Ula Formation (new)
(From NPD Bulletin no. 3)
Vestland Group
Name
From the Ula Field in Norwegian Block 7/12. The name was first proposed by
Bailey et al., (1981) to describe a sequence of Oxfordian-Kimmeridgian marine
sands overlying the non-marine Bryne Formation (then termed the Haldager Formation) in the Ula Field, but is extended here to a wider geographic area and strati-graphic range.
Well type section
Norwegian well 7/12-2
(BP) in the Ula Field, from 3378.5 to 3531.5 m,
coord N 57°06'41.34", E 02°50'50.73" (Fig. 32).
Well reference sections
Norwegian well 2/1-2
(BP) from 3316 m to 3346.5 m, coord N 56°57'30.76", E03°12'23.07" (Fig. 33).
Thickness
152 m in the type well and 30.5 m in the reference well.
Lithology
In the type well the Ula Formation is a generally massive, fine to medium grained, grey sandstone. A thin, dark grey siltstone is present in the basal part of the formation. The sandstones are arkosic to subarkosic, glauconitic and micaceous. Sorting and angularity vary between individual units of the formation. Bivalve shells and belemnite debris occur, often concentrated in thin lag deposits. Thin, nodular calcite-cemented bands are common.
Within the Ula Field the Ula Formation can be subdivided into a number of units on the basis of large scale coarsening upward and fining upward cycles (Bailey et al.,op.cit.).The sandstones are extensively bioturbated throughout, and this usually obliterates smaller scale sedimentary features. However, in rare zones, parallel or low angle inclined lamination and planar cross bedding are preserved.
Boundaries
The base of the Ula Formation usually occurs where the marine sandstones pass
downwards into the non-marine sandstones/shale/coal sequence of the
Bryne Formation. This boundary is often difficult to
establish on log characteristics alone. In the region of the Ula Field there
is a gamma ray break between the low values of the Ula Formation and the
higher values of the more argillaceous Bryne Formation (e.g. in the type
well 7/12-2). Elsewhere, where the Bryne Formation contains cleaner, more
massive sands, the base of the Ula Formation is picked at the top of the
highest penetrated coal band. The top of the Ula Formation is easily
recognized where the sands give way to the shales of the
Tyne Group.
Distribution
The Ula Formation is developed around the eastern flanking "highs" of the
Central Graben, in particular on the south-western flank of the Southern
Vestland Arch. It passes basinwards into marine shales but is often
recognizable as a very thin sandstone. It becomes thin or absent over the
"highs". Tongues of similar sands occur locally in the Tyne Group mudstones
(e.g. 3/5-2 from 3175 m to 3182.5 m). Comparable formations in lithofacies
and partly in age occur both in the Sleipner Area (the
Hugin Formation) and in the Fiskebank and Egersund
Sub-Basins (the Sandnes Formation).
Occurrences of formation tops in wells
Age
Oxfordian to Ryazanian. In the region of the Ula Field the sands are Oxfordian to Early Vol-gian in age. Around the fringes of the Jseren and Mandal Highs and locally on the Southern Vestland Arch, developments of the formation may be as young as Middle/Late Volgian or possibly Ryazanian.
Depositional environment
The sands of the Ula Formation are generally shallow marine in origin although the type of marine environment probably varies from area to area. In the Ula Field the depositional environment of the sands is particularly difficult to establish due to the unusual thickness of the formation and the scarcity of sedimentary structures; the sands have at this location variously been called shoreface, offshore bar and tidal sand wave deposits (Bailey et al., 1981):
