Kosmos Energy Ltd. said Dec. 17 it will not make an offer for Tullow Oil after both companies confirmed discussions of a possible all-stock deal last week.

Kosmos gave no reason for breaking off the acquisition talks, which the companies had said were at a preliminary stage, the companies had said on Dec. 12.

However, after closing at $3.71 per share on Dec. 11, Kosmos shares had fallen 15.5% at $3.14 per share following the announcement.

Following the news, shares in Tullow Oil fell about 4%, while Kosmos stock rose by around 10%, Reuters reported.

Kosmos has a market value of approximately $1.75 billion; Tullow Oil has a market value is about US$483 million (£381 million).

Dallas-based Kosmos owns deepwater assets focused on the offshore Atlantic Margins, including the U.S. Gulf of Mexico (GoM), Ghana and Equatorial Guinea.

London-based Tullow Oil’s producing assets are concentrated in West Africa, with production offshore Ghana, Gabon and Cote d’Ivoire.