What Is Shale?
Shale is a fine-grained, earthy, sedimentary rock rather like clay, but, unlike clay, it is formed in thin layers. It is usually harder than clay and does not so easily dissolve in water. It is normally colored gray, yellow, green or red. Shales are made up of sediments that collected hundreds and thousands of years ago on the beds of lakes and oceans.
Some of them contain other ingredients than clay minerals and may be called sandy shale, limy shale and so on. They are used a great deal in the making of bricks, tiles and Portland cement. The Rocky Mountains are an example of shale rock which contains a great amount of oil. But the cost of extracting it is high.
Black organic shales are the source rock for many of the world’s most important oil and natural gas deposits. These shales obtain their black color from tiny particles of organic matter that were deposited with the mud from which the shale formed. As the mud was buried and warmed within the earth, some of the organic material was transformed into oil and natural gas.
The oil and natural gas migrated out of the shale and upwards through the sediment mass because of their low density. The oil and gas were often trapped within the pore spaces of an overlying rock unit such as sandstone. These types of oil and gas deposits are known as “conventional reservoirs” because the fluids can easily flow through the pores of the rock and into the extraction well.
Although drilling can extract large amounts of oil and natural gas from the reservoir rock, much of it remains trapped within the shale. This oil and gas is very difficult to remove because it is trapped within tiny pore spaces or adsorbed onto clay mineral particles that make up the shale.
The process in the rock cycle which forms shale is called compaction. The fine particles that compose shale can remain suspended in water long after the larger particles of sand have deposited. Shales are typically deposited in very slow moving water and are often found in lakes and lagoonal deposits, in river deltas, on floodplains and offshore from beach sands. They can also be deposited in sedimentary basins and on the continental shelf, in relatively deep, quiet water.
‘Black shales’ are dark, as a result of being especially rich in unoxidized carbon. Common in some Paleozoic and Mesozoic strata, black shales were deposited in anoxic, reducing environments, such as in stagnant water columns. Some black shale contains abundant heavy metals such as molybdenum, uranium, vanadium, and zinc. The enriched values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after sedimentation or to slow accumulation from sea water over long periods of sedimentation.
Shale is distinguished from other mudstones because it is fissile and laminated. “Laminated” means that the rock is made up of many thin layers. “Fissile” means that the rock readily splits into thin pieces along the laminations.
Some shale has special properties that make them important resources. Black shales contain organic material that sometimes breaks down to form natural gas or oil. Other shales can be crushed and mixed with water to produce clays that can be made into a variety of useful objects.