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Abstract
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The
Middle Jurassic Oseberg Delta, Northern North Sea:
A Sedimentological and Sequence
Stratigraphic Interpretation1T. Muto and R. J. Steel3 |
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ABSTRACT
The Aalenian Oseberg Formation
(0-80 m thick) is an important reservoir unit in the Middle Jurassic Brent
Group in the northern North Sea, consisting of multiple sets of sandy Gilbert-type
deltas. Small-scale (1.5-10 m) fining-upward units seen in the gamma-ray
log correspond with individual delta sets, as independently confirmed by
steepening-upward trends seen in the dip log. Within each set, the steep
foreset slopes typically show thinly bedded sandstone facies (avalanche
grain flows), whereas the lower foreset slopes, toesets, and bottomsets
are formed largely by massive sandstone facies (sandy debris flows). On
an intermediate scale (up to 40 m), the gamma-ray logs show both fining-upward
and coarsening-upward trends through stacked delta sets, and these trends,
traceable between wells, are interpreted in terms of decelerating and accelerating
rates of relative sea level rise, respectively.
The relative abundance of the sandy debris-flow
deposits reflects a periodic and significant instability of the delta's
upper foreset slope, probably during times of increased water depth in
front of the delta. The normal progradation of individual Gilbert-type
sets, however, is likely to have been along a subhorizontal topography
during periods of little or no change in water depth. The long-term change
to produce the observed vertical stacking of deltaic sets was one of a
generally rising relative sea level. Modeling of the sea level rise in
a steplike
©Copyright
1997. The American Association of Petroleum Geologists. All rights reserved. 1Manuscript received February
9, 1996; revised manuscript received October 9, 1996; final acceptance
February 21, 1997.
2Department of Geology, Nagasaki
University, 1-14 Bunkyomachi, Nagasaki 852, Japan.
3Department of Geology and
Geophysics, University of Wyoming, Laramie, Wyoming 82071.
The present study
was financially supported in part by a 29th IGC Research Grant-in-Aid (to
Muto). We are grateful to Norsk Hydro for access to Oseberg well logs and
cores. We also express our thanks to AAPG reviewers K. T. Biddle, D. W.
Houseknecht, O. Martinsen, and T. Elliott for their critical reading and
numerous helpful comments; however, the ideas and interpretations presented
herein are entirely our own. |
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