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ABSTRACT

Relatively few studies have offered criteria for recognition of subsurface incised-valley fills using conventional 2D seismic data. This paper describes how sequence stratigraphic analysis of regional surveys acquired across the Barents shelf of Norway first indicated the presence of seismic anomalies interpreted to be incised-valley fills. These anomalies were later tested by wildcat drilling. Detailed sedimentologic and stratigraphic studies of cores, logs, and cuttings in the objective interval support the incised-valley fill interpretation.

On regional seismic surveys, the main valley-fill is a relatively subtle, U-shaped, flat-bottomed, transparent anomaly truncating flanking reflections. The valley-base unconformity is recognizable as a high-amplitude, discontinuous reflection. The valley-fill is capped by a high-amplitude, continuous, flat to slightly inclined reflection probably representing a transgressive surface. Valley width ranges from 1 to 5 km and thickness of infilling is estimated to be 60 to 80 m. The paleovalley can be traced with certainty for 25 km in a NESW direction, an orientation which is consistent with the known Early Jurassic/Late Triassic paleogeography of the Barents shelf. The valley-fill trends parallel with seismically-observed, prograding highstand deltaic complexes.

The seismic interpretation was later confirmed by drilling and coring of Well 7226/11-1, which penetrated an objective section including a 30-m thick, medium- to coarse-grained, Late Rhaetian to Hettangian age sandstone succession unconformably overlying siltstones and shales of Early Rhaetian age. Sedimentary structures and vertical sequences in the cored sandstone interval indicate deposition in a multiple-channel (braided) fluvial system. The underlying siltstone interval, located below the incised-valley fill, originated in an alluvial plain/lacustrine setting. Biostratigraphic analysis indicates that the unconformity separating the two units approximates a lacuna of one to four million years (210 to 214 Ma). Dipmeter analysis also confirms the interpreted infilling-succession.

Identification of this paleovalley fill has important ramifications for exploration on the Barents Shelf. Valley-fill sandstones display some of the best reservoir characteristics found to date in Mesozoic strata of the Barents Shelf. High permeabilities, porosities, net/gross sandstone ratios, and appreciable areal dimensions (over 12,000 ac or 4860 ha.) make these attractive targets in other locations where overlying seals have maintained their integrity during regional Tertiary and Pleistocene uplift events.

While documented amounts of valley entrenchment here exceed 50 m, the Haq global sea level cycle chart shows only Type 2 sequence boundaries in the probable time frame for this case of incision event. The apparent discrepancy may be the result of relatively low rates of differential subsidence across the Norsel High or possible syndepositional tectonic uplift during this period. Thus, this case study highlights influence of local factors (such as subsidence) versus more global controls (eustasy) on the architecture of basin-filling strata.

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