Maybe. Probably not. How can you tell? Stick your finger in it (or a stick, if you’re squeamish).
If it immediately “heals” – that is the shiny film closes back up again – it really IS oil. If it doesn’t, it’s a naturally occuring bacteria or other natural source. According to the USGS, these kinds of films “can be found anywhere that ground water, which lacks oxygen and carries iron and manganese, discharges into a stream.” I assume ponds are likely candidates, too. The article goes on to say:
Certain bacteria, the oxidizers, fix oxygen onto iron and manganese. Other bacteria, the reducers, remove the oxygen. In fixing or removing oxygen, some are getting energy and others are performing other life functions. Bacteria have been involved in the iron and manganese cycles for billions of years.
So, don’t worry about your pond being polluted until you stick your finger in it!