March 21 (Reuters) - Chinese energy firm CNOOC Ltd posted a 12.6% fall in 2023 net income after a record high a year earlier as hydrocarbon prices fell though a focus on cost control and reserve expansion helped.

The state-controlled offshore oil and gas specialist reported a net profit of 123.8 billion yuan ($17.20 billion), a company filing to the Hong Kong Stock Exchange showed on Thursday.

Its oil and gas output rose 8.7% to 678 million barrels of oil equivalent (boe), topping its target of 650-660 million boe.

Historically one of the industry's lower-cost explorers and producers, its all-in production cost fell to $28.83 per barrel from $30.39.

Capital expenditure rose by 37% to a record 137.35 billion yuan.

Net proven reserves stood at about 6.78 billion boe at end-2023, up from 6.24 billion a year earlier, as it maintained a reserve life of more than 10 years for a seventh consecutive year. Its reserve replacement ratio stood at 180%.

CNOOC is a top contributor to China's domestic oil production growth as state-owned oil companies tackle geologically more complex and more costly resources to counter a steep decline at mature basins.

This month, CNOOC announced two large finds each with 100 million tons of oil equivalent proved in place - the deepwater Kaiping South Oilfield in the Pearl River Delta and Qinhuangdao 27-3 off Bohai Bay.

In 2024, the company said it plans to focus on increasing reserves and production "with a greater emphasis on gas exploration" and forecast "steady growth" in both reserves and production.

($1 = 7.1994 Chinese yuan renminbi) (Reporting by Colleen Kristen Howe in Beijing and Chen Aizhu in Singapore; editing by Jason Neely)