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FORTIES FIELD, BLOCK 21/10, 22/6A, NORTH SEA, UK

1. PROVINCE GEOLOGY 1.1 LOCATION OF NORTH SEA The North Sea (56°00’ N, 4°00’ E) is the site of Western Europe’s largest oil and natural gas reserves. The North Sea Graben of north-western Europe is considered in the World’s Energy Project Province 4025 and is located entirely offshore with the territorial waters of Denmark, Germany, Netherlands, Norway and the United Kingdom. The oil and gas accumulations of the province are considered as part of the single petroleum system known as the Kimmeridgian Shales Total Petroleum System (TPS). During the period of extension and rifting, the source rocks of the Kimmeridgian Shales TPS were deposited in Late Jurassic to earliest Cretaceous time. (Gautier, D.L., 2005)

Figure 1. Location of North Sea (Modified from: Gautier, D.L., 2005)

1.2 STRATIGRAPHY OF THE PROVINCE The oil and gas reserves in the North Sea can be better understood by looking at the fundamentals of the extensional tectonics and failed rifting during the period of Late Jurassic and earliest Cretaceous. The geological history of North Sea can be subdivided into three parts depending on the main episode of rifting or basin cycles. These history also look at the period of sediment deposition before, during and after each of episode of rifting together also considering the volcanic activity within the respective area. North Sea is one of the classic examples of failed rift which was initiated during the rifting preceding Devonian to Preliminary Eocene period. (Jayasekera, A.J.,1999)

North Sea

Norway

United

Kingdom

Netherlands 0 170 kms

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Figure 2. Stratigraphic Summary of the North Sea Graben Province (Source:

Jayasekera, A.J.,1999)

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2. GEOLOGICAL CHARACTERISATION OF THE FORTIES FIELD The Forties field was the first major oil field developed in the British sector of the North Sea. The field is located predominantly in block 21/10 and lies some 180 Km (112 miles) ENE of Aberdeen. The Forties Field, discovered in October 1970, is located in the North Sea has been estimated that the field has an aerial coverage of approximately 35 square miles containing an estimated 1.8 billion barrels of recoverable oil based on present day economic evaluations and recovery techniques. The field was developed using 5 platforms (Alpha, Bravo, Charlie, Delta and Echo) with first oil being produced in 1975. Until May 1997, 2.2 billion barrels of oil is being produced by with a further estimation of 280 million barrels of oil still recoverable from the field. (Koehler, A.M et al., 1975)

Figure 3. Location of the Forties field, North Sea

3. STRATIGRAPHY OF THE FORTIES FIELD The hydrocarbons of this field are all accumulated in the Paleocene sandstone within an anticline structure. The Forties Member consists of fine- to coarse-grained, moderately to poorly sorted, locally pebbly sandstones that are commonly massively bedded and homogenous, or display dewatering structures. Beds of muddy, extremely poorly sorted, pebbly sandstone are also encountered, with rework in the stratigraphic of this field. The Lista Formation is attributed to the non marls, bio turbinated and poorly laminated greenish grey clay stones and mudstones which are stratigraphically located between the Sele and Lista Formations.

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Primary structures include planar lamination and sporadic small scale cross-lamination. Bed thickness is highly variable. Distal facies display repetitive successive units that rarely exceed two meters in thickness, whereas proximal facies often include units of 20 m or more. The inter-bedded mudstones and siltstones are generally carbonaceous, micaceous, poorly sorted and crudely laminated layers within the reservoir rocks of the field. (Koehler, A.M et al., 1975)

Figure 4. Distribution of the channel bodies within the Forties field.

Figure 5. Cross Sectional view of the Forties Field indicating the reservoir stratigraphic positions of the channel complexes.

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In the above diagram, each of the channel complexes has got considerable internal infrastructures and the detail of the relative positions of the channel complexes also change along the trend of the field. Each of the channel complexes represents a long period of erosion and deposition which has resulted in a complicated internal fill. Generally, net-to-gross is high (greater than 85%) but especially at the top of each complex there is frequently a lower net-to-gross section. In much of the field, it is believed that the boundaries to channel complexes form effective baffles to fluid flow. This is probably associated with mudstone deposition along the erosional base of the complexes. (Koehler, A.M et al., 1975) Faulting is recognised within the field and while it is insufficient to offset entire reservoir sections, there is good evidence from well behaviour and 4D seismic that these faults frequently form barriers or baffles to fluid flow. 4. HISTORY OF THE FORTIES FIELD It was discovered in 1970, when an exploration well encountered hydrocarbons in Palaeocene sandstone within an anticline structure. Four appraisal wells were drilled during 1970-1971, revealed a large reservoir at a depth of about 7,000 feet (2,100 m). Hence it was marked as the first and largest major oil field discovery in the United Kingdom sector of the North Sea. It was named as the Forties field after the sea area in which it lies. The field began producing oil in September 1975 and was officially inaugurated by Her Majesty on November 3, 1975. The Stock Tank Oil Initially in Place (STOIIP) has been calculated as 4343 MMBBL, and original reserves estimated as 2470 MMBBL, representing an overall recovery of 57%. The field came off plateau production of 500 000 BOPD in 1981, and by mid-1989 production had declined to about 250 000 BOPD. Forties ranks as the biggest single oil accumulation in UK waters, spanning some 28,000 thousand acres and it contains oil in place of as much as 5 billion barrels. Within years of being brought on stream by BP in 1975, output rose to as high as 500,000 b/d, but then fell steadily through the 1980s and 1990s.When Apache bought a 97% interest in the field from BP in 2003, the field was producing less than 50,000 b/d and its remaining proved reserves were estimated at just 147.6 million barrels. In April 2003, operatorship was transferred from BP after Apache acquired a majority interest in the field (96.14% working interest, rising soon after to 97.14%). Cumulative production up to 2003 had reached 2.5 billion barrels of oil. Since Apache has taken over, production through to the end of 2006 has totalled 75.3 million bbl oil while booked proved reserves at year end 2006 are 35% higher than when the field was acquired. Through to the end of 2006, Apache has invested $1.24 billion of capital in the field. (Cheterman, A. et al, Sept 2009)

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Figure 6. Production split of the forties field since 1997 showing the contributions from Apache drilling, workovers and interventions on the entire field life until 2007. 5. PETROLEUM SYSTEM OF THE FORTIES FIELD

• Reservoir Structure: It is situated close to the junction of the Central Viking and Witch Ground Grabens

• Source: Jurassic Kimmeridge clay formation • Trap: Forties is primarily a simple four way dip closed anticline overlying the

forties Montrose ridge. Depth to the crest is 2030 m subsea and vertical relief is about 190 m. reservoir seal is provided by the overlying conformable mudstone of the Sele formation. The Charlie Sand is effectively isolated from the remainder of the field, and part of the reserves in this unit appears to lie below the structural spilt point, stratigraphically trapped by a lateral facies change. Oil migration has occurred upwards from the Kimmeridge Clay source rock within the buried rift valley, and has then travelled along the sand layers. Some of the migrating oil has then been trapped within the dome-shaped beds of sandstones above the underlying hump.

• Reservoir: The Forties reservoir occurs in thick upper Paleocene sands deposited as a major sand rich submarine fan sequence. The reservoir consists of several stacked sandstone bodies. Based on seismic mapping integrated with the well log and bio-stratigraphic data, a new gross reservoir zonation has been defined, consisting of 5 zones as shown below.

• Reservoir Drive Mechanism: The recovery mechanism is bottom aquifer drive supported by peripheral seawater injection.

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Figure 7. Cross-sectional view of the Forties Field

6. RESERVOIR FACIES AND DEPOSITIONAL ENVIRONMENT The field is a four-way dip-closed anticline that overlies the Forties-Montrose Ridge and is close to the junction of the Central, South Viking, and Witch Ground Grabens as shown in the figure below. The reservoir occurs in the thick Upper Paleocene sandstones deposited as a sand-rich submarine fan sequence, probably of slope-apron or ramp type. (Cheterman, A. et al, Sept 2009) The association with grey, often laminated mudstone indicates that the bottom waters were anoxic. The presence of green and red mudstones is ascribed to the reworking from local highs.

Oil Water Contact

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Figure 8. Forties field tectonic and depositional setting

7. DRILLING AND SEISMIC PROGRAMS Seismic characterization states that top of the Forties Member is often associated with a strong to moderate positive amplitude event that can be correlated locally to sub-regionally. Sometimes it is interfering with the Base Balder Formation reflector making it more difficult to pick. The base of the Forties Member is often visible as a moderate to weak variably negative to positive acoustic amplitude event, and can be correlated sub regionally. The Forties Member can usually be identified by the presence of seismic tricks. Its thickest development follows axial parts of the Central Graben. (Vaughan, O., Sept 2007) To the east wedge shaped geometry may be seen in cross section, believed to represent distal thinning of submarine fan systems. In the area around the Cod Field a seismic convex up geometry can be seen at top and the bottom reflector is rather flat. Internally can sometimes be seen dipping reflectors. This may represent a mixture of submarine fan systems and pro-grading slope and shallow marine systems.

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Figure 9. Example wells from early in field life (left) showing a typically thick oil column and a well from Apache drilling phase (right) showing a thinner oil column in a more inter bedded reservoir section.

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Figure 10. Fluid change volume for 2005-2000 time steps in the uppermost 10 milliseconds of the reservoir. Significant increase in the water volume is shown in dark blue, minimal change in yellow. The sweep pattern along the channel complexes can be seen at this scale and when examined in more detail, sweep associated with aquifer influx or water injection to producer is highlighted. 8. FUTURE OPPORTUNITIES A number of projects are in progress such as further drilling rig upgrades and increasing gas lift volumes on the "deep" gas lift platforms. In addition to these projects, it has been recognised that with a large stock of remaining drilling targets there is an opportunity to assess future projects in an increasingly integrated way across the entire field. The redevelopment of Forties to date has at times been reactive, but has now created a period of greater stability. This is allowing the next level of issues such as plant throughput capacity, optimized power generation and supply, water injection volumes, and drilling slot constraints to be addressed in a more holistic way. Given what has been learned over the preceding years, it is certain that Forties will continue as an important producing field for many years to come. (Brand, P.J. et al, April 1996)

Sweep along Bravo

Channel Complex

Sweep along

Charlie Channel

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9. RESERVOIR AND FLUID CHARACTERISATION & FACILITIES

RESERVOIR CHARACTERIZATION

FORMATION FORTIES

AGE PALEOCENE

AREA, SQ. MILES 36

DEPTH TO CREST, FT SUBSEA 6.6603

INITIAL OIL/WATER CONTACT, FT SUBSEA 7.2746

MAXIMUM OIL COLUMN, FT 614

AVERAGE POROSITY 0.27

RANGE 0.1 TO 0.36

PERMEABILITY

AVERAGE, MD 700

RANGE, MD 30 TO 4000

NET/GROSS RATIO AVERAGE 0.65

INITIAL PRESSURE AT 7.136 FT SUBSEA, PSIG 3215

TEMPERATURE AT 7.136 FT SUBSEA, F 204.8

FLUIDS CHARACTERIZATION

OIL GRAVITY, API 37

OIL VISCOSITY, AT 3.215 PSIG AND 204.8 F 0.76

PRODUCTION GOR (MAIN FIELD) SCF/STB 300

BUBBLE POINT AT 204.8 F, PSIG 1,142 TO 1,390

WATER SALINITY, PPM CL 55,000

OIP, MMSTB 4,196

RECOVERY FACTOR % 59

REMAINIG RESERVESMMSTB 218

FACILITIES

NUMBER OF WELL SLOTS 142

NUMBER OF CONDUCTORS DRIVEN 111

NUMBER OF WELLS 103

PEAK MONTH DEC 1978

PEAK MONTH PRODUCTION B/D 557,000

PLATEAU PRODUCTION B/D 500,000

CURRENT PRODUCTION B/D 111,000

CURRENT WATER PRODUCTION B/D 246,000

CURRENT WATER CUT % 69

CURRENT INJECTION B/D 390,000

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10. REFERENCES

1. Gautier, D.L. (2005), Bulletin 2204-C: “Kimmeridgian Shales Total Petroleum System of the North Sea Graben Province”, U.S. Department of the Interior and U.S. Geological Surve, Reston, Virginia, 1-20.

2. Jayasekera, A.J., Goodyear, S.G (March 1999), “The Development of Heavy

Oil Fields in the United Kingdom Continental Shelf: Past, Present and Future”, Society of Petroleum Engineers, SPE 54087, 1-16.

3. Gringarten et al. (October 2000), “A Petroleum Engineering Educational Model Based on the Maureen Field UKCS”, Society of Petroleum Engineers, SPE 64311, 1-17.

4. Jones, R.D., Lurie, P., Hibbert, E.D. (Sept 2007), “Design, Planning, Implementation & Management of a Multi-Lateral Well on the BP Forties Field: A North Sea Case History”, Society of Petroleum Engineers, SPE 38494, pp 1-15.

5. Brand, P.J., Clyne, P.A., Kirkwood, F.G., Williams, P.W., (April 1996), “The Forties Field: 20 Years Young”, JPT, pp 280-291.

6. Cheterman, A., Vaughan, O. and Plahn, S. (Sept 2009), “Forties: Creating a Long Range Development Plan for a large, mature, Complex North Sea Oil Field”, SPE127605, pp 1-12.

7. Vaughan, O., Jones, R. and Plahn, S. (Sept 2007), “Reservoir Management Aspects of the Rejuvenation of the Forties Field, UKCS”, SPE 109012, pp 1-20.

8. Koehler, A.M et al. (1975), “Development of North Sea Forties Field Platform Concept”, Offshore Technology Conference, OTC 2245, Texas 75206, pp 1-12.