Trying to turn resources into terrain features - advice?

[Corbeau]

Basically, I'm trying to figure out how to turn oil into a terrain feature for oil spills and similar effects. Note that I'm not trying to do this on the fly during a game - I just want a new terrain feature that looks a lot like oil. Problem is, I can't figure out how - rearranging the XML tags gives very unsatisfying results (oil slicks centered on tile corners and below the terrain level, making them invisible on land).

Any ideas or advice?

 

[Corbeau]

Nobody? C'mon, it's got to be possible...