An oil-exploration ship contracted by Exxon Mobil suspended operations off the coast of Guyana over the weekend after it was approached by a Venezuelan navy vessel in a renewal of a century-old border dispute.

The Venezuelan navy did not board the ship, a Norwegian vessel that was conducting seismic testing in Guyanese waters 90 miles from the internationally recognized border Saturday morning. The ship sailed away to safer waters.

The episode occurred only hours after the shaky Guyanese coalition government lost a no-confidence vote in the National Assembly late Friday, beginning what promises to be a volatile national election campaign. Meanwhile, President David A. Granger is battling cancer as his government falters.

Guyana has become one of the hottest oil patches in the hemisphere since Exxon Mobil started finding oil in 2015, which has so far amounted to five billion barrels of reserves. The discoveries have rekindled charges by Venezuela that it was cheated by an 1899 Paris arbitration tribunal and that Caracas rightfully claims nearly two-thirds of its English-speaking neighbor.