(Reuters) - Warren Buffett's Berkshire Hathaway Inc <BRKa.N> said on Sunday it agreed to buy Dominion Energy Inc's <D.N> natural gas transmission and storage business for $4 billion in cash, expanding its energy operations while allowing Dominion to focus on its utilities operations.

Berkshire's energy unit, Berkshire Hathaway Energy, is buying Dominion Energy Transmission, Questar Pipeline and Carolina Gas Transmission; 50% of the Iroquois Gas Transmission System, and 25% of the Cove Point liquefied natural gas facility in Maryland.

The transaction includes more than 7,700 miles (12,390 km) of natural gas storage and transmission pipelines and about 900 billion cubic feet of gas storage.

Berkshire Hathaway Energy will also assume $5.7 billion of Dominion's debt, giving the transaction a $9.7 billion enterprise value. It expects a closing in the fourth quarter, pending regulatory approvals.

"We are very proud to be adding such a great portfolio of natural gas assets to our already strong energy business," Buffett said in a statement.

The transaction does not include Dominion's interest in the Atlantic Coast Pipeline.

Dominion and Duke Energy Inc <DUK.N> announced separately on Sunday they would cancel that project, saying delays and uncertain costs threatened its economic viability.

(Reporting by Maria Ponnezhath in Bengaluru and Jonathan Stempel in New York; Editing by Peter Cooney)