Certain noncancelable leases are classified as capital leases, and the leased assets are included as part of "Properties, plant and equipment, at cost." Such leasing arrangements involve tanker charters, crude oil production and processing equipment, service stations, office buildings, and other facilities. Other leases are classified as operating leases and are not capitalized. The payments on such leases are recorded as expense. Details of the capitalized leased assets are as follows:

At December 31
2008 2007*
Upstream $491 $482
Downstream $399 $551
Chemical and all other 171 171
Total 1,061 1,204
Less: Accumulated amortization 522 628
Net capitalized leased assets $539 $576

Rental expenses incurred for operating leases during 2008, 2007 and 2006 were as follows:

Year ended December 31
2008 2007 2006
Minimum rentals $2,984 $2,419 $2,326
Contingent rentals 6 6 6
Total 2,990 2,425 2,332
Less: Sublease rental income 41 30 33
Net rental expense $2,949 $2,395 $2,299

Contingent rentals are based on factors other than the passage of time, principally sales volumes at leased service stations. Certain leases include escalation clauses for adjusting rentals to reflect changes in price indices, renewal options ranging up to 25 years, and options to purchase the leased property during or at the end of the initial or renewal lease period for the fair market value or other specified amount at that time.

At December 31, 2008, the estimated future minimum lease payments (net of noncancelable sublease rentals) under operating and capital leases, which at inception had a noncancelable term of more than one year, were as follows:

At December 31
Operating Leases Capital Leases
Year:
2009 $503 $97
2010 463 77
2011 372 77
2012 315 84
2013 288 59
Thereafter 947 154
Total $2,888 $548
Less: Amounts representing interest and executory costs (110)
Net present values 438
Less: Capital lease obligations included in short-term debt (97)
Long-term capital lease obligations $341