The nomination of Exxon Mobil's chief executive for a top post in president-elect Donald Trump's cabinet is casting a fresh spotlight on U.S. oil investments in some of Africa's most corrupt and authoritarian regimes.

Rex Tillerson, whose 41-year career at Exxon culminated in a decade as CEO until his resignation at the end of last month, has been nominated as secretary of state in the administration of Mr. Trump, who takes office on Friday.

Under his leadership, Exxon has invested heavily in the oil-rich countries of Angola, Equatorial Guinea, Chad and Nigeria. In each of those countries, human-rights groups have questioned Exxon's murky dealings with notoriously corrupt governments.

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Of the four Sub-Saharan African countries where Exxon has major investments, only Nigeria allows free and fair elections. The others are dominated by an autocratic leader or ruling party that tightly controls the election process, allowing no real opposition. And in all four countries, the oil wealth has generally benefited a small coterie of powerful insiders, especially those in the government's inner circle, while the rest of the population remains in poverty.

Mr. Trump rarely speaks about Africa, and his policies toward the continent are unclear. But some analysts believe the Tillerson nomination is a foreshadowing of a Trump policy that will emphasize U.S. commercial and security interests in Africa, at the expense of democracy and good governance.

At his Senate nomination hearing last week, Mr. Tillerson said little about Africa. He spoke briefly about U.S. foreign aid for Africa, heaping praise on a U.S. program known as PEPFAR – the President's Emergency Plan for AIDS Relief. Since its creation in 2003, PEPFAR has provided life-saving drugs to millions of Africans who could otherwise not afford the medicine.

But a list of Africa-related questions to the State Department, submitted by the Trump transition team, is highly negative in its tone about PEPFAR and other U.S. aid programs. The list, reported by The New York Times, asks: "Is PEPFAR worth the massive investment when there are so many security concerns in Africa? Is PEPFAR becoming a massive, international entitlement program?"

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Analysts based in Africa have voiced concern that the Trump administration could neglect Africa, cutting U.S. aid programs and tearing up trade agreements that benefit Africa, while turning a blind eye to regimes that entrench corruption and undermine democracy.

Civil-society groups such as Oxfam and Global Witness have criticized Mr. Tillerson's record in Africa. His company's "questionable transactions" in several African nations "have contributed to entrenching poverty, fuelling instability and violating human rights," said a statement this month by Global Witness, an independent London-based group that investigates the natural-resources sector.

Perhaps the best example of U.S. business support for an African dictatorship is in Equatorial Guinea, the third-biggest oil producer in Sub-Saharan Africa, where Exxon is the biggest oil investor. The country is sometimes called the "Kuwait of Africa," with the richest per-capita income in the continent, and the World Bank classifies it as an "upper-middle income" nation – yet the vast majority of its population lives in desperate poverty, with an income of less than $2 a day and one of the world's worst life expectancies, because the oil money is concentrated in the hands of a single family.

Equatorial Guinea has been ruled by the same dictator, Teodoro Obiang Nguema Mbasogo, for more than 37 years. His son, Teodorin Obiang, is the vice-president, and has become fabulously wealthy, with a lavish lifestyle that includes Bugatti and Ferrari sports cars, private jets, yachts, Paris and Malibu mansions, French Impressionist paintings, and a crystal-studded glove that once belonged to Michael Jackson. He went on trial in France this month on corruption charges for allegedly looting nearly $115-million (U.S.) from his own country.

Yet throughout all this, Exxon has continued to do a profitable business in Equatorial Guinea, recently signing a new oil-exploration deal with the country.