LONDON—Glencore PLC on Thursday said it would write down the value of its oil assets in Chad by $790 million and cut its capital expenditure this year to preserve cash, as the commodities group battles with weaker prices.

The Baar, Switzerland-based trader and producer of commodities said it would take an impairment charge in Chad after significantly reducing the number of drilling rigs and changing the capital budget of its operations amid weaker oil prices. Glencore acquired the Chad assets through its roughly $1.5 billion Caracal Energy purchase in July last year.

It also said it would reduce this year's overall capital expenditure budget to $6 billion from a previous range of $6.5 billion to $6.8 billion to preserve funds, as it reported mixed output for the first half of the year and reduced its guidance for full-year output of certain commodities including copper and coal.

Glencore, whose shares traded largely unchanged in response to the news, isn't alone in taking impairment charges and reducing spending on large projects to retain cash and maintain dividends amid weaker commodity prices.

Brent crude oil down by more than 50% over the past 12 months and some metals are trading at multiyear lows, with copper hitting a six-year low on Tuesday.

Last week at its interim results Rio Tinto PLC was hit by a $400 million write-down and cut its capital expenditure to $5.5 billion from $7 billion. Anglo American PLC has also written down the value of its assets and reduced spending, while analysts expect BHP Billiton Ltd to cut spending when it announces full-year results on Aug. 25. BHP already said it would write down the value of its U.S. onshore petroleum business by $2 billion.

Glencore, which has the biggest exposure to copper of the large diversified miners, said its own sourced copper production fell 3% to 730,900 tons in the first half of the year, compared with the same period a year earlier, due to lower output from its operations in South America.

Zinc output, another large earnings driver after copper for the company, rose 12% annually to 730,300 tons in the first half, mainly because of the ramp-up of expansion projects in Australia.

Glencore's own sourced coal production, another key revenue driver, fell 4% to 68.7 million tons in the first half primarily because of the market-driven production cutbacks.

The trader didn't provide any details about its closely watched marketing division, its largest earnings driver last year. The company is due to report first-half results on Aug. 19.

Write to Alex MacDonald at alex.macdonald@wsj.com

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