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NORLEX


Forties Member

Rogaland Group, Sele Formation

Unit definition

The Forties Member is attributed to the intra Sele Formation sandstones in subarea SW in Figs. 12 and 13.

Name

The name "Forties Formation" was introduced by Deegan and Scull (1977). Mudge and Copestake (1992) reduced the unit to member status within the Sele Formation.

Derivatio nominis

The name is from the Forties Field, UK Block 21/10 (Deegan and Scull 1977).

Type well

UK 21/10-1. Depth 2131.5-2314 m RKB. Coordinates N 5743'50.370", E 0058'29.190". Cores:

Reference wells

Norwegian well 7/11-1 (revised by Dreyer et al, 2004) (Fig. 103). Depth 2904-3069 m RKB. Coordinates N 5704'15.60", E 0226'24.40". No cores.

Norwegian well 1/2-1 (Fig. 104). Depth 3121-3287 m RKB. Coordinates N 5653'15.07", 0228'35.70". Cores: Cores 1-8.

Norwegian well 1/3-6 (Fig. 105). Depth 2914-2984 m RKB. Coordinates N 5656'14.92", E 0242'20.81". Cores: Core 1.

Fig. 103. Well 7/11-1 Composite log Rogaland Group. Stratigraphic position of the Forties Member is outlined in stratigraphic column to the right.

Fig. 104. Well 1/2-1 Composite log Rogaland Group. Stratigraphic position of the Forties Member is outlined in stratigraphic column to the right.

Fig. 105. Well 1/3-6 Composite log Rogaland Group. Stratigraphic position of the Forties Member is outlined in stratigraphic column to the right.

Composition

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 reworked Sele and Lista Formation mudstone clasts.

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 interbedded mudstones and siltstones are generally carbonaceous, micaceous, poorly sorted and crudely laminated (Knox & Holloway, 1992). A core photo example from well 1/2-1 is shown in Fig. 106 and a core description log from well 7/11-3 in Fig. 107.

Fig. 106. Core photo from well 1/2-1. Graded beds of turbiditic origin, displaying horizontal lamination, and convolute lamination. Photo from NPD Fact pages at http://www.npd.no.

In the southern part of UK Quad 23 and northern part of Q30 Dreyer et al. (2004) have recognized several shallow marine to non marine indicators (Figs. 108 and 109):

These observations are incompatible with the accepted idea that deep water anoxia dominated in the entire basinal area during deposition of the entire Sele Formation, and give strong indications that local areas in the southern Norwegian North Sea Basin were shallow and oxic at least for a short time during the Late Thanetian-Early Ypresian.

Fig. 107. Core description example from the upper parts of the Forties Member well 7/11-3.

Fig. 108. Possible shallow marine indicators from the Forties Member, Well UK23/27-1 Pierce Field, close to the Norwegian-UK border. Core photographs show inferred paleosols, vertical burrows of Diplocraterion type and pedogenically altered/freshwater influenced carbonates. Wireline log to the left shows position of shallow marine indicators relative to sandstone beds of partly turbiditic origin.

Fig. 109. Sedimentary structures showing deposition under fluctuating flow regime. It seems possible that the undulating appearance with cut and fill of offshooting pinch and swell laminas can be explained by influence of oscillatory currents generated by wave activity.

Wireline log characterization

On a larger scale the Forties Member displays a coarsening upwards trend with decreasing gamma-ray values and increasing velocity, reflecting a progradational system.

Lower boundary

Where the Forties Member rests on the Mey Member its lower boundary is defined by a decrease in velocity into the sandstones of the Forties Member. When the Forties Member rests on the Sele or Lista Formation the boundary is characterised by an upwards increase in gamma-ray readings and decrease in velocity.

Upper boundary

The upper boundary is defined as the break between the Forties sandstones and the shales of the Sele Formation above. The log response changes upwards fom low gamma-ray readings and high velocity to high gamma-ray readings and low velocities in the Sele Formation.

Thickness

The Forties Member is variable in thickness: 70 m in well 1/3-6, 73 m in reference well 7/11-1, 165 m in example well 1/2-1 and 182.5 m in type well UK 21/10-1. In the Central Graben it reaches a thickness of more than 200 m, as in reference well UK 21/10-2, where it reaches 262 m.

Seismic characterization

Top of the Forties Member

The top of the Forties Member is often associated with a strong to moderate positive amplitude event that can be correlated locally to subregionally. Sometimes it is interfering with the Base Balder Formation reflector making it more difficult to pick.

Base of the Forties member

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 thicks. Its thickest development follows axial parts of the Central Graben. To the east a 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 (Dreyer et al. 2004). This may represent a mixture of submarine fan systems and prograding slope and shallow marine systems. A seismic line from south of the Cod Field to the Ula Field is shown in Fig. 110. An example from east of Flyndre is shown in Fig. 111.

Fig. 110. Seismic section through the Breiflabb Basin and the Ula structure. Interval with inferred presence of Forties Member is outlined.

Fig. 111. Seismic cross section along the axis of channelised Forties Member in blocks 1/5 and 1/6.

Age

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

Biostratigraphy

Being contained in the Sele Formation, the age of the Forties Member is bounded by biostratigraphy and age assignments for the Sele Formation. See description for the Sele Formation in Subchapter 6.1

Correlation and subdivision

The Forties Member is grossly considered as being deposited by one pulse, reflecting the S1 (lower Sele Formation), and is roughly time equivalent to the Hermod S1 of the Viking Graben. Sandstones of S2 (Upper Sele Formation), the so-called Cromarty sandstones in the UK, do not seem to be developed in the Norwegian part of the Central Graben.

Geographic distribution

The Forties Member is distributed in the area from the Outer Moray Firth and into the northern part of the Central Graben, where it appears to reach into the area northwest of the Ekofisk Field (Fig. 12, 13 and 102).

Occurrences of member tops in wells

Depositional environment

According to Knox & Holloway (1992), the sands were shed southeastwards off the shelf areas of the East Shetland Platform and the Inner Moray Firth, and deposited within a submarine sand fan system, probably of slope-apron or ramp type (Stewart, 1987). 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.

According to Dreyer et al. (2004) the palynofacies of the Forties Member of the Central Graben in UKQ23 and 30 and Norwegian Q1 and Q7 varies between normal to low salinity, and anaerobic to aerobic. The depositional environments range from slope and basin plain environments to near shore and even non-marine environments. Hydrocarbon discoveries with Forties Member as reservoir



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