input offset voltage of an op amp
If your signal is 0 V to 0.5 mV, the input offset could be 4 times your signal (2 mV/0.5 mV). In practice, the problem is that if you want to have a high gain to amplify your signal (e.g. 500x), then the input offset will have a significant impact on the output, as it will be amplified as well and it will end up being a voltage range that you cannot use. If you are only interested in the AC component, you can have a band-pass filter to remove the offset, but the voltage range will still be limited by your op-amp output voltage range - (input offset*gain). 

For very small signals like this, you should look into instrumentation or differential amplifiers with low input voltage offset and make a two-stage amplifier. For the second op-amp, the input signal will be larger enough, thus, you can choose one with more relaxed specifications.