A small but noticeable spill of diesel fuel into the Mystic River reported on Saturday near the Everett ExxonMobil Pipeline Company terminal is being investigated by the Coast Guard.

Coast Guard officials determined that a sheen of fuel was on the surface of the water in an area about 800 feet long and 20 feet wide.

Coast Guard cutters patrolled the ocean shoreline in the area of the Everett terminal scouring the area in order to find the cause of the spill.

“It is not known how many gallons spilled into the ocean,” said Coast Guard spokesman, Petty Officer James Rhodes.

The Coast Guard apparently received a call from employees of ExxonMobil at 1:23 p.m. Saturday.

The Coast Guard at the time responded immediately and released information indicating the sheen appeared to be coming from upriver.

ExxonMobil officials insisted that the spill, which was reported by ExxonMobil, was not theirs.

“We have examined our facilities and have found no issues with them at this time,” said Patty Errico of ExxonMobil.