Why are oil rigs in the middle of the ocean?

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We put the rigs where the oil is!

There aren’t any rigs in the “middle” of the ocean, but it is fairly common to find major oilfields over 150 km off the coast. This happens because:

• Shallow seas often had the correct conditions for oil formation millions of years ago. Specifically, something like an algae bloom has to die and sink into oxygen-free conditions on the sea floor, then that organic material gets buried and converted to rock over geologic time.
• The continental shelf downstream of a major river delta is a great place for deposition of loose, sandy sediments that make good oil reservoir rocks.

These two types of rock — organic-rich source rock and permeable reservoir rock — must be deposited in the correct order in the same place for there to be economically-viable oil reservoirs. Sometimes, we find ancient shallow seas (or lakes) on dry land. Sometimes, we find them underneath modern seas. In that latter case, you get underwater oil and offshore oil rigs.

In the “middle” of the ocean, the sea floor is primarily basaltic crust generated by volcanic activity at the mid-ocean ridge. There’s no source of sufficient organic material for oil source rock or high-permeability sandstone for reservoir rock. So there is no oil. Which is fine, because the water is too deep to be very practical to drill on the sea floor anyway. (Possible, but not practical.)