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 57°36'54.5", E 04°18'57.7" (Fig. 36).
Well reference section
Norwegian well 8/1-1
(Phillips) from 2551 m to 2606 m, coord N 57°51'43.53", E03°12'27.64" (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.
