Cnooc Ltd. (883) sold $2 billion of bonds denominated in U.S. dollars in its first offering in more than a year.

China's biggest offshore oil and gas explorer issued $1.5 billion of 3.875 percent, 10-year notes and $500 million of 5 percent, 30-year bonds, both at a relative yield of 190 basis points more than similar-maturity Treasuries, according to data compiled by Bloomberg.

Cnooc's success in discovering new fields will help the company achieve its goal of increasing annual production by as much as 10 percent, it said in a statement. Spending increased 58 percent in the first quarter, helping the Beijing-based company make five new discoveries, it said. Proceeds from the bond sale will be used for general corporate purposes, according to a company announcement to the Hong Kong stock exchange dated April 25.

Moody's Investors Service ranked the notes at Aa3, its fourth-highest level of investment grade. Barclays Plc, BOCI Asia Ltd., and Citigroup Inc. are managing the 144A/Reg S sale, the company said in the statement to the exchange.

CNOOC last sold bonds in January 2011, issuing $1.5 billion of 4.25 percent, 10-year notes at a spread of 100 basis points and $500 million of 5.75 percent, 30-year debt at 120 basis points. A basis point is 0.01 percentage point.