Petroleum products and coal are the USA's main source of energy, accounting for about 92%. While filling up gas tanks and enjoying gas fireplaces, most people aren’t aware of the geological processes that have created this valuable resource. The type of hydrocarbon that forms deep underneath the surface of the earth is determined by the kind of sediment deposited and geologic setting, the kind of organic matter trapped and the changes in this organic matter.
Fossil Fuels Form in Sedimentary Basins
The first requirement for the formation of fossil fuels is a sedimentary basin. Sedimentary basins are formed when the repeated transgression and regression of seas and lakes deposit numerous layers of sediment. As these sediments are deposited, the remains of dead animals, plants, algae, plankton and bacteria are trapped within the layers. This organic matter is buried within numerous layers of sedimentary deposits, which eventually become sandstones, limestones and shales.
The conversion of organic matter takes time, pressure and heat. Organic matter is deposited on the surface of the Earth, in underwater environments – either as swamps are flooded or as marine life dies and falls to the bottom of the ocean. As the various layers of sediment are deposited on top, the load gets heavier and squeezes down on the organic matter below. As the compaction proceeds, the carbon compounds within the organic matter are transformed into a substance called kerogen that contains a range of hydrocarbons.
The Formation of Oil and Natural Gas
Oil and natural gas form within sedimentary rocks that contain organic matter originating from small marine life such as phytoplankton, algae and bacteria. This organic matter is rich in proteins, lipids and carbohydrates – compounds found in oil and natural gas. The trap rock – the rock that holds the hydrocarbon – in this setting is usually shale, which is made up of very fine sediment that is deposited in deep-water environments such as lakes and seas.
Most oil and natural gas around the world is found in layers that were deposited during the Cenozoic era about 50 million years ago. During this time, much of the coastal United States was underneath the ocean. This has resulted in rich deposits of oil and natural gas along the Gulf Coast and along the east and west coasts.
The Formation of Coal
Coal forms within sedimentary rocks that contain organic matter originating from plant life such as trees and bushes. This kind of organic matter is rich in resins, waxes and lignins, which tend to remain solid even after million of years. The trap rock for coal is usually sandstone, which is indicative of an environment where water was transported quickly and the strong currents were able to deposit larger pieces of sediment such as coarse sand.
Most of the world’s coal is found in layers of sediment that were deposited during the Carboniferous period, when plant life was emerging and vast swamps covered most of the Earth’s land. In fact, the name “Carboniferous” itself refers to the abundance of plant-life fossils found in sedimentary rocks deposited at this time. During a sub-period of the Carboniferous period, called the Pennsylvanian period, a portion of land - which is now the majority of the United States interior – was repeatedly flooded by an encroaching sea. As a result of this, the swamp that covered this area was buried under multiple layers of sediment. In the present day, this region of the United States has a rich layer of coal buried underneath it.
Source:
Dott, Robert H. and Prothero, Donald R. Evolution of the Earth. NY: McGraw-Hill, 1994.
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