Where is this from? Which country does it refer to?
I suspect "endowment" refers to a sum of money, and if the country is an oil-producing nation, then it could well be money the country has from selling hydrocarbons (oil and gas), similar in nature, perhaps, to Norway's sovereign wealth fund that was created so the entire country could benefit from the oil reserves discovered off the Norwegian coast.
"Hydrocarbon endowment" is a phrase intended to imply that the hydrocarbons in a country are a form of wealth that has been inherited by the country.
This is a technical term used in geology to describe hydrocarbon deposits in geological formations, and how they may be distributed in the structures. Recently the phrase has started to be used more widely in non-technical contexts.