Play Concept Of The Draugen Field Biology Essay

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This paper describe the proven petroleum system or "play concept" of the Draugen field in the Haltenbanken area on the mid-Norwegian shelf. The Draugen field was discovered by Norske Shell in 1984 on the Trondelag Platform with the exploration well 6407/9-1. Several reservoir rocks have been identified in the exploration phase. However, Draugen field is producing oil mainly from Rogn Formation (informally the name Frøa is used) of Late Jurassic age and from Garn Formation of Middle Jurassic age. Today the field has in total five subsea production wells and six subsea water injection wells, where only two injection wells are in use, The production started in 1993 and till now the field produced approximately 125.5 mil Sm3 of oil. The field is located in water depth of approximately 250 meters and is buried 1600 meters beneath the sea-bottom. The geometry of the reservoir is extended some 20 kilometers in longitudinal direction (South to North) by 6 kilometer (East to West). Rogn and Garn Formations are forming a low relief anticlinal structure, with a vertical closure of approximately 50 meters. Main formation properties are porosity 27%, net to gross ratio 97% and high permeability of 4 Darcy. Norske Shell is the operator company of the field.

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Draugen field is regarded as one of the key discoveries in the Norwegian Sea due to previously not envisaged Upper Jurassic Formations. The main formation is the Rogn Formation which is interpreted to be a shallow marine sand bar on the Trondelag platform in Late Jurassic with very good reservoir properties and aquifer support. Furthermore, Garn Formation sandstones of Middle Jurassic are forming a smaller reservoir in the field.

Introduction

The purpose of this paper is to describe the proven petroleum system or "play concept" of the Draugen field in the Haltenbanken area on the mid-Norwegian shelf. The Draugen field was discovered by Norske Shell in 1984 on the Trondelag Platform with the exploration well 6407/9-1. Several reservoir rocks have been identified in the exploration phase. However, Draugen field is producing oil mainly from Rogn Formation (informally the name Frøa is used) of Late Jurassic age and from Garn Formation of Middle Jurassic age. In total 6 wells were drilled as part of the exploration and appraisal phase and production started in 1993. The field is located in water depth of approximately 250 meters and is buried 1600 meters beneath the sea-bottom. The geometry of the reservoir is extended some 20 kilometers in longitudinal direction (South to North) by 6 kilometer (East to West) (Figure 1). Rogn and Garn Formations are forming a low relief anticlinal structure, with a vertical closure of approximately 50 meters.

Draugen field is a high quality reservoir, contains the most prolific oil production well, with current production of approximately 0.25 mil Sm3 per month (Norwegian Petroleum Directorate. The NPD's Fact-pages, 2010). The main formation properties are porosity 27%, net to gross ratio 97% and high permeability of 4 Darcy. Norske Shell is the operator company of the field. The field is producing from seven production wells in the center of the field and another five water injection wells at the northern and southern end of the field are used for pressure support. Water is mainly injected in order to keep reservoir pressure close to its initial value which, therefore, is the main reason why the field has no primary gas cap.

Figure 1. Draugen field (Norwegian Petroleum Directorate. 2010).

Source rock

The source formation of oil in the Draugen field is the same for both the reservoirs. This is the Upper Jurassic Kimmeridge clay equivalent of the Spekk Formation. This formation is the main source for oil accumulations in the Haltenbanken - Trondelag area. In addition, it is believed that other organic carbon-rich formations may have also charged the same area with hydrocarbons. These sources formations are belonging to Paleocene, Lower Cretaceous, Jurassic and Permian sequences (Gautier 2000).

Anoxic bottom water conditions (Stagnant bottom) were interpreted as the depositional environment of the Spekk shales. The lithology of this formation is mainly Claystone and has average total organic carbon (TOC) 7% of Kerogen type II (Goesten and Nelson 1992). The main source formation is widely spread in the area relative to the Draugen field; it can be found as the surrounding formation of the reservoir formation as well as in big accumulations in the neighbor area of the Halten Terrace.

Maturation/Migration

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Spekk formation (source rock) within the Trondelag Platform area is interpreted as immature oil source, nevertheless the same source in the adjacent deeper area of Halten Terrace is maturated appropriately and its vitrinite reflectance (maturation) is Ro=1.1. Primary migration started 65 Ma. According to Goesten and Nelson (1992) the present depth to top of source is 4000 meters and the thickness of the source is some 100 meters.

Reservoirs

Rogn Formation of late Jurassic age is the main oil-bearing formation. Furthermore, on the west side of the Draugen field Garn Formation of Middle Jurassic age is containing oil as well. Both reservoirs have good characteristics. Below a more detailed description is given for each reservoir.

Garn Formation

Garn Formation is located below the Rogn Formation and extends throughout all Draugen field with a constant thickness of 110 m. Only the top part is oil bearing with an oil column of approximately 18 m at its higher part (Figure 5). This Middle Jurassic Formation is comprised mainly by two sequences. The upper one contains from bottom to top silt shales with thin, bioturbated sand layers, passing into fine-grained cross-bedded sandstones (Provan 1992). Reservoir properties from this sequence are porosity variation of 33% to 36% and average range of permeability 0.2 to 7 Darcy. There is limited information for the bottom sequence in this area due to lack of coring. Nevertheless, this sequence is comprised of two coarsening upward sequences, at the base there are 65 to 75 m of thick sequence of basal silty shales and becoming sandier upward, passing to coarse grained poorly sorted massive sands at the top (Goesten and Nelson 1992). This sequence is not present in all wells drilled in Draugen field. Average porosity of this unit is 29% and the permeability range between 5 to 10 Darcy.

The Garn Formation is interpreted as progradational sequence of shallow marine to coastal deposits.

Rogn Formation

There are several interpretations of the Rogn Formation, Goesten and Nelson (1992) have described the mineralogy and reservoir geology, the palynostratigraphy and palynofacies of the unit have been described by Van der Zwan (1989, 1990).

The interval between the reservoir quality sands of the Rogn and Garn Formations which consist of bituminous shales has been interpreted by Goesten and Nelson (1992), as part of the Rogn Formation and as part of the Spekk Formation by (Provan 1992). In general, Rogn Formation forms a coarsening upward sequence which can be categorized in two subdivisions with extensions of both intervals throughout all Draugen field. In the lower division we can recognize a normal transition from the underlying shales of the Spekk formation of silty claystones and sands to strongly bioturbated fine grained sandstones. These can also be observed moving towards the central part of the field from west and east sides. Porosity of this interval ranges from 26% to 30% and permeability varies from 0.7 to 2 Darcy, the later value of the permeability corresponds in the upper most part of the sub-unit.

The uppermost unit of the Rogn Formation comprises unconsolidated sandstones with fine to medium grain size. The sorting of this formation is moderate and contains abundant dispersed coarse grains. In general, the sands are massive and bioturbated, however it is observed scarce low angle cross bedding. This unit thins towards the west side of the field and it pinches out.

The very good to excellent reservoir properties of this unit have been explained as that limited compaction and diagenesis have taken place with regards to the depositional history. Therefore, high porosities of 27% to 32% exist in the field accompanied with permeabilities that vary from 6 up to 10 Darcys.

Figure 2. Fault system and structure map Rogn Formation (Goesten and Nelson 1992).

Seal

The seal for the main reservoir of the Draugen field, Rogn Formation (Oxfordian - Kimmeridgian) is also the source Spekk Formation. This is due to the fact that Rogn Formation developed as lens within the Spekk Formation (Figures 4 % 5). For that reason, Spekk Formation is regarded as the cap rock as wells as the seat seal for the main reservoir of the Draugen field. Rogn Formation is characterized as a low relief anticline; the sides are sealed on the north south by dip closure and on the east and west by a combination of dip closure and pinch out of the formation (Figures 7 & 8).

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The underlying Garn Formation is sealed by Spekk formation as well, at least where the unit has oil accumulations and is regarded as a separate reservoir of the Draugen field.

Trap

Draugen field is an exceedingly low relief north to south trending anticline which extends on an area of approximately 20 km by 6 km (Figure 2). The top/ceiling of the structure is found on a depth of 1600 m subsea. The maximum vertical closure is of about 50 m this exists due to the highest structural relief point at the north central part of the field. It can be seen in figure 1 that there is a southern extension of the field due to a lower relief southern lobe by a structural saddle which has a smaller vertical closure of 17 meters.

It is also notable the system of faults that run the Draugen field. A northeast to southwest trending of faults is running the greater area of Haltenbanken (Figure 3). This trend is also followed in the Draugen field but with relatively smaller faults (Figure 2). These minor normal faults are observed in a northeast to southwest direction, the throw of these faults is small, less than 15 meters. These faults are observed only at the lower part of the Rogn Formation extended to deeper zones. Only some of the faults are penetrating the Rogn Formation of the flacks of the system on the east and west of the formation.

Figure 3. Map showing fault trends in Haltenbanken area (Goesten and Nelson 1992).

Figure 4. South - North structural cross section of Draugen Field (Goesten and Nelson 1992).

Figure 5. East - West structural cross section of Draugen Field (Goesten and Nelson 1992).

Discussion

Aquifer Support / Development plan

Back in the initial phase of the development of this field a pressure support by injection of water was pointed as the optimal production solution. This was supported from geophysical evaluations of 2D data and results from the exploration and appraisal wells. The field has been developed with placing the production wells on central positions of the field close to the crest and all injection wells on the south and north parts (Figure 4). Aquifer support on the field is present form the extension of the high reservoir quality sands of the Rogn Formation on the south and north part of the field below the oil water contact. In addition, the underlying Garn Formation was interpreted as main aquifer support of the Rogn Formation even though communication between these two formations is quite restricted by the development of the Spekk Formation. The northern aquifer was believed not to be very high, but during 2005 the water injection in the north part of the field was ceased due to technical reasons without having a great impact on the production (Norwegian Petroleum Directorate. The NPD's Fact-pages, 2010). A simple explanation for these could be that the faults on this part of the field to provide a better communication between Rogn and Garn Formations.

Conclusion - Recent Developments

Draugen field is regarded as one of the key discoveries in the Norwegian Sea due to previously not envisaged Upper Jurassic Formations. The main formation is the Rogn Formation which is interpreted to be a shallow marine sand bar on the Trondelag platform in Late Jurassic with very good reservoir properties and aquifer support. Furthermore, Garn Formation sandstones of Middle Jurassic are forming a smaller reservoir in the field.

Currently field's exploitation is been done with a concrete fixed facility at 2509 meters water depth. The field has in total five subsea production wells and six subsea water injection wells, where only two injection wells are in use.

The current Draugen oil reserve estimates are 145 mil Sm3 total recoverable and 19.7 mil Sm3 are the remaining oil reserves (Norwegian Petroleum Directorate. The NPD's Fact-pages, 2010).

References List / Bibliography

NORWEGIAN PETROLEUM DIRECTORATE. THE NPD's FACT-PAGES, 2010. Field: Draugen. [online]. Stavanger. Available from: http://www.npd.no/engelsk/cwi/pbl/en/field/all/43758.htm. [Accessed 8 April 2010].

NORWEGIAN PETROLEUM DIRECTORATE. 2010. Draugen. [online]. Stavanger. Available from: http://www.npd.no/factmaps. [Accessed 10 April 2010]

GAUTIER, D.L., 2000. Halten terrace - trondelag Platform Assessment Unit 40170101. p

[online]. Reston, VA: U.S. Geological Survey World Petroleum Assessment 2000 - Description and Results U.S. Geological Survey World Energy Assessment Team. Available from: http://energy.cr.usgs.gov/WEcont/regions/reg4/P4/tps/AU/au401711.pdf. [Accessed 8 April 2010].

GAUTIER, DONALD L., 2005, Kimmeridgian Shales Total Petroleum System of the North Sea Graben Province: U.S. Geological Survey Bulletin 2204-C, 24 p.

GOESTEN, M.J.B.G., NELSON, P-H., 1992. Draugen Field, Norway, North Sea basin, Haltenbanken area, In Structural traps, VI: AAPG Treatise of Petroleum Geology, Atlas of Oil and Gas Fields, A-24, p. 37-54.

PROVAN, DONALD M.J., 1992. Draugen oil field, Haltenbanken Province, Offshore Norway. In Giant oil and gas fields of the decade 1978-1988: AAPG memoir 54, p. 371-382.

VAN der ZWAN, C.J., 1989. Palynostratigraphical principles as applied in the Jurassic of the troll and Draugen fields, Offshore Norway, in Correlations in Hydrocarbon exploration: Norwegian Petroleum Society, London, Graham and Trotman, p. 357-365.

VAN der ZWAN, C.J., 1990. Palynostratigraphy and palynofacies reconstruction of the Upper Jurassic to Lowermost Cretaceous of the Draugen field, offshore mid Norway: Review of Paleobotany and palynology, v. 62, p. 157-186.

OTTESEN, D., RISE, L., ANDERSEN, E.S., BUGGE, T. & EIDVIN, T.: Geological evolution of the Norwegian continental shelf between 61°N and 68°N during the last 3 million years. Norwegian Journal of Geology Vol. 89, pp. 251-265. Trondheim 2009, ISSN 029-196x.

DALLAND, A., WORSLEY, D., and OFSTAD, K., eds., 1988. A lithostrategraphic scheme for the Mesozoic and Cenozoic succession offshore mid- and northern Norway. NPD-Bulletin No 4. Oljedirektoratet Biblioteket. [online]. Stavanger. Available from: http://www.npd.no/Global/Norsk/3-Publikasjoner/NPD Bulletin/NPD_BulletinNo4.pdf. [Accessed 3 April 2010]