25th International Meeting on Organic Geochemistry IMOG 2011

P-087
Petroleum geochemistry of the Stord Basin, Norwegian North
Sea
Mark Bastow
Applied Petroleum Technology AS, Kjeller, Norway (corresponding author:mb@aptec.no)
The Stord Basin centred on Quadrants 26/27 and
31/32 in the Norwegian North Sea remains virtually
unexplored, due largely to the shallow depth of burial
of the Jurassic section and consequent concerns
regarding maturation levels for petroleum generation.
For wells drilled around the margins of the basin,
available data and results of new geochemical
analyses have been combined in order to identify
potential source rocks and the challenges for working
petroleum systems within the basin.
The Draupne Formation is generally organically rich
with good oil source potential and around the margins
of the Stord Basin is generally early mature to
marginally middle mature for oil generation and
immature for significant gas generation. Well 26/4-1
demonstrates extension of this source rock interval
into the Stord Basin, where the Draupne Formation
may attain marginal middle maturity for oil generation
but with low transformation ratio and consequently
with significant risk for expulsion of economic
hydrocarbon volumes. Atypical oils from the
Norwegian North Sea have been suggested to be
derived from locally developed carbonate facies of the
Draupne Formation. These may generate oil at lower
maturation level compared to typical siliclastic source
rock facies and may be important for the prospectivity
of areas, including the Stord Basin, that have
undergone limited subsidence and burial.
The Heather Formation is interpreted to have mainly
gas source potential but is known to have localized oil
source potential on the Horda Platform and the
development of similar source facies within the Stord
Basin cannot be discounted.
The Drake Formation generally has fair to good gas
source potential. Within thick Drake Formation
sections wireline logs indicate thin intervals with
possible oil source potential that can be correlated
between wells. In study wells around the margins of
the Stord Basin, the Drake Formation is generally
early to marginally middle mature for oil generation
and immature for significant gas generation. Within
the Stord Basin the Drake Formation is likely to attain
middle maturity for oil generation.
Coals within the Ness/Sleipner Formations and thin
coals within the Statfjord Formation are unlikely to
attain maturity for significant gas generation within the
Stord Basin.
Uplift and erosion of the eastern margin of the Stord
Basin is indicated, in the south by well 17/3-1 and in
the north mainly by wells 31/6-3 and 32/4-1.
Estimated thicknesses of section missing estimated
are about 1000m. The timing of uplift and erosion is
late, post-dating deposition of the Hordaland Group
and pre-Quaternary Nordland Group and followed by
deposition of sediments of Quaternary age.
Well 26/4-1 reported weak oil shows in the Hugin and
Statfjord Formations. Core samples from the Hugin
Formation indicate no significant oil stain and very
weak, possibly diesel contamination of the core. To
date, discoveries and significant shows of
thermogenic hydrocarbons on the Utsira High (well
25/6-1) and on the Horda Platform (Troll Field) are
related to migration from source rocks outside the
Stord Basin. Discovery well 17/3-1 tested gas of
biogenic origin in the Sandnes Formation.
3D modelling of seismic depth maps would enable
maturity mapping of potential source rock intervals,
indicate possibilities for both expulsion and migration
within the Stord Basin, and also model temperatures
for evaluation of both degradation risk in shallow
prospective reservoir intervals and phase separation
due to uplift and erosion.
230