 "Reserves growth" also commonly occurs in previously existing fields, as the characteristics of the reservoir become better understood, as fields are extended laterally, or new oil and gas reservoirs are found in existing fields.Reserve growth may also take place due to technological and economic changes.In Russia, reserves categories A, B, and C correspond roughly to developed producing reserves, undeveloped reserves (development is approved), and discovered resources without a firm plan to develop yet, respectively; the designation ABC corresponds to discovered resources.