The distribution of Cretaceous source rocks in the Norwegian and Barents Sea. Charnock, M.A. et al Crittenden, S.

1. The distribution of potential Cretaceous source rock horizons in the
Norwegian and Barents Sea
Charnock, M. A.1
, Badics, B.1
, Augustson, J.H. 2
and Crittenden, S. 3
1) Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen
2) Norsk Hydro, N-9480 Harstad
3) SC Independent Consultant, Chagford, Devon, United Kingdom
The stratigraphic and geographic distribution of
potential source rock horizons within the Cretaceous
System in both the Norwegian and Barents Sea areas
are interpreted and described from an extensive well
database. Three Cretaceous organic-rich horizons
are known in the North West Europe region in
addition to the main source rock horizon that spans
the Jurassic/Cretaceous boundary. These are the
Cenomanian/Turonian (Oceanic Anoxic Event II
OAEII), early Aptian (OAEI) and Barremian
/Hauterivian. These potential source rock horizons
are interpreted and described. Stratigraphic horizons
are calibrated with detailed biostratigraphy and TOC
values are reviewed based on observed
measurements and calculated Passey logs.
The Cenomanian/Turonian boundary (K50)
sequence is developed within the ‘upper’ Lange
Formation in the Norwegian Sea and Kveite
Formation of the Barents Sea. In both areas it is not
a distinct, formally recognised litho-unit in standard
schemes. It is often poorly defined on wireline logs,
has <2% TOC values and has been regularly mis-
identified on biostratigraphy due to extensive
reworking of microfossils. This study indicates that
in the Mid Norway area this horizon and most of the
underlying Cenomanian stage is generally absent
over a wide area of the Halten Terrace and
Trøndelags Platform e.g. 6506/12-1 due to either
non-deposition or an early Turonian erosional event.
Where it has been interpreted with confidence e.g.
Selje High, Dønna Terrace and western margin of
the Halten Terrace e.g. 6506/11-3 it shows no
appreciable thickness variation compared to the
Blodøks Formation in the North Sea (average 20m)
and in the outer Halten Terrace and Sklinna Ridge
area it is commonly associated with Cenomanian
and Turonian Lange sandstones. However, well data
are generally lacking in the deeper parts of the
Vøring and Møre Basins where it is below most of
the current well penetration.
In the Barents Sea the time equivalent section is
rarely preserved being either truncated by younger
Cretaceous or Tertiary rocks. The gross distribution
and pattern conforms to the model proposed by
Kuhnt and Wiedmann (1995) for the North Atlantic
in which there was both a reduction in frequency of
organic rich shales and TOC values across the
Cenomanian/Turonian boundary in both deep sea
and shelf/slope environments away from low
latitudes. Furthermore, Mann and Zweigel have
recently questioned the value of this interval as a
source rock in the Vøring Basin on the basis of
locally high sedimentation rates.
In the Lower Cretaceous interval of the Norwegian
Sea within the Lyr Formation are distinct, high –
gamma ray peaks with good TOC values at two
horizons of early Aptian (k18 sequence) and early
Barremian/late Hauterivian age (k15) that show
TOC values as good and in some instances better
than those of Cenomanian/Turonian. The problem
with these horizons, however, is that at least around
the Halten Terrace area; these are generally thin,
condensed units that are often in close proximity to
the underlying Spekk Formation. Thicker sequences
in the Barents Sea can be biostratigraphically
calibrated to time equivalent intervals within the
basal Kolmule/Kolje and Knurr formations e.g.
7120/2-2 and 7321/8-1.
Work is currently progressing on the distribution
and significance of these horizons in this area.
May, 2004.
Reference
Kuhnt, W. and Wiedmann, J. 1995. Cenomanian –
Turonian Source Rocks: Palaeobiogeographic and
Paleoenvironmental Aspects. In Huc, A-Y. (ed.).
Paleogeography, Paleoclimate and Source Rocks.
AAPG studies in Geology 40, 213-232.