As many as 7,000 people were evacuated from their homes in Port Arthur, Tex., after a leaking crude oil tank at a Chevron plant created a noxious cloud of gas with traces of hydrogen sulfide, officials said. More than 20 people were treated and released at hospitals for minor reactions, most for fume inhalation, officials said. Chevron officials said they were monitoring air quality in the area of the leak since it was discovered in the early afternoon, and described the gaseous release as a minimum health hazard. "Most crude oils in Texas have sulfur in them, and that means their gas containes hydrogen sulfide . . ." said Chevron spokesman Art Spencer. Hydrogen sulfide is a flammable, poisonous gas with an odor of rotten eggs. The odor got so strong that officials decided to evacuate people downwind of it, he said.