HOUSTON, Dec 7 (Reuters) - Global crude-oil refiners will not be able to follow a single roadmap to lower carbon emissions, and each region will adopt its own approach, energy company executives said at a Houston conference on Tuesday. Refiners could be among the most exposed to the industry shift to cleaner fuel, with higher costs for emissions controls coupled to declining demand for transportation fuels as more vehicles shift to electricity. “We need a unique road map based on our region,” said Anibor Kragha, executive secretary of the African Refiners and Distributors Association, at a panel during the World Petroleum Congress. African refineries, he said, account for a far smaller amount of greenhouse gases than do refineries in the United States and Europe. “There is no one size fits all,” agreed Mark Nelson, a Chevron Corp (CVX.N) executive vice president and head of its downstream and chemicals business. Future refineries must be designed to produce fuels from plants, animal fats and recycled plastics as well as the century-old process of breaking down crude oil into gasoline and diesel, Nelson said. “The integration will get more sophisticated,” he said. Increased digitalizaton of refineries could lead to reduced numbers of maintenance workers in the plants, said Gabriel Szabo, executive vice president for downstream with Hungarian oil and gas company MOL Europe.