b'Changes in the Standardized Measure for Discounted Cash FlowsEquity MethodInvestmentU.S. Gulf of EquatorialGhana Mexico Guinea Total(In millions)Balance at December 31, 2015 . . . . . . . . . . . . . . . . . . . . . . $1,169 $$$1,169Sales and transfers 2016 . . . . . . . . . . . . . . . . . . . . . . . . . (191) (191)Net changes in prices and costs . . . . . . . . . . . . . . . . . . . . (653) (653)Previously estimated development costs incurred duringthe period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 225Net changes in development costs . . . . . . . . . . . . . . . . . . 4 4Revisions of previous quantity estimates . . . . . . . . . . . . . . 65 65Net changes in Ghanaian tax expenses(1) . . . . . . . . . . . . . 143 143Accretion of discount . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 145Changes in timing and other . . . . . . . . . . . . . . . . . . . . . . (61) (61)Balance at December 31, 2016 . . . . . . . . . . . . . . . . . . . . . . $ 846 $$$ 846Purchase of minerals in place . . . . . . . . . . . . . . . . . . . . . 146 146Sales and transfers 2017 . . . . . . . . . . . . . . . . . . . . . . . . . (451)(16) (467)Extensions and discoveries . . . . . . . . . . . . . . . . . . . . . . . . 21 21Net changes in prices and costs . . . . . . . . . . . . . . . . . . . . 485 485Previously estimated development costs incurred duringthe period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6Net changes in development costs . . . . . . . . . . . . . . . . . . (388) (388)Revisions of previous quantity estimates . . . . . . . . . . . . . . 415 415Net changes in tax expenses(1) . . . . . . . . . . . . . . . . . . . . (8) (8)Accretion of discount . . . . . . . . . . . . . . . . . . . . . . . . . . . 98Changes in timing and other . . . . . . . . . . . . . . . . . . . . . . (53)Balance at December 31, 2017 . . . . . . . . . . . . . . . . . . . . . . $ 971 $$ 130 $1,101Purchase of minerals in place . . . . . . . . . . . . . . . . . . . . .1,4871,487Sales and transfers 2018 . . . . . . . . . . . . . . . . . . . . . . . . . (545) (117) (287) (949)Extensions and discoveries . . . . . . . . . . . . . . . . . . . . . . . .Net changes in prices and costs . . . . . . . . . . . . . . . . . . . . 1,154408 1,562Previously estimated development costs incurred duringthe period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 105Net changes in development costs . . . . . . . . . . . . . . . . . . 18129 210Revisions of previous quantity estimates . . . . . . . . . . . . . . 485574 1,059Net changes in tax expenses . . . . . . . . . . . . . . . . . . . . . . . (565)(136) (701)Accretion of discount . . . . . . . . . . . . . . . . . . . . . . . . . . . 11230 142Changes in timing and other . . . . . . . . . . . . . . . . . . . . . . (358)(357) (715)Balance at December 31, 2018 . . . . . . . . . . . . . . . . . . . . . . $1,540 $1,370 $ 391 $3,301(1) The Company was a tax exempt company incorporated pursuant to the laws of Bermuda atDecember 31, 2017 and 2016. The Company was not subject to future income tax expense relatedto its proved oil and gas reserves levied at a corporate parent level. Accordingly, the CompanysStandardized Measure for the years ended December 31, 2017 and 2016, respectively, only reflectthe effects of future tax expense levied at an asset level.156'