The uncertainty depends chiefly on the amount of reliable geologic and engineering data available at the time of the estimate and the interpretation of these data.
The relative degree ofuncertainty may be conveyed by placing reserves into one of 2 principal classifications, either proved or unproved.
Reserve status categories define the development and producing status of wells and reservoirs:
- Developed reserves are expected to be recovered from existing wells including reserves behind pipe
- Producing reserves subcategorized as producing are expected to be recovered from completion intervals which are open and producing at the time of the estimate
- Non-producing reserves subcategorized as non-producing include shut-in and behind-pipe reserves
- Undeveloped reserves are expected to be recovered from new wells on undrilled acreage, from deepening existing wells to a different reservoir or where a relatively large expenditure is required to (a) recomplete an existing well or (b) install production or transportation facilities for primary or improved recovery projects
Nearly 80% of the world’s oil reserves are in the Organization of the Petroleum Exporting Countries (OPEC).
The top 5 countries in the world by oil reserves are Venezuela, Saudi Arabia, Canada, Iran and Iraq.