrier it can’t cross. This layer captures water and keeps it at an unnatural level. The water is still trying to get down, but the non-porous rock layer prevents it.
This process can be both positive and negative. On the plus side, this creates mountain lakes and water supplies that are out of the ordinary for the area. This access to water is vital for some plants and animals, allowing life to thrive in areas it otherwise wouldn’t be able to. On the downside, this water is often unable to filter properly and can taste bad or contain harmful bacteria. These contaminants would normally be filtered out by the downward journey of the water.
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