An all-stock deal could be announced as soon as Wednesday, the person said.
Bloomberg News reported about the deal earlier. Pioneer and Exxon did not immediately respond to a Reuters request for comment.
Pioneer, which closed at $237.41 on Tuesday at a market valuation of about $55.35 billion, is the third-largest oil producer in the Permian basin after Chevron Corp (NYSE:CVX) and ConocoPhillips (NYSE:COP).
Exxon in talks to pay over $250 per share for Pioneer