What's been drilled, found in world's deepwater areas

Eloi Dolivo
Petroconsultants SA
Geneva

The first wildcat drilled in deep water for oil exploration predates by two years the onset of the Deep Sea Drilling Project.

Pan American abandoned the Pan Am-1OE Tors Cove D-52 well 300 km southeast of Newfoundland in 710 m of water in 1966. The DSDP started in 1968 and managed the drilling of more than 1,100 holes before being succeeded by the Ocean Drilling Program in 1985. Until 1973, deepwater oil exploration remained a Newfoundland curiosity; just six wells were drilled at the time without commercial success.

The term deep water in this article relates to water more than 200 m deep. Ultradeep water refers to depths exceeding 500 m.

This information comes from the Petroconsultants IRIS21 database as of December 1996 and for the Gulf of Mexico from the U.S. Minerals Management Service. These data are available on various Excel spreadsheets, by region and worldwide, for those readers who would like to investigate in more detail.

All mentions of reserves refer to proven and probable recoverable reserves. However, the only reserves figures published for the Gulf of Mexico are indeed from the MMS, and they cover only the producing fields.

The oil prices are based on the Brent average price of the year in money of the day, not corrected to December 1996 U.S. dollars.

More firsts

With the exception of Shell in the South Java basin, it was not until 1973 that oil price increases prompted explorers to check whether proven petroleum plays on the shelf extended into deep water (Fig. 1 [35,435 bytes]).

Apart from the first wells in the Gulf of Mexico, Trinidad & Tobago, and the arctic, which turned out to be gas discoveries, exploration took two to six years to mature from first well to first discovery (Fig. 2 [33,044 bytes]).

Shell discovered Cognac oil field in the Gulf of Mexico Mississippi Canyon area in 260 m of water shortly after another U.S. operator discovered a gas field in nearly that water depth in Mississippi Canyon Block 148 in 1974. Cognac through early 1997 has produced about 85% of its 180 million bbl of initial recoverable reserves.

Shell drilled the Astarte Marine 1 in the Gulf of Guinea off Gabon in 1974; the well, drilled to TD 2,303 m in 644 m of water, is a duster. However, Phillips A-01X oil discovery off Cote d'Ivoire in 1980 triggered the first 3D seismic in sub-Saharan Africa in 1981.

Elf drilled the first deepwater wildcat in the western Shetlands in the North Sea. The 205/20-1 was abandoned in 1974 in 215 m of water. Mobil found oil in the 211/13-6 well in the Viking graben; the well bottomed at TD 3,700 m in Triassic Banks sandstones.

Esso first ventured into deep waters of the Mediterranean in 1975 and abandoned as dry the NDO B-1. Chevron made the first discovery there in 1977 when the Montanazo D1 in the Ebro basin yielded 9,504 b/d of 32° gravity oil from Jurassic strata.

In the Indian Ocean region, Esso abandoned the RSO B95 1 at TD 4,022 m in the Red Sea in 1976. Indian state company ONGC made the first discovery in the area, in the Krishna-Godavari basin. The G 01-1 wildcat tested 676 b/d of 31.2° gravity oil on a 32/64 in. choke from a 5 m sandy interval in Pliocene. Some 50 m lower, another 5 m interval flowed gas and condensate.

The wildcat is one of two discoveries in the Indian Ocean region. The other is ONGC's G 02-2 in 1983.

Shell is the pioneer of deepwater drilling in Southeast Asia. Its drilled Alveolina 1 in 1972 to 2,163 m in basement in 352 m of water. It was to be nine years until Cities Service at last made a discovery, Galoc 1, in the region. It tested 1,712 b/d of 35.3° gravity oil from Miocene sandstones. Oil shows were logged in more than 300 m of limestones below 2,669 m. Seismic showed a 40.5 sq km structure at the time.

It was again Shell that first drilled a deepwater wildcat in Australia in 1973. The Mermaid East 1 bottomed in Jurassic silty sandstones interspersed with coal of the Canning basin at TD 4,048 m. Esso discovered oil in Hapuku 1 in the Gippsland basin in 1975.

By 1976-78, exploration- ists were also proving the ultradeepwater, or more than 500 m (Fig. 3 [39,660 bytes]). Between one and six years later, and in any case before the 1986 oil price collapse, ships were drilling in more than 1,000 m of water (Fig. 4 [38,395 bytes]).

Brazilian state company Braspetro made the first discovery in 1,155 m as Rio de Janeiro Submarino (RJS) 366 flowed 530 b/d of 19° gravity oil from an interval topping 2,450 m in the Turonian to Holocene Carapebus mass flow deposits. The well bottomed at TD 3,401 m. Two other intervals remained untested following formation damage. By 1996, Shell's 1987 Coulomb MC 657-1 is the deepest discovery. It sits in the Gulf of Mexico in 2,286 m of water.

Table 1 [19,208 bytes] shows the onset of deepwater exploration by region.

The time perspective

Fig. 5 [28,196 bytes] shows the number of deepwater and ultradeepwater wildcats year by year. This number neatly follows the price of oil until 1992, sometimes with the few years' lag needed for a prospect to mature to the drilling stage.

The 1984-85 maximum followed by three years the oil price peak due to delays in interpreting data and availability of drilling units.

Since 1993, the number of wells increased independently of the price of oil, mainly as a result of the falling price of developing deepwater resources. As quoted from P. Delaittre from Elf Aquitaine in a presentation at the 1997 French technicians meeting, "Development investment in ultradeep waters of the Gulf of Mexico is of the same order of magnitude as in shallower water. Capital and operating costs amount to some $5/bbl."

Another striking feature of the recent trend is the growing number of wells drilled in ultradeep water. Of the 24 deepwater wildcats drilled in 1996, 14 were in more than 500 m of water.

The success ratio has steadily risen from an initial 10-20% in the 1970s to 30-40% (Fig. 6 [30,318]). The figure also shows three to four year cyclic trend of success and failure. Time and tools are missing for conducting a detailed analysis of this trend, but guidelines for such work are available (Table 2 [11,670 bytes]).

Since 1992, the success ratio tends to be more stable as deepwater emerged as a ground with its own plays (mainly gravity flows from the shelf) rather than being only the extension of plays proven on the shelf, the main game the majors chased in deep waters in the late 1970s-early 1980s.

Deepwater today

The North Sea, Gulf of Mexico, and Latin America-which holds the largest share of ultradeepwater wildcats-lead the pack in the number of wildcats drilled in deep water ( Fig. 7 [29,932 bytes], Fig. 8 [30,656 bytes]).

Other than five wells, Brazil's state Petrobras has drilled all deepwater wildcats in Latin America, mainly in the Campos and Santos basins. The Gulf of Guinea off Africa, with at least the same potential as the Brazil deep offshore, has barely reached 40% of Petrobras' drilling effort in the Brazilian deepwater.

In terms of success ratio, Petrobras definitely leads the pack with a success ratio exceeding 50%. This performance may be due to a good continuity of the plays from shelf to deep waters.

The arctic Norwegian and Russian sectors follow with 42%, a figure that could be attributed to the early discovery of 3 tcf Snohvit gas field in 1984. A 33% success ratio rewards explorers in the Gulf of Mexico and just over 24% of those in the North Sea.

Exploring the deep waters is barely more risky than exploring any other kind of terrain (Table 3 [4,535 bytes]). As per the end of 1996, the number of wildcats drilled in water more than 1,000 m deep is 111, of which 24 are in the Gulf of Mexico and 37 in Brazil's Campos and Santos basins.

Deepwater reserves

Reserves discussed are proved plus probable except for the Gulf of Mexico, where the only available figures are proved recoverable reserves in producing fields only.

The slope of cumulative discovered oil in deep water worldwide has generally increased, especially if the Gulf of Mexico flat performance is redressed to an amount comparable to North Sea and South America (Fig. 9 [22,994 bytes]).

The first major reserve increment is the discovery of Troll and Snorre gas and condensate fields in the North Sea Norwegian sector. Norsk Hydro discovered Troll's 1.4 billion bbl of liquids and 47 tcf of gas in Malm Viking Group sandstones of the Horda platform. Oil reserves are exploited since 1995, while gas has fed the Norwegian grid since 1991. Meanwhile, Saga found Snorre in Triassic fluviatile sandstones of the East Shetlands basin. It took 13 years for the operator to bring on stream the 1.35 billion bbl of oil and 350 bcf of gas reserves.

The next milestone in massive reserves growth of deep waters (Fig. 10 [37,299 bytes]) is the discovery by Petrobras between 1984 and 1989 of Albacora, Marimba, Marlim, Marlim Sul, and Barracuda fields. All those accumulations hold their reserves in the Upper Cretaceous Carapebus sandstones deposited in a mass flow deposits environment. Worth noting is the nascent Gulf of Guinea curve, in the mirror image of the Brazilian geologic setting.

In 30 years, deepwater operators found no less than 21 billion bbl and 88 tcf of recoverable reserves-more than 37 billion bbl of oil equivalent-of which they have produced less than 4%. These figures are 2.3% and 1.9%, respectively, of oil and gas remaining reserves in the world.

Elf's estimates of deepwater reserves published in December 1996 indicate a 75 billion bbl potential, one third liquids.

Apart from probable reserves in producing fields and all reserves in undeveloped discoveries of the Gulf of Mexico, the North Sea holds 54% of the reserves, of which more than half are in the two giants Snorre and Troll. The largest liquids reserves outside of the Gulf of Mexico are in Brazilian deep waters: 41% of the oil reserves in deep water lie in Brazil, of which 46% are in Marlim and Marlim Sul fields only.

Production has started to deplete stocks to some extent only in the Gulf of Mexico and Brazil (Fig. 11 [25,724 bytes]).

The South Atlantic

An article on such an exciting exploration pitch does not deserve a conclusion because the game is just beginning. However, the South Atlantic case is worth a few observations.

Here is a much simplified comparison between the Campos basin in Brazil and the Lower Congo and Kwanza basins in Angola (Fig. 12 [41,443 bytes]). Both sides of the Atlantic show the same tectonic events creating the same sedimentary patterns, in particular the Malembo mass flow deposits with an assigned Oligocene age in Angola, and the Carapebus "turbiditic" sands (in fact many of them are deepwater fans) disseminated from the Upper Cretaceous to the Miocene. As mentioned above, the Carapebus hosts most of the Brazilian giant fields; in Angola, Shell's Bengo find and Elf's giant Girassol lie in the Malembo formation.

Apart from 11 wells, most of them drilled in the late 1970s-early 1980s and scattered from Honduras to Chile, all deepwater wildcats in Latin America were drilled by Petrobras in the Campos and Santos basins. Of those 141 wells, Petrobras drilled 87 in more than 500 m of water.

Petrobras is probably the company in the world with the largest experience in drilling in deep water. The number of wells is completely independent of oil price, with a rapid increase in the number of wildcats drilled in ultradeep water (Fig. 13 [66,040 bytes]). Market economics was not until recently a driving criterion in the selection of prospects.

By contrast it took more than 15 years to realize the potential of the mirror geological setting on the other side of the Atlantic, the Gulf of Guinea (Fig. 14 [36,465 bytes]). The number of wildcats depends closely on the oil price. The late 1970s-early 1980s fever is an urge to check whether shelf plays extend into deep water.

The 1986 oil price collapse cooled explorers' enthusiasm. However, a better understanding of the deposition of deepsea mass flows, the improvement in price and technology of 3D seismic, the cheaper alternatives to develop deepwater fields, and the Brazilian example urged majors to go and have a second and most successful look. And at least in Congo, shelf plays extend into deep water, as the example of the Moho oil discovery in the Albian carbonates rift shows.

The case is a fine example of what a dynamic state company can afford to do (regardless of economics) and also shows the bright future of Gulf of Guinea deep waters.

The reader is welcome to use the following tables on Petroconsultants' website:

• exploil.xls
  Country, megaregion, basin, well name, content, initial operator, objective age, completion year, bottomhole age, TD in meters, status, deviated/vertical, number of rig days, and water depth in meters.
• gmwildc.xls
  Spud year, company name, field name, field code, well number, type, status (interpreted), and water depth in meters.
• fieldswld.xls
  Country, megaregion, basin, field name, discovery date, water depth in meters, ultradeep/deep/unknown, reservoir lithology, current operator, original operator, production status, reservoir younger age, reservoir older age.

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