I have a new AR-15 with 24" bull barrell and it shoots great.  No problems at all.  Yesterday I noticed what appears to be exhaust gas residue coming out on the top of the barrell where the gas block meets the SS barrell.  The residue is going out forward of the gas block towards the muzzle.  The residue is about 3-4mm in length and appears like blowing high pressure gas through a black oily substance...like blowing an air hose through a high consistency oil making streaks.  This is my first AR-15 and I don't know much about this.  Is this normal?

Thanks