I should also note that the idea of blocking the hole with gravel isn't exactly a bad one. You're right to point out that if you use big pebbles for your gravel the oil will just squeeze through the abundant empty spaces. So... use finer gravel!
The pressure will try to push the gravel out. So maybe make it really sticky gravel, so that it glues itself together.
Now there are three things to say about this plan:
(a) we call this sticky, very fine gravel concrete and we use it all the time;
(b) a fun educational point: Even with really fine gravel, oil can still diffuse through it. Oil diffuses through "solid" rock all the time. The oil underground is actually trapped inside rock. The rock that holds the oil is relatively porous; the oil is kept underground because above that rock is a layer of denser rock that oil can't diffuse through very well.
(c) In case it isn't obvious yet: The reason your idea still doesn't work very well is that the first cupful of wet concrete you pour onto the hole will promptly get flushed away by the pressure of the leaking oil. ;) As will every successive cupful of wet concrete. Unfortunately, a giant slab of solid concrete must necessarily start out life as a thin layer of wet concrete, so the only way to put a concrete seal over a hole that is actively leaking is to cast it someplace else and then drop it on the hole.
Well this was why I didn't suggest pouring concrete into it...I was assuming that the cap would be cast at the same time as the cylinder :)
The only point of the gravel (rather than sand) is to disperse the flow sufficiently that maneuvering is easier - same way you could wade across a stream but would be knocked over by the same volume of water aimed at your from a firehouse.
The pressure will try to push the gravel out. So maybe make it really sticky gravel, so that it glues itself together.
Now there are three things to say about this plan:
(a) we call this sticky, very fine gravel concrete and we use it all the time;
(b) a fun educational point: Even with really fine gravel, oil can still diffuse through it. Oil diffuses through "solid" rock all the time. The oil underground is actually trapped inside rock. The rock that holds the oil is relatively porous; the oil is kept underground because above that rock is a layer of denser rock that oil can't diffuse through very well.
(c) In case it isn't obvious yet: The reason your idea still doesn't work very well is that the first cupful of wet concrete you pour onto the hole will promptly get flushed away by the pressure of the leaking oil. ;) As will every successive cupful of wet concrete. Unfortunately, a giant slab of solid concrete must necessarily start out life as a thin layer of wet concrete, so the only way to put a concrete seal over a hole that is actively leaking is to cast it someplace else and then drop it on the hole.