The Bulgarian refinery's year-on-year change in throughput was positively affected by maintenance works at the refinery in the first quarter of 2018, which lowered its production at the time, Lukoil said in a statement earlier this year.
Outside Russia, Lukoil also operates refineries in Romania and Italy.
In the third quarter alone, the throughput of Lukoil's refineries in Bulgaria, Romania and Italy increased by 8.1% compared with the previous quarter, to 6.6 million tonnes.