This excerpt taken from the YPF 6-K filed Nov 3, 2009.
f. Accounting for asset retirement obligations
SFAS No. 143, Accounting for Asset Retirement Obligations (SFAS No. 143), addresses financial accounting and reporting for obligations associated with the retirement of tangible long-lived assets and the associated asset retirement cost. The standard applies to legal obligations associated with the retirement of long-lived assets that result from the acquisition, construction, development and normal use of the asset. SFAS No. 143 requires that the fair value of a liability for an asset retirement obligation be recognized in the period in which it is incurred, if a reasonable estimate of fair value can be made. The asset retirement obligations liability is built up in cash flow layers, with each layer being discounted using the discount rate as of the date that the layer was created. Measurement of the entire obligation using current discount rates is not permitted. Each cash flow layer is added to the carrying amount of the associated asset. This additional carrying amount is then depreciated over the life of the asset. The liability is increased due to the passage of time based on the time value of money (accretion expense) until the obligation is settled. The activity with respect to retirement obligations under U.S. GAAP is detailed in Note 8.c.
Argentine GAAP is similar to SFAS No. 143, except for a change in the discount rate which is treated as a change in estimates, so the entire liability must be recalculated using the current discount rate, being the change added or reduced from the related asset.