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
2007 2006*
* 2006 conformed to 2007 presentation.
Upstream $482 $461
Downstream $551 $550
Chemical and all other 171 2
Total 1,204 1,013
Less: Accumulated amortization 628 548
Net capitalized leased assets $576 $465

Rental expenses incurred for operating leases during 2007, 2006 and 2005 were as follows:

Year ended December 31
2007 2006 2005
Minimum rentals $2,419 $2,326 $2,102
Contingent rentals 6 6 6
Total 2,425 2,332 2,108
Less: Sublease rental income 30 33 43
Net rental expense $2,395 $2,299 $2,065

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, 2007, 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:
2008 $513 $103
2009 478 106
2010 430 83
2011 347 85
2012 293 91
Thereafter 1,106 347
Total $3,167 $815
Less: Amounts representing interest and executory costs (315)
Net present values 500
Less: Capital lease obligations included in short-term debt (94)
Long-term capital lease obligations $406