MARTINEZ -- A chemical cloud that leaked from the Shell refinery in Martinez dissipated Tuesday night as plant workers tried isolating the faulty equipment.

The leak was reported at about 8:30 p.m. somewhere in one of the plant's processor units, said Randy Sawyer of Contra Costa Health Services. It released a chemical cloud that rose upward but did not drift outside the plant's site, Sawyer said.

Some oil spray was determined to have drifted onto nearby Pacheco Boulevard, Sawyer said, with drops visible on white cars.

Sawyer said that the processor unit had been shut down, but that workers were being prevented from identifying and isolating the faulty piece of equipment because of fumes emanating from hot oil on the ground at the site.

An employee who was burned by hot oil in the accident was taken by ground ambulance to Doctors Medical Center in San Pablo, Sawyer said.

Contra Costa Health Services has issued a health advisory for the area surrounding the plant, but has not given a shelter-in-place order, Sawyer said.

The exact chemical makeup of the cloud is unknown, but Sawyer said it was probably some kind of fuel gas, which is similar to natural gas. The gas might have hydrogen sulfide mixed in with it, said Sawyer. That would cause it to smell like rotten eggs, and could exacerbate pre-existing breathing conditions such as asthma.

A representative from Shell could not be reached for comment.