Oil: End Product or Still Changing
Date: Fall 2012
If oil is not extracted and left in the ground, will it transform into something else under the pressure? Are there still chemical reactions happening or is oil quite stable and will not change anymore?
I am not sure anyone really knows but there are conditions that suggest continuing reactions. (1) The composition of oil from various sources does not have the same chemical composition. (2) Given the high temperatures and pressures changes, it is unlikely that the equilibrium conditions are the same everywhere.
A good question. There is little no direct data on this question because of the time involved in any changes is probably too long. But from observations in existing rocks, we can make some pretty good guesses.
If the oil is heated up by tectonic events it will actually "crack" (that is break into simpler molecules) and leave behind natural gas and possibly some tar like residue. If it gets too hot, the hydrocarbons are destroyed and you get carbon dioxide.
In the oil business, the maximum temperature the rock has reached is called its thermal maturity. It is investigated by looking for kerogen in the source rocks (usually in outcrops or in cores). Source rocks are the rocks whose carbon content goes on to create the oil and gas. Shale is the main type of source rock. By looking at the color of the kerogen one can tell the maximum temperature the rock has experienced. If the temperature was too high, the kerogen is very dark and you can expect to find carbon dioxide. Somewhat lighter kerogen color indicates temperatures that favor gas. Even lighter kerogen is an indicator temperatures that favor oil. BUT, if the rocks were not heated enough and the kerogen is too light, the area is called immature and probably has no oil or gas at all. It takes heat to create and move oil and gas.
We also know that many oil and gas traps "leak". That is parts of the oil reservoir come to the surface along faults or they simply migrate towards the surface through the rocks. In that case we expect to see just oil in the reservoir since any gas has escaped. The more the seepage, the gooier the stuff left behind.
Lastly, there is the action of microbes. Yes, we have found live oil eating microbes at some pretty extreme depths as of late. Exactly how much these microbes do to the oil is still under investigation, but we are sure that they do little to help.
Kind of a long answer, but I hope it helps.
Continued pressure and heat could change hydrocarbons (oil) into graphite (coal, which is mostly carbon and not as much hydrogen), and then into diamonds (which is all carbon).
Greg (Roberto Gregorius)
It depends on the conditions underground. If it is subjected to higher temperatures and pressures, it will decompose to more thermodynamically stable products, which would be methane and carbon or carbon dioxide, depending on what other materials are present. If the petroleum seeps to the surface, it eventually is broken down by aerobic bacteria. If it stays underground in without oxygen or high temperatures, it will not do much.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
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