Sotra Member (New)
Rogaland Group, Lista Formation
Unit definition
The Sotra Member is attributed to the intra Lista Formation
sandstones in subarea NW in Figs. 12 and 13.
Name
The Sotra Member is defined for the first time in this study. This member status is
attributed to sandstones of intra Lista Formation with an eastern provenance in the Sogn
Graben and the Stord Basin.
Derivatio nominis
The Sotra Member is named after one of the largest islands in Hordaland.
Type well
Norwegian well 35/8-3
(New, Fig. 91). Depth 1665-1709 mRKB. Coordinates N 61°21'05.35",
E 03°32'02.63". No cores.
Fig. 91. Well 35/8-3 Composite log Rogaland Group. Stratigraphic position of the Sotra
Member is outlined in stratigraphic column to the right.
|
Reference wells
Norwegian well 35/3-1 (New,
Fig. 92). Depth 1392-1422 m RKB, major body of Member.
Several thinner sandstone beds in interval 1422-1540 m RKB. Coordinates N 61°50'41.89",
E 03°43'41.36".
Norwegian well 35/11-3 S.
Depth 1905-1997 mRKB. Coordinates N 61°10'59.27", E 03°20'18.47". Core 1.
Fig. 92. Well 35/3-1 Composite log Rogaland Group. Stratigraphic position of the Sotra
Member is outlined in stratigraphic column to the right.
|
Composition
The Sotra Member consists of clear to white, sometimes grey quartzitic sandstones. Grain
size is fine to very coarse, but predominantly medium and sorting poor to moderate, with
angular to subrounded grains. The sandstones have traces of pyrite, glauconite and
mica, and are occasionally calcite cemented.
There are few cores taken from the Sotra Member, and thus there is limited information
about variability in facies. A core photo example from Norwegian well 35/11-3 S is shown
in Fig. 93.
Fig. 93. Example of massive, clean, structureless sandstones of the Sotra Member
35/11-3. Photo from NPD Fact Pages at http://www.npd.no.
|
Wireline log characterization
Wire line log response of the Sotra Member is blocky to serrated, representing thick
clean sandstones and series of thinner sandstone layers in alternation with mudstones.
Sometimes zones of high velocity sonic readings and high values on density logs are
seen, which seem to correspond to zones of calcite cementation in the sandstones.
In some cases content of glauconite in the sands increases their natural radioactivity
and thereby makes it difficult to assess the sand amount and the distinction between the
Sotra Member and the Lista Formation from gamma logs. In these cases their identification
must rely on other logs such as density, neutron and resistivity logs.
Lower boundary
The Sotra Member overlies the Lista and sometimes the
Våle Formation, and the boundary
is characterised by lower gamma readings and increased velocity upwards from the Lista
and Våle Formation below.
Upper boundary
The Lista Formation usually overlies the Sotra Member, and the
boundary is usually
characterised by higher gamma-ray readings and lower velocity upwards into the Lista
Formation.
Thickness
The Sotra Member is often seen as scattered stringers of m scale and up to some tens of
meters. Thickness of the Sotra Member is 44 m in Type well 35/8-3, 92 m of blocky sand in
reference well 35/11-1 and 132 m of blocky sand in 31/2-19 S.
Seismic characterization
To the east in the distribution area the sandstones belonging to the Sotra Member
typically occur in westwards dipping layers in westwards thinning wedges (prograding
slope) of the Rogaland Group on the Måløy Terrace and the
Horda
Platform, partly
stretching into the Sogn Graben. In some cases a blocky log response corresponds to
mounded geometries, especially in the lower part of the wedge.
West of the wedge, in what is believed to represent a basin floor setting, channels and
lobe like geometries sometimes stand out as thickness increase of the Lista Formation,
and can be interpreted as the presence of sands belonging to the Sotra Member. Fig. 94
shows a seismic section through Norwegian well 35/11-1, with the Sotra Member shown.
Fig. 94. Seismic Cross section through southern parts of Quadrant 35 and well
35/11-1. Inferred presence of Sotra Member is outlined.
|
Age
Late Middle to Late Paleocene (Late Selandian to Early Thanetian)
Biostratigraphy and age
Being contained in the Lista Formation, the age of the Sotra
Member is bounded by
that of the Lista Formation. The Lista Formation belongs in the foraminiferal Zone
NSR2A - 2B, A. ruthvenmurray - R. pauperum (Gradstein & Bäckström, 1996), and
the
dinocyst Zones D3b - D4 in Luterbacher et al. (2004). The age is Late Selandian
through Thanetian, late Middle through Late Paleocene. See also description for the
Lista Formation in Subchapter 5.1
Correlation and subdivision
The Sotra Member is divided into three units: Sotra L1, Sotra L2 and Sotra L3,
corresponding to Lista L1, L2 and L3 (se Subchapter 5.1 on
Lista Formation).
Geographic distribution
The Sotra Member is present from the area south of 62°N, stretching south to the
Horda Platform, and westwards into the Sogn Graben (Figs. 12, 13 and 72). It is not
known whether the Sotra Member is stretching further southwards into the Sogn Graben
due to lack of wells in that area.
Occurrences of member tops in wells
Depositional environment
In eastern areas the Sotra Member was deposited as gravity flow sands in a prograding
slope setting sometimes in submarine slope fans. Further west in the Sogn Graben they
were deposited in a basin floor setting (Fig 72).
