Seismic Stratigraphy and Facies of the Frigg Fan Complex
J. E. McGovney, B. J. Radovich, 1985. "Seismic Stratigraphy and Facies of the Frigg Fan Complex", Seismic Stratigraphy II: An Integrated Approach to Hydrocarbon Exploration, Orville Roger Berg, Donald G. Woolverton
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The Frigg submarine fan complex (Ypresian) is composed of five m ajor depositional sequences (fan lobes) defined by lap-out relationships on seismic sections. The West Frigg and Odin lobes were sourced from the southwest and the East Frigg, Lower and Upper Northeast Frigg lobes were probably sourced from the east and northeast. The major gas reservoir in the Frigg field consists of the West Frigg and East Frigg lobes. The gas reservoir at the Odin field consists of the Odin and Upper Northeast Frigg lobes. Even though the lobes are of slightly different ages, they form a single, laterally continuous reservoir. Reservoir continuity is provided by stacking and extensive lateral migration of channel sands during deposition as well as downcutting of channels into prograded sheet sands.
Channel sands and prograded sheet sands form two distinct facies that can be recognized on seismic sections and that have distinctive well-log characteristics. The channel sand facies is expressed on seismic sections as a high-frequency, low-amplitude, mounded to discontinuous reflections. The facies tends to occur in the thicker portions and near the top of individual lobes, mostly above the gas/oil contact. Channel sands in well logs have a “blocky”, low gamma response, range in thickness from 10 to 100 m (33 to 328 ft), and are separated by 2- to 5-m (6.6- to 16.4-ft) thick shales. The sands are interpreted as amalgamated channel-fill grain flow deposits or turbidites.
Prograded sheet sands are expressed on seismic sections by concordant and downlapping low-frequency, continuous, medium- to-high amplitude reflections. The facies occurs in fan fringe and outer fan environments where individual sheet shands, 5 to 20 m (16.4 to 65.6 ft) thick, are separated by equally abundant but thinner shale and calcareous shale beds. Groups of sheet sands may display a coarsening upward gamma response (that is, lower gamma values in upper sands of the group), interpreted to result from fan progradation.