(1) Biogenerated Oil Theory
Most geologists believe that oil, like coal and natural gas, is gradually formed by long-term compression and heating of ancient organic matter. According to this theory, oil is formed by the changes of prehistoric marine animals and algae corpses. Plants on land generally form coal. After a long geological period, these organic substances are mixed with silt and buried under thick sedimentary rocks. Under the high temperature and high pressure underground, they gradually transform, first forming waxy oil shale, and then degenerating into liquid and gaseous hydrocarbons. Because these hydrocarbons are lighter than the nearby rocks, they penetrate upward into the nearby rocks until they penetrate into the tightly impermeable and hollow rocks above. The oil accumulated in this way forms an oil field. People can get oil from oil fields by drilling and pumping water.
Geologists call the temperature range of oil formation "oil window". If the temperature is too low, oil cannot be formed; if the temperature is too high, natural gas will be formed. Although the depth of oil formation varies around the world, the "typical" depth is 4 to 6 kilometers. Because oil will penetrate into other rock formations after formation, the actual oil field may be much shallower. Therefore, the formation of an oil field requires three conditions: rich source rocks, permeable channels and rock structures capable of accumulating oil.
(2) Abiotic oil generation theory
Thomas Gold, an astronomer of abiotic oil generation theory, developed on the basis of the theory of Russian petroleum geologist Nikolai Kudrya. According to this theory, there is already a lot of carbon in the earth's crust, and some of this carbon exists naturally in the form of hydrocarbons. Hydrocarbons are lighter than water in rock crevices, so they permeate upward along rock crevices. Biomarkers in petroleum are caused by thermophilic microorganisms living in rocks. It has nothing to do with oil itself.
This theory is supported by only a few geologists. It is generally used to explain the unexplained oil inflow in some oil fields, but this phenomenon rarely occurs. The abiotic oil generation theory cannot explain that more than 99% of the world's oil is stored in sedimentary rocks, and the oil in those non-sedimentary rocks can also be explained as migrating from sedimentary rocks elsewhere. Similarly, abiotic oil generation theory cannot explain the widespread distribution of biomarkers such as sterane, γ-alkane, phytane, hopane, terpenoids and lighter isotopes in petroleum.
It is very expensive to exploit oil, and it may also bring damage to the environment. Offshore oil exploration and exploitation will cause interference to the marine environment. In particular, the excavation work to clean the seabed has the greatest damage to the environment. The crude oil or refined oil leaked after the tanker accident has caused serious damage to the fragile coastal ecosystems in Alaska, Galapagos Islands, Spain and many other areas.
When oil burns, it releases carbon dioxide into the atmosphere, which leads to global warming. Oil releases less carbon dioxide per unit energy than coal, but more than natural gas. However, it is particularly difficult to reduce carbon dioxide emissions caused by burning oil as transportation fuel. Generally, only large power plants can be equipped with carbon dioxide absorption devices, and a single vehicle cannot be equipped with such devices.
Although renewable energy is an option now, how much oil can be replaced by renewable energy and the possible environmental damage caused by renewable energy itself are still uncertain and controversial. Renewable energy sources such as sunlight, wind energy and geothermal energy can't replace oil as transportation energy with high energy density. In order to replace oil, these renewable energy sources must be converted into electricity (in the form of storage batteries) or hydrogen (through fuel cells or internal combustion engines) to drive vehicles. Another scheme is to use liquid fuel (ethanol, biodiesel) generated by biomass energy to drive vehicles, but the current technology can not make biomass fuel environmentally friendly enough. In a word, it is not easy to replace oil as the main transportation energy.