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I did asked this question the other day: Why are salt domes suitable for the final disposal of nuclear waste?

@Weiss posted the following illustration as a part of the answer:

enter image description here

Source: https://antisocialnetwork-2labz.blogspot.com/2017/04/rock-salt-for-dummies-freelance-files_15.html

I wonder why there is oil on the sides of the salt dome and if it is being extracted?

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good observation! The very simple answer is the same as for nuclear waste. The protective anhydrite layer over the salt dome became denser over time more dense. Therefore, the oil rises from below but remains on the sides (comparable to opening a balloon under an umbrella).

Whether this oil is being extracted: yes, salt domes are always being searched for in oil exploration, as they usually have oil on their flanks. A sad example is the exploration drilling at Lake Peigneur. In this example, a salt works was built in a salt dome. Oil exploration damaged the protective layer and the salt became wet. As a result, parts of the salt mine collapsed.

enter image description here

Source: https://www.reddit.com/r/submechanophobia/comments/bc8qyp/diagram_of_the_lake_peigneur_disaster_check_out/

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    $\begingroup$ There are videos about that disaster. $\endgroup$
    – Fred
    Commented Jul 16, 2022 at 9:58
  • $\begingroup$ There have been a few such accidents on the Gulf Coast. There was fairly active salt dome/pinnacle , oil production in MI. $\endgroup$
    – blacksmith37
    Commented Jul 16, 2022 at 21:29
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    $\begingroup$ @Weiss Re "The salt domes are very dense" This phrasing is confusing to me, maybe you could clarify. Isn't it the case that salt has a lower density (~2g/cm³) than the layers of sedimentary rock above it (~2.5g/cm³), causing it to rise, leading to the formation of salt domes? At least that is what I seem to remember from my high school days. $\endgroup$
    – njuffa
    Commented Jul 16, 2022 at 22:06
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    $\begingroup$ @njuffa I think very non-porous is what was intended. $\endgroup$
    – Dan Is Fiddling By Firelight
    Commented Jul 16, 2022 at 23:28
  • $\begingroup$ @njuffa oh, yeah I didn´t make it clear, I didn´t mean the salt ;) I fixed it $\endgroup$
    – Weiss
    Commented Jul 17, 2022 at 7:33
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There are 2 main reasons that oil is found near salt domes:

  1. The permeability of the salt is extremely low, meaning fluids can't flow through it, and
  2. The geometry of the salt dome is such that it can trap oil beneath it.

Oil and gas, being less dense than water, "wants" to migrate upward through the rock strata from the source rock (which is almost always shale) where it was generated. Occasionally it will encounter geologic features, such as a salt dome, which trap it. The hydrocarbons get trapped because they can't move downward, they can only move up. They can't move downward because everything else around it (rock and water) has higher density.

The other poster who alluded to the density of the anhydrite is sort of right, but for the wrong reason. The salt has very low density (relatively) and thus it also wants to move upward through the rock strata, and as it moves upward it also expands due to there being less stress on it. This gives it the geometry that it needs to form an effective trap for oil and gas.

Conventional oil explorationists also look for anticlines (think upside down bowls), faults, and stratigraphic pinchouts which can also trap hydrocarbons.

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