Unravelling the nature of deep-marine sandstones through the linkage of seismic geomorphologies to sedimentary facies; the Hermod Fan, Norwegian North Sea - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 646

Unravelling the nature of deep-marine sandstones through
the linkage of seismic geomorphologies to sedimentary facies;
the Hermod Fan, Norwegian North Sea

BJØRN KÅRE LOTSBERG BRYN* † aNd MaRK aNdREW aCKERS † *
* Norwegian Energy Company, Nykirkebakken 2, 4013 Stavanger, Norway
† Present address: Centrica Energi, Rosenberggata 99, 4003 Stavanger, Norway
(E-mail: [e-mail])

ABSTRACT

The Late Palaeocene to Early Eocene deep-marine Hermod Sandstone Member of the
Sele Formation in the Norwegian North Sea has been studied in a mid to outer subma-
rine fan setting by using a combination of seismic reflection data and wells. a complex
geological setting provided by a network of intricate sand fairways, in places affected
by substantial post-depositional sediment remobilisation, makes the Hermod Member
a challenging hydrocarbon reservoir to develop in the North Sea. The application of
geophysical techniques resulted in convincing images of subtle details that are rarely
documented in deeply buried submarine fan systems. These details include levee-
confined channels, avulsions and braided frontal splays. Four main feeder channels
enter the study area from west-north-west and interact differently with basin structure
controlled by extensional faults. Structurally controlled channels make sharp turns,
have wide avulsion angles, align with substratum faults and tend to build complex
frontal splay zones. Non-confined channels cross substratum faults, build small frontal
splays and display common avulsions close to the channel end, probably due to fre-
quent channel shifting in response to slope advantage to the side of their frontal splays.
Post-depositional sediment remobilisation structures have also been evaluated and
candidate pockmarks were found to be closely associated with steep-sided mounds. It
is proposed that blocking of pockmark craters resulted in the trapping of gas in the
Hermod sands and the triggering of sand remobilisation. Tens of metres of clean sand-
stone, with no apparent systematic vertical grain-size grading and a sharp contact to
underlying and overlying shale, occurs both in remobilised and apparently in situ
sediments. It is further suggested that sand and mud became segregated during trans-
port with clean sand concentrated in channels and frontal splays which later became
parent beds for sand injection to mounds. The result is a mostly homogeneous sand-
stone type with a multifaceted 3d geometry.

Keywords: North Sea, seismic geomorphologies, submarine fans, turbidites, injectites,
sediment remobilisation.

INTRODUCTION decades of further exploration, appraisal and

characterisation before it could be concluded that
some of the smaller fields were commercial. For
instance, the Balder Field did not start production
until 1999 and the British Gannet F structure,
where oil was discovered in 1969, began produc-
tion in 1997. The long delay before some of the
earliest discoveries came on stream was not only a
function of oil price and development cost but
was also related to the time it took to gather

Gravity-driven sand influxes during the Palaeocene
and Eocene have deposited the reservoir bodies
of several oil and gas fields in the North Sea
(ahamadi et al ., 2003 and references therein). The
first Norwegian oil discovery was made in 1967 in
this play type in what is today the Balder Field.
Whereas some of the earliest discoveries came on
stream in the 1970s (e.g. the Frigg Field), it took