How is petroleum crude generated in the Earth over time?

Answer 1

Crude oil is created by a natural process known as catagenesis. Organic material is trapped in sediment, and as it is gradually buried under more sediment it is warmed by heat from the earth and converted to oil and natural gas.

The organic material is composed primarily of phytoplankton (tiny plants) and zooplankton (tiny animals) that live in the ocean or in lakes. Some plant material is also trapped in sediments, generally carried into the ocean by rivers. In some instances this process may take place at the bottom of fresh water lakes. In order for the material to be preserved, there must be anoxic conditions within the sediments, meaning that there is no oxygen. The sediments are typically carried into a basin by rivers, but another environment where organic sediments are trapped are mudflats known as sabkhas, and some other lagoonal settings. The common idea that dinosaurs contribute to oil is wrong, because the bodies of large animals rot and decompose quickly, and the organic parts are almost never preserved. Most of the organisms that go into oil are microscopic in size.

As this organic-rich sediment is buried under river deltas or other depositional processes, it will eventually reach what is called the oil window. This is the area of the subsurface where the temperature exceeds about 60 degrees Celsius (140 degrees F.). At that temperature, the organic material in the rock begins to convert to oil. As the sediment continues to be buried deeper and deeper it may warm to the point it reaches the gas window, about 160 degrees Celsius (320 F.), and will begin to form gas. If it stays this hot long enough, most of the oil will change to natural gas. If it gets too hot, both the oil and gas will eventually convert to graphite. This process of catagenesis can be duplicated in a laboratory.

One of the ideas that has been presented here is the hypothesis of abiotic oil generation. It has been proposed that oil may form from carbon deep within the earth's mantle. If this happens, it is extremely rare, and it is easy to verify that the vast majority of oil and natural gas do not form this way using isotopic testing, and the visible microscopic evidence of phytoplankton and zooplankton remaining in crude oil. Also, if the abiotic oil generation hypothesis were true, oil would be found in metamorphic and igneous rocks that originate deep within the earth. Oil is never found in these types of rock, so most geologists conclude that the abiotic process must be very insignificant. I don't know where the other answerer gets the idea that it is found there, as he doesn't give any sources for his information, and his explanation about clay is some sort of a combobulation of unrelated concepts (oil expulsion has nothing to do with the formation of oil). Oil is found in sedimentary rocks such as sandstone, limestone and shale. Some shale, known as a "source rock" is so rich in oil that it can be crushed and burnt. That is because shale is one of the original places where the remains of the plankton and plants were trapped and stored.

Reference:
Hunt, John M., 1996, Petroleum Geochemistry and Geology, New York: W. H. Freeman and Company, 743 pp.

Also see my answer to a question about the amount of time it takes for oil to form:
http://answers.yahoo.com/question/index;_ylt=AvalPg6ZWwN.S2inAcRBXefsy6IX?qid=20061013142708AA6mprf

Answer 2

The theory of biogenenic origin of petroleum is just that, a theory. The presence of petroleum in igneous bodies many miles from any source and much deeper than any source argues strongly for an abiogenic origin, yet old ideas die hard. Petroleum did not come from dead dinosaurs. Petroleum is probably the result of methane migrating upward and being consumed by deep zone of bacteria that feed off the methane. Some of the residual methane and hydrocarbon bi-products migrate upward until they are trapped by so sort of geologic trap. Obviously much of it escapes into the atmosphere where it is quickly oxidized to CO2. The old theory which I learned in school was that petroleum was generated in clays. Because pressures are not adequate to account for its formation, a theory was developed that the petroleum produced its own pressure as it was forming. Once a certain pressure was generated with the clay pore space, the pore space opened and the oil flowed out of the clay eventually. There are problems with this theory. This theory developed because most geologists thought that the earth formed from a melt and therefore all the hydrocarbons would have been destroyed. This is no longer the dominant theory yet the old tired theory of the formation of oil, coal, methane, and methane hydrates has not been modernized IMO except in a few countries such as Russia.

Answer 3

Carbon based animals and plants were buried over time. The weight of the rock on top of them created pressure , along with the high temperature of the earth breakdown the plants and animals into oil. Thats why oil is typically found near marine settings (because of the many plants and animals that were buried many years ago. Even today oil is continuing to form. Its just takes a long time

Answer 4

Masses of plant material (plus the odd dinosaur) were gradually buried by geological processes. Over time, the heat and pressure gradually converted them into crude petroleum.

Answer 5

That relies upon who you ask. some geologists have self assurance it became produced from inorganic carbon interior the crust over 5 billion years in the past and has been created continuously ever because. _____ >>>> there are no longer any geologists that have self assurance the earth is that >>>>> previous (It’s 4.5 billion years previous). hi, enable's no longer chop up hairs. what's 0.5 a billions years right here or there? I stand corrected on the age element. yet my unique factor maintains to be, that there are geolgists who have self assurance that oil began forming tremendously plenty as quickly because of the fact the Earth shaped, and actually as quickly because it had cooled sufficient to solidify to any degree.

Answer 6

Dead dinosaurs and forrests decomposing quietly, deeply.