The loonie was trading 0.5pc lower at 1.2621 to the greenback, or 79.23 U.S. cents, having touched its weakest intraday level since March 31 at 1.2634.
The DoublePoint Energy deal is Pioneer's second big purchase and the fourth multi-billion shale deal this year. In January, Pioneer closed a $4.5 billion, all-stock purchase of Parsley Energy, giving it one of the largest positions in the Permian Basin, the top US shale field.