Home / A Guide to Oil Shale, Coal, and Other Sedimentary Rocks

A Guide to Oil Shale, Coal, and Other Sedimentary Rocks


When rocks are formed from dead organisms and/or rocks that currently exist, they are called sedimentary rocks. They can be divided into three types: clastic, organic, and chemical. Clastic sedimentary rocks are rocks that form from broken pieces of other rocks. Organic sedimentary rocks are formed from the remains of once living creatures and plants. Chemical sedimentary rocks are composed of mineral crystals. These crystals are made from dissolvable elements that are formed by a chemical process and typically found in water. In addition to sedimentary rocks, there are two other major types of rocks – igneous and metamorphic. Rocks that have undergone a significant change to their original form due to factors such as high pressure, high heat or hot fluids that are mineral rich are called metamorphic rocks. This type of rock may be foliated or non-foliated. Igneous rocks are rocks that are formed by the solidification of magma that comes from within the Earth. Igneous rocks may be intrusive or extrusive. Intrusive rocks are formed beneath the Earth’s surface from magma that is slow to cool. Extrusive igneous rocks are formed near the surface of the Earth, or just at the surface. With this type of rock the magma cools very quickly. This article will review sedimentary rocks and some of the most common types of rocks within that category.


Shale is made up of sediment grains that have settled into defined layers on the ocean’s floor. The grains mix with organic matter that is found on the floor of the ocean. When this organic matter and clay compact together, shale is formed. Oil shale is a type of shale in which kerogen combines with organic sediment during formation. Oil shale can be found in saltwater and freshwater lakes and marine basins. This type sedimentary rock is the most commonly found sedimentary rock in the world.


The formation of coal takes ages to complete and it is the oldest of the fossil fuels. It is formed when the remains of plants, such as peat, undergo bacterial decay followed by changes from compaction due to burial, the passage of time and heat. As it undergoes this process, changes occur as it releases methane and increasingly becomes more carbon-rich. Coal is dark in color, usually black; however, certain types of coal may also be brown. Coal reserves are found around the world, within one hundred feet of the surface. There are several types of coal including lignite which is the lowest ranking, bituminous coal, anthracite, and finally graphite, which is the highest ranking.


Limestone is typically found in waters that are shallow and warm. Common areas where it can be found include the Persian Gulf, the Caribbean Sea, Gulf of Mexico, the Indian archipelago and the Pacific Ocean Islands. It can be chemical or organic. When organic, it has formed from organic materials such as coral, shell and fecal debris. When limestone is a chemical sedimentary rock it is formed from the carbonate in the lake or ocean. Chemical limestone may also be evaporative and found in caves.


Sandstone has a medium grain and is typically gray or pale yellow, but the color depends on the environment and the materials that formed it. It may also have a red or brown color. It is formed by the compression and cementing of silica and sediment grains. The grains are transported and deposited by wind or water, which effects how it forms. For example, when deposited by winds, sand dunes may form.

Rock Salt

Rock salt is the common name for halite. It is a chemical sedimentary rock and is an evaporite. It is formed from the evaporation of enclosed bodies of water where they form salt beds. Salt beds are generally underground and can be found in the U.S., Canada, the U.K., and Pakistan. Rock salt may also be found in salt domes which are layers of salt that are pushed up towards the surface by the underground salt beds. These domes are often found underground in hard layers. It is formed by large bodies of salt water that has evaporated, leaving salt behind. It is most often colorless, but may at times have a blue, gray, or even an orangish appearance. Naturally it has a very salty taste.

Iron Ore

When oxygen and iron combine in fresh or marine water they cause a chemical reaction which helps form iron ore. As a result, iron ore is considered a chemical sedimentary rock. It is typically found in the form of hematite or magnetite. Iron ores vary when it comes to their coloring. Some may be dark gray while others may be rust colored, purple, or yellow.


Flint is a type of chemical sedimentary rock. It is a variety of chert and is often referred to as such. It differs from chert, however, in that it is darker in color. Flint is typically a black or dark gray, but may also be tan as well. It is formed with combustible materials and will spark when struck. This rock is made of fine grained quartz, or cryptocrystalline quartz, and is made during the chemical changes that occur during the formation of sedimentary rock. It can be found as nodules in chalk or some marine limestone. Historically, flint was used to make weapons due to its ability to retain its sharp edges.


Breccia is a clastic sedimentary rock formed when grains are transported short distances prior to their deposit in valleys or streams. They may be transported by mud slides, avalanches or water. Breccia is characterized by the shape of the fragments of boulders, pebbles and other rocks, which are angular as opposed to rounded. The fragments are also typically greater than 2mm and may vary in color.

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