12 Studies in Geology 56

Copyright ©2007 by The American Association of Petroleum Geologists. DOI: 10.1306/1240889St561638

Executive SummarySince initial exploration success more than a century ago in onshore California, more than 872 discoveries with total reserves

>137 billion barrels oil equivalent (BBOE) have been made in deep-water deposits. A rapid increase in such discoveries began in the 1970s and more than 350 are expected in this decade based on historic trends.

More than 80% of these discoveries are in offshore basins. North America dominates the number of total discoveries as well as reserves but there is an increasing number of African and Brazilian discoveries and fields. Reservoir rocks range in age from Ordovi-cian to Pleistocene. However, more than 90% are in Cretaceous or younger rocks. Passive margins have been the setting for most of these accumulations, and most are in slope depositional environments. However, recent technological advances allowing drilling in deeper waters will lead to more discoveries in lower slope to basin depositional environments. Further statistics on hydrocarbons, reservoir drive, trap types, porosity, and hydrocarbon column heights are discussed from a commercially available database compiled by Cossey and Associates Inc.

5 A Global Overview of Fields and Discoveries in Deep-water DepositsStephen P. J. Cossey1 and Joseph R. J. Studlick2

1Cossey and Associates Inc., Durango, Colorado, USA 2Maersk Oil Inc., Houston, Texas, USA

IntroductionWhen many people think “deep-water” oil and gas, they think “current-day offshore waters greater than 183 m (600 ft) or 460 m

(1500 ft).” However, not all oil and gas found in deep-water reservoirs is offshore today. A sizeable proportion of the reserves in such reservoirs have been found in onshore basins. In fact, the first discoveries in these deposits were discovered onshore in California more than a century ago. Many people also think deep-water reservoirs are “young.” Although most are young “geologically,” the oldest deep-water reservoirs found to date are in Ordovician rocks of onshore Libya, and prolific producers are found in the Pennsylvanian deep-water reservoirs of onshore West Texas.

Geographic, geologic, and other characteristics of fields and discoveries (FADs) in deep-water deposits have been analyzed from published data. Cossey and Associates Inc. has compiled these data during the last eleven years into a searchable, commercially available database. “Discoveries” are defined as oil and gas accumulations that have not been developed or are to be developed (i.e., have not yet been produced). From this published data, 872 FADs have been identified. This estimate of the number of FADs is conservative because some FADs have not been documented, especially as some countries (e.g., Former Soviet Union, China, and others) do not report such data.

Geographic Distribution of Fields and DiscoveriesOffshore dominates the global inventory of FADs (Figure 1) because many more offshore basins are present and have been explored.

On a continent basis, almost 90% of FADs are in North America, Africa, and Europe (Figure 2). On a country basis, the U.S., U.K., and Brazil contain almost 70% of FADs (Figure 3). However, note that the U.S. has a disproportionate number of FADs.

Historical SummaryThe exploration and production (E&P) industry has a long history of exploring and developing deep-water deposits. The earliest

discovery was in 1890 at Coalinga Field in the San Joaquin Basin, onshore California (International Petroleum Encyclopedia, 2003). A subsequent discovery was made in the Los Angeles Basin at Whittier in 1896 (Lindblom, 1975). In the early 1900s, discoveries were made in onshore Trinidad at Tabaquite Field (Geological Society of Trinidad and Tobago, 2006) and more discoveries were made in the Los Angeles and Ventura Basins. The first discovery of a deep-water reservoir in the Permian Basin was in 1925 at Wheat Field (Galloway et al., 1983). In the onshore portion of the Gulf of Mexico, deep-water reservoirs were discovered at Barataria, Louisiana, in 1939 (Ventress and Smith, 1984) and at Starks, Louisiana, in 1943 in the prolific Hackberry trend (Cossey and Jacobs, 1992). The earliest offshore discovery was in 1951 in Brighton Marine Field, Trinidad (Ablewhite and Higgins, 1968). Figure 4 shows the number of FADs discovered by decade.

Figure 1. Percentages of onshore and offshore fields and discoveries in deep-water deposits. N = 872.

Figure 2. Percentages of fields and discoveries in deep-water deposits by continent. North America has more than 50%. N = 875 (3 FADs straddle con-tinental boundaries).

Figure 3. Graph of the number of FADs by country. Note that Russia has three FADs and all countries below it on the graph have one or two.

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A Global Overview of Fields and Discoveries in Deep-water Deposits13

Geographic and Age Distribution of ReservesReserve estimates are available on 608 FADs and total 136.7 BBOE recoverable. Offshore basins dominate this total with 101.4

BBOE (74%). On a continent basis, North America, Africa, and Europe contain 86% of these global reserves (Figure 5). A compilation of reserves by country (those countries with reserves >1.0 BBOE) is shown in Figure 6.

As seen in Figure 7, there are few FADs (only 9%) in reservoirs older than Cretaceous. Miocene reservoirs are found in 15 of the 31 countries in the database and comprise one-quarter of the total number of FADs. More than 50% of the FADs are in young rocks — Miocene and younger. The Oligocene appears to have a very low number of FADs and may be under-explored or has fewer deep-water sediments. A graph of reserves vs. geologic age (Figure 8) shows a very similar distribution to number of FADs vs. geologic age (Figure 7).

Figure 4. Graph of the number of FADs in deep-water deposits versus the decade when discovered. Note the significant and sus-tained increase starting in the 1970s result-ing from E&P efforts in the North Sea and U.S. Gulf of Mexico and later activities in offshore West Africa and Brazil. Data extrap-olation suggests the current decade will have >350 FADs, in agreement with the trend starting in the 1970s. N = 746 as 126 FADs (of 872 total) did not have the discovery year noted in published data.

Figure 5. Recoverable reserves (BBOE) by continent.

Figure 6. Recoverable reserves by country (>1.0 BBOE).

Figure 7. Graph of geologic ages of FADs (N = 631 as 241 FADs do not have public geologic age information).

Figure 8. Graph of reserves versus geologic age (N = 631). Reserves of 16.0 BBOE cannot be assigned on 241 FADs, as geologic ages are not known.

Viewing the number of FADs and reserves on a percentage basis versus geologic age shows some interesting trends (Table 1). Note that on the basis of reserves versus number of FADs, Pleistocene- and Pliocene-aged reservoirs are under-represented whereas the Miocene, Oligocene, and Eocene Ages are over-represented. Average reserves per FAD show that Oligocene- and Eocene-aged FADs have the highest values.

The largest reserves reported for a discovery or field is for Chicontepec Field(s), onshore Mexico at 12.3 BBOE (Busch, 1992). The largest reported offshore field is Roncador, Brazil, with 3.1 BBOE of recoverable reserves (Rangel et al., 1998). The smallest onshore or offshore FADs are two onshore California fields, Cantua Creek and Horse Meadows, each with estimated reserves of only 200 MBOE (Hall, 1981).

The shallowest FAD is Tabaquite, Trinidad, at 61 m (200 ft) subsea (Ablewhite and Higgins, 1968) and the deepest is the Tahiti discovery (Green Canyon 640), Gulf of Mexico, between 7315 and 8230 m (24,000–27,000 ft) subsea (Yip et al., 2005).

Figure 5. Recoverable reserves (BBOE) by continent.

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14 Studies in Geology 56

Figure 9. Pie chart of FADs versus deposi-tional setting. Slope deposits are the dominant setting, which also reflects exploration drilling in shallower waters until the 1990s. DW = deep water.

Figure 10. Graph of FADs (N = 820) versus tectonic setting showing that 70% are in passive margin settings, predominantly in the Gulf of Mexico, offshore West Africa, and offshore Brazil.

1The term “high rate, high ultimate (HRHU) reservoirs” was coined by Shell Oil workers and is discussed in the Weimer and Pettingill articles (this volume, Chapters 4, 117). HRHU reservoirs were defined in 1997 as those able to produce at sustained rates of >10 thousand BOE per day and recover >20 million BOE (G. Steffens, 2006, personal communication). Such reservoirs would require fewer wells and rapidly recoup sunk finding and develop-

ment (F&D) costs.

Traps, Hydrocarbons, and ReservoirsThe hydrocarbon column height (HCCH) is a critical factor in evaluating the risk and potential reserves of a prospect. Although data

is limited (N = 189), a few very large HCCHs have been reported (Figure 11). The largest reported is 1326 m (4350 ft) at Lobster Field, Ewing Bank 873, U.S. Gulf of Mexico (Edman and Burk, 1998). Data on reservoir drive are also limited (N = 126). However, water drive accounts for approximately half of the drive mechanisms in the data set (Figure 12). A larger, more representative sample of FADs (N = 387) versus trap type is available (Figure 13). The data does not allow the differentiation desired (i.e., structural, stratigraphic, or combination [structural/stratigraphic] trapping) as “stratigraphic” here means “having a stratigraphic component.” However, a strati-graphic component of trapping — in combination with structure or alone — is significant.

Figure 11. Graph of hydrocar-bon column heights (N = 189). Although 94% are <609 m (2000 ft), some very large columns have been reported.

Figure 12. Data on 126 FADs indicate about half have water-drive reservoirs.

Figure 13. Graph of trap types (N = 387). Data was gathered as “stratigraphic” or “structural.” Thus, “stratigraphic” includes pure stratigraphic trapping and combination structural/strati-graphic trapping. Pure stratigraphic traps are expected to be the smallest group of the three trap types if such data were compiled.

Table 1. Number and reserves on a percentage basis versus geologic age (for those FADs that had geo-logic age assigned). Average reserves per FAD gives a flavor of “richness” by geologic age.

Geologic AgeNumber

(%)Reserves

(%)

Reserves per FAD

(MMBOE)

Pleistocene 9.1 3.9 83

Pliocene 19.0 14.9 151

Miocene 25.0 30.4 233

Oligocene 4.3 7.6 340

Eocene 8.1 14.3 337

Paleocene 8.9 8.2 176

Cretaceous 16.4 14.8 173

Jurassic 4.5 4.0 170

Triassic 0.6 0.0 0

Permian 3.2 1.1 67

Carboniferous 0.6 0.3 82

Devonian 0.0 0.0 0

Silurian 0.0 0.0 0

Ordovician 0.2 0.5 560

Depositional and Tectonic SettingsLimited data is available on the depositional setting of FADs (N = 179). However, we believe the distribution shown in Figure 9 is

representative globally. All FADs have also been classified by tectonic setting (Figure 10). Passive margin settings have been a significant target of E&P

because of past drilling in “shallower” water and the abundance of high rate and high ultimate reservoirs1 in that setting.

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A Global Overview of Fields and Discoveries in Deep-water Deposits15

Although published data on reservoir rock thickness and properties are not widely available, a representative chart (N = 291) of porosity is possible (Figure 14). The highest porosity reservoirs (>35%) are shown in Table 2.

Table 2. Highest porosity deep-water reservoirs.

Reservoir Porosity Age

Reef Shark, Main Pass 61, GOM 38% Pliocene–Pleistocene

Boi, OPL 213, offshore Nigeria 35% Pliocene–Miocene

Marquette, Green Canyon 52, GOM (Schneider and Clifton, 1995) 35% Pliocene

Harding, CS 9/23B, U.K. North Sea (McKay et al., 1998) 35% Paleogene

Dos Cuadras, onshore California (Goodhue and Cavit, 1992) 35% Pliocene*

*Highest onshore globally. GOM = Gulf of Mexico.

The thickest hydrocarbon pay reported is at the Trident prospect, offshore Gulf of Mexico, with >274 m (900 ft) of oil and gas pay (Blickwede et al., 2004).

Hydrocarbon-type data are available on most FADs. Almost two-thirds of the FADs are oil (Figure 15). Of the oil and condensate reservoirs (excluding gas reservoirs), almost three-fourths have gravities between 20 and 40 degrees API (Figure 16). The lowest reported gravity is at Navarro prospect, Green Canyon 37, Gulf of Mexico at 8.6° API. The highest gravities are reported for condensates of 66° and 68° API at Nile, U.K. 914, and Ozona SW, onshore Texas, respectively.

ConclusionsA significant number of FADs and associated reserves have been found in deep-water deposits. Most are offshore in passive margin

settings. FADs have been found in 31 countries and on all continents except Antarctica. However, almost 45% of the reserves are in North America. The U.S. has the most FADs, 466 of 872 globally, and the largest reserves (47 BBOE) due to the many onshore and offshore basins and the long history of, and extensiveness of, E&P activities.

Exploration of deep-water deposits began in onshore California more than a century ago. However, less than 20% of deep-water deposit reserves are onshore. With increased exploration of the U.S. Gulf of Mexico and North Sea beginning in the 1970s and offshore West Africa in the 1990s, an increasing trend of discoveries in deep-water deposits has occurred. We expect more than 350 discoveries in deep-water deposits, exclusively offshore, will be made in this decade.

ReferencesAblewhite, K., and G. E. Higgins, 1968, A review of Trinidad, W. I., oil development and the accumulations at Soldado, Brighton

Marine, Grande Ravine, Barrackpore-Penal, and Guayaguayare: Transactions of the 4th Caribbean Geological Conference, p. 41–74.

Blickwede, J. F., J. G. Hawkins, J. W. Hidore, E. Doré, M. Damm, M. J. DiMarco, B. L. Gouger, G. Jones, J. Sanclemente, A. Trevena, and S. Walden, 2004, The Trident discovery: Play opener of the Perdido Foldbelt, deepwater northwestern Gulf of Mexico: AAPG International Conference Program Book, Cancun, Mexico, p. A8.

Busch, D. A., 1992, Chicontepec Field — Mexico, in N. H. Foster and E. A. Beaumont, eds., Stratigraphic Traps III, Atlas of Oil and Gas Fields: AAPG Treatise of Petroleum Geology, p. 113–128.

Cossey, S. P. J., and R. E. Jacobs, 1992, Oligocene Hackberry Formation of southwest Louisiana: Sequence stratigraphy, sedimentology and hydrocarbon potential: AAPG Bulletin, v. 76, p. 589–606.

Edman, J. D., and M. K. Burk, 1998, An integrated study of reservoir compartmentalization at Ewing Bank 873, offshore Gulf of Mexico, SPE 49246: Society of Petroleum Engineers Annual Technical Conference, New Orleans, Louisiana, USA, p. 653–668.

Galloway, W. E., T. E. Ewing, C. M. Garrett Jr., N. Tyler, and D. G. Bebout, 1983, Atlas of major Texas oil reservoirs: Bureau of Eco-nomic Geology, Austin, Texas, USA, 131 p.

Geological Society of Trinidad and Tobago, 2006, www.gstt.org (accessed April 21, 2006).Goodhue, C., and D. Cavit, 1992, Dos Cuadras Field: Shallow horizontal wells breathe new life into old field (abstract): AAPG Bul-

letin, v. 76, p. 419–420.Hall, E. A., 1981, The Horse Meadows oil field, in M. H. Link, R. L. Squires, and I. P. Colburn., eds., Simi Hills Cretaceous turbidites,

southern California, Pacific Section: SEPM, Fall Field Trip Guidebook, p. 107–110.International Petroleum Encyclopedia, 2003: PennWell Corporation, Tulsa, Oklahoma, 250 p. Lindblom, R. G., 1975, The Whittier Oil Field (central area), in J. N. Truex, ed., A tour of the oil fields of the Whittier fault zone, Los

Angeles basin, California: Pacific Section AAPG Field Trip Guidebook, April 1975, p. 25–41.McKay, G., C. Bennett, C. Price-Smith, and W. McLellan, 1998, Harding, A field case study: Sand control strategy for ultra high pro-

ductivity and injectivity wells, SPE 48977: Society of Petroleum Engineers Annual Technical Conference, New Orleans, Louisiana, p. 133–147.

Rangel, H. D., P. R. Santos, and C. M. S. P. Quintaes, 1998, Roncador Field, a new giant in Campos basin, Brazil: 1998 Proceedings no. 008876, Offshore Technology Conference, v. 1, no. 30, p. 579–588.

Schneider, W. J., and H. E. Clifton, 1995, Green Canyon 184 upper slope turbidites, in R. D. Winn and J. M. Armentrout, eds., Tur-bidites and Associated Deep-Water Facies: SEPM Core Workshop no. 20, Houston, p. 55–73.

Ventress, W. P. S., and G. W. Smith, 1984, The “E” Sand of Barataria Field, southeastern Louisiana — A Miocene deep-water, hydro-carbon reservoir: Gulf Coast Section-SEPM Foundation Research Conference, December 2–5, p. 103–108.

Weimer, P., and H. Pettingill, 2007, Global overview of deep-water exploration and production, in T. H. Nilsen, R. D. Shew, G. S. Steffens, and J. R. J. Studlick, eds., Atlas of deep-water outcrops: AAPG Studies in Geology 56, p. 7–11.

Weimer, P., and H. Pettingill, 2007, Deep-water exploration and production: A global overview, in T. H. Nilsen, R. D. Shew, G. S. Steffens, and J. R. J. Studlick, eds., Altas of deep-water outcrops: AAPG Studies in Geology 56, CD-ROM paper, 29 p.

Yip, F., J. Pear, and P. Siegele, 2005, The subsalt Tahiti Field discovery, Green Canyon 640: Opening another deepwater frontier: Hous-ton Geological Society Bulletin, October 2005, p. 11–13.

Figure 14. Chart of reservoir poros-ities (N = 291). Although about three-fourths are in the range of 10–30%, almost one-fourth have poros-ities >30%.

Figure 15. Chart of percentages of FADs when assigned by hydrocarbon type (N = 816).

Figure 16. Chart of API gravities for oils and condensates (N = 405).

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