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If there were an earthquake that somehow cause a crack in the earth and allowed a significant amount of magma to flow into a very large oil deposit. With that high amount of explosive energy that would be produced, could it cause an earthquake or even a volcano? Does this usually happen?

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    $\begingroup$ It wouldn't be explosive, as petroleum doesn't explode, or even burn, unless mixed with air. Of course if it's a big enough volcano and deposit, you get a replay of the Permian-Triassic Extinction event. $\endgroup$ – jamesqf Feb 10 '17 at 3:25
  • $\begingroup$ Somehow related: The oil window, the temperature range when oil is formed. If temperature is lower, the kerogen stays in the pores and if temperature is higher, gas is generated. oilandgasgeology.com/oil_gas_window.jpg $\endgroup$ – Tactopoda Feb 10 '17 at 12:24
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If there were an earthquake that somehow cause a crack in the earth and allowed a significant amount of magma to flow into a very large oil deposit.

My understanding from this is that you are making two assumptions, which are not necessarily true:

  1. That earthquakes open cracks allowing magma to flow. It may actually the other way around: when magma flows, it cracks rocks under pressure, causing minor earthquakes. Large earthquakes, from the kind that destroys things and kills people, are usually unrelated to magmas.
  2. That there is magma everywhere underneath the crust, and it's deeper than oil deposits. Again, not necessarily true. The overwhelming majority of the crust and the mantle are solid. There are localised areas where melting occurs (the location of active volcanos is a sign of that). Usually, these locations are not where oil deposits occur. And even then, magma chambers can be very close to the surface. In some cases about 1 km below the surface or less. Oil deposits can be several km deep.

That said, what will happen if it does actually happen?

With that high amount of explosive energy that would be produced, could it cause an earthquake or even a volcano?

Not really. Injecting magma into oil may cause some of it to boil or evaporate, causing expansion and possibly fracturing. But the magma is already fracturing the rock anyway. The oil itself is not going to burn or explode, because this requires oxygen that is not readily available underground. The oil can reduce the iron in the magma, thus being oxidised to CO2 (and forming metallic iron). This is probably not going to make much of a difference if the oil was decomposed to methane, which is another gas. This might ignite if carried to the surface with the magma, but I doubt that it's going to do anything more spectacular the magma itself will.

Does this usually happen?

No.

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An oil deposit is gravel and rock mixed with oil, capped by rock - like this:

enter image description here

My guess is that if somehow, likely from below or sideways, magma would intrude into this deposit, oil (and posibly water) would evaporate. The pressure rises, slowing the magma intrusion. If enough magma and enough heat is provided, the vapor pressure of the oil may be enough to crack the rock layer at places, oil would escape and condense in the shale layer.

My gut feeling is that we are talking about processes that take years to decades to play out.

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