State-run Ghana National Petroleum Corp (GNPC) is studying an agreement by Exxon Mobil to buy Kosmos Energy's stake in the Jubilee field in Ghana before passing it to the energy ministry for its perusal, a Ghanaian government source said on Wednesday.'GNPC is required to look a the entire deal as negotiated -- the idea is to ensure that it is the best offer not only in monetary terms but also it should come with the technical expertise,' the government source involved in the energy sector told Reuters.Sources close to the matter said on Tuesday Exxon had agreed to buy a stake which analysts have previously valued at up to $5 billion. The sources did not say how much Exxon would be paying.Private equity-backed Kosmos put its interests in the field, which it co-owns with UK-based oil explorer Tullow Oil and Houston-based Anadarko Petroleum on the market earlier this year, industry sources previously said.Source: Reuters