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.