LONDON—France’s Total SA said it will sign a deal on Monday that completes a $1 billion investment in a giant Iranian gas field, capping months of negotiations over the first big move by a Western oil company into the country in years.

Total will be the lead operator in a partnership with China National Petroleum Corp. and Iran’s Petropars to develop South Pars—a gas field under the Persian Gulf that is one of the world’s largest. Iran’s oil ministry said the contract would be signed Monday afternoon.

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