Oil reserves

A map of world oil reserves, 2013.

Oil reserves are the amount of technically and economically recoverable oil. Reserves may be for a well, for a reservoir, for a field, for a nation, or for the world. Different classifications of reserves are related to their degree of certainty.

The total estimated amount of oil in an oil reservoir, including both producible and non-producible oil, is called oil in place. However, because of reservoir characteristics and limitations in petroleum extraction technologies, only a fraction of this oil can be brought to the surface, and it is only this producible fraction that is considered to be reserves. The ratio of reserves to the total amount of oil in a particular reservoir is called the recovery factor. Determining a recovery factor for a given field depends on several features of the operation, including method of oil recovery used and technological developments.^[1]

Based on data from OPEC at the beginning of 2013 the highest proved oil reserves including non-conventional oil deposits are in Venezuela (20% of global reserves), Saudi Arabia (18% of global reserves), Canada (13% of global reserves), and Iran (9%).^[2]

Because the geology of the subsurface cannot be examined directly, indirect techniques must be used to estimate the size and recoverability of the resource. While new technologies have increased the accuracy of these techniques, significant uncertainties still remain. In general, most early estimates of the reserves of an oil field are conservative and tend to grow with time. This phenomenon is called reserves growth.^[3]

Many oil-producing nations do not reveal their reservoir engineering field data and instead provide unaudited claims for their oil reserves. The numbers disclosed by some national governments are suspected of being manipulated for political reasons.^[4][5]

Classifications

Schematic graph illustrating petroleum volumes and probabilities. Curves represent categories of oil in assessment. There is a 95% chance (i.e., probability, F95) of at least volume V1 of economically recoverable oil, and there is a 5-percent chance (F05) of at least volume V2 of economically recoverable oil.^[6]

All reserve estimates involve uncertainty, depending on the amount of reliable geologic and engineering data available and the interpretation of those data. The relative degree of uncertainty can be expressed by dividing reserves into two principal classifications—"proven" (or "proved") and "unproven" (or "unproved").^[6] Unproven reserves can further be divided into two subcategories—"probable" and "possible"—to indicate the relative degree of uncertainty about their existence.^[6] The most commonly accepted definitions of these are based on those approved by the Society of Petroleum Engineers (SPE) and the World Petroleum Council (WPC) in 1997.^[7]

Proven reserves

Proven reserves are those reserves claimed to have a reasonable certainty (normally at least 90% confidence) of being recoverable under existing economic and political conditions, with existing technology. Industry specialists refer to this as P90 (that is, having a 90% certainty of being produced). Proven reserves are also known in the industry as 1P.^[8][9]

Proven reserves are further subdivided into "proven developed" (PD) and "proven undeveloped" (PUD).^[9][10] PD reserves are reserves that can be produced with existing wells and perforations, or from additional reservoirs where minimal additional investment (operating expense) is required.^[10] PUD reserves require additional capital investment (e.g., drilling new wells) to bring the oil to the surface.^[8][10]

Until December 2009 "1P" proven reserves were the only type the U.S. Securities and Exchange Commission allowed oil companies to report to investors. Companies listed on U.S. stock exchanges must substantiate their claims, but many governments and national oil companies do not disclose verifying data to support their claims. Since January 2010 the SEC now allows companies to also provide additional optional information declaring "2P" (both proven and probable) and "3P" (proven + probable + possible) provided the evaluation is verified by qualified third party consultants, though many companies choose to use 2P and 3P estimates only for internal purposes.

Unproven reserves

An oil well in Canada, which has the world's third largest oil reserves.

Unproven reserves are based on geological and/or engineering data similar to that used in estimates of proven reserves, but technical, contractual, or regulatory uncertainties preclude such reserves being classified as proven.^[11] Unproven reserves may be used internally by oil companies and government agencies for future planning purposes but are not routinely compiled. They are sub-classified as probable and possible.^[11]

Probable reserves are attributed to known accumulations and claim a 50% confidence level of recovery. Industry specialists refer to them as "P50" (i.e., having a 50% certainty of being produced). These reserves are also referred to in the industry as "2P" (proven plus probable).^[8]

Possible reserves are attributed to known accumulations that have a less likely chance of being recovered than probable reserves. This term is often used for reserves which are claimed to have at least a 10% certainty of being produced ("P10"). Reasons for classifying reserves as possible include varying interpretations of geology, reserves not producible at commercial rates, uncertainty due to reserve infill (seepage from adjacent areas) and projected reserves based on future recovery methods. They are referred to in the industry as "3P" (proven plus probable plus possible).^[8]

Russian reserve categories

In Russia, reserves categories A, B, and C1 correspond roughly to proved developed producing, proved developed nonproducing, and proved undeveloped, respectively; the designation ABC1 corresponds to proved reserves. The Russian category C2 includes probable and possible reserves.^[12]

Strategic petroleum reserves

Many countries maintain government-controlled oil reserves for both economic and national security reasons. According to the United States Energy Information Administration, approximately 4.1 billion barrels (650,000,000 m³) of oil are held in strategic reserves, of which 1.4 billion is government-controlled (m³=cubic meters). These reserves are generally not counted when computing a nation's oil reserves.

Resources

Unconventional oil resources are greater than conventional ones.^[13]

Cumulative oil production plus remaining reserves and undiscovered resources. United States not included.

A more sophisticated system of evaluating petroleum accumulations was adopted in 2007 by the Society of Petroleum Engineers (SPE), World Petroleum Council (WPC), American Association of Petroleum Geologists (AAPG), and Society of Petroleum Evaluation Engineers (SPEE). It incorporates the 1997 definitions for reserves, but adds categories for contingent resources and prospective resources.^[6]

Contingent resources are those quantities of petroleum estimated, as of a given date, to be potentially recoverable from known accumulations, but the applied project(s) are not yet considered mature enough for commercial development due to one or more contingencies. Contingent resources may include, for example, projects for which there are no viable markets, or where commercial recovery is dependent on technology under development, or where evaluation of the accumulation is insufficient to clearly assess commerciality.

Prospective resources are those quantities of petroleum estimated, as of a given date, to be potentially recoverable from undiscovered accumulations by application of future development projects. Prospective resources have both an associated chance of discovery and a chance of development.

The United States Geological Survey uses the terms technically and economically recoverable resources when making its petroleum resource assessments. Technically recoverable resources represent that proportion of assessed in-place petroleum that may be recoverable using current recovery technology, without regard to cost. Economically recoverable resources are technically recoverable petroleum for which the costs of discovery, development, production, and transport, including a return to capital, can be recovered at a given market price.

"Unconventional resources" exist in petroleum accumulations that are pervasive throughout a large area. Examples include extra heavy oil, oil sand, and oil shale deposits. Unlike "conventional resources", in which the petroleum is recovered through wellbores and typically requires minimal processing prior to sale, unconventional resources require specialized extraction technology to produce. For example, steam and/or solvents are used to mobilize bitumen for in-situ recovery. Moreover, the extracted petroleum may require significant processing prior to sale (e.g., bitumen upgraders).^[6] The total amount of unconventional oil resources in the world considerably exceeds the amount of conventional oil reserves, but are much more difficult and expensive to develop.

Estimation techniques

Example of a production decline curve for an individual well

The amount of oil in a subsurface reservoir is called oil in place (OIP).^[10] Only a fraction of this oil can be recovered from a reservoir. This fraction is called the recovery factor.^[10] The portion that can be recovered is considered to be a reserve. The portion that is not recoverable is not included unless and until methods are implemented to produce it. ^[11]

Volumetric method

Volumetric methods attempt to determine the amount of oil in place by using the size of the reservoir as well as the physical properties of its rocks and fluids. Then a recovery factor is assumed, using assumptions from fields with similar characteristics. OIP is multiplied by the recovery factor to arrive at a reserve number. Current recovery factors for oil fields around the world typically range between 10 and 60 percent; some are over 80 percent. The wide variance is due largely to the diversity of fluid and reservoir characteristics for different deposits.^[14][15][16] The method is most useful early in the life of the reservoir, before significant production has occurred.

Materials balance method

The materials balance method for an oil field uses an equation that relates the volume of oil, water and gas that has been produced from a reservoir and the change in reservoir pressure to calculate the remaining oil. It assumes that, as fluids from the reservoir are produced, there will be a change in the reservoir pressure that depends on the remaining volume of oil and gas. The method requires extensive pressure-volume-temperature analysis and an accurate pressure history of the field. It requires some production to occur (typically 5% to 10% of ultimate recovery), unless reliable pressure history can be used from a field with similar rock and fluid characteristics.^[11]

Production decline curve method

Decline curve generated by decline curve analysis software, utilized in petroleum economics to indicate the depletion of oil & gas in a petroleum reservoir. The Y axis is a semi log scale, indicating the rate of oil depletion (green line), and gas depletion (red line). The X axis is a coordinate scale, indicating time in years and displays the production decline curve. The top red line is the gas decline curve, which is a hyperbolic decline curve. Gas is measured in MCF (thousand cubic feet in this case). The lower Blue line is the oil decline curve, which is an exponential decline curve. Oil is measured in BBL (Oil barrels). Data is from actual sales, not pumped production. The dips to zero indicate there were no sales that month, likely because the oil well did not produce a full tank, and thus was not worth a visit from a tank truck. The upper right legend (map) displays CUM, which is the cumulative gas or oil produced. ULT is the ultimate recovery projected for the well. Pv10 is the discounted present value of 10%, which is the future value of the remaining lease, valued for this oil well at $1.089 million USD.

The decline curve method uses production data to fit a decline curve and estimate future oil production. The three most common forms of decline curves are exponential, hyperbolic, and harmonic. It is assumed that the production will decline on a reasonably smooth curve, and so allowances must be made for wells shut in and production restrictions. The curve can be expressed mathematically or plotted on a graph to estimate future production. It has the advantage of (implicitly) including all reservoir characteristics. It requires a sufficient history to establish a statistically significant trend, ideally when production is not curtailed by regulatory or other artificial conditions.^[11]

Reserves growth

Experience shows that initial estimates of the size of newly discovered oil fields are usually too low. As years pass, successive estimates of the ultimate recovery of fields tend to increase. The term reserve growth refers to the typical increases in estimated ultimate recovery that occur as oil fields are developed and produced.^[3]

Estimated reserves by country

Trends in proved oil reserves in top five countries, 1980-2013 (date from US Energy Information Administration)

BBL = barrel of oil

Countries with largest oil reserves

Most of the world's oil reserves are in the Middle East.^[17]

Summary of Proven Reserve Data as of 2012^[2]

—	Country	Reserves[18] 109 bbl	Reserves 109 m3	Production[19] 106 bbl/d	Production 103 m3/d	Reserve/ Production Ratio1 years
1	Venezuela	296.50	47.140	2.1	330	387
2	Saudi Arabia	265.40	42.195	8.9	1,410	81
3	Canada	175.00	27.823	2.7	430	178
4	Iran	151.20	24.039	4.1	650	101
5	Iraq	143.10	22.751	3.4	540	163
6	Kuwait	101.50	16.137	2.3	370	121
7	United Arab Emirates	97.80	15.549	2.4	380	156
8	Russia	80.00	12.719	10.0	1,590	22
9	Libya	47.00	7.472	1.7	270	76
10	Nigeria	37.00	5.883	2.5	400	41
11	Kazakhstan	30.00	4.770	1.5	240	55
12	Qatar	25.41	4.040	1.1	170	63
13	China	25.40	4.038	4.1	650	15
14	United States	25.00	3.975	7.0	1,110	10
15	Angola	13.50	2.146	1.9	300	19
16	Algeria	13.42	2.134	1.7	270	22
17	Brazil	13.20	2.099	2.1	330	17
	Total of top seventeen reserves	1,324.00	210.499	56.7	9,010	64
Notes: 1 Reserve to Production ratio (in years), calculated as reserves / annual production. (from above)

It is estimated that between 100 and 135 billion tonnes of the world's oil reserves have been used between 1850 and the present. ^[20]

OPEC countries

Since OPEC started to set production quotas on the basis of reserves levels in the 1980s, many of its members have reported significant increases in their official reserves.^[21][22] There are doubts about the reliability of these estimates, which are not provided with any form of verification that meet external reporting standards.^[21] The following table illustrates these rises.

OPEC countries

oil reserves of OPEC 1980–2005

The sudden revisions in OPEC reserves, totaling nearly 300 bn barrels, have been much debated.^[23] Some of it is defended partly by the shift in ownership of reserves away from international oil companies, some of whom were obliged to report reserves under conservative US Securities and Exchange Commission rules.^[21][24] The most prominent explanation of the revisions is prompted by a change in OPEC rules which set production quotas (partly) on reserves. In any event, the revisions in official data had little to do with the actual discovery of new reserves.^[21]

Total reserves in many OPEC countries hardly changed in the 1990s.^[21] Official reserves in Kuwait, for example, were unchanged at 96.5 Gbbl (15.34×10^⁹ m³) (including its share of the Neutral Zone) from 1991 to 2002, even though the country produced more than 8 Gbbl (1.3×10^⁹ m³) and did not make any important new discoveries during that period.^[21] The case of Saudi Arabia is also striking, with proven reserves estimated at between 260 and 264 billion barrels (4.20×10¹⁰ m³) in the past 18 years, a variation of less than 2%,^[21] while extracting approximately 60 billion barrels (9.5×10⁹ m³) during this period.

Sadad al-Huseini, former head of exploration and production at Saudi Aramco, estimates 300 Gbbl (48×10^⁹ m³) of the world's 1,200 Gbbl (190×10^⁹ m³) of proven reserves should be recategorized as speculative resources, though he did not specify which countries had inflated their reserves.^[25] Dr. Ali Samsam Bakhtiari, a former senior expert of the National Iranian Oil Company, has estimated that Iran, Iraq, Kuwait, Saudi Arabia and the United Arab Emirates have overstated reserves by a combined 320–390bn barrels and has said, "As for Iran, the usually accepted official 132 billion barrels (2.10×10¹⁰ m³) is almost one hundred billion over any realistic assay."^[26] Petroleum Intelligence Weekly reported that official confidential Kuwaiti documents estimate reserves of Kuwait were only 48 billion barrels (7.6×10^⁹ m³), of which half were proven and half were possible. The combined value of proven and possible is half of the official public estimate of proven reserves.^[22]

In July 2011, OPEC's Annual Statistical Review showed Venezuela's reserves to be larger than Saudi Arabia's.^[27][28]

Prospective resources

Arctic prospective resources

Location of Arctic Basins assessed by the USGS

A 2008 United States Geological Survey estimates that areas north of the Arctic Circle have 90 billion barrels (1.4×10¹⁰ m³) of undiscovered, technically recoverable oil and 44 billion barrels (7.0×10⁹ m³) of natural gas liquids in 25 geologically defined areas thought to have potential for petroleum. This represented 13% of the expected undiscovered oil in the world. Of the estimated totals, more than half of the undiscovered oil resources were estimated to occur in just three geologic provinces—Arctic Alaska, the Amerasia Basin, and the East Greenland Rift Basins. More than 70% of the mean undiscovered oil resources was estimated to occur in five provinces: Arctic Alaska, Amerasia Basin, East Greenland Rift Basins, East Barents Basins, and West Greenland–East Canada. It was further estimated that approximately 84% of the oil and gas would occur offshore. The USGS did not consider economic factors such as the effects of permanent sea ice or oceanic water depth in its assessment of undiscovered oil and gas resources. This assessment was lower than a 2000 survey, which had included lands south of the Arctic Circle.^[29][30][31]

Unconventional prospective resources

In October 2009, the USGS updated the Orinoco tar sands (Venezuela) value to 513 billion barrels (8.16×10¹⁰ m³).^[32]

In June 2013 the U.S. Energy Information Administration published a global inventory of estimated recoverable tight oil and tight gas resources in shale formations, "Technically Recoverable Shale Oil and Shale Gas Resources: An Assessment of 137 Shale Formations in 41 Countries Outside the United States." The inventory is incomplete due to exclusion of tight oil and gas from sources other than shale such as sandstone or carbonates, formations underlying the large oil fields located in the Middle East and the Caspian region, off shore formations, or about which there is little information. Estimated technically recoverable shale oil resources total 335 to 345 billion barrels.^[33]

See also

• Decline curve analysis
• Global strategic petroleum reserves
• Oil exploration
• Peak oil
• Petroleum Industry
• Strategic Petroleum Reserve

Energy and resources:

• Petro-aggression
• Energy security
• World energy resources and consumption
• List of countries by natural gas proven reserves

References

External links

• OPEC Annual Statistical Bulletin
• Energy Supply page on the Global Education Project web site, including many charts and graphs on the world's energy supply and use
• Oil reserves (most recent) by country
• Statistical Review of World Energy BP Statistical Review of Energy 2013

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