Agat Member

Cromer Knoll Group, Rødby Formation

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

Introduction

The Agat Member of the Rødby Formation is a Lower Cretaceous stratigraphic unit that belongs to the Cromer Knoll Group (Fig. 1). Economic importance as a reservoir rock is proven in the Agat gas discovery in block 35/3. It is a well-established unit with no need for major revision. However, it has not been extensively documented and therefore poorly understood, as there are discrepancies concerning age determinations, interpretations of the depositional environment, and well correlations in the literature. Minor changes in the definition of the boundaries are introduced here, and the main characteristics of the member are documented. This review includes a discussion of the literature and work performed by RWE Dea prior to 2006. Subsequently, the original Agat Formation has been reassigned as a member of the Rødby Formation for consistency with other detached sandstone units of deep water aspect re-evaluated by Norlex.

Figure 1. Cretaceous stratigraphy of the northern North Sea

Unit definition

Name: Agat Member or Agatleddet

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.

Lithology

In the original definition Isaksen & Tonstad (1989) gave a detailed account of the lithology of the Agat Member (word Formation changed to Member in this text):

"“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."

Sample depository

Palynological preparations

Slides available from type well 35/3-4 for the interval 660-4043 m (NPD)

Core photographs

A suite of core photographs illustrates the main characteristics of the reservoir interval in wells 35/3-5, 35/3-4 and 35/3-2.

Thin-sections

Unavailable

Thickness

In the type section of well 35/3-4 the member measures 197 m thick, and in the reference well 35/3-5 it spans 386 m. In the 35/3 block gross thickness varies within this range. In the Gjøa area wells 35/9-3 and 36/7-3 encountered sand packages assigned to the Agat Member, with 44 m and 100 m gross thickness, respectively.

Geographical distribution

The geographical distribution and regional setting of the Agat Member is seen in figs. 2 and 3. The unit is hitherto only known in blocks 35/3, 35/9, 36/7, 6204 and 6205.

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.

Occurrences of member tops in wells

Type well

Well name: 35/3-4

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

Reference well

Well name: 35/3-5

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

Upper and lower boundaries

The boundaries are defined by Isaksen & Tonstad (1989) at the base and top of the sandstone interval as suggested by the gamma-ray and velocity logs, but this does not exactly coincide with the lithology log illustrated by them. The lower limit in the type section is now more accurately placed at the base of the massive sandstones, which occurs at 3542 m instead of 3589 m (Fig. 4). As seen in this figure both density and velocity logs exhibit changes that result in an acoustic impedance contrast. Likewise, in the reference section well 35/3-5 (Fig. 5) the base is slightly shifted up from originally 3620 m to 3605 m. The upper limit in both well sections occurs at a high gamma-ray spike now viewed as representing a maximum flooding surface, and is maintained here. The boundaries picked for wells 35/3-1 and 35/3-2 (Fig. 6) are in agreement with these criteria.

Well log characteristics

Figs. 4-6 show the main logs for wells 35/3-4, 35/3-5, 35/3-2 and 35/3-1. The Agat Member normally has a distinct expression that shows up well in the gamma ray log. In some cases the density log shows better contrasts, as in well 35/3-1 at the base of the unit. The array of logs illustrated here is suitable for recognizing the base of the sands that mark the boundaries of the unit.

Figure 6. Wireline log expression of the upper and lower boundaries of the Agat Member

Seismic characteristics

Type seismic section: Fig. 7 (cross line 1505 of 3D survey GP3D93R02) shows the type seismic section that ties to type well 35/3-4. It illustrates well the top and base of the Agat Member, the BCU plus three additional intra Agat markers. An in-line over the same well is presented in Fig. 8. For reference well 35/3-5 cross line 2129 (Fig. 9) and in-line 555 (Fig. 10) are illustrated, and are called here reference seismic sections. In addition a random section tying both wells 35/3-4 and 35/3-5 (Fig. 11) helps to illustrate the complexity of the seismic facies and deformation. Seismic sections over reference well 35/3-5 were published by Gulbrandsen (1987: fig. 5) and Shanmugan et al. (1994: fig. 28), while the Millennium Atlas (Copestake et al., 2003: fig. 12/20b) shows a line tying three key wells.

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

Biostratigraphy

The first biostratigraphic reports are from Robertson Research (well 35/3-1), where they stated an Albian-Aptian age for the Agat sands. Based on the understanding of the zonal use of more than 20 foraminiferal and dinocyst events in the Agat region, fine-tune correlations in the reservoir sand interval were performed. The Lower Cretaceous foraminiferal zonation compiled by Gradstein et al. (1999) is reproduced here in Fig. 12. The reservoir sands mainly extend from the Recurvoides/Glomospira Zone, middle Albian through the Hedbergella delrioensis LCO Zone, late early Cenomanian - early middle Cenomanian. The top of the Albian is approximated at the top of the Osangularia schloenbachi Zone, and the top of middle Albian at the transition from the Recurvoides/Glomospira Zone into the Uvigerinammina una Zone (Fig. 12).

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.

Age

Middle – Late Albian.

Correlation

Different correlations for the classical wells of the Agat field were published by Gulbrandsen (1987), Skibeli et al. (1995) and Bugge et al. (2001). Yet another version appears in the Millennium Atlas (Copestake et al., 2003) with some more detail, particularly the non-extension of the Agat sands from the type area east to well 36/4-1. It was early stated that the sands lack pressure communication between wells 35/3-2 and 35/3-4, supposing the existence of a stratigraphic boundary (Gulbrandsen, 1987).

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.

Depositional environment

No other aspect of the Agat Member has been more debated than the depositional environment. The first sedimentological model by Gulbrandsen (1987) interpreted submarine fans with multiple point sources at the shelf break, with eastern provenance. Core photographs have only been published by Shanmugan et al. (1994, 1995) and Skibeli et al. (1995). Some of them are reproduced here in Fig. 17, where diverse facies are overprinted by secondary features such as injected sands or possible glide planes. According to them the main mechanism of sedimentation implies plastic flows, particularly slumps and sandy debris flows on an upper slope setting. This was objected by Nystuen (1999), who interpreted the dominant depositional process of the sandstones in the Agat wells to have been by turbidity currents, probably within a channel system.

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.

References

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. 

Figures

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.