that seems strange if the crust is only abt 3 mi thick in the oceans how do we get away with drilling at least 5 miles below the sea floor in the near middle of the Gulf of Mexico and still have low enough temperatures for liquid hydrocarbons.
Much of the Gulf is part of the continental shelf. Plus, the 3 mile average is found closer to a ridge like the Mid - Atlantic ridge. That’s the problem with averages that they gloss over a lot of detail. Continental shelves are part of the continental crust and are geologically similar to the continental crust. The ocean floor is basically heavy basalt.