Definitions, Sources and Explanatory Notes

 Category:   Crude Reserves & Production
 Topic:   Crude Oil Proved Reserves


Key Terms Definition
Acquisitions The volume of proved reserves gained by the purchase of an existing fields or properties, from the date of purchase or transfer.
Adjustments The quantity which preserves an exact annual reserves balance within each State or State subdivision of the following form:

Adjustments + Revision Increases - Revision Decreases - Sales + Acquisitions + Extensions + New Field Discoveries + New Reservoir Discoveries in Old Fields - Report Year Production = Published Proved Reserves at End of Report Year

These adjustments are the yearly changes in the published reserve estimates that cannot be attributed to the estimates for other reserve change categories because of the survey and statistical estimation methods employed. For example, variations as a result of changes in the operator frame, different random samples or imputations for missing or unreported reserve changes, could contribute to adjustments.
Crude Oil A mixture of hydrocarbons that exists in the liquid phase in natural underground reservoirs and remains liquid at atmospheric pressure after passing through surface separating facilities. Crude oil may also include:

  • Small amounts of hydrocarbons that exist in the gaseous phase in natural underground reservoirs but are liquid at atmospheric pressure after being recovered from oil well (casinghead) gas in lease separators, and that subsequently are comingled with the crude stream without being separately measured.
  • Small amounts of nonhydrocarbons produced with the oil.

  • When a State regulatory agency specifies a definition of crude oil which differs from that set forth above, the State definition is to be followed.
    Extensions and Discoveries Additions to a county's proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reserves through additional drilling in periods subsequent to discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields.
    Extensions The reserves credited to a reservoir because of enlargement of its proved area. Normally the ultimate size of newly discovered fields, or newly discovered reservoirs in old fields, is determined by wells drilled in years subsequent to discovery. When such wells add to the proved area of a previously discovered reservoir, the increase in proved reserves is classified as an extension.
    New Field Discoveries The volumes of proved reserves of crude oil, natural gas and/or natural gas liquids discovered in new fields during the report year.
    New Reservoir Discoveries in Old Fields The volumes of proved reserves of crude oil, natural gas, and/or natural gas liquids discovered during the report year in new reservoir(s) located in old fields.
    Production, Crude Oil The volumes of crude oil which are extracted from oil reservoirs during the report year. These volumes are determined through measurement of the volumes delivered from lease storage tanks, (i.e., at the point of custody transfer) with adjustment for (1) net differences between opening and closing lease inventories, and for (2) basic sediment and water. Oil used on the lease is considered production.
    Proved Reserves of Crude Oil Proved reserves of crude oil as of December 31 of the report year are the estimated quantities of all liquids defined as crude oil, which geological and engineering data demonstrate with reasonable certainty to be recoverable in future years from known reservoirs under existing economic and operating conditions.

    Reservoirs are considered proved if economic producibility is supported by actual production or conclusive formation test (drill stem or wire line), or if economic producibility is supported by core analyses and/or electric or other log interpretations. The area of an oil reservoir considered proved includes: (1) that portion delineated by drilling and defined by gas -- oil and/or gas -- water contacts, if any; and (2) the immediately adjoining portions not yet drilled, but which can be reasonably judged as economically productive on the basis of available geological and engineering data. In the absence of information on fluid contacts, the lowest known structural occurrence of hydrocarbons is considered to be the lower proved limit of the reservoir.

    Volumes of crude oil placed in underground storage are not to be considered proved reserves.

    Reserves of crude oil which can be produced economically through application of improved recovery techniques (such as fluid injection) are included in the "proved" classification when successful testing by a pilot project, or the operation of an installed program in the reservoir, provides support for the engineering analysis on which the project or program was based.

    Estimates of proved crude oil reserves do not include the following: (1) oil that may become available from known reservoirs but is reported separately as "indicated additional reserves"; (2) natural gas liquids (including lease condensate); (3) oil, the recovery of which is subject to reasonable doubt because of uncertainty as to geology, reservoir characteristics, or economic factors; (4) oil that may occur in undrilled prospects; and (5) oil that may be recovered from oil shales, coal, gilsonite, and other such sources. It is necessary that production, gathering or transportation facilities be installed or operative for a reservoir to be considered proved.
    Revisions Changes to prior year-end proved reserves estimates, either positive or negative, resulting from new information other than an increase in proved acreage (extension). Revisions include increases of proved reserves associated with the installation of improved recovery techniques or equipment. They also include correction of prior report year arithmetical or clerical errors and adjustments to prior year-end production volumes to the extent that these alter reported prior year reserves estimates.
    Sales The volume of proved reserves deducted from an operator's total reserves when selling an existing field or property, during the calendar year.

    For definitions of related energy terms, refer to the EIA Energy Glossary.


    "Form EIA-23L, "Annual Report of Domestic Oil and Gas Reserves (County Level Version)":  Survey Form |  Statistical Details.

      Explanatory Notes

  • Miscellaneous includes Arizona, Missouri, Nevada, New York, South Dakota, Tennessee, and Virginia.
  • The production estimates in this table are based on data reported on Form EIA-23L, "Annual Survey of Domestic Oil and Gas Reserves." They many differ from the official Energy Information Administration production data for crude oil contained in the Petroleum Supply Annual.
  • Related data and background information are published in the U.S. Crude Oil and Natural Gas Proved Reserves Annual Report.
  • Maps of Selected State Subdivisions.