Todd, I’d guess that SDOT would be the first line of contact.
How long has it been like that? I wonder if it’s from a vehicle that had a massive oil leak, or if it’s leftover from an accident, and didn’t properly/thoroughly get cleaned up?
If it’s just normal oil residue from vehicles from our recent dry spell, it seem like it should wash away after a couple of good rains.
Not that that makes it any easier or safer for you to negotiate on your bike until it actually does wash away.
In any regard, be safe!
Mike