Reserves (gas, oil) are the amount of a subsurface resource in the Earth defined according to geological exploration data and during the development process.

Reserves are measured in volume and mass units; natural gas – in cubic meters, oil – in tons, gas condensate – in grams per cubic meter. Reserves reliability depends on the geological complexity of the structure, on the scope of geological exploration work performed and its degree of detail. Reserves are characterized by certain profitability of their extraction, processing and use, which depends on the field location, size, complexity of geological and production conditions and other natural, technical and economic conditions.

The first reserves classification in the former USSR was worked out in 1928 by the Geological Committee commission. In accordance with the confidence principle (exploration maturity) reserves were classified into 3 categories:

  • A (prepared);
  • B (explored);
  • C (probable).

Changing conditions led to periodical alteration of reserves classification. Upon that, requirements for certain reserve categories became more specific. Requirements for reserves of low categories became more stringent in order to use them in reservoir engineering and investments rationale. The role of high category reserves gradually decreased. Unexplored oil and gas resources were included into the classification. New classifications improved reserves reliability, accelerated exploration and reduced the costs of their preparation for commercial development.