Trade Routes

What constitutes an ocean trade route? I had a town across an ocean form my main empire one tile of which produces oil. As soon as I created the town I built a harbour and I could oil was available in my empire. A few moves later the oil is no longer available. Is this because some - but not all - of the intervening ocean squares are now within the boundaries of another civ as they've jst expanded?

I assume they are so I build an airport in my capital and in this far flung town. Now I know its not realistic to import oil by air but the civilopedia does say it creates a trade route - but I still don't have oil available!

That is very strange. Are you sure the oil isn't disconnected on the island, or simply depleted?
You could check the trade advisor, there's a list of unconnected cities there.

can you post a savegame please?

Regardless of the ocean, the airport should give you a trade route.

Did the oil deplete? If you build your city on top of the oil, it may be hard to see thus you may not have noticed it went away.

sounds to me like the oil ran out.

No - the oil did not runout - but the road between it and the city disappeared. I can't believe anything other than a mistaken pillage. Fortifying on top of the square wouyldn't destroy the road would it?

@emjay: thanks for the clarification. This explains everything Fortifying doesn't destroy roads... It's a clever move to defend the resource...