Note E. Asset retirement obligations
The Company's asset retirement obligations represent the estimated present value of the estimated cash flows the Company will incur to plug, abandon and remediate its producing properties at the end of their productive lives, in accordance with applicable state laws. Market risk premiums associated with asset retirement obligations are estimated to represent a component of the Company's credit-adjusted risk-free rate that is utilized in the calculations of asset retirement obligations.
The Company's asset retirement obligation transactions during the years ended December 31, 2013, 2012 and 2011 are summarized in the table below:
| Years Ended December 31, | ||||||||||
| (in thousands) | 2013 | 2012 | 2011 | |||||||
| Asset retirement obligations, beginning of period | $ | 86,261 | $ | 59,685 | $ | 43,326 | ||||
| Liabilities incurred from new wells | 6,338 | 7,729 | 7,178 | |||||||
| Liabilities assumed in acquisitions | 593 | 29,113 | 527 | |||||||
| Accretion expense for continuing operations | 6,047 | 4,187 | 2,444 | |||||||
| Accretion expense for discontinued operations | - | 1,004 | 529 | |||||||
| Disposition of wells | - | (24,614) | (463) | |||||||
| Liabilities settled upon plugging and abandoning wells | (3,447) | (1,261) | (686) | |||||||
| Revision of estimates | 5,801 | 10,418 | 6,830 | |||||||
| Asset retirement obligations, end of period | $ | 101,593 | $ | 86,261 | $ | 59,685 | ||||