[stratomorph]

A "properly sized" conventional explosive would be impractically enormous: nukes are measured in kiloton or megaton yields. There is no way to effectively detonate (say) twenty million pounds of conventional explosives all at once, never mind the non-trivial challenge of emplacing that amount. As I understand it, you'd place it below the sea floor, into the well, so the surrounding water would be shielded by all the rock above the blast site. For the same reason, there would be no fallout plumes, because all that solid mass would prevent the newly-created radioactive material from going anywhere. And it can't ignite the reservoir, because there's no oxygen to burn down there.

All that said, it's still probably not a good idea. Hopefully it doesn't become the only idea.

[Ixiaus]

An excellent point about oxygen, that had slipped my mind.