Fair Value Measurements

As more fully discussed in Note 13 to the Consolidated Financial Statements included in EOG's 2016 Annual Report, certain of EOG's financial and nonfinancial assets and liabilities are reported at fair value on the Condensed 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 September 30, 2017 and December 31, 2016 (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 September 30, 2017
Financial Assets:
Natural Gas Swaps	$	—	$	1	$	—	$	1
Natural Gas Options/Collars	—		3		—		3
Financial Liabilities:
Crude Oil Basis Swaps	$	—	$	5	$	—	$	5

At December 31, 2016
Financial Assets:
Natural Gas Options/Collars	$	—	$	1	$	—	$	1
Financial Liabilities:
Crude Oil Swaps	$	—	$	36	$	—	$	36
Natural Gas Swaps	—		4		—		4
Natural Gas Options/Collars	—		22		—		22

The estimated fair value of commodity derivative contracts 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 $258 million were written down to their fair value of $93 million, resulting in pretax impairment charges of $165 million for the nine months ended September 30, 2017. Included in the $165 million pretax impairment charges are $138 million of impairments of proved oil and gas properties and other property, plant and equipment for which EOG utilized an accepted offer from a third-party purchaser as the basis for determining fair value. In addition, EOG recorded pretax impairment charges of $23 million for a commodity price-related write-down of other assets.

EOG utilized average prices per acre from comparable market transactions as the basis for determining the fair value of unproved properties received in non-cash property exchanges. See Note 4.

Fair Value of Debt. At September 30, 2017 and December 31, 2016, EOG had outstanding $6,390 million and $6,990 million, respectively, aggregate principal amount of senior notes, which had estimated fair values at such dates of approximately $6,620 million and $7,190 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.