Washington/Oslo/Winnipeg — In January, Occidental Petroleum announced it had accomplished something no oil company had done before: It sold a shipload of crude that it said was 100% carbon-neutral.

While the two-million-barrel cargo to India was destined to produce more than a million tonnes of planet-warming carbon over its life cycle, from well to tailpipe, the Texas-based driller said it had completely offset that impact by purchasing carbon credits under a UN-sponsored program called Corsia...