Fair Value Measurements

As more fully discussed in Note 13 to the Consolidated Financial Statements included in EOG's 2017 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 June 30, 2018 and December 31, 2017 (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 June 30, 2018
Financial Assets:
Natural Gas Swaps	$	—	$	—	$	—	$	—
Natural Gas Options	—		2		—		2
Crude Oil Basis Swaps	—		83		—		83
Financial Liabilities:
Crude Oil Swaps	$	—	$	261	$	—	$	261

At December 31, 2017
Financial Assets:
Natural Gas Swaps	$	—	$	2	$	—	$	2
Natural Gas Options/Collars	—		6		—		6
Financial Liabilities:
Crude Oil Swaps	$	—	$	38	$	—	$	38
Crude Oil Basis Swaps	—		19		—		19

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 $163 million were written down to their fair value of $131 million, resulting in pretax impairment charges of $32 million for the six months ended June 30, 2018. Included in the $32 million pretax impairment charges are $21 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 June 30, 2018 and December 31, 2017, EOG had outstanding $6,390 million aggregate principal amount of senior notes, which had estimated fair values at such dates of approximately $6,419 million and $6,602 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.