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Oil and gas
Oil and gas are hydrocarbons which are used by man as fuel and as a basis for various industrial applications. These resources are extracted on a large scale through drilled holes in the Earth's crust. The origin of oil and gas reserves are millions of years back in geological history.
Oil and gas originates from mostly marine sediments from different geological periods in which large amounts of organic material has accumulated of descending dead organisms (mostly plankton) from the water column. This could only happen in anoxic conditions on the sea floor, otherwise the organic material would have decayed. Often these deep-sea sediments, but also lacustrine and swamp deposits occur as source rock. Under anoxic conditions, the organic material is converted by anaerobic bacteria into kerogen. For example, oil shale or tar sands are rocks with kerogen. New techniques are tried to extract shale gas or oil directly from these rocks. Deeper in the Earth's crust , the temperature and the pressure is higher than at the surface . After burial of the kerogen containing sediments deeper in the Earth's crust, oil is formed by the rising temperature to a hundred degrees Celsius. If the temperature continues to rise, natural gas can be formed from the kerogen. In general you can say that the type of organic material in the source rock determines whether oil or gas is formed. Natural gas therefore is formed from a different kind of organic material than oil.
Both oil and natural gas are lighter than water and stone and are squeezed upwards through porous rock by the pressure . Many oil and gas travelled in this way to the surface, and thereby escaped into the atmosphere and reabsorbed in the carbon cycle of the Earth. Today there are still natural sources of oil on the surface in many places on Earth, or there are escaping gas bubbles. In some cases the pressed upward oil was stopped by an impermeable layer which caused the oil or gas to accumulate in a reservoir. Often oil and gas are found together in a resevoir, with the gas shallower because it is lighter than oil. Important for a reservoir is that the oil or the gas cannot escape through the sides. There must therefore a fully closed structure be present in the substrate.
An anticline fold can be a good structure for a reservoir. Tilted layers can also form a reservoir when the upper end ends on a fault with impermeable rock on the other side (see image). A reservoir can only occur if there is a source rock is present from which oil and gas can be released, a porous reservoir rock in which oil and gas can accumulate and a sealing layer. An example of the reservoir rock is porous sandstone. And sealing layer can, for example be shale or rock salt . Moreover, the temperature of the source rock in the geological history should have been hot enough to have formed oil and gas. This occurs on average at depths of 1.5 to 4 kilometers depth in the Earth's crust.
Oil and gas is extracted through deep drill holes in the Earth's crust. Sometimes the oil comes up spontaneously, but often it needs to be pumped. This depends on the viscosity of the oil. The thickness of the oil depends on the amount of hydrogen atoms with respect to the carbon. Gas comes up much more spontaneously. Drilling can be done on land but is also daon on oil rigs at sea. Due to advancement of the drilling technique, it is possible to drill in increasingly deeper water.
Boring for new oil in Ballsh, Albania. Photo: Herman Zevenberg
Oil and gas reservoirs are detected by seismic surveys. Through artificial vibration waves (e.g. an explosion) and by recording the reflections in the subsurface with many receptors , the subsurface can be mapped. Harder layers and sharp layer boundaries reflect better. Together with stratigraphic data from boreholes, spots in the surface can be located where a source rock , reservoir rock and a sealing layer is present. However, many of these resevoirs are too small to be economically recoverable. Also after test drilling it is discovered that the quantities are too small to proceed with extraction.
Due to the large-scale extraction and combustion of fossil fuels, man has a big impact on the environment with the high CO2 emissions . This contributes to climate change . Also, the extraction and transportation of oil and gas causes local pollution. This can be very disruptive to the environment even though oil and gas are natural products.
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