* Maersk buys Gulf of Mexico oilfield stakes from Devon

* Maersk to pay $1.3 billion in cash for assets

* Maersk sees further investments of around $4 bln over time

* Devon's 2010 Gulf of Mexico capital budget down $400 mln

(Adds more details, Maersk CEO's comments)

COPENHAGEN, Dec 22  - Shipping and oil group A.P. Moller-Maersk (MAERSKb.CO) has agreed to buy Devon Energy Corp's (DVN.N) stakes in the Cascade, Jack and St. Malo oilfields in the Gulf of Mexico for $1.3 billion, the firms said on Tuesday.

Maersk will pay $1.3 billion for the deepwater assets, the Danish and the U.S. firms said in separate statements.

Maersk will invest a further $1.3-1.8 billion in the fields ahead of on-stream production. Over time, the investments will probably reach $4 billion, Maersk said.

Maersk, which has said it wants to expand its oil operations, said its share of recoverable resources from the fields was probably more than 200 million barrels of oil equivalent.

Christian Nagstrup, analyst at Sydbank said it was a very big investment for Maersk: 200 million barrels of oil equivalents is to be added to the around 135 million that was their share in 2008.

Nagstrup said the move may solve the firm's problems with falling production. It has long been clear that they would do one thing or another once done with investments in Qatar, and this is a really big step.

The deal involves a 50 percent stake in the Cascade field and a 25 percent stake in each of the Jack and St. Malo fields.

The Petrobras-operated (PETR4.SA) (PBR.N) Cascade field is expected to start production in 2010 and the Chevron-operated (CVX.N) Jack and St. Malo fields are expected to start production in 2014, Maersk said.

We have said that we wanted to invest in Maersk Oil and Gas, said Maersk's Chief Executive Officer Nils Smedegaard Andersen told Reuters.

Smedegaard said he hoped for further finds in the area, where Maersk is already active. Hopefully (we can) also produce other things that we find out there in the same area.

Maersk is currently evaluating finds at its Buckskin deepwater field in the Gulf. I believe Cascade will be the first field in production in the area, said Smedegaard.

Devon in November announced plans to sell its Gulf of Mexico and international assets for as much as $7.5 billion and instead focus on its onshore fields in the United States and Canada. [nN16505198]

Data rooms for all of the remaining divestiture assets will be open in the first quarter of 2010, Devon said on Tuesday.

Devon said it would use the post-tax $1.1 billion proceeds from the sale to reduce debt. It said the sale would reduce Devon's previously announced 2010 capital budget for the Gulf of Mexico by around $400 million. [ID:nWNBB3675]

The firms said the deal was subject to a waiver of preferential purchase rights held by other partners in the fields, and regulatory approvals.

We cannot rule out that those who are already engaged in these fields want to buy more shares, Smedegaard said.

The firms expect to close the deal early in 2010. (Additional reporting by Henriette Jacobsen; Editing by Greg Mahlich)