Brunei is wealthy largely because of its petroleum resources.
Petroleum in this context includes crude oil and the natural gas often produced alongside it. Brunei, officially Brunei Darussalam, sits on the northern coast of Borneo and has substantial offshore hydrocarbon reserves. Revenue from extracting and exporting these resources has funded the country’s high incomes, public services, and extensive welfare system.
Oil exploration began in the late nineteenth century, and commercial production developed during the twentieth century. Brunei Shell Petroleum, a partnership between the Brunei government and Shell, became the country’s principal oil and gas producer. Liquefied natural gas is also a major export, so “natural gas” is a defensible related answer—but the stored answer, petroleum, is the broader canonical term.
Brunei’s dependence on hydrocarbons also creates an economic challenge. Oil and gas prices and production levels can strongly affect national revenue, which is why the government has pursued diversification through sectors such as downstream energy, tourism, food, technology, and services.