Dear (forget my nationality for few moments) please just ponder over the links which you provided.
They mentions Shell and BP are amongst the top customers/consumers of the oil (petro products).
They don't mention anywhere that they are running oil extraction/development/exploration programme in Iran.
Buying crude oil is different and running oil/gas exploration is a a different game. In the first, money would be paid (remittances) after the oil is delivered/ or advance remittance on the basis of pro-forma invoice. But, again the amount shall be limited to the consignment amount etc.
Running an exploration program is in altogether different ballpark. To begin with, In first stage, billions would be invested to locate the oil/gas wells and than billions would be spend on erecting the required infrastructure to harness the oil (if you succeeded to find oil in the first place else kiss your billions goodbye).
Even break even is usually years away. Needless to say, actual managers and planners who conceive/run these programmes are usually paranoid and take every precaution to ensure profitability/viability.
Though as a token of thanks, for refusing to bomb Yemen (or in other words for not poking their nose in the matters which do not concern them), Iranians should perhaps let Pakistan develop these oil fields......