Leaks?

I've been noticing 2 different areas my car has been leaking underneath. There aren't any puddles (the driveway is only stained) and when I look, nothing is dripping so they must be slow leaks. One is smaller and located close near the front of the car in the center, I put white cardboard under it and the next day there way a tiny bit of black/grey fluid. Maybe medium consistency, I'd assume oil(?) But I don't know exactly where it would be dripping. The other leak is a bit bigger and pretty much by the gas tank. Pretty close to the passenger rear tire. It looked dark in color and had a real faint smell of gas (which I have smelled on occasion when starting my car). Can these be simple fixes or dangerous and expensive. Advice please. :/