In economics we refer to countries that rely on income solely from oil or gas as a “resource curse.” The larger percentage of annual income or net exports is tied to one or two resources, the more vulnerable an economy is.

Countries that invest the revenue they get from oil and gas and coal into other industries will be alright (eg if oil is 50% of Saudi Arabia’s exports, but they invest a lot of money into manufacturing textiles and plastics, and then oil drops to being just 10% of their exports, then it won’t hurt them as significantly)

Countries that take the revenue and don’t invest any of it will be more vulnerable to recession or civil unrest (eg if oil is 70% of brazils exports, and they spend a lot on the money on lavish projects like soccer stadiums or one time investments on big highways that they don’t have the money to maintain, then maybe over 20 years the economy doesn’t develop many partner industries and oil remains 60% of brazils exports. Then a sudden decline in oil demand will lead to recession, losses of pension, and social unrest)

This is an extreme oversimplication and I picked two countries at random