The Ormen Lange turbidite systems: sedimentary architectures and sequence structure of sandy slope fans in a sediment-starved basin - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 608

The Ormen Lange turbidite systems: sedimentary architectures and
sequence structure of sandy slope fans in a sediment-starved basin

RODMAR RAVNÅS* † , ANDREW COOK* ‡ , KRISTOFFER ENGENES*, HARRY GERMS* § ,
MARTIN GRECULA*, JOSTEIN HAGA*, CRAIG HARVEY* ǁ and JAMES A. MACEACHERN ¶
* AS Norske Shell, PO Box 40, 4098 Tananger, Norway
† Department of Petroleum Engineering, University of Stavanger, Stavanger, Norway
‡ Current address: Shell International Exploration and Production, NL 2596 HR, den Haag, Netherlands
§ Current address: Shell International Exploration and Production, TX 77025-1299 Houston, Texas, United States
ǁ Current address: Qatar Shell Service Company W.L.L,
¶ Department of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada, V5A 1S6

ABSTRACT

The reservoirs of the giant Ormen Lange field, located off the Møre coast, consist of
deep-marine turbidites within the Upper Maastrichtian Springar Formation and the
Danian Egga sandstone unit. The two sandstone units are interpreted in terms of inner to
middle and outer parts of two, stacked lower slope sandy fan systems, respectively. The
two fan systems were sourced from an uplifted hinterland; the tectonic rejuvenation of
this is related to rift-margin uplift in response to phased late Campanian to Danian rifting
within the western Møre Basin. Rift-margin uplift was accompanied by basinward tilting
and draped folding over deep-seated basement highs along the Møre Margin, which
produced a stepped slope margin with local slope minibasins and terraces. Each phase
of repeated hinterland uplift, basin-margin tilting and sandy fan development represents
a time span of a few million years. The Springar and Egga turbidite systems evolved
through fill-spill processes on the slope terraces or minibasin(s). Within an individual
minibasin or slope terrace, an early phase of 'infilling' or smoothing of inherited topog-
raphy appears to have been followed by a more organised backfilling or upslope onlap-
ping combined with forestepping or outbuilding of the sandy fan across the individual
terraces or minibasins. Compensationally stacked channel storeys and lobe storeys form
the fundamental architectural elements and probably reflect major flooding events in the
hinterland or drainage basin. Multistorey channel belts represent further outbuilding of
the local terrace or minibasin-filling fan, with sediment bypass and delivery to further
downslope minibasins and terraces. The uppermost parts of the fan systems appear less
organised and contain disorganised, slumped and gravitationally re-sedimented units
alternating with multistorey channel-fills. These less-organised facies associations
formed in response to an early stage or initial tilting of the basin margin slope. Final
backstepping and return to predominantly hemipelagic to pelagic slope deposition is
attributed to renewed upslope or hinterland structuring, which resulted in capture of
sand in proximal subbasins.

Keywords: Slope topography, turbidite systems, megasequences, turbidite architectures,
controlling factors.

INTRODUCTION systems infill intra-slope subbasins or slope ter-

races ('steps') that are significantly larger than that
of the turbidite systems themselves. However,
slope turbidite systems also possess their own
unique characteristics and variability that contrasts

Slope turbidite systems possess many similarities
to base-of-slope and inner basin-floor turbidite sys-
tems, especially in situations where the turbidite