The mature stage of Petroleum generation, which occurs at depths of about 760 to 34,880 metres, includes the full range of hydrocarbons produced within the oil window. Often a significant amount of thermal methane gas is generated along with the oil. Below 2,900 metres, primarily wet gas (gas containing liquid hydrocarbons) is formed. Many theories have been put forward to explain the formation of Petroleum in nature, such as:
Carbide Theory
This theory was originally suggested by Mendeleeff and supported by Moissan, Sabatier and Senderens. It regards Petroleum as inorganic origin being formed by the action of steam or water on metallic carbides in the inner portion of the earth’s crust. The carbide theory describes the formation of Petroleum by the following steps:
(i) Formation of carbides: The molten metals in the earth’s hot interior came in contact with coal, and carbides were formed. Thus,
(ii) Action of steam with carbides: Carbides reacted with steam under high pressure and at high temperature, forming hydrocarbons:
(iii) Reduction of unsaturated hydrocarbons. The unsaturated hydrocarbons formed by step (ii) are reduced with hydrogen in the presence of a metallic catalyst at high temperatures. The hydrogen needed for reduction is obtained by the action of hot metals with steam.
(iv) Polymerization. Unsaturated hydrocarbons polymerized in the presence of hot metals form aromatic hydrocarbons, cycloalkanes, and higher open-chain hydrocarbons. 2-Butene will again be hydrogenated, forming butane
There is no doubt that the Carbide theory explains the formation of Petroleum rationally, but there are severe objections against it, and at present, this theory commands little consideration. The two main facts, which go against it, are:
(1) Natural Petroleum contains sulphur and nitrogen compounds, chlorophyll, haemin, etc. The Carbide theory fails to explain their presence in Petroleum as they are all essentially of organic origin.
(2) Petroleum contains optically active compounds. According to the Carbide theory, the natural mode of formation of Petroleum is laboratory oriented, and we know that we cannot get optically active substances by ordinary synthetic methods in the laboratory,
Engler's Theory
Engler (1900) discovered that a product similar to natural Petroleum could be obtained by the destructive distillation of fish blubber. It contained sulphur and nitrogen compound commonly found in Petroleum and was also optically active. This provides direct experimental evidence of the production of Petroleum from sea animals by some natural process akin to destructive distillation. This can happen if there is a volcano by the seaside. The volcanic gases contain sulphur dioxide, which dissolves in seawater. Thus fish and other sea animals coming towards this region would die because of suffocation. Over time, a huge accumulation of dead animals formed this way would be covered by the earth’s crust; under the influence of heat from the volcano, decomposition sets in, resulting in a petroleum deposit.
Another fact that lends weight to Engler’s theory is that mineral oil is always found in company with salt and brine, which must have been obtained from seawater.
Modern views
It has been described above that Petroleum is very probably of animal origin. The presence of compounds derived from haemin, the pigment of the red blood corpuscles, and the discovery of fossils in areas are enough proof to support this theory.
Many oils contain compounds derived from chlorophyll, the green of the plants, thus pointing to the vegetable origin of Petroleum. Maybe some upheaval brought a forest well under the earth’s crust where the plants were converted to Petroleum by the action of heat and water. This view is supported by the fact that coal is found in the neighbourhood of oil deposits. The oil’s vegetable origin also accounts for Burma oil’s high resin content.
Because of the above evidence, it appears very likely that while some oils are of animal origin, others are of vegetable origin.
Astronomers have recently established that the atmosphere of the large planets, including Jupiter and Saturn, consists chiefly of methane. This has led to the idea that our earth’s crust was swathed in huge quantities of methane at one time. Since methane is converted to higher hydrocarbons by the action of ultraviolet and radioactive rays, it has been conjectured that radioactive substances in the earth’s crust may have transformed methane into Petroleum.
