Chapman, who has helped build BG from a UK-focused player into a $74bn (£46.6bn) international oil and gas giant, will step down before the end of 2013, the group said.
Chris Finlayson, managing director for Europe and central Asia, chief financial officer Fabio Barbosa and Martin Houston, managing director for the Americas and head of liquefied natural gas, will be given additional duties to test their suitability for the top role, a BG spokesman said.
“We have identified three internal candidates... This is all part of an orderly succession plan,” he said.
Finlayson will take over BG Advance, the unit responsible for BG’s exploration strategy, capital projects and IT & technology capability.
Barbosa, who replaced the man who was previously seen as Chapman’s likely successor, Ashley Almanza, less than a year ago, will also work on strategy and portfolio development.
Martin Houston will become chief operating officer in addition to his existing roles.