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 manner can account for the rarity of topsets. The progressive eastward truncation of the underlying Drake Formation on the Horda Platform by the Oseberg Formation suggests tectonic uplift and a lowstand of sea level immediately prior to the development of the Oseberg deltas. The Oseberg Formation is thus seen as a lowstand prograding wedge that developed during an interval of relatively high sediment supply and a variably rising sea level, culminating in the latest Aalenian flooding seen in the base of the overlying Rannoch Formation. The dip-log data, integrated with cores and other logs, are critical to correctly interpreting the reservoir's internal architecture. The slope-readjustment model predicts that the development of stacked massive reservoir sandstones of Oseberg type will occur preferentially in local areas of high subsidence or syndepositional faulting, or where there is deeper water in the lee of already existing fault scarps.

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