More specifically, West Texas Intermediate Crude futures for early May delivery are sub-zero. Late-May delivery is more expensive.
This is in part a reflection of the fact that the oil producers have already been paid for the output and are contractually obligated to deliver it, but no one actually really has a use for it once it is there, and it will cost money to transport or store it.
It can't just be producers being contractually obligated to deliver it, can it? If that was the case, couldn't they close out their obligation by buying back some of these futures (at a profit), and "delivering" it to themselves by reducing production? So it must be that reducing production itself is not possible or too expensive for this to make sense.
This is in part a reflection of the fact that the oil producers have already been paid for the output and are contractually obligated to deliver it, but no one actually really has a use for it once it is there, and it will cost money to transport or store it.