CHANGES IN STANDARDIZED MEASURE OF DISCOUNTED FUTURE NET  CASH FLOWS RELATING TO PROVED OIL AND GAS RESERVES For the years ended December 31 (Millions of dollars) 2003 2002 2001 Standardized measure of discounted futur net cash flows at beginning of year $ 7,085 $ 5,056 $ 6,828 Changes during the year Sales and transfers of oil and gas produced during year net of  production costs (2,291) (2,964) (2,840) Development costs incurred during year 1,082 1,095 1,182 Net changes in prices and production costs applicable to future production 796 5,767 (4,346) Net change in estimated future development costs (726) (546) (838) Extensions and discoveries (including improved recovery) of oil and  gas reserves, less related costs 261 287 521 Revisions of previous oil and gas reserve estimates 622 (939) 231 Purchases (sales) of minerals in-place, net (469) (247) 1,186 Accretion of discount 945 796 1,087 Net change in income taxes 72 (1,701) 1,943 Revision in rate or timing of future production and other changes (360) 481 102 Total (68) 2,029 (1,772) Standardized measure of discounted futur net cash flows at end of year $ 7,017 $ 7,085 $ 5,056 63