In your opinion, which source is likely to be the most accurate in each country’s oil reserves? I see that information on this is highly variable. It just may be that it is impossible to even answer such a question.
The Bible of oil is the World Statistical Report that BP has published for about 70 years, each June. They outsourced the storage of the spreadsheet last year to something called the Energy Institute, who just publish the data BP provides.
They lay out all the methodology and how they get info from govt and “secondary sources”.
You’ll find the spreadsheet here:
https://www.energyinst.org/statistical-review/resources-and-data-downloads
Get the .xslx file, not .pdf. It’s easier to navigate.