Snorre Field reservoir

Morad S., Bergan M., Knarud R., and Nystuen J. P. (1990) Albitization of detrital plagioclase in Triassic reservoir sandstones from the Snorre Field, Norwegian North Sea. J. Sedim. Petrol. 60, 411-425. 

The simulated present reservoir pressure (Fig. 5c) indicates significant decrease in effective stress, but below estimated fracture pressure (0.8-0.85 X geostatic pressure) in the Snorre field. An important factor in controlling the estimated porosities in the reservoir sands, due to the... 

Another possible explanation of the low GORs in the Field is the imperfect seal over the reservoir. Shows of petroleum resembling the Snorre population have been found several hundreds of metres over the reservoir interval (Caillet 1993 Leith et al. 1993 Leith Fallick 1995), even though the field probably has never reached fracture pressure. It is clear that the poor cap rock quality on the relatively shallow and underfilled Snorre structure makes this a perfect candidate for a Type 3 trap as explicitly classified by Sales (1997), i.e. the GOR of the Snorre petroleum charge is determined more by the cap rock properties than by SR quality and its kinetic properties. The oil inferred by Leith et al. (1993) may have leaked from the reservoir as the cap rock section has very low pore entry pressures and micro fracturing is also observed in the cap rock section. One can therefore speculate that the leakage may be a prerequisite to maintain black oil in the Snorre Field. 

The Snorre Field is located in the northern part of the Tampen Spur (Fig. 7). Major reservoir units are the Triassic Lunde Formation (northern part of the field) and the lower Jurassic Statljord Formation (southern part of the field). The reservoir contains an undersaturated black oil and has a generally low GOR (62-160 Sm /Sm ), typically increasing towards the north. The Snorre Field represents a rotated fault block cut by NNE-SSW and NE-SW faults during Upper Jurassic rifting (Horstad et al. 1995). The main source rock in the area is the Upper Jurassic Draupne Formation, a prolific Type II marine source rock. The underlying Heather Formation is also locally developed as a Type II, oil prone source rock. The main kitchen area of the oil encountered in the Snorre Field is situated in the basin directly east and south of the field (Horstad et al. 1995 Skeie et al. this volume). The Snorre... 

Fig. 8. Simulated extent of kerogcn transformation in the kitchen area of Snorre Field. The accumulation of undersaturated oil in the reservoir.

Table 5. Comparison of reservoir fluid properties simulated for the Snorre reservoir as a function of time and natural fluid properties from Snorre Field...

The two extremes, using instantaneous and cumulative phase predictions discussed above, provide only the framework for the total variability, which can be expected in the reservoir filling history studied here. Unravelling the evolution of petroleum fluid compositions in the Snorre Field through time would require a model resolution far exceeding what can be handled in reasonable computing time. The approach shown allows, however, a prediction of fluid properties, which is much closer to the natural fluid compositions than previously possible. This compositional kinetic scheme is the first of its kind to allow reasonable petroleum phase behaviour assessment in the simulation of basin evolution and hydrocarbon migration. 

D basin modelling of the Snorre Field (Norway) filling history, using the developed compositional kinetics, resulted in an excellent correlation between predicted and reported reservoir fluid properties for the present-day situation, and indicate that the reservoir contained a two-phase system prior to the latest Plio-Pleistocene burial and overpressuring event. 

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