U.S.coal producers have written off nearly $20 billion in asset value from theirbooks since coal markets began to tank several years ago, reflecting the loftyexpectation coal executives had at the time that never materialized.

Accordingto an S&P Global Market Intelligence analysis of producers' financialstatements, the lion's share of the charge-offs occurred in 2015, a year whereseveral major producers under assault from market and regulatory forces filedfor bankruptcy courtprotection. A sizeable chunk of write-offs also in 2012 when metallurgical coalmarkets shrank a year after several large producers made major in the space.

,which bet big onthe met coal market when it purchased Central Appalachian producer Massey Energy for$8.5 billion in 2011, wrote off the largest amount of assets among covered U.S.producers. Alpha, which filedfor bankruptcy in 2015, has written-off $9.4 billion in asset value since 2010,including $5.33 billionin 2015 alone.

, which made asimilar bet on met coal markets, wrote off $4.02 billion between 2010 and 2015.And Peabody EnergyCorp., the nation's largest producer, has written off $2.89 billionin value over that time. Most of Peabody's impairments were in 2015.