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ALMATY, Nov 20 (Reuters) - Output at Chevron-led Tengizchevroil (TCO), Kazakhstan's largest oil producer, fell by 1.5 percent year-on-year in the January-September period to 19.8 million tonnes (158 million barrels), the company said on Thursday.
TCO, which develops the mammoth Tengiz onshore oilfield in western Kazakhstan, boosted its output to a record 27.1 million tonnes last year from 24.2 million in 2012.
The company gave no reason for the output decline in the first nine months of this year. Senior Kazakh energy officials said last month that TCO was undergoing planned repair and maintenance work which was due to end this month.
TCO's output is crucial for keeping Kazakhstan's oil production at 81.8 million tonnes this year and next. Output at the huge Kashagan oilfield in the Caspian Sea, which was halted due to gas leaks shortly after its launch in September 2013, is unlikely to restart before the second half of 2016.
Last month Kazakhstan gave the go-ahead to an expansion project to boost TCO's output to 38 million tonnes by 2019.
Chevron holds 50 percent of TCO, while Exxon Mobil owns 25 percent, Kazakh state oil company KazMunaiGas 20 percent and Lukarco, controlled by Russia's Lukoil, the remaining 5 percent. (Reporting by Mariya Gordeyeva; Writing by Dmitry Solovyov; editing by Jason Neely and Pravin Char)