What is the fine-grained topic of the following text: Lukoil, which is now Russia's largest oil company, and Bashneft are currently drilling for oil in the Trebs oil field in the Nenets Autonomous District which is estimated to hold 153 million tons of oil. Only return the topic.
Lukoil and Bashneft drilling for oil in the Trebs oil field in the Nenets Autonomous District.