Influence an ally to build an oil well that destroys another improvement

There's a city-state allied to me that just found oil. At least, I can see oil in their lands since I developed the relevant technology.

I'd like them to build a Well improvement over that tile, so that they'll connect the oil to my trade network.

But there's another improvement there (Trading Post), which would be destroyed if they built the well. They seem hesitant to build over it. There's an idle Worker just sitting in their city.

How can I get that well built?

I've tried taking one of my workers there, but I can't build outside of my territory.

I've tried taking a combatant there, but I can't pillage a non-enemy either.

War or culture bombs would not be worth it.

I'm about to build planes, perhaps I can bomb an ally?

Any other ideas?

AI players, including city states, will always upgrade strategic resources when possible- even if that'd mean removing a tile improvement.

If the city state isn't building an oil well, it's because they don't have the technology they need. I'm assuming you're the only civ with biology; city states won't have access to it until a second player researches it.

The easiest choice would of course be to invade the city state. Claim they have WMDs or you're bringing them freedom if it makes you feel any better.

As Studoku mentioned, if they do not have the technology they will not be able to improve the resource. However, if you want to have that oil right now, you can check each turn and try to improve it for 200 gold through the "Send gift" option in the City State's diplomacy menu - this will immediately improve the oil into a well so you do not have to wait for the worker to build the well.