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NORLEX


Tau Formation

(From NPD Bulletin no. 3)

Boknfjord Group

Name

From a village within the Boknfjord area in Rogaland. The formation represents only the lowermost, radioactive part of the Borglum Member as defined by Deegan and Scull (1977).

Well type section

Norwegian well 9/4-3 (Conoco) from 2400 m to 2437 m, coord N 5736'54.5", E 0418'57.7" (Fig. 36).

Well reference section

Norwegian well 8/1-1 (Phillips) from 2551 m to 2606 m, coord N 5751'43.53", E0312'27.64" (Fig. 37).

Thickness

11 m in the type well and 55 m in the reference well.

Lithology

The Tau Formation consists of dark grey to black, pyritic, fissile, organic rich slightly to non-calcareous shales.

Boundaries

This formation is highly radioactive and the boundaries are characterized by prominent log breaks. The underlying Egersund Formation is less radioactive and has a lower interval transit time than the Tau Formation. The upper boundary may be more gradational but shows a marked increase in radioactivity compared to the overlying unit.

Distribution

The formation is confined to the central part of the type area of the Boknfjord Group, and grades laterally into the Etarglum unit southwards and eastwards.

Occurrences of formation tops in wells

Age

Kimmeridgian to Early Volgian.

Depositional environment

The Tau Formation was deposited in an anaerobic marine environment with high organic productivity and restricted bottom circulation.

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