The Stack Exchange question wondering how much of the petroleum ever formed made it into reservoir rocks got me pondering what becomes of the organic material that isn't trapped in reservoir rock.

I imagine it continues downward and potentially stays/becomes more diffuse than is found in reservoir rock. But (understanding that less is probably known about the intimate details for deeper in the Earth)... what are the theories on where and how it would wind up?

Would there be a layer it would tend to collect at due to stratification/gravity? Would the bulk of it wind up in different compounds due to potentially much longer time (conceivably long into the future), different rock compositions, and different pressures/temperatures? Or would it just gradually spread throughout the mantle into minute concentrations?

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    $\begingroup$ With no sealing formation to contain it; over a relatively long time it would migrate from source formation to the surface. There are numerous oil and gas seeps reaching the surface. $\endgroup$ Sep 15 at 21:04
  • $\begingroup$ So as solid it moves downward (if nothing else than by deposition?), then when it becomes a liquid it moves upward by... capillary action?? Treat me like I'm someone who knows very little about such topics, as honestly I don't (I probably learned a little about it in intro to geology once upon a time... but 20 years later it's receding deep into the lost caves of my mind these days) $\endgroup$ Sep 16 at 2:05
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    $\begingroup$ No solids, liquids and gas , depending on pressure and temperature. Some very viscos to some very low viscosity. Crude oils are all different . Generally it does not move down because there is a large likely hood that there is water/brine ,below it. It is not in caves ,it is in the rock. That is why fracking may be necessary to permit movement. $\endgroup$ Sep 16 at 2:15
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    $\begingroup$ I agree with @blacksmith37, the layer it would tend to collect at due to density is... the surface, as petroleum is less dense than rock. If it's not trapped, it would never go down to the mantle, but rather go up to the surface by buoyancy. See tar pits: en.wikipedia.org/wiki/Tar_pit $\endgroup$ Sep 16 at 15:02

1 Answer 1


Oil originally trapped in an oil reservoir, can, over millions of years, migrate laterally from one location to another, sometimes many tens of kilometers from the original reservoir. We may think of present-day reservoirs as static and unchanging, but even now processes forcing the migration of the oil are continuing. Oil and petroleum products can escape through horizontal and vertical fractures and fissures and ultimately discharge at the surface as vapors and liquids. Vapors most easily penetrate such fractures and as they escape, they leave a residue by product of concentrated oil or tar. This concentrated by product may be too viscous to migrate and will largely remain stationary. If the residue is not highly viscous, the product may migrate with facility along fractures and also be discharged at the surface. Most oil discharged to the surface becomes evapoconcentrated as volatile components escape into the atmosphere. This thickened oil is tar, the most famous occurrence being the La Brea tar pits, in Los Angeles.

Oil products can also become concentrated and solidify as an asphalt, called asphaltite or gilsonite. This product can be exposed at the surface and has an appearance like coal. Upon closer examination, however, asphaltite may appear as a soft, light density, glassy material, that spalls with concoidal fractures like obsidian. Gilsonite will burn, and can be refined into heavy oil and other useful petroleum products. The refining process, however, is noted for flaring methane as a by-product of the refining process. Gilsonite is typically found in oil shale.

Keep in mind that in many instances, oil at depth may be within a reservoir that is very hot. The high temperature is due primarily to the geothermal gradient. This heat can cause the volatilization of oil thereby driving the volatile products through fissures and along faults to escape at the surface. The now thickened oil can also force open these same fissures and planes along faults and be deposited there. In the case of gilsonite, the thickened petroleum can form sills between bedding planes as well as dikes along fissures and faults. Erosion can later expose these deposits at the land surface.


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