Our U.S. natural gas properties primarily consist of our non-operated Marcellus shale gas interests located in northeastern Pennsylvania where we hold an interest in approximately 60,000 net acres.
Our U.S. natural gas production averaged 112,547 Mcf/day in 2013, representing approximately 39% of our total natural gas production. As a result of the acquisition of additional working interests in northeastern Pennsylvania during the fourth quarter of 2013, exit production from the Marcellus was approximately 171,083 Mcf/day.
Proved plus probable U.S. natural gas reserves were 600.9 Bcf at December 31, 2013, an increase of 168% from year-end 2012, and represented 25% of our total proved plus probable reserves.
In 2013, $79 million was invested in our interests in the Marcellus.
We plan to spend between $110 million and $130 million on drilling projects in our Marcellus properties in the northeast region of Pennsylvania in 2014. We expect Marcellus production to grow from 95 MMcf/day in 2013 to over 140 MMcf/day in 2014, which would represent approximately 40% of our anticipated natural gas production.