Giant US oil fields evaluated by trap type, reservoir parameters, other properties

June 12, 2000
Oil fields with over 100 million bbl of ultimate recovery-cumulative production plus remaining reserves-are considered giants, the gleam in the eye of any explorationist. These fields account for only 362 of the more than 31,000 oil fields of all sizes in the US.

Oil fields with over 100 million bbl of ultimate recovery-cumulative production plus remaining reserves-are considered giants, the gleam in the eye of any explorationist. These fields account for only 362 of the more than 31,000 oil fields of all sizes in the US. However, their cumulative production is 60% of the US total, and they hold 50% of remaining US reserves.

The only statistics available to the author for many of these fields are depth of production, lithology, type trap, proved acres, reservoir thickness, and reservoir age. Is there any information in these properties that might explain why the reserves in some fields are so much larger than other fields? The giant fields were studied by these properties without reference to their location.

Discovery dates of US giant fields considered here range from Bradford field in Pennsylvania in 1871 to Ewing Bank 873 field in the Gulf of Mexico in 1991.

Half of the giants, 186 fields, had been discovered by the end of 1937. Half of the 126 billion bbl total giant-field reserves, a total of 63 billion bbl, had been discovered in 164 fields by the end of 1934.

The best single year for discovery of new fields was 1938 with 17 giant fields. The best single-year reserve discovery was 1967, when 10.7% of the total giant field reserve was discovered in five giant fields. Probably most surprising is that over 84 giant fields were found before the introduction of seismic prospecting.

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The largest number of fields, 48 or 13% of the total, were discovered between 1935-40, and the largest reserves, 20.7 billion bbl, 16% of total giant-field reserves, were discovered between 1965-70 (Fig. 1). There is a general agreement between the number of fields discovered and percent of reserves.

Giant fields range in depth from 500 to 15,500 ft with an average depth of 5,168 ft. The largest number of fields in any 1,000-ft interval is 66 fields at 3,000- 4,000 ft. Those 66 fields also contain the maximum reserves in any one depth interval, 28 billion bbl or 22% of the total reserves in giant fields. Largest average giant-field reserves by depth are 1.4 billion bbl at depths less than 1,000 ft.

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Again there is general agreement be- tween the number of fields discovered by depth and the percent of reserves discovered (Fig. 2). Average acres/thickness by depth show a similarity in the 3,000-4,000 ft range but differences at 5,000-6,000 ft, 8,000-9,000 ft, and 10,000-11,000 ft.

The proved area in giant fields ranges from 700 to 1,723,000 acres with an average of 24,199 acres. The most important size is 5,000-10,000 proved acres with 108 fields and 29 billion bbl, 23% of the giant field reserves. Largest average area by depth is 57,912 acres in 25 fields at depths of 1,000-2,000 ft. Smallest average area by depth is 2,250 acres in one field at depths of 15,000-16,000 ft. Largest proved area is 1,723,000 acres in Panhandle field with 1.2% of the reserves.

Largest average reserves in a single acreage interval are the 4 billion bbl in four giant fields with 50,000-55,000 proved acres. Smallest average reserves are 104 million bbl in one field at a depth of 12,000-13,000 ft.

Reservoir thickness in giant fields averages 238 ft with a range from 10-2,100 ft. The largest number of giant fields in one thickness interval is 76 fields with thickness of 50-100 ft and 16 billion bbl reserves, 13% of the total. The largest reserves of 20 billion bbl, 16% of the total, are in 74 reservoirs with 10-50 ft thickness.

Largest average reserves by thickness are in two fields with reservoir thickness of 450-500 ft and average reserves of 6.6 billion bbl. Smallest average reserves are in one field with thickness of 55 ft and reserves of 418 million bbl.

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Giant fields range in size from 100 million bbl to 13 billion bbl (Fig. 3). When divided into 100-million-bbl increments the smaller giant fields dominate as might be expected: A total of 210 fields, 58% of the total, in the 100-to-200 million bbl interval have 22% of the giant field reserves. These 210 fields have an average depth of 5,521 ft, average area of 14,582 acres, and average thickness of 189 ft. Second largest number is the 59 fields between 200 million and 300 million bbl with 11% of the reserves. These fields have an average depth of 4,941 ft, average proved area of 19,456 acres, and average thickness of 279 ft.

At present 44 giant fields have either been depleted or additional production has not been reported. Cumulative production for a total of 345 giant fields has been over 50% of their total reserves, leaving 17 giant fields with over 50% of their reserves remaining to be produced.

Reservoir lithologies

Generalized lithologies of giant-field reservoirs include carbonates, clastics, and mixed carbonate and clastic reservoirs.

Carbonate reservoirs in 91giant fields have 24% of the giant-field reserves, clastic reservoirs in 244 fields have 59%, and mixed clastic and carbonate reservoirs in 27 fields have 17%.

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The percent fields/reserves by lithology and the average proved acres/average reserves by lithology show great similarity, but the average reserves in the combination carbonate and clastic reservoirs are much higher than in the other two lithologies (Fig. 4).

Depths of carbonate reserves range from 1,300-15,500 ft with an average of 4,808 ft, depths of clastic reservoirs range from 500-11,320 ft with an average 5,251 ft, and depths of the mixed reservoirs range from 2,800-10,800 ft with an average of 5,630 ft.

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Largest number of giant fields with carbonate reservoirs by depth is 19 fields at 3,000-4,000 ft (Fig. 5). The largest number of fields with clastic reservoirs by depth is 36 fields at 3,000-4,000 ft, and the largest number of giant fields in mixed reservoirs by depth is seven at 3,000-4,000 ft.

Maximum reserves in carbonate reservoirs by depth are 18 reservoirs at 4,000-5,000 ft with 27% of the reserves in carbonate reservoirs and 7% of the total reserves. Maximum reserves in clastic reservoirs by depth are in 36 reservoirs at 3,000-4,000 ft with 26% of the reserves in clastic reservoirs and 15% of the total reserves. Maximum reserves in mixed reservoir lithologies by depth are in three reservoirs at 8,000-9,000 ft with 59% of the reserves in mixed lithologies and 10% of the total reserves.

Average field size for carbonate reservoirs is 336 million bbl, average size for clastic reservoirs is 304 million bbl, and average field size for mixed reservoir lithologies is 817 million bbl.

Proved area in carbonate reservoirs averages 27,738 acres, proved area in clastic reservoirs averages 17,426 acres, and proved area in mixed reservoirs averages 78,339 acres.

Reservoir thickness in carbonate reservoirs averages 131 ft, thickness in clastic reservoirs averages 280 ft and thickness in the mixed type reservoirs average 135 ft.

Types of traps

Three types of traps are recognized in giant fields: stratigraphic, structural, and combination (stratigraphic and structural).

The 45 stratigraphic-trap fields contain 17% of the giant-field reserves with an average of 484 million bbl. The 281 structural trap fields contain 62% of the reserves, average 278 million bbl. The 36 combination traps contain 21% of the reserves, average 750 million bbl.

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The percentages of giant fields/ reserves by type of trap are very similar, but the average reserves in combination traps are much higher than in the other two trap types (Fig. 6). Average reservoir thickness/reserves are close in structural traps but not in the other two types.

Stratigraphic traps in giant fields range from 500-15,500 ft in depth with an average of 4,516 ft. Structural traps range in depth from 1,240-l2,000 ft with an average of 5,343 ft. Combination traps range from 900-10,000 ft in depth and average 4,611 ft.

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Largest number of giant fields in stratigraphic traps by depth intervals of 1,000 ft is 12 fields between 3,000-4,000 ft with 37% of the reserves in stratigraphic traps (6% of total giant-field reserves, Fig. 7). Largest number of giant fields in structural traps is 49 fields at depths of 4,000-5,000 ft with 18% of the reserves in structural traps (11% of total reserves). Largest number of giant fields with combination traps is seven fields at 1,000-2,000 ft with 8% of the reserves in combination traps (2% of total reserves) and seven fields at depths of 3,000-4000 ft with 17% of the reserves in combination traps (4% of total reserves).

Maximum reserves in stratigraphic traps are found at depths of 3,000-4,000 ft with 37% of the reserves in stratigraphic traps (6% of total reserves). Maximum reserves in structural traps are found at 3,000-4,000 ft with 19% of the reserves in structural traps (12% of total reserves). Maximum reserves in combination traps are found at 8,000-9,000 ft with 49% of the reserves in combination traps (10% of total reserves).

Field reserves average 484 million bbl for stratigraphic traps, 278 million bbl for structural traps, and 749 million bbl for combination traps. Proved area averages 46,447 acres for stratigraphic traps, 19,748 acres for structural traps, and 32,245 acres for combination traps. Reservoir thickness averages 155 ft for stratigraphic traps, 247 ft for structural traps, and 202 ft for combination traps.

Reservoir age

Ages of the reservoirs in the US giant fields range from Cenozoic to Paleozoic. Five reservoirs cross the Mesozoic and Paleozoic boundary and are referred to as Mesozoic/Paleozoic in age. The totals:

Cenozoic reservoirs: 183 giant fields containing 41% of giant-field reserves.

Mesozoic reservoirs: 28 fields containing 10% of the reserves.

Mesozoic/Paleozoic reservoirs: five fields containing 1% of the reserves.

Paleozoic reservoirs: 146 giant fields containing 48% of giant-field reserves.

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The percent fields/reserves are very similar by age, but there is little correlation in the average acres/average reserves and the average thickness/average reserves (Fig. 8).

Depths of Cenozoic reservoirs in giant fields range from 500-11,700 ft and average 5,498 ft. Twenty-three of the Cenozoic reservoirs are found at 4,000-5,000 ft and another 23 reservoirs are found at 5,000-6,000 ft. Depths of Mesozoic reservoirs range from 1,000-15,500 ft and average 5,175 ft with six fields producing from 4,000-5,000 ft.

Mesozoic/Paleozoic reservoirs range in depth from 1,500-9,600 ft and average 4,825 ft with two fields producing from 7,000-8,000 ft. Depths of Paleozoic reservoirs range from 1,000-12,000 ft and average 5,168 ft with 35 fields producing from depths of 3,000-4,000 ft.

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Twenty-two Cenozoic giant fields between 3,000-4,000 ft have 22% of the Cenozoic giant field reserves and 9% of the total reserves. Four reservoirs of Mesozoic age at depths of 3,000-4,000 ft have 46% of the Mesozoic reserves and 5% of the total reserves (Fig. 9).

One reservoir of Mesozoic/Paleozoic age at depths of 1,000-2,000 ft has 66% of the reserves of that age and .5% of the total reserves. Nine reservoirs of Paleozoic age at depths of 8,000-9,000 ft have 25% of the Paleozoic reserves and 12% of the total reserves.

Proved acres in Cenozoic fields average 8,158 acres, proved acres in Mesozoic reservoirs average 19,830 acres, proved acres in Mesozoic/Paleozoic fields average 9,825 acres, and proved acres in Paleozoic reservoirs average 45,507 acres.

Summary

Depth of production in giant fields ranges from 500-15,500 ft, proved area ranges from 700 to over 1.7 million acres, reservoir thickness ranges from 10-2,100 ft, and reservoir age ranges from Cenozoic to Paleozoic.

Most giant fields produce from clastic reservoirs. Most giant fields are in structural traps. Most giant field reservoirs are Cenozoic in age, and most giant-field reserves are in Paleozoic reservoirs. To sum up, giant fields can and do occur under almost any of the conditions discussed.

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Once the presence of oil fields has been established in a basin, the next question is why some fields are larger than others. Proved area in the 14 largest giant fields totals 2.3 million acres with an average of 165,657 proved acres, 14 times greater than the 14 smaller fields with a total of 165,675 acres and an average of 11,834 acres (Fig. 10).

Reservoir thickness in the largest fields totals 4,590 ft with an average of 328 ft, three times greater the smaller fields with a total of 1,445 ft and an average of 103 ft. The largest fields have a smaller total depth of 53,900 ft with an average of 3,850 ft, the smallest fields have a larger total depth of 67,100 ft with an average of 4,793 ft.

Total reserves in the 14 largest fields are 41.8 billion bbl with an average of 2.9 billion bbl, almost 30 times larger than the 1.4 billion bbl, and average size of 100 million bbl in the 14 smallest fields.

Studies appear to indicate that, if sufficient porosity and permeability are present, proved acreage has a definite effect on the size of reserves in giant fields; it may well be one of the most important factors. Reservoir thickness appears to also have an effect, although not nearly as important. Deeper depth may reduce the size of the fields or this may be the result of less drilling density at greater depths.

The Author

F.R. Haeberle, a consulting geologist, moved to Delaware, Ohio, after 20 years in Dallas. He has worked for Standard of Texas, Atlantic Refining Co., Mobil, Mayfair Minerals, and J.J. Lynn Oil Division. He holds BS and MS degrees in geology from Yale University and an MBA degree in finance from Columbia University Graduate Business School. E-mail: [email protected]