11. Fair Value Measurements

As more fully discussed in Note 13 to the Consolidated Financial Statements included in EOG's 2014 Annual Report, certain of EOG's financial and nonfinancial assets and liabilities are reported at fair value on the Consolidated Balance Sheets. The following table provides fair value measurement information within the fair value hierarchy for certain of EOG's financial assets and liabilities carried at fair value on a recurring basis at March 31, 2015 and December 31, 2014 (in millions):



	Fair Value Measurements Using:
	Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
At March 31, 2015
Financial Assets
Natural Gas Options/Swaptions	$	—	$	86	$	—	$	86
Crude Oil Swaps	—		103		—		103
Crude Oil Options/Swaptions	—		141		—		141

At December 31, 2014
Financial Assets
Natural Gas Options/Swaptions	$	—	$	100	$	—	$	100
Crude Oil Swaps	—		121		—		121
Crude Oil Options/Swaptions	—		244		—		244

The estimated fair value of crude oil and natural gas derivative contracts (including options/swaptions) was based upon forward commodity price curves based on quoted market prices. Commodity derivative contracts were valued by utilizing an independent third-party derivative valuation provider who uses various types of valuation models, as applicable.

The initial measurement of asset retirement obligations at fair value is calculated using discounted cash flow techniques and based on internal estimates of future retirement costs associated with property, plant and equipment. Significant Level 3 inputs used in the calculation of asset retirement obligations include plugging costs and reserve lives. A reconciliation of EOG's asset retirement obligations is presented in Note 6.

Proved oil and gas properties and other assets with a carrying amount of $7 million were written down to their fair value of $2 million, resulting in pretax impairment charges of $5 million for the three months ended March 31, 2015. Significant Level 3 inputs associated with the calculation of discounted cash flows used in the impairment analysis include EOG's estimate of future crude oil and natural gas prices, production costs, development expenditures, anticipated production of proved reserves, appropriate risk-adjusted discount rates and other relevant data.

Fair Value of Debt. At March 31, 2015 and December 31, 2014, EOG had outstanding $6,890 million and $5,890 million, respectively, aggregate principal amount of debt, which had estimated fair values of approximately $7,370 million and $6,242 million, respectively. The estimated fair value of debt was based upon quoted market prices and, where such prices were not available, other observable (Level 2) inputs regarding interest rates available to EOG at the end of each respective period.