a barrel of oil equivalent basis, 32% of the Corporation’s December 31, 2003 worldwide proved reserves are undevel- oped. The estimates of the Corporation’s proved reserves of crude oil and natural gas (after deducting royalties and operating interests owned by others) follow: The Corporation’s net oil and gas reserves have been estimated by independent consultants DeGolyer and MacNaughton. The reserves in the tabulation below include proved undeveloped crude oil and natural gas reserves that will require substantial future development expenditures. On (a) Includes the impact of changes in selling prices on production sharing contracts with cost recovery provisions and stipulated rates of return. In 2003 such revisions were immaterial. In 2002 revisions included reductions of approximately 44 million barrels of crude oil and 26 million Mcf of natural gas relating to higher selling prices. In 2002 revisions also reGLYPH<223>ected reductions in reserves on GLYPH<222>elds acquired in the LLOG and Triton acquisitions. (b) Includes the reclassiGLYPH<222>cation of reserves to GLYPH<210>Africa, Asia and otherGLYPH<211> from GLYPH<210>Equity InvesteesGLYPH<211> as a result of the consolidation of the CorporationGLYPH<213>s interest in the JDA. (c) Includes additions and reductions to reserves from asset exchanges. (d) Includes 32% of crude oil reserves and 43% of natural gas reserves held under production sharing contracts. These reserves are located outside of the United States and are subject to different political and economic risks. (e) Excludes 443 million Mcf of carbon dioxide gas for sale or use in company operations. 61 OIL AND GAS RESERVES Crude Oil, Condensate and Natural Gas Liquids Natural Gas (Millions of barr els) (Millions of Mcf) Africa, Africa, United Asia and  Equity World- United Asia and  Equity World- States Europe other Total Investees wide States Europe other Total Investees wide Net Proved Developed and  Undeveloped Reserves At January 1, 2001 156 419 180 755 11 766 552 945 310 1,807 320 2,127 Revisions of previous  estimates 3 (1) 4 6 (1) 5 31 (25) (17) (11) 46 35 Improved recovery GLYPH<209> 34 GLYPH<209> 34 GLYPH<209> 34 GLYPH<209> 27 GLYPH<209> 27 GLYPH<209> 27 Extensions, discoveries and  other additions 9 18 8 35 GLYPH<209> 35 62 196 33 291 GLYPH<209> 291 Purchases of minerals in-place 22 1 190 213 13 226 227 GLYPH<209> 10 237 493 730 Sales of minerals in-place GLYPH<209> GLYPH<209> GLYPH<209> GLYPH<209> GLYPH<209> GLYPH<209> GLYPH<209> (1) GLYPH<209> (1) (25) (26) Production (28) (63) (18) (109) (2) (111) (155) (131) (10) (296) (7) (303) At December 31, 2001 162 408 364 934 21 955 717 1,011 326 2,054 827 2,881 Revisions of previous  estimates (a) (10) 7 (73) (76) (5) (81) (82) (16) 8 (90) (81) (171) Extensions, discoveries and  other additions 13 11 15 39 GLYPH<209> 39 69 24 31 124 3 127 Sales of minerals in-place (3) (1) (6) (10) GLYPH<209> (10) (29) (43) GLYPH<209> (72) GLYPH<209> (72) Production (24) (61) (34) (119) (2) (121) (136) (124) (15) (275) (13) (288) At December 31, 2002 138 364 266 768 14 782 539 852 350 1,741 736 2,477 Revisions of previous  estimates (a) 8 8 33 49 GLYPH<209> 49 (8) 14 (25) (19) GLYPH<209> (19) Extensions, discoveries and  other additions 1 6 4 11 GLYPH<209> 11 3 81 4 88 GLYPH<209> 88 Purchases of minerals in-place (c) 8 GLYPH<209> 14 (b) 22 (6) (b) 16 21 GLYPH<209> 1,023 (b) 1,044 (405) (b) 639 Sales of minerals in-place (c) (8) (20) (81) (109) (7) (116) (103) (13) (157) (273) (316) (589) Production (20) (53) (22) (95) (1) (96) (92) (134) (23) (249) (15) (264) At December 31, 2003 127 305 214 646 GLYPH<209> 646 (d) 360 (e) 800 1,172 2,332 GLYPH<209> 2,332 (d) Net Proved Developed Reserves At January 1, 2001 140 353 80 573 9 582 476 842 111 1,429 199 1,628 At December 31, 2001 144 318 196 658 7 665 580 709 111 1,400 220 1,620 At December 31, 2002 113 294 140 547 8 555 450 631 154 1,235 221 1,456 At December 31, 2003 105 249 111 465 GLYPH<209> 465 297 518 633 1,448 GLYPH<209> 1,448