Aniya,

"I don't know" might not be very satisfying but if it's true then that's the answer. As far as "other tools" go, yes....they are available and would generate a reasonable result. But there are no tools which can make up for the lack of basic data. The KSA has a very good estimate of their remaining reserves as well as future production rates. They've had some of the best (and most expensive) reservoir modelers working on Ghawar and its other fields for decades. But they are not willing to share those answers. If they were allow others to analyze the production histories we would all know the answer. But they won't. Compare that policy to those of US public corporations who are required by law to present independent third party audits of their reserves which must conform to very strict criteria.

Ranging the risk to any value is fine but one must have a credible baseline to work with. Making a wild and unsupported estimate and then assigning a heavy risk to your answer is deceptive. The MSM and the public will generally use your answer and then conveniently forget to mention the qualifying risk. With regards to the "big picture" the KSA won't even admit how much oil has been produced from its fields since production began or even how much each produced last month. Even if you magically knew how much oil was in Ghawar initially you couldn't even tell today if it were 40% depleted or 70%.

Exactly.

Of course the executives who want to base their decision on such studies want to hear more than "I don't know", otherwise they wouldn't have hired you. Finally they want to know if they shall drill/build/invest or not, so you should give them a clear answer. But normally even under big uncertainties I always could draw conclusions from my data. These could result in a range of values and perhaps an average, and if the client preferred for exaple a conservative statement he could chose the lowest value.

Of course there should never be inconsistencies like in the WEO's chapter of Saudi Arabia:

"There are 6.2 billion barrels of reserves in 11 other known fields in Saudi Arabia that have yet to be developed — the highest in the world. Each of them holds reserves of at least 150 million barrels and the average is 450 million barrels."

Classical maths says: 11 fields x 450 million = 4.95 billion.

If these data weren't mixed up by mistake then the report could have said something like this:
The exact reserves are unknown. 6.2 billion barrels of reserves are reported in (source a), however (source b) has 11 fields of 450 million barrels each. So we estimate the total resource is probably in a range between 4.95 and 6.2 billion barrels.