Major South Korean refiners are expected to have suffered their worst ever performance in 2014 due to relentless slides in oil prices, with their combined operating loss reaching over 2 trillion won ($1.84 billion), according to guidance reports Thursday.

The combined operating loss of the four refiners -- SK Innovation, GS Caltex, S-Oil and Hyundai Oil Bank -- reached 1.2 trillion won in the first three quarters of last year, and they are expected to post an additional loss of over 1 trillion won in the final quarter alone due to increased losses on their crude inventories.

The dour outlook was attributable to the falling oil prices, which were battered especially after the Organization for Petroleum Exporting Countries in late November refused to cut supply in a price war against rising U.S. shale producers.

Dubai crude was trading at $93.52 on Oct. 1, but it shed about $40 to slip to $53.60 on Dec. 31, undervaluing inventory by local refiners, which heavily rely on imports from the Middle East.

SK Innovation Co., the nation's largest refiner, was expected to have posted over 500 billion won of operating loss in the October-December period, which is more than the accumulated operating loss of 406 billion won logged through September, guidance reports by multiple brokerage firms showed.

No. 2 GS Caltex Co. was also forecast to have logged an operating loss of some 400 billion won during the fourth quarter of last year, and S-Oil's operating loss in the last three months was estimated at 200-300 billion won.

Hyundai Oil Bank Co., whose 94 percent of sales coming from cracking margins, posted 179.2 billion won of operating profit through September, but it is expected to have barely avoided losses in the last quarter, they showed.

In the upcoming earnings season later this month, SK Innovation is expected to turn to red for the first time in 37 years, while GS Caltex and S-Oil are forecast to see their first losses in six years and 34 years, respectively, industry watchers said. (Yonhap)