The world’s largest oil companies are reporting underwhelming first-quarter profits as an array of geopolitical challenges and weaker prices around the world slowed recent progress in generating excess cash and crimped margins for processing oil into fuel. Sanctions in Venezuela, production cuts in Canada and lower natural-gas prices in Asia took a toll on Exxon Mobil Corp., Chevron Corp. and other companies. The business of refining crude, one of the most reliable profit centers in the industry during the last five years,…