From a WSL July 10, 2014 article included in a state dept email, Hunter Biden's gas experience was aquired by joining the Burisma board.
"Hunter Biden has experience in natural gas, having joined the board of Ukrainian gas producer Burisma Holdings Ltd. earlier this year. Burisma is controlled by a former top security and energy official for deposed President Viktor Yanukovych."
https://foia.state.gov/Search/results.aspx?searchText=burisma&beginDate=&endDate=&publishedBeginDate=&publishedEndDate=&caseNumber=