Suncor Energy Inc. is close to acquiring Petro-Canada for about $15 billion in stock, uniting two of Canada's biggest oil companies at a time of tepid world demand for oil, according to people familiar with the matter.

A price of $15 billion would represent a roughly 30% premium for Petro-Canada. The all-stock deal would allow the companies to merge while conserving cash, a sign of the pressure some oil producers face amid the plunge in crude prices since last summer.

Spokeswomen for both companies declined to comment. A deal announcement was expected to come as early as Monday, though the ...