In North America, it's fracked oil making up for declines in conventional crude oil production. Over the past 20-ish years fracked oil, which is mostly too lig

In North America, it's fracked oil making up for declines in conventional crude oil production. Over the past 20-ish years fracked oil, which is mostly too light to cost effectively refine into diesel but makes nice gasoline, has made up most of the increase in oil production. At the same time, conventional crude oil production has been slowly declining. Conventional crude is quite easy to refine into diesel. Heavy/sour crude replacements are more expensive to refine into diesel because more work is needed; also preexisting refineries need refitting to support it. The net result is that diesel production hasn't kept up with the wider percentage increase in oil-based fuels and refinement is more expensive because it's a choice between cheaper oil which requires more work and costly refits, or less-available and therefore more expensive base oil. Thusly diesel is more expensive due to cost of production and reduced relative supply.