It's far from clear how much oil they even sold to Exxon.
Yes it is far from clear .....
Well you could always sort of look at Exxon refineries that deal with Venezuelan oil to give you some sort of idea , though of course that wouldn't work since there would be that little problem called joint ownership of the facilities , in which case to get the Exxon total you would have to subtract the partners proportion from the total amount of oil .
The partner in this case being errrrr....Venezuelas national oil company .