CALGARY, Alberta - The "vast majority" of workers at Syncrude's oil sands plant in northern Alberta had returned to work on Wednesday after a fire the previous day, said a company spokesman who declined to say whether production at the facility had been affected.

Access to the 350,000-barrel-per-day Mildred Lake upgrading facility, however, was still restricted, spokesman Will Gibson said. He added that Syncrude was investigating the cause of Tuesday's fire.

Synthetic crude prices jumped again on Wednesday morning, hitting a nine-month high of $1.40 per barrel over the West Texas Intermediate benchmark, according to Shorcan Energy brokers. On Tuesday light synthetic crude for April delivery settled at 80 cents per barrel over WTI, having rallied hard in the last hour of trading.

Syncrude, Canada's largest single source of synthetic crude, is majority-owned by Suncor Energy and operated by Imperial Oil.

An explosion rocked the facility on Tuesday at around 2 p.m. local time, prompting an evacuation, employees on site at the time said. Syncrude is situated about 40 kilometers (25 miles) north of the oil sands hub of Fort McMurray.

One worker was injured in the fire and was taken to hospital in serious but stable condition, according to Alberta Health Services.

Tuesday's fire and the expected disruption to production echoes problems with Syncrude's operating performance before Suncor Energy acquired a majority share in the plant last year.

The facility has been plagued in the past by outages and below-capacity output, but has been running at around 100 percent utilization since the second half of 2016, helped by a long period of maintenance in May and June when it was shut during wildfires in Fort McMurray and the surrounding area.

Before the fire on Tuesday Syncrude had a turnaround due to start in late April and last until June.