The ananlysis of petroleum systems explains the current distribution of oil and gas fields and permits the appraisal of undrilled prospects when associated with other geological technical tools. Assuming the existence of a good size trap , with an adequate reservoir, efficint cap rock and well -defined geometry the basic question to be applied in all cases is whether the trap will be water , gas or oil bearig and what kind and amount of oil light or heavy it will contain. that is the question I will address in the first part of my talk . in the second part we will look at a few ways to increase the probability of discovery by improving our methods. to illustrate my point i will review the two most prolific petroleum systems of the zagros foldbelt and its contiguous offshore namely the palaeozoic petroleum system that caused the deposition of the huge gas reserves discovered in permo-triassic carbonate reservoirs dalan and kangan formations in coastal fars and offshore , and the middle cretaceous to early mioicens system that charged the asmari and bangestan reservoirs of the dezful embayment giant anticlines representing som 90% of iran oil reserves.