A host of law firms around the globe, including Davis Polk & Wardwell, Herbert Smith, Stikeman Elliott, Blake Cassels & Graydon, and Paul, Weiss, Rifkind, Wharton & Garrison, are advising on China National Offshore Oil Corp.’s latest foray into Canada.

The state-owned, Beijing-based company has agreed to pay $15.1 billion to acquire Canadian oil and gas company Nexen Inc. As part of the deal, CNOOC has already agreed to assume an additional $4.3 billiion of Nexen’s debt.

The deal is subject to regulatory and shareholder approvals, and is expected to close in the fourth quarter. The agreement also includes a breakup fee of $425 million that would be paid to Nexen in the event that Chinese regulators do not approve the deal. CNOOC is also entitled to the same amount should Nexen’s board withdraw its support for the deal.

Davis Polk is advising CNOOC with a team that includes New York partners George Bason Jr. and Leonard Kreynin, as well as Beijing partner Howard Zhang and Hong Kong partner Kirtee Kapoor. The firm’s New York antitrust partner Ronan Harty is also advising, while partners Paul Chow and Antony Dapiran are providing Hong Kong law advice. Partner John Reynolds III in the firm’s Washington D.C. office is advising on matters related to the Committee on Foreign Investment in the United States.

The firm also advised CNOOC on a $2 billion debt issue on the Hong Kong Stock exchange in April.

British firm Herbert Smith and Canadian firm Stikeman Elliot are also representing CNOOC. The London-based firm is advising CNOOC on U.K. and European Union aspects of the deal with a team led by London partners James Quinney and Simon Tysoe. Last year, CNOOC also turned to Herbert Smith for counsel on its joint $2.9 billion acquisition of stakes in three Ugandan oil exploration projects.

Stikeman Elliot is providing Canadian law advice, and its team includes partners William Braithwaite and John Ciardullo in Toronto and Christopher Nixon and Bradley Grant in Calgary.

Nexen turned to Toronto-based Blake Cassels for counsel. The firm is leading the deal with a team that includes Calgary-based corporate partner Pat Finnerty, securities partners Jeff Bakker, Ross Bentley, John Eamon, Shlomi Feiner and Michael Gans, and competition partner Jason Gudofsky.

Paul Weiss Hong Kong partners Jeanette Chan, Andrew Foley, and Edwin Maynard are also advising Nexen.

Nexen’s board of directors has turned to Burnet, Duckworth & Palmer securities partner Grant Zawalsky for legal advice. The company has also engaged Calgary-based Richard A. Shaw Professional Corp., for counsel.

Morrison & Foerster New York partners Jonathan Melmed and Spencer Klein and Hong Kong-based partner Venantius Tan are advising BMO Nesbitt Burns, financial adviser to CNOOC.

The acquisition will give CNOOC access to oil and gas producing fields around the Gulf of Mexico and the North Sea, as well as to oil sands in the Alberta province. CNOOC has already established a presence in Canada by acquiring Canadian oil sands firm OPTI Canada for $2.1 billion after OPTI filed for bankruptcy last year.  At that time, CNOOC turned to Gowling Lafleur Henderson for counsel. Canadian firms Macleod Dixon and Bennett Jones advised Calgary-based OPTI.

E-mail: jseah@alm.com