Question for Ya or similar, page-2

Oil migration into the upper targets from below is fault dependent. If there are no hydrocarbons in the upper targets it either means there are no hydrocarbons below, that the faults weren't suitable for oil migration into those traps, that the traps don't have proper seals (although they'd probably find residual oil in that case) or that the reservoir is not what they expected.
In the sense that a best case scenario has oil in all targets having migrated from below, then the chance of that is indeed lowered. In the sense that oil in the upper targets is diagnostic of oil in the bottom targets then not having that diagnostic means the risk is higher.

However as the main proportion of oil is in the bottom targets that are more ideally placed for charge, then whether it's a buying opportunity rests on what the price falls to vs what people consider the new risk to reward ratio to be. Given that a third of the potential oil is in the upper targets the SP should drop at least that amount in theory. The chance of success also changes as the upper targets had the higher COS's.
I'm not going to suggest that the SP behave rationally according to the maths given it's barely behaving normally at all, but with two thirds of the potential oil in the lower targets at some point the SP becomes a buying opportunity.
I have to add though, that to me the maths says it's a buy just for the lower targets right now at the current price let alone after a duster in the upper targets so it's all very wishy-washy and hand wavy.

Ya would probably be able to enlighten on how much greater risk there is if there is nothing in the upper targets.