Fiskebank Member (revised)

updated to follow: Stratigraphic Guide to the Rogaland Group, Norwegian North Sea. Harald Brunstad, Felix M. Gradstein, Jan Erik Lie, Øyvind Hammer, Dirk Munsterman,  Gabi Ogg, and Michelle Hollerbach. Newsletter on Stratigraphy, vol 46/2 pp137-286, 2013.

Rogaland Group, Sele Formation

Unit definition

The Fiskebank Member is attributed to the the Intra Sele Formation sandstones in Subarea SE in Fig. 1, where it is found in the Norwegian Danish Basin, the Siri Canyon Area and in the eastern parts of the Central Graben in the Gyda-Ula Area.

 


Fig 1: Location map of the Members of the Sele Formation.

 

Name

The former Fiskebank Formation defined by Deegan and Scull (1977) is hereby downgraded to the Fiskebank Member. The Fiskebank Member has also been attributed to the intra Sele Formation sandstones of the Siri Canyon (Ahmadi et al., 2003). Although Schioeler et al. (2007) have redefined the Fiskebank Member to the Kolga Member at the Danish side of the Siri Canyon, we keep the name Fiskebank Member for the Norwegian side in this area.

Derivatio nominis

Named after the Fiskebank (Fisher bank), offshore southern Norway. Named by Deegan & Scull (1977).

Type well

Norwegian welll 9/11-1: 1483 to 1335m (new definition), coordinates N 57°00'41.40", E 04°00'33.52" (Fig. 117). No cores.

Fig. 117. Well 9/11-1 Composite log Rogaland Group. Stratigraphic position of the Fiskebank Member is outlined in stratigraphic column to the right.

Reference wells

Norwegian well 8/9-1 (Fig. 118): 1315 to 1375 mRKB (new definition), coordinates: 57°26'27.28" N, 3°51'03.48" E. No cores

DK well Siri-3 (Kolga Member seen from seismic to be continous with the Fiskebank Member Norway), (Fig. 119). Depth 2206.4-2036.1 mRKB. Coordinates N 56°30'34.92", E 05°03'48.27". Cores: Core 2, 2065-2120 mRKB.

DK Well: Nini-3, 1717.2-1700.4 mRKB. Coodinates: N56°41'31.96", E05°24'12.35" (Fig. 119). Cores: Core 1, 1684-1765 mRKB.

Fig. 118. Well 8/9-1 Composite log Rogaland Group. Stratigraphic position of the Fiskebank Member is outlined in stratigraphic column to the right.

Fig. 119. Well Composite logs wells DK Nini-3 and Siri-3. Position of the Fiskebank Member is outlined in the stratigraphic column to the left. Modified from Schioeler et al. (2007) to fit definitions made in the nomenclature for the Norwegian North Sea (this paper).

Composition

The Fiskebank Member consists of dark grey to grey brown very fine grained sandstones, but varies from medium to silt sized and is remarkably clean (<0.1% detrial clay). It has subrounded grains, is fairly uniform, well sorted, micaceous, partly micromicaceous, friable, and with calcareous cement in places. There is a high content of black to dark glauconite pellets (15-25%, Hamberg et al, 2005) that are found as rounded pellets of the same grain size as quartz grains. Pyritic grains may also be present. Locally sandstones may be cemented by calcite or chlorite.

In Danish cores the sandstones of the Fiskebank Member (Kolga Member close to the Norwegian Border) are observed as generally massive, homogenous and with few structures. Faint deformed lamination and dish and pillar structures are rather common, and witness post sedimentary water movement in sediments. There are few signs of primary sedimentary structures in sandstones, although a few examples of faint cross bedding seem to occur.

Sandstone intrusions are frequently found associated with the upper boundary of sandstones bodies, often with an abundance of angular and tabular mudstone clasts (Hamberg et al., 2005).

Wireline log characterization

The Fiskebank Member is often associated with lower gamma ray response than the surrounding shales of the Sele Formation (and possibly upper parts of the Lista Formation). However, it is sometimes difficult to distinguish from the shales by gamma logs because of the content of radioactive grains of mica and glauconite in the sandstones. Thus other logs such as neutron, density and resistivity logs are sometimes more reliable for detecting the sands. Often a coarsening upwards trend can be inferred from the logs.

Upper boundary

The Fiskebank Member is usually overlain by the Sele Formation, and the boundary often marks an abrupt change from sandstones to dark shales. In this case the boundary is generally seen as an upwards increase in gamma-ray response and a decrease in velocity.

In some cases the Balder Formation directly overlies the Fiskebank Member. In these cases the boundary may be seen as an upwards decrease in gamma-ray response and an increase in velocity readings.

 

Lower boundary (revised)

The lower boundary of the Fiskebank Member is identified by an upwards transition from the shales of the Sele or the Lista Formation below to the coarser sediments of the Fiskebank member above.

In the Siri Canyon the log response is characterised by a sharp upwards change from high gamma-ray readings and low velocity in the shales to low gamma-ray readings and high velocity in the Fiskebank sandstone. In the north and east the boundary is often not well defined on the gamma-ray log, and must sometimes be picked by use of sonic log in combination with other logs such as resistivity and caliper logs to pin point the shale-sandstone interboundary.

 

Thickness

The Member is 82 m in the type well, 60 m in the reference well 8/9-1, 17m in DK Well Nini-3 and 30 m in Siri-3.

Seismic characterization

In the Siri Canyon area and in the western parts of the Fiskebank Member there seems to be a correlation between thick development of the Sele Formation interval from seismic and presence of sandstones. From seismic mapping of the Sele Formation interval, sandstones can be inferred by the observation of elongated thickness anomalies and from seismic cross sections by the presence of a lenticular to mounded shape. A seismic line through the Siri Canyon and the Nini Discovery is shown in Fig. 120.

Fig. 120. Seismic line from the axis of the Siri Canyon thorough the Nini discovery. Presence of the inferred Fiskebank Member is highlighted.

Age

Latest Paleocene-Earliest Eocene (Late Thanetian-Earliest Ypresian).

Biostratigraphy

Being contained in the Sele Formation, the age of the Fiskebank Member is bounded by biostratigraphy and age assignments for the Sele Formation. See description for the Sele Formation.

Correlation and subdivision

Deposits of the Fiskebank Member occur both above and below the intra Sele Formation Apectodinum spp. Acme biomarker. Hence, using this criterion the Fiskebank Member may be subdivided in a lower unit, Fiskebank S1 and an upper unit, Fiskebank S2.

Geographic distribution

The Fiskebank Member is encountered in the Norwegian - Danish Basin, the eastern parts of the Central Graben, possibly the Søgne Graben and south eastwards into the Siri Canyon (Fig. 102).

Depositional environment

The coarsening upwards trend seen in the eastern parts of the Fiskebank Member probably represents shelf to basin margin deposits laid down by a mixture of shallow marine to turbiditic processes on a prograding slope.

The western and south western parts of the Fiskebank Member was deposited by gravitational flows in sand-rich basin-floor to toe-of-slope fans systems. The gravity flows were probably a result of a combination of high density sandy turbidity currents and sandy debris flow. The high glauconitic content suggests that there was a significant contribution of shelf sands.

Discoveries with the Fiskebank Member as reservoir



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