When I read about "geologists" making these wildly optimistic forecasts about oil and gas reserves, I wonder how many of them have ever actually worked with the oil industry.

Decades ago, oil exploration was a rather random process - companies would drill where oil had already been found, and would often find that if they drilled else where that they wouldn't find oil.

Eventually a detailed understanding of the requirements for oil and gas to occur was acquired as detailed in the cartoon below.  Source rocks are one of the most important pre-requisites - if a basin / area has no source rock then there will be no oil or gas.  The source rock then has to be buried to a depth where the temperature is in the range 90 to 150 C.  Too shallow / cool - no oil or gas.  Too deep - CO2.  And so on.

However, even armed with this understanding, companies would still drill lots of dry holes - normally the result of one of these 5 factors failing.  For example, seal failure, no reservoir etc.

The bottom line is that oil and gas is hard to find - even with todays array of technology.

The reason that the oil in Saudi Arabia is all clustered in one region is that this is the area where all 5 factors have combined to produce oil fields.  It is the same in the North Sea, where fields are concentrated along the graben axes - where source rocks are buried deeply enough.

So those who look at a map of Saudi Arabia and observe that much of the area is unexplored need to be aware of the fact that geologists will have studied these areas and probably for multiple raesons decided that they are not prospective.

One of the more sobering aspects of your article is the fact that a 600 mmbbl oil field has made the news in Russia.  It is analagous to the 100 mmbbl field discovery making headlines in the UK.  The media have no sense of perspective - but unfortunately they have a lot of influence - and very short memories.