Vergara, L.1, H. Brunstad1, T. Nordlie1, M. A. Charnock2 & F.M. Gradstein3
1. RWE Dea Norge, Karenslyst allé 2-4, N-0212, Oslo
2. StatoilHydro, PO Box 7190, N-5020 Bergen
3. Geological Museum, POB 1172 Blindern, N-0318 Oslo
Figure 1. Cretaceous stratigraphy of the northern North Sea
Derivatio nominis: Isaksen & Tonstad (1989) defined the unit after the Agat gas-condensate field in block 35/3.
Publication: ISAKSEN, D. & TONSTAD, K. 1989. A revised Cretaceous and Tertiary lithostratigrahic nomenclature for the Norwegian North Sea. Norwegian Petroleum Directorate Bulletin, 5, 1-59.
"“In the type well the member consists of white to light grey, fine- to medium-grained moderately to well-sorted sandstones alternating with grey claystones. The sandstones are usually micaceous and glauconitic and sometimes contain small amounts of pyrite. The sandstones in the type well are carbonate- and silica-cemented in zones. In the reference well, the upper part of the member consists of medium- and coarse-grained to pebbly sandstones and conglomerates alternating with dark grey claystones. The conglomerates are both matrix- and grain-supported. The claystones are often found as 0.5-5 m thick layers between the sandstones. They are dark grey, usually calcareous and contain varying amounts of siltstone. They may occasionally pass into light grey, micaceous, calcareous and glauconitic siltstones."
Slides available from type well 35/3-4 for the interval 660-4043 m (NPD)
Figure 2. Paleobathymetric reconstruction of the northern North Sea, prior to the deposition of the Agat Member. A rugged relief and largely exposed basin at the beginning of the Cretaceous (Base Cretaceous Unconformity, BCU) was the result of regional Late Jurassic-Early Cretaceous rifting.
Figure 3. Paleobathymetric reconstruction of the northern North Sea, following the deposition of the Agat Member. The former relief was filled during the thermal subsidence phase and a more gentle morphology between basin and slope was created, whereas the ancestral shelf back-stepped to the East and is mostly absent today.
WGS84 coordinates: 61°51'54.54" N 3°52'26.99" E (see Fig. 2)
UTM coordinates: 6859631.80 N 545989.90 E
UTM zone: 31
Operator: Saga Petroleum
Completion date: 06.06.1981
Status: P&A G/C W
Interval of type section: The type section is from 3542 m (originally 3589 m, see below) to 3345 m. See Fig 4.
Figure 4. Type section for the Agat Member
WGS84 coordinates: 61°47'46.71" N 3°54' 44.01" E (see Fig. 2)
UTM coordinates: 6851990.69 N 548099.85 E
UTM zone: 31
Operator: Saga Petroleum
Completion date: 31.03.1982
Status: P&A dry
Interval of type section: The reference section is from 3605 m (originally 3620 m, see below) to 3219 m. See Fig 5.
Figure 5. Reference section for the Agat Member
Figure 6. Wireline log expression of the upper and lower boundaries of the Agat Member
Figure 7. Type seismic section for the Agat Member: cross line
Figure 8. Type seismic section for the Agat Member: in-line
Figure 9. Seismic section through reference well: cross line
Figure 10 Seismic section through reference well: in-line
Figure 11. Seismic section through type and reference wells of the Agat Member
Figure 12. Lower Cretaceous foraminiferal zonation of Gradstein et al. (1999).
Gradstein & Agterberg (1998) first brought forward that H. delrioensis FCO & LCO are useful to correlate the Agat sands and further north, separate more scattered Rødby sands, assigned to the lower Cenomanian, from more massive Rødby sands, assigned to middle Cenomanian Turonian or Coniacian. The planktonic LCO event, observed in 14 wells, evidently is a lull in sand sedimentation. In the 35 block (Agat) area, the H. delrioensis FCO event occurs in the uppermost O.schloenbachi Zone with the LO of Ovoidinium scabrosum, Upper Albian, whereas the H. delrioensis LCO event marks the top of the H. delrioensis LCO zone, at the transition from lower to middle Cenomanian that here also includes the LO’s of Rhombodella paucispina, Plectorecurvoides alternans and Lingulogavelinella jarzevae. In terms of local sands, the H. delrioensis FCO event in the Agat area appears abruptly almost at the top of the massive Agat sands of Albian age in well 35/3-4, and the LCO event occurs above some more Agat sands of possible Early Cenomanian age in 35/3-5. Hence, the stratigraphic range of common to abundant Hedbergella delrioensis is in the upper half of the Agat sands (Gradstein, wr. com., 2000).
The age determinations are summarized in the biostratigraphic well correlation of Fig. 13. Hydrocarbon bearing sands in the 35/3-2 and 35/3-4 wells are of Middle - Late Albian age, and more than 100 m thick (net) in 35/3-4. The upper massive sands in the 35/3-5 well, devoid of hydrocarbons, are of Albian through possibly Early Cenomanian age. In the most landward wells 36/1-1 and 1-2, the Albian is absent. Understanding of reworking in these wells is crucial to age determinations. Reworked Aptian assemblages occur in well 35/3-1 (3935 to 4085 m), while reworked Jurassic palynomorphs occur in a lower section (4120-4141m), and also in Albian sands of well 35/3-5, together with Lower Cretaceous dinoflagellate cysts. Apart from this, the base of the Agat sand is diachronous, responding to deposition first in the East followed by slightly younger sand dispersal to the West (Fig. 13).
In the Gjøa area well 35/9-3 penetrated a reservoir section (2657.5 -2701.5 m) of Late to Middle Albian. Later, well 36/7-3 encountered a massive sand reservoir section (2532-2632 m) dated Late Albian. Both intervals are attributable to the Agat Member and occur within shales or marls of the Rødby Formation.
Note that the Lower Albian is missing in all wells of block 35/3. The break occurs in wells 35/3-1 at 4085 m, in well 35/3-2 at 3726 m, and in well 35/3-5 at 3620 m (Gradstein, writ. com., 2000). It roughly coincides with the base Agat and can be related to an erosional vacuity due to an important sea-level drop followed by lowstand deep-water sands in these wells. Biostratigraphic analyses of well 35/3-4 carried out by Mobil (Skibeli et al., 1995; fig. 16), postulate a division of the Agat Member into three sequences bounded by maximum flooding surfaces. The oldest one at 3565 m coincides with the major break where the Lower Albian is absent.
Figure 13. Biostratigraphic correlation showing diachroneity in the onset of the deposition of the Agat Member.
Incorporation of biostratigraphical, seismic and cyclostratigraphical tools have been crucial in correlating the wells to much more detail than the one presented here. Fig. 13 shows the biostratigraphic well correlation discussed earlier, whereas Fig. 14 shows the wells with the major horizons correlated with use of 3D seismic. The latter was indispensable in unraveling biostratigraphic artifacts caused by natural reworking of microfaunas, as in the interval 3935-4085 m of well 35/3-1 containing reworked Aptian material. Note that the seismic correlation supports the well correlation and the previously discussed Early Albian break is in good accordance with the reflectors, consistently occurring just below the base Agat. An additional cyclostratigraphic high-resolution correlation (Enres, internal report RWE-Dea) is in agreement with the Early Albian gap previously determined by biostratigraphy.
Figure 14. 3D seismic correlation of the Agat Member in block 35/3.
Fig. 15 shows a sequence stratigraphic well correlation based on the most relevant bounding surfaces, which coincide with intra-Agat horizons shown in the seismic sections of Figs. 7-11. This results in a division of the unit into four informal zones. The correlation is strongly supported by 3D seismic. For example, zone 1 in well 35/3-5 onlaps the base Agat sequence boundary and is absent in well 35/3-4, as seen in the seismic line of Fig. 11. An important implication of the new chronology supported by the well correlations is that the Agat Member is not coeval with the Sola Formation, essentially of Aptian age, as previously illustrated by Isaksen & Tonstad (1989: fig. 11). Due to its restriction to the Middle-Late Albian it can be viewed as the sandstone unit that interfingers with the background shales of the Rødby Formation (also Bugge et al. 2001). This can be seen in Fig. 16, which shows the chronostratigraphic relationships of the Agat Member in the type area. Note the Early Albian hiatus between preserved pre-Albian section of the Cromer Knoll Group. In the basinal setting of the Sogn Graben an expanded and conformable Cretaceous section is predicted.
Figure 15. Sequence stratigraphic correlation of the Agat Member.
Figure 16. Chronostratigraphic section of the Agat Member.
Figure 17. Core photographs of the Agat Member showing diverse sedimentary structures.
Depositional models have ascribed the lack of pressure communication between wells 35/3-2 and 35/3-4 to primary depositional features. Bugge et al. (2001) presented a model where slide scars from small-scale slumping and sliding created accommodation space for preservation of isolated sand bodies transported by turbiditic currents. Core descriptions have been performed in all wells. An example of them is shown in Fig. 18 and the photographs in “Core photographs”. Cores in well 35/3-5 are interpreted as high-density turbidites deposited close to the main fairway. In general, the facies associations in the wells show predominantly turbidite deposition interacting with debris flows.
Figure 18. Core descriptions of the reference section of the Agat Member.
The 3D palaeobathymetric reconstructions (Figs. 1, 2) give a good picture of the basin configuration in the Early Cretaceous. The agglutinated foraminiferal assemblages retrieved from intervening shales in the Albian of the Agat wells are indicative of bathyal conditions (Gradstein, wr. com., 2000), in line with a slope setting. Seismic facies extracted from 3D surveys in conjunction with isopach anomalies have constrained our depositional model by more accurately outlining the fan distribution. The upscaled regional depositional model (Fig. 19) shows the generalized extension of the fans of the Agat Member on the slope of the Måløy terrace. This is roughly in agreement with the model of Copestake et al. (2003; Fig. 12/20). Its hypothetical extension into the Sogn Graben, as postulated by Shanmugan et al. (1984) and Skibeli et al. (1995), remains to be proven by drilling.
Figure 19. Regional depositional model for the Agat Member.
BUGGE, T., TVEITEN, B. & BÄCKSTRÖM, S. 2001. The depositional history of the Cretaceous in the northeastern North Sea. In: Martinsen, O. & Dreyer, T. (eds.) Sedimentary Environments Offshore Norway – Paleozoic to Recent. Norwegian Petroleum Society Special Publication, 10, 279-291
COPESTAKE, P., SIMS, P., CRITTENDEN, S., HAMAR, G., INESON, J., ROSE, P. & TRINGHAM, M. (2003) Lower Cretaceous. In: Evans, D. et al. (eds.) The Millennium Atlas. The Geological Society of London, 191-211.
GULBRANDSEN, A. 1987. Agat Field. In: Spencer, M.A. (ed.) Geology of the Norwegian oil and gas fields: Graham and Trotman, London, 363-370.
GRADSTEIN, F.M. & AGTERBERG, F.P. 1998 Uncertainty in Stratigraphic Correlation. In: Gradstein, F., Sandvik, O. & Milton, D. (eds.) Sequence Stratigraphy – Concepts and Applications. Elsevier Publishing Company 9-29.
GRADSTEIN, F.M., KAMINSKI, M.A. & AGTERBERG, F.P. 1999. Biostratigraphy and paleoceanography of the Cretaceous seaway between Norway and Greenland. Earth Science Reviews 46, 27-98.
ISAKSEN, D. & TONSTAD, K. 1989. A revised Cretaceous and Tertiary lithostratigrahic nomenclature for the Norwegian North Sea. Norwegian Petroleum Directorate Bulletin, 5, 1-59.
NYSTUEN, J. P. 1999. Submarine sediment gravity flow deposits and associated facies: core examples from the Agat Member. Extended Abstracts Bergen Conference. Norwegian Petroleum Society, 211-215.
SHANMUGAN, G., LEHTONEN, L. R., STRAUME, T., SYVERSTSEN, S. E., HODGKINSON, R. J. & SKIBELI, M. 1994. Slump and Debris-Flow dominated Upper Slope Facies in the Cretaceous of the Norwegian and Northern North Seas (61-67ºN): Implications for Sand Distribution. American Association of Petroleum Geologists Bulletin, 78, 6, 910-937.
SHANMUGAN, G., BLOCH, R.B., MITCHELL, S.M., BEAMISH, G.W.J., HODGKINSON, R.J., DAMUTH, J.E., STRAUME, T., SYVERTSEN, S.E. & SHIELDS, K.E. 1995. Basin-floor fans in the North Sea: sequence stratigraphic models vs. sedimentary facies. American Association of Petroleum Geologists Bulletin 79, 4, 477-512.
SKIBELI, M., BARNES, K., STRAUME, T., SYVERSEN, S. E. & SHANMUGAN, G. 1995. A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea. In: Steel, R. et al. (eds.) Norwegian Petroleum Society Special Publication, 5, 389-400.
1 . Cretaceous stratigraphy in the northern North Sea (Tampen Spur – Måløy Terrace area) showing the position of the Agat Member.
2 . Regional setting and geographic distribution of the Agat Member on a 3D palaeobathymetric reconstruction at 137 my (Base Cretaceous Unconformity).
3 . Regional setting on a 3D Palaeobathymetric reconstruction at 90 my (top Lower Cretaceous).
4 . Main logs of well 35/3-4, type section of the Agat Member.
5 . Main logs of well 35/3-5, reference section of the Agat Member.
6 . Main logs of wells 35/3-1 and 35/3-2.
7. Type cross line section over well type section 35/3-4; CL1505; 3D survey GP3D93R02.
8. Type in line section over well type section 35/3-4; IL472; 3D survey GP3D93R02.
9. Reference cross line section over well type section 35/3-5; CL2129; 3D survey GP3D93R02.
10. Reference in line section over well type section 35/3-5; IL555; 3D survey GP3D93R02.
11. Random 3D line through wells 35/3-4 and 35/3-5; 3D survey GP3D93R02.
12. Lower Cretaceous biostratigraphic zonation, after Gradstein et al. (1999).
13. Biostratigraphic well correlation; after F. Gradstein, internal report RWE-Dea. Numbers indicate sampled intervals. Location of wells in Fig. 2.
14. 3D well correlation. Location of wells in Fig. 2.
15. Sequence stratigraphic correlation. Map with location of wells is taken from fig. 2.
16. Chronostratigraphic diagram.
17. Examples of sedimentary structures that illustrate the facies complexity; after Skibeli et al. (1994); Shanmugan et al. (1994).
18. Example of core descriptions carried out for well 35/3-5. 19. Regional depositional model.