From 10-K:

As of December 31, 2008, our total estimated proved reserves were 103.6 MMBoe, of which approximately 75 percent were natural gas and
25 percent were crude oil. As of December 31, 2007, our total estimated proved reserves were 142.2 MMBoe, of which approximately 59
percent were natural gas and 41 percent were crude oil. The decrease in reserves was primarily due to lower commodity prices at the end of 2008
($45 per Bbl for oil and $5.62 per Mcf for natural gas) compared to prices at the end of 2007 ($96 per Bbl for oil and $7.48 per Mcf for natural
gas). During 2008, we added proved reserves totalling 8.2 MMBoe from additions. This equates to 121 percent of our production for 2008. The
reserve additions were offset by negative economic and technical revisions of 40.5 MMBoe.

Question:
How come, when the price goes down, "proved" reserves disappear?