The increase in year-end 2012 estimated proved reserves is primarily a result of our horizontal development project in the Wolfcamp oil shale resource play. Year-end 2012 estimated proved reserves included 60.1 MMBoe attributable to the Wolfcamp shale play, compared to 24.2 MMBoe at year-end 2011, representing a 149% increase.The increase in proved reserves was partially offset by the reclassification of 8.9 MMBoe of proved undeveloped reserves to probable undeveloped. These reserves are attributable to vertical Canyon locations in southeast Project Pangea. Due to our horizontal Wolfcamp development project, including pad drilling, postponement of these deeper locations beyond five years from initial booking is necessary to integrate their development with the shallower Wolfcamp and Wolffork zones. As a result of lower natural gas and NGL prices during 2012, we also recorded 2.4 MMBoe of price revisions. The following table summarizes the changes in our estimated proved reserves during 2012.
|Oil (MBbl)||NGLs (MBbl)||Natural Gas (MMcf)||Total (MBoe)|
|Balance – December 31, 2011||18,051||29,123||178,807||76,975|
|Extensions and discoveries||21,993||8,639||49,372||38,861|
|Balance – December 31, 2012||37,252||29,100||174,760||95,479|
|Proved developed reserves at December 31, 2012||8,816||11,761||73,178||32,774|