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Inventories
6 Months Ended
Jun. 30, 2020
Inventory Disclosure [Abstract]  
Inventories Inventories
Inventories consisted of the following:

 Millions of Dollars
 June 30
2020
December 31
2019
Crude oil and petroleum products$4,806  3,452  
Materials and supplies330  324  
$5,136  3,776  
Inventories valued on the last-in, first-out (LIFO) basis totaled $4,701 million and $3,331 million at June 30, 2020, and December 31, 2019, respectively. The estimated excess of current replacement cost over LIFO cost of inventories amounted to approximately $1.4 billion and $4.3 billion at June 30, 2020, and December 31, 2019, respectively.