Halten Terrace Lower and Middle Jurassic inter-rift megasequence analysis: megasequence structure, sedimentary architecture and controlling parameters - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 215

Halten Terrace Lower and Middle Jurassic inter-rift megasequence
analysis: megasequence structure, sedimentary architecture
and controlling parameters

RODMAR RAVNÅS* § , KARi BERGE*, HEATHER CAMPBELL*, CRAiG HARVEY* †
and MiKE J. NORTON* ‡
* AS Norske Shell, PO Box 40, 4098 Tananger, Norway
§ Department of Petroleum Engineering, University of Stavanger, Stavanger, Norway
† Qatar Shell Service Company W.L.L, 1st Floor, Al Mirqab Tower, Corniche Road, West Bay, PO Box 3747, Doha, Qatar
‡ Premier Oil, 23 Lower Belgrave Street, London

ABSTRACT

The Lower to Middle Jurassic of the Halten Terrace, that is the Åre (uppermost parts),
Tilje, Tofte-Ror, ile and Garn Formations, comprises three megasequences: the Tilje,
the Tofte-ile and the Garn megasequences. These megasequences have a common
architectural motif and stratigraphic structure with a basal coarsening or 'sandying'
upwards motif, a central sand-rich unit and an upper part of more varied stratigraphy.
The lower progradational and parts of the central aggradational segments of these
megasequences represent infilling and progradation of predominantly basin-marginal
clastic wedges sourced from the Fennoscandian hinterlands. These segments formed
during an interval of slowly rising through still-stand to falling and renewed rise
of base-level, representing highstand normal through forced to lowstand normal
regressions. The upper aggradational and retrogradational segments show a varied
architectural motif that formed in response to increased basinal subsidence rates and
mild structuring of the area. They are characterised by back-stepping of the rift-
marginal clastic wedges and the presence of locally developed clastic systems formed
around tectonically rejuvenated, intra-basinal highs. These upper parts or segments of
the megasequences portray a sedimentary architecture mimicking mixed non-marine-
shallow-marine syn-rift infills. The Lower to Middle Jurassic megasequences represent
a series of depositional systems of varied size, origin and provenance. Larger hinter-
land-sourced fluviodeltaic systems dominated during progradational and aggrada-
tional stages, whereas more localised systems become increasingly important during
the retrogradational stages. Repeated low-order relative sea-level fall, attributed to
broad upwarpings of the rift-margin and its hinterland, forced the basin-marginal
fluviodeltaic systems basinward to produce the repeated forced regressive to lowstand
wedges, resulting in the formation of high-quality sheet-like reservoir bodies in the
core of the megasequences. The uplift of rift-margins appears to be linked to and to
represent the initial stages of basin-scale structuring, preceding and heralding
subsequent minor rifting phases.

Keywords: megasequences, Halten Terrace, Lower and Middle Jurassic, deltaic to
shallow marine, sedimentary architectures.

INTRODUCTION and retreat of basin-marginal, fluvio-deltaic

depositional systems. Such infill patterns have
been argued to form a common motif within both
syn-rift infills (Ravnås et al ., 2000) as well as the
infill of prolonged periods between successive
rift-episodes (Steel, 1993); the so-called inter-rift

Rift-basins often have a cyclic infill pattern defined
by a series of stacked clastic wedges (Fig. 1). These
clastic wedges are defined by alternations of sand-
ier and muddier lithologies, related to the advance