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 2-million-barrel cargo to India was destined to produce more than a million tons of planet-warming carbon over its lifecycle, from well to tailpipe, the Texas-based driller said it had completely offset that impact by purchasing carbon credits.

Such credits are financial instruments generated by projects that reduce or avert greenhouse gas emissions such as mass tree plantings or solar power farms. The projects’ owners can sell the credits to polluting companies, who then use them to make claims of offsetting their carbon emissions.