Kosmos Energy and GALP Energia will perform joint seismic studies in three offshore oil blocks in Sao Tome and Principe starting in January 2017, the island state’s petroleum agency said April 7.

The survey, expected to last six months, will cover roughly 12,800 sq km in exploration blocks 6, 11 and 12 off the coast of the former Portuguese colony, said Orlando Sousa Pontes, director of the petroleum agency.

The cost of the project was not immediately clear. U.S.-based Kosmos and Portugal’s Galp were not immediately available for comment.

The tiny African country has 19 oil blocks in its exclusive economic zone and an additional joint exploration zone with Nigeria. It is located in the resource-rich Gulf of Guinea but has yet to find any commercially viable oil.

Kosmos said on April 2 it had acquired a 65% stake in block 12, adding to stakes in three other blocks. One of those blocks, Block 6, is operated by GALP Energia.