6. In the 21st century, prior to the secession of South Sudan, what had been Sudan's most valuable known natural resource?
From Quiz The Geography of Sudan
Answer:
Petroleum
Petroleum, or crude oil, had become Sudan's major export commodity and largest natural resource. Following oil exploration in the 1970s and 1980s, often interrupted by civil war in Sudan, the country began exporting oil products in 1999. Oil became the dominant export, accounting for at least two-thirds of export revenues. Estimates vary greatly, but by 2009, the Sudanese government estimated that oil production was up to about 520,000 barrels a day. Oil reserves have been estimated at five billion barrels. Sudan also has extensive natural gas reserves, as well as other natural resources such as uranium, chromium, gold, silver, manganese, lead, zinc, and copper. However, South Sudan, which seceded from Sudan in 2011, has about 80 percent of the oil reserves. Sudan's economy has suffered greatly from this change.