COMSTOCK RESOURCES, INC. AND SUBSIDIARIES NOTES TO CONSOLIDATED FINANCIAL STATEMENTS — (Continued) The following table summarizes the changes in the Company’s total estimated liability: Reserve for Future Abandonment Costs at beginning of the year ..... New wells placed on production ........................... Changes in estimates .................................... Liabilities settled and assets disposed of .................... Accretion expense ...................................... Reserve for Future Abandonment Costs at end of the year .......... The Company has stock-based employee compensation plans under which stock awards, comprised of restricted stock, stock options and performance share units, are issued to employees and non-employee directors. The Company follows the fair value based method in accounting for equity-based compensation. Under the fair value based method, compensation cost is measured at the grant date based on the fair value of the award and is recognized on a straight-line basis over the award vesting period. Excess tax benefits on stock-based compensation are recognized as an adjustment to additional paid-in capital and as a part of cash flows from financing activities. The Company presently operates in one business segment, the exploration and production of oil and natural gas. The Company accounts for derivative financial instruments (including certain derivative instruments embedded in other contracts) as either an asset or liability measured at its fair value. Changes in the fair value of derivatives are recognized currently in earnings unless specific hedge accounting criteria are met. The Company estimates fair value based on a discounted cash flow model. The fair value of derivative contracts that expire in less than one year are recognized as current assets or liabilities. Those that expire in more than one year are recognized as long-term assets or liabilities. If the derivative is designated as a cash flow hedge, changes in fair value are recognized in other comprehensive income until the hedged item is recognized in earnings. In 2013, the Company had two purchasers of its oil and natural gas production that accounted for 51% and 36% of total oil and gas sales. In 2012, the Company had two purchasers of its oil and natural gas production that accounted for 42% and 27% of total oil and gas sales. In 2011, the Company had two purchasers of its oil and natural gas production that accounted for 49% and 14% of total oil and gas sales. The loss of any of these customers would not have a material adverse effect on the Company as there is an available market for its oil and natural gas production from other purchasers. F-14 Major Purchasers Derivative Financial Instruments and Hedging Activities Segment Reporting Stock-based Compensation 2011 2012 2013 (Inthousands) $ 6,674 $13,256 $16,387 417 833 1,083 5,839 2,900 (3,324) (56) (1,281) (558) 382 679 946 $13,256 $16,387 $14,534