gizmo, I have something very similar that I see in my pond from time to time. It is usually associated with an area where the dirt is very orange. I do have some old mine run-off, and I am certain that the color and "oil-looking" stuff is associated.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."