Round disk with lights

Was working night shift at Fuller Gas Plant in 1991, saw round object could see lights yellow green, maybe blue, going around, didn't really see the color of the object itself, just the lights. It hovered/moved very slowly over gas plant, but the distance it covered it must have been faster than I thought. I didn't hear any sounds, and nothing out of the ordinary happened at the gas plant. I watched it for about 5 minutes before it dissapeared suddenly, As it was moving, I called another co-worker at the water station, but he didn't notice it. It was travelling from west- northwest to south south-east direction. It dissapeared just when it passed over my head. It was a dark night, no clouds.
Witness indicates that the date of the event is approximate.  PD