Fig. 1. Situation map showing the aerial extent of Cenozoic deposition and the main hydrocarbon occurrences of the North Sea Basin. The box shows the location of the study area given in Fig. 2. The contours represent total Cenozoic sedimentary thicknesses in metres.

Fig. 2. Location Map showing the position of the study area on a promontory of the East Shetland Platform on the western margin of the Viking Graben and adjacent to the East Shetland Basin and Bruce-Beryl Embayment. The map also highlights the easterly limit of progradation of the Dornoch deltaic system, the extent of which is shown by the stipple ornament, and the location of the Frigg Field and its satellites. The approximate location of the seismic line used in Fig. 4 and the area depicted in Fig. 6 and Fig. 16 are also shown.

Fig. 3. Lithostratigraphic Nomenclature used to describe the Paleogene Stratigraphy of the North Sea (Modified after Deegan, 1977; Isaksen; Knox; Mudge and Mudge; Mudge, personal communication 2000). The informal term, Bressay Sandstone, is introduced here to describe the sedimentary units found within a pronounced incised valley complex.

Fig. 5. Generalised cartoon depicting how the stratigraphy in the Bressay area correlates with the sedimentary fill in the neighbouring deep-water basin depocentre.

Fig. 4. W–E trending interpreted regional seismic line across the Ninian and Alwyn area showing progradational deltaic sediments ascribed to the Dornoch Formation. The line depicts the easterly limit to progradation of the Dornoch delta, which is sometimes referred to as the Ninian Delta in the East Shetland Basin by virtue of its maximum development in a position above the large, oil-bearing Jurassic tilted fault block bearing the same name. In contrast to the Bressay area, there is a notable absence of major incision of the Dornoch Formation in the Ninian area.

Fig. 6. Location map for the Bressay Area. The positions of the exploration boreholes, the course of the seismic lines (Fig. 7, Fig. 8 and Fig. 9) and the aerial extent of the 3-D seismic data from which the time-slices have been derived ( Fig. 10) are all shown.

Fig. 7. W–E trending regional seismic line and line drawing interpretation through well 9/2-1 and the projected trajectory of well 9/3-4. The line shows the remnant incised nature of the Dornoch delta progradation and subsequent drape by the Balder Formation. The easterly limit of the delta lies approximately 5 km west of the buried Jurassic graben margin on the East Shetland Platform. The seismic line also demonstrates the role that Cenozoic contraction had in creating the structural closure around the 9/2-1 discovery. Like with the subtle stratigraphic traps described in this paper, recognition of other similar contractional features may also enhance the prospective potential of the Cenozoic in the East Shetland Platform.

Fig. 8. Detailed 3-D seismic line and line drawing interpretation of a representative dip line through well 9/3-2 showing how the nature of erosion into the progradational Dornoch Formation (stippled) created palaeogeomorphic relief and hence, controls the subsequent extent and length of the hydrocarbon column. The occurrence of a shallow, ENE-dipping erosion surface interpreted to represent the bounding surface between the two main (highstand and forced regressive) progradational episodes.

Fig. 9. 3-D seismic line and line drawing interpretation of a representative NW–SE trending random line through wells 9/3-2, 9/3-3 and the Bressay Discovery to illustrate the relationship between the incised Dornoch Formation and the younger, Bressay Sandstone. The line is interpreted to illustrate the effects of compactional relief above a deeply-incised valley. The sediments contained within the clastic incised valley fill are informally termed the “Bressay Sandstone” within this paper. The seismic line also shows how the compactional relief has controlled the subsequent extent and length of the hydrocarbon column. The occurrence of gas chimneys above the level of the Dornoch Formation attests to hydrocarbon charge having first filled and then breached the sedimentary unit.

Fig. 10. Interpreted time slices through the 3-D seismic volume in the Bressay area: (a) shows the easterly extent of Dornoch delta progradation, its erosional upper boundary, which effectively defines the limits to the 9/3 Discovery and the compactional relief generated above the Bressay channel system; (b) depicts the trace of the NE–SW trending compressional fault (F-F) and its associated hangingwall anticlinal closure, which was drilled by the 9/2-1 well.

Fig. 11. Summary line drawing interpretations of the stratigraphic relationships in the Bressay area. The diagrams show linked NW–SE and SW–NE trending cross-sections. The colour ornaments are consistent with those used in Fig. 16.

Fig. 12. Well-correlation panel for the Bressay Area. The correlation has been flattened to a marker approximately 15 m above the top of the Balder Formation in order to demonstrate the original erosional relief of the Bressay incised valley, which exceeds 250 m in well 3/28-2. Location of the section is given in Fig. 16.

Fig. 13. Representative core photographs showing the main character of the three recognisable sedimentary facies in well 3/28-4. (a) Tuffaceous Mudstone Facies; (b) Heterolithic Facies; (c) Pebbly Sandstone Facies.

Fig. 14. Schematic cartoon to illustrate the six main stages in the stratigraphic development and evolution of the Bressay area. The montage records the progradational history of the Dornoch Formation, the creation of erosional relief, its subsequent incision by the Bressay channel complex and final drape and compactional relief.

Fig. 15. Schematic view (from the SSE) of the interpreted palaeogeography of the East Shetland Platform during the development of the Bressay incised valley system. Ideas for the palaeogeography shown have been partially derived from examples previously published by Rosenthal and Zaitlin.

Fig. 16. Depth map showing the extent of the Paleogene palaeogeomorphic and incised valley play types in blocks 3/27, 3/28 and 9/3. The line of cross-section refers to the well-correlation panel shown in Fig. 12. The colour ornaments used are also consistent with those used in Fig. 11. The depth conversion applied used an average interval velocity model based on the wells shown on the basemap.