This term refers to the net amount at which an asset (reserves of oil and gas in this case) is carried on the books of account. Under SEC rules oil and gas companies are only allowed to book reserves from oil and gas finds that are considered readily available or from conventional sources. In addition, they are required to report their reserves on the basis of the price of oil and gas that they are selling or plan to sell as of the last day of the year for which they are reporting. US companies cannot book reserves until they have demonstrated that the reserves are marketable. Many international companies have more latitude