L.F. IvanhoeExploration consultant Santa Barbara, Calif.George G. LeckiePetroconsultants SA London
This article provides the first known summary of all global oil and gas fields. All reported fields are included.
Definitions and semantics had to be adjusted between different sources of data. These regional field numbers will suffice until superseded by improved detailed data on individual countries.
The tables summarize data on Latin America, western Europe, Africa, Middle East, East Asia, Australasia from a previous OGJ article1 and combine them with new data on the rest of the globe: eastern Europe, C.I.S., China, Canada, the U.S. lower 48 states, and Alaska.
Oil fields include conventional crude oil and associated condensate, while gas fields include conventional natural gas plus natural gas liquids/condensate and non-associated gas.
Field grades, estimated ultimate recoveries, and field sizes follow common international oil company usage.
The earlier article by the same authors1 included a table of the numbers and sizes of the known (discovered, producing, and abandoned) oil and gas fields in all parts of the world except Communist nations and North America.
STUDY'S RATIONALE
Oil company explorationists and government planners concerned with future oil and gas supplies need realistic numbers on the sizes and distribution of known fields to help compare regions and estimate what sizes of fields may still be expected in any nation or oil province.
Clearly regional summaries ate not substitutes for details on individual countries or basins when evaluating the local prospectivity of any area or oil province, each of which must be appraised on its own merits. For example, Latin American prospectivity ranges from that of superoily Venezuela to still nonoily Paraguay.
The authors hope that publication of these regional summaries will encourage others to release similar and more detailed data on individual nations and basins.
The world had about 41,164 known oil fields as of Dec. 31, 1989, of which 31,385 were located in the U.S. and 9,779 in the rest of the world.
Oil fields are not created equal. There are many tiny fields and few big ones. The large fields are by far the more important for global oil supplies.
Fig. 1 summarizes the total number of oil fields known in the world vs. the total estimated ultimate recovery from Table 1.
Two supplementary bar graphs on Fig. 1 break down the cumulative oil discovered-1,214 billion bbl-vs. cumulative number of oil fields discovered by field grades for the world excluding the U.S.
The authors' total estimated ultimate recovery for all known oil fields is 1.397 trillion bbl.
Data on U.S. fields come from the U.S. Department of Energy/Energy Information Administration.2 Data on the rest of world are from the 1991 Petroconsultants SA international field data files3 used for this study and from the Canadian government.
The 370 known giant (grade AAA and larger) fields, each with estimated ultimate recoveries of 500 million bbl or more, contain a combined 1.041 trillion bbl or 74% of global EUR. The 1,331 major fields (including the 370 giants), each with EURs of 100 million bbl or more, contain a combined 1.314 trillion bbl or 94% of the total discovered oil.
All of the globe's 39,833 fields smaller than major size contain only about 86 billion bbl of oil. The world's 28,589 known tiny fields with EURs of less than 1 million bbl/field were omitted from Fig. 1 to show details of the larger fields. The total EUR of the tiny fields is less than 10 billion bbl.
NATURAL GAS FIELDS
In most parts of the world, gas is still less economic than crude oil because of its comparative lack of usage flexibility and much higher transport costs.
Fewer gas than oil fields are reported in nonindustrialized regions of the world. This probably is due to the current lower economic/utility value of gas rather than to the existence of fewer gas fields.
Access to established pipelines and urban markets allows many tiny U.S. lower 48 and Canadian gas fields to be declared discoveries that would be ignored and abandoned as untested wildcats in remote land and offshore areas.
No other fuels, such as gas, coal, or wood, can be meaningfully converted to realistic barrels of oil equivalent on the sole basis of their calorific content due to the inherent flexibility of oil's usage; nothing else comes close to oil's convenience.
Steam plant boiler gas might be better converted to tons of coal equivalent rather than to BOE.
Since natural gas is less valuable than crude oil, gas was converted to BOE for EUR/field grades at the practical economic ratio of 10,000 cf/bbl rather than at the usual calorific based ratio of 5,800 cf/bbl. Use of 10,000 cf/bbl allows conversion of billions/trillions of cubic feet of gas by simply shifting the decimal point so that, for instance, 1 tcf of gas equals 100 million bbl of oil and a major gas field.
EURs of gas in Table 2 are reported in trillions of cubic feet rather than in BOE.1
Natural gas liquids/condensate fields were categorized as BOE where separable but otherwise gas rather than oil fields in this study. Some global oil EUR estimates by others include NGL and/or gas as BOE and consequently are higher than the authors' numbers for oil alone.
To facilitate comparisons between regions, the relative frequency vs. size percentages of the tabulated fields can be plotted for each region as log-probability graphs of EUR volumes vs. percentages of fields larger than for oil/gas fields.
When graphed in this manner, the fields fall along semi-straight geological distribution subparallel lines. Log-log plots of EUR vs. field size sequences are also useful.4
Another graph shows that in the Middle East the average (i.e. median field is 100 million bbl) is some 20 times as large as in Latin America-where the median field contains 5 million bbl-and 1,000 times as large as in eastern Europe-where the median field holds 100,000 bbl (Fig. 2).
The Persian Gulf, with median field size of 370 million bbl, is the world's oiliest region. The median field size for the non-U.S. rest of the world is about 3 million bbl.
In the anomalous U.S. lower 48, the abnormal number of 29,650 very small fields reduces the median field size to about 100,000 bbl in spite of oily provinces like Texas. This is an excellent example of the limitations of regional averages.
ADJUSTED FIELD NUMBERS
Sizes of oil and gas fields in any region or oil province vary log-normally.
There are few giants but many tiny fields so the number of fields should theoretically increase steadily as the fields become smaller. Consequently a shortage of very small fields is evidence that such are underreported if the region/nation/basin is being actively explored for oil and gas.
In the rare superoily areas small prospects are uneconomic and do not get drilled. Hence the shortage of very small known fields there very likely reflects a lack of wildcat drilling for and testing of minor features.
Old nonproducing fields are regularly omitted from computerized data banks, which consequently have a bias toward fields that are larger and/or producing.
In many countries there is a substantial difference between the higher total number of known (discovered, producing, and abandoned) fields in official long term government reports and the lower totals that are included in commercial computerized data banks that tend to concentrate on the most important, largest producing fields and commonly winnow out the tiny and abandoned fields.
Such was found to be the case for the U.S., the former U.S.S.R., and eastern Europe where published summaries exist, and the same situation is suspected in other nations.
The authors' totals include all known fields on any published lists. The higher totals of the published reports were accepted as correct.
It is believed that the data bank numbers are valid for the largest fields, that data bank numbers are reasonably reliable for the intermediate sized commercial fields, but are commonly too low for the very small fields as large as 10 million BOE.
Data bank totals were subtracted from the published field totals and any shortages in the data bank numbers were assumed to all be for tiny fields of 5 million BOE or smaller.
The resulting number of such fields (ND = no detailed data) may not be precise, but the final numbers of the very small fields vs. the published totals are considered to be realistic.
The field totals of Tables 1 & 2 are deemed 90-99% accurate, depending on region. In any case, the total volume of oil or gas in the many tiny fields is much less than in the few giant fields.
CPE COUNTRIES
These countries include the People's Republic of China and the former centrally planned economies (CPE) of eastern Europe-Albania, Bulgaria, East Germany, Hungary, Poland, Romania, and Yugoslavia-and the C.I.S.
All of these CPE nations treated their oil reserves as state secrets, so details have been difficult to assemble.
Gas reserves, by contrast, were not usually classified. Orders of magnitude of EURs of the largest producing oil fields can be appraised indirectly from their cumulative and present production rates. Such research estimates are a major source of current Petroconsultants international field data files on CPE nations. Lists with the names of all known fields were published for eastern Europe and the former U.S.S.R. during the late 1980s.
China is the weakest link in this study. China's numbers are from scattered scientific papers, trade journals, scout reports, and so on. China's supergiant Daqing is a complex rather than a field.
As elsewhere, many very small fields that are included in published reports are not yet included in any computerized data bank's or are abandoned.
CANADIAN FIELDS
Canadian provincial and national petroleum statistics are probably the world's most complete.
Unfortunately for this study, their databases were all set up to analyze production and reserves by pools rather than by the broader term fields.
Volunteer nongovernment geologists and computer analysts converted Canada's pools to field numbers for this study so they could be compared with the rest of the world (see acknowledgments).
Canadian data banks vary slightly between years and agencies but generally match publications.
Canadian arctic frontier (the mainland territories of Yukon and Northwest Territories, Mackenzie Delta-Beaufort Sea, and Arctic Islands) fields were allocated on the basis of provincial best current estimates for significant discoveries by the National Energy Board.
U.S. LOWER 48 FIELDS
The combined estimated ultimate recovery of all known oil fields in the contiguous U.S. is 183 billion bbl and of gas fields is 906 tcf.2
Due to lack of information on very old fields, DOE's report omits all Appalachian oil and gas fields, an estimated 3,100 fields, including Pennsylvania's giant Bradford field,5 and another 6,000 identifiable fields with no volumetric data.
These 9,100 fields were included in the authors' Tables 1 & 2 as estimated 4,500 oil and 4,600 gas fields as no detailed data. These fields were added to the U.S. lower 48 numbers to reconcile these with published data from other countries. Inclusion or omission of these 9,100 very small fields would not substantially change the total EUR of oil or gas in the lower 48.
The vast difference between the numerous tiny oil fields in the U.S. vs. the rest of the world is an amazing contrast that requires explanation.
The many oil operators in the U.S. lower 48 were presumably neither brighter nor duller than elsewhere in the world, so the differences in wildcat drilling and tiny fields discovered must be due to some unique economic/legal factor in the U.S. that concentrated drilling here.
SMALL U.S. FIELDS
The conterminous 48 states is by far the most explored region on earth with a large majority of the world's fields, 76% of the known oil fields and 76% of the known gas fields.2
Many of these fields are very small and in many parts of the globe would have been abandoned as noncommercial or left undrilled.
That they were exploited is due not only to the huge local U.S. market for petroleum products but more importantly to the highly developed oil infrastructure of towns, roads, pipelines, service centers, and so on that resulted from the unique U.S. oil business that developed after Col. Drake's 1859 discovery at Titusville, Pa.
Many of the business practices that resulted in the numerous independent oil companies of the onshore lower 48 states evolved from the mineral rights in most of the flatland basins being owned by descendants of the original farmers that homesteaded there in the 1800s. As a result the royalty on any oil discovered was paid to the landowners and not to the crown.
Furthermore, the constitutionally independent U.S. law courts were not legally involved and consequently were politically impartial in any contractual lawsuits brought to resolve royalty obligations between the landowners/lessors and oil companies/lessees.
The critical legal factor was the ownership of the royalties, not the ownership of the judges. Cooperation between strongly motivated oil well promoters, land-men, drillers, etc., many operating on nontechnical hunches, and the farmers was insured by the latter's ownership of the mineral/oil rights in the onshore U.S. lower 48, all of which combined to get many wells drilled and many tiny fields discovered.
SMALL FIELD IMPORTANCE
In the U.S. 48, very small fields with EURs of only 1 million bbl are defined by the American Association of Petroleum Geologists as significant fields, but such discoveries would be abandoned in Alaska and in many foreign countries.
Comparable wildcat drilling rates and discoveries of very small fields are unlikely to ever be attained anywhere else in the world.
After 130 years of active oil and gas exploration, the onshore U.S. lower 48 basins have been badly fished out, and the nation is now in a very mature stage of petroleum development with few large fields expected to still be discovered at economic depths.
In the future U.S. petroleum engineers with their more expensive enhanced oil recoveries from known fields will gradually replace the new field explorationists.
In the offshore U.S. 48, the mineral/oil rights are owned by the state or federal government, so the politics, economics, and operating rules are much different than onshore, which causes small prospects to be left undrilled at sea. In this study, the offshore U.S. 48 statistics were not separated from the onshore numbers.
Offshore U.S. 48 exploration began in earnest in the Gulf of Mexico and off Southern California soon after the development of computerized digital seismic surveys and marine production technologies in the 1950s. The obvious fields on the continental shelves have been found wherever U.S. or state government lands were open for leasing.
ALASKA FIELDS
Alaskan and U.S. 48 field numbers are separated to show clearly the difference in current prospectivity of the two regions.
Failure to distinguish Alaska from the U.S. lower 48 skews the exploration statistics for both areas. Alaskan mineral rights are held by the government as they are outside the U.S.
Alaska has many of the other characteristics of major companies' foreign operations in less developed countries. There characteristics include lack of infrastructure, low population, remoteness, severe operating problems, high costs, extreme climate, and technical rather than nontechnical exploration.
Consequently, very small oil and gas fields have not yet been deliberately explored for in Alaska, where they are still uneconomic. Modern petroleum exploration resulted in the Kenai peninsula/Cook Inlet discoveries in southern Alaska in the mid-1950s and in arctic Alaska's supergiant Prudhoe Bay field in 1968.
All of Alaska's onshore and offshore basins have now been studied by professional explorationists, and with the exception of the Arctic National Wildlife Refuge, the potential giant plays have had some initial testing.
FIELD SIZE SIGNIFICANCE
Acorns are found near oak trees, and oil fields are found in oil provinces.
The relative oiliness (prospectivity for oil and gas) of any region, nation, or basin is determined by the size of the known fields. The oily areas stand out.
More than half of the world's nations produce no oil. The giant fields are normally found first in any technical stage of exploration.
The rare superoily areas such as the Persian Gulf and Venezuela with numerous giant fields are of excellent hunting quality. The poor hunting regions such as eastern Europe have the highest percentages of very small fields.
However, any large geographic region will contain areas of both good and poor hunting quality, each of which must be appraised on its own merits.
A qualitative appraisal often is sufficient to outline prospective areas of interest. It is commonly forgotten that some of our grandfathers were excellent oil finders using nothing more than hunches, trends, surface geology, and the crudest analog seismic techniques.
Past explorationists did a very competent job, and the presently known oil and gas producing areas include every known geological basin that has been accessible for 130 years. International majors, independents, and government oil companies have been operating all over the world for more than 45 years since World War II, offshore since the 1950s, and have been using geochemistry and digital seismic methods onshore and offshore since 1965.
Lack of drilling is not necessarily due to a lack of knowledge-it may indicate a shortage of good local economic prospects. An area is no longer virgin if geological/seismic surveys have been run there and the sizes/natures of the sedimentary basins are outlined.
Offshore shelves can be quickly evaluated by digital seismic surveys, and only a few holes are needed to condemn the commercial prospects of a remote offshore basin if geochemical results are unfavorable.
Modern petroleum exploration is a very efficient process, and political/leasing delays often consume more time than the necessary geophysical work and wildcat drilling.
Maturity of a basin's oil prospects comes quicker now than it did in the old days. Digital seismic surveys decreased the average finding time required to discover a new basin's five largest fields from 14 years in the 1940s to 6 years in the 1970s (if all of the basin was politically accessible/open for leases).
Modern 3-D digital seismic surveys give a much more accurate delineation of the structure and stratigraphy for early estimation of the final EUR than did older methods, particularly offshore. Large stratigraphic trap fields now seem to be possibilities rather than probabilities.
If large commercial sized stratigraphic oil fields like East Texas field were common, geologists should have discovered more of them in the highly drilled U.S. lower 48 or in other areas via digital seismic bright spots during the last 30 years.
Analysis of the distribution of European oil and gas reserves showed that less than 2% of all Europe's reserves were held in purely stratigraphic traps; it is not likely that the situation is much different elsewhere in the world.6
The world has now reached a semimature stage of petroleum exploration. We now have several years of hindsight plus the industry's disappointing new field discovery record during the world's great drilling surge between 1978-86.
The results are sobering. No new major oil provinces were found or developed. To date, many lightly drilled basins were found to be barren or only marginally commercial (U.S. Atlantic Coast, Gulf of Alaska, Bering Sea, Alaskan Arctic Ocean, Northwest Australian Exmouth plateau, South China offshore, Alaskan ANWR, etc.).
The C.I.S. and China are no longer virgin areas. New giant field discoveries are now practically restricted to the superoily megaprovinces of the OPEC Persian Gulf and OPEC Venezuela. Few giant discoveries are being made in the critical non-OPEC oil exporting countries.
Time is on OPEC's side.
ACKNOWLEDGMENTS
The authors appreciate the assistance of many persons without whose help these data could not have been assembled. Special thanks are due to: Canada-Richard Proctor and Jack Century, Calgary, and Dwight's Energydata Inc., Oklahoma City; eastern Europe, C.I.S., China-Arthur A. Meyerhoff, Tulsa; China-Irfan Taner, Tulsa.
REFERENCES
1. Ivanhoe, L.F., and Leckie, G.G., Data on field size useful to supply planners, OGJ, Apr. 29, 1991, pp. 73-74.
2. U.S. Department of Energy/Energy Information Administration Geologic distribution of U.S. oil and gas, Report DOE/EIA-0057, July 1992, 137 pp.
3. Petroconsultants SA, World production and reserves statistics 1990, database including information on 13,500 oil and gas fields outside North America, 1991.
4. Ivanhoe, L.F., Oil/gas potential in basins estimated; OGJ, Dec. 6, 1976, pp. 154-155.
5. Burns, P.J., and Claus, R.C., An overview of hydrocarbon production in Appalachian basins-a glance at the forest, OGJ, Aug. 12, 1983, pp. 116-122.
6. Leckie, G.G., and Chew, K.J., The discovered hydrocarbon reserves of Western Europe, in Generation, accumulation and production of Europe's hydrocarbons, EAPG Spec. Pub. 1, A.M. Spenser, ed., Oxford University Press, 1991, pp. 1-23.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
