Our U.S. natural gas production is primarily derived from our Marcellus shale gas properties in northeast Pennsylvania. These properties averaged 83 MMcf/day of natural gas during the third quarter, well ahead of our planned 2013 exit rate of 75 MMcf/day. We continue to be encouraged by strong well performance. We invested $23 million in the Marcellus during the third quarter, and year-to-date funds flow has increased significantly to approximately $48 million. Well costs have also improved, declining approximately 20% from our original budget expectations.
In November, we added additional interests in our core Marcellus properties for approximately US$153 million before closing adjustments. The acquisition includes 17,000 net acres of land in existing leases in the northeast region of Pennsylvania with approximately 42 MMcf/day of natural gas production.
We now have about 60,000 net non-operated acres in the Marcellus, and the region we are focused on has some of the best prospectivity for natural gas across the entire play.
We plan to spend approximately $80 million with our partners in our Marcellus properties in the northeast region of Pennsylvania in 2013. This drilling program is expected to result in the majority of our core non-operated acreage to be held by production by the end of 2013.
We have approximately 1.3 trillion cubic feet of best estimate contingent resources associated with our acreage position in the Marcellus at December 31, 2012.