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Supplemental information on oil and gas producing activities (unaudited) - Reserve information (Details) - gal
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Supplemental information on oil and gas producing activities (unaudited)      
Opening balance 2,011 2,002 1,770
Proved reserves, Revisions of previous estimates [1] 9 63 315
Proved reserves, Improved recovery 93 81 139
Proved reserves, Purchases 0 48 0
Proved reserves, Extensions and discoveries 17 57 12
Proved reserves, Sales 0 0 (3)
Proved reserves, Production (247) (240) (231)
Closing balance 1,883 2,011 2,002
Proved developed reserves: Opening balance 1,376 1,370 1,297
Proved developed reserves: Closing balance 1,435 1,376 1,370
Proved undeveloped reserves: Opening balance 635 632 473
Proved undeveloped reserves: Closing balance 448 635 632
Gas      
Supplemental information on oil and gas producing activities (unaudited)      
Opening balance 2,828 3,151 2,921
Proved reserves, Revisions of previous estimates [1] (165) (104) 431
Proved reserves, Improved recovery 9 21 107
Proved reserves, Purchases 0 50 0
Proved reserves, Extensions and discoveries 0 33 0
Proved reserves, Sales 0 0 (4)
Proved reserves, Production (326) (323) (304)
Closing balance 2,346 2,828 3,151
Proved developed reserves: Opening balance 2,174 2,561 2,636
Proved developed reserves: Closing balance 2,007 2,174 2,561
Proved undeveloped reserves: Opening balance 654 590 285
Proved undeveloped reserves: Closing balance 339 654 590
Oil      
Supplemental information on oil and gas producing activities (unaudited)      
Opening balance 1,515 1,449 1,257
Proved reserves, Revisions of previous estimates [1] 38 81 240
Proved reserves, Improved recovery 91 77 120
Proved reserves, Purchases 0 39 0
Proved reserves, Extensions and discoveries 17 52 12
Proved reserves, Sales 0 0 (3)
Proved reserves, Production (190) (183) (177)
Closing balance 1,471 1,515 1,449
Proved developed reserves: Opening balance 995 921 834
Proved developed reserves: Closing balance 1,083 995 921
Proved undeveloped reserves: Opening balance 520 528 423
Proved undeveloped reserves: Closing balance 388 520 528
[1] Represents changes in previous proved reserves, upward or downward, resulting from new information (except for an increase in a proved area), usually obtained from development drilling and production history or result from changes in economic factors.