IRVING, Texas (

TheStreet

) -

Exxon Mobil

(XOM) - Get Report

has agreed to buy a $4 billion stake in an oil field off the coast of Ghana to get a foothold in the major new oil-producing region, the

Wall Street Journal

reports.

The deal, which is Exxon's first major purchase in a decade, isn't yet finalized and is subject to change. An outstanding issue in the deal could be that the Ghanaian national oil company has the right to increase its stake.

The deal appears to highlight that Exxon believes oil prices will rise over the long term. Some energy analysts have asserted that weak demand for the fuel can't support even the current price of $70 a barrel, the

Journal

notes.

Crude oil is trading Wednesday at $71.19, up 31 cents.

The seller is Dallas-based Kosmos Energy which has entered into a binding agreement with Exxon for its 23.5% stake, the

Journal

reports. Other stakeholders in the oilfield - Jubilee -- include

Anadarko Petroleum

(APC) - Get Report

and

Tullow Oil

, which has a 34.7% stake.

It isn't clear which company would be the operator of Jubilee, which is expected to begin producing oil in 2010, the

Journal

notes.

A spokesman for Exxon declined to comment for the

Journal

.

This article was written by a staff member of TheStreet.com.