Journal of the Geological Society, London, Vol. 149, 1992, pp. 13-26, 12 figs, Printed in Northern Ireland Tectonic and climatic control of Triassic sedimentation in the Beryl Basin, northern North Sea L. E. FROSTICK’,T. K. LINSEY’ & I. REID2 ‘Postgraduate Research Institute for Sedimentology, The University, Reading RG6 2AB, UK 2Department of Geography, Birkbeck College, University of London, Malet Street, London WClE 7HX, UK, Abstract: The Beryl Basin (Embayment) occupies a central position along the Viking Graben of the northern North Sea and has been a hydrocarbon play since the early 1970s. Detailed analysis reveals a complex half graben that was established during a rift-phase of development that occurred in the Early Triassic (Teist Formation), after which, a long period of thermal subsidence (Lomvi and Lunde Forma- tions) provided accommodation forin excess of 1OOOm of sediment. The Teist Formation sediments are complex, they include shales, sands and conglomerates, and their superimposition on Zechstein salts is indicative of both uplift and the development of a moderate relief. They give way to the comparatively clean and occasionally pebbly sands of the Lomvi Formation,for which an analysis of both bedding and texture suggests depositionby a westerly-directed river system draining the hanging-wall ramp. The Lunde Formation records the spreading influence of a bajaddplaya complex and its eventual conversion to a more persistent lake. The growing importance and eventual dominance of the succession by lacustrine sediments gives clear indication a of change in climate, and this has been attributed to the rapid northward drift of Pangea during the Triassic period. Triassic sediments of the northern North Sea are geographi- The present study focuses attention on the Beryl Embay- cally extensive and attain considerablethickness (Lervik et al. ment, which sits on the eastern margin of the East Shetland 1989; Steel & Ryseth 1990). They occur across most of the Platform between the North and SouthViking Graben (Fig. 1). region, from the MureBasin in the north to theLing Graben in The area containsseveral productive oil fields, including Beryl the south and from the onshore and seabed outcrops of the and Bruce, whose reservoirs were discovered in theearly 1970s Inner Moray Firth in thewest to the deepsub-surface deposits and areMiddle Jurassic (Brennandet al. 1990). As a result, the of the StordBasin off the Norwegian coast in the east. In some area has been drilled extensively and much of the data ob- areas, e.g. the Horda Platform, thicknesses of up to 1858m tained is now in the public domain. Seismic data arealso more have been proven(well N31/6-1). Yet the nature of theTriassic readily available than in areas of current prospect. basins and the factors that controlled sediment character are With this comparative wealth of information, it has been poorly and only partly understood. There are several reasons possible to explore the patterns of Triassic basin development for this. First, the main exploration targets of the northern and to infer the events and processes controlling the changing North Sea lie in younger rocks and, consequently, few wells nature of the sedimentary fill. penetrate more than a couple of hundred metres into Triassic sediment. Second, the Triassic sequence consists almostexclu- sively of continental redbeds in which biostratigraphical con- Structural setting trol is either poor, makinguse of sparse palynomorphs(Bertel- Post-Triassic structural evolution of the area is well known as sen 1974; Warrington et al. 1980; Lervik et al. 1989), or it is a result of oil exploration aimed at Jurassic, Cretaceous, and entirely absent. This presents major problems of correlation in Tertiary plays (Heritier et al. 1979; Rochow 1981). But the any attempt at basin analysis. earlyrift-phase that belongs to the Triassicperiod is only To date, most papers that dealwith the Triassic succession sketchily understood. Thekey to thisearly history ofthe basin of the region have consisted of broad surveys. These provide lies undoubtedly in careful analysis of the Triassic fill using useful overviews of Triassic sedimentation, but lack any de- all available well-log, core and seismic data. tailed analysis of sequences and lateral facies changes in in- The Beryl Embayment is situated at the southern endof the dividual basins. Recent examples include those of Lerviket al. North Viking Graben which hasan extensionalhistory (1989) and Fisher & Mudge (1990). The Inner Moray Firth extending backat least as far as the Trias (Whitemanet al. 1975; Basin is a notable exception. Here, access to bothonshore out- Zeigler 1982b; Glennie 1990). Beryl has a complex half graben cropsand offshoreexploration and production well data structure inwhich the north-south East Shetland Fault acts as gives a valuable advantage. Asa result, there havebeen several the major boundary on westernits margin (Fig.1). It occupies a detailed sedimentological studies, although not all have been 200 km by 180 km hanging-wall block which is fragmented by aimed at basin-wideanalysis (Peacock 1966; Clemmensen a number of antithetic andsynthetic faultsinto a series of tilted 1987; Frostick et al. 1988; Naylor et al. 1989).Besides the sub-blocks. Although the western margin of the basin is well- Moray Firth, the other area that hasreceived close attention is defined by the boundary fault, its eastern margin is more diffi- that of Tampen Spur in the Norwegian Sector, where R0e & cult to establish. The basin extends across the international Steel (1985) haveexamined theinterplay of rivers and Median Line into the Norwegian sector.But there, databecome shorelines at the edge of the Late Triassic/Early Jurassic con- sparseand structural information isless available. In this tinent. study, thebasin is defined as extending toa set of NNE-SSW- 13 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/149/1/13/4891751/gsjgs.149.1.0013.pdf by guest on 01 October 2021 14 L. E. FROSTICK ET AL. 160N East Shetland c .-0, r N25/8-1 \ \ -KEY \ ’r Major faults (after Jakobbsen et al.) f \ *%.Medianline 0 D ry well -$- Dry ‘\ P roduction well0 Production \ 0 0 G as and Gas oil well \\\.XN0 Fig. 1. Index maps of the Beryl Basin p,--- showing the pattern of faults according Oilwell __ + I\\ to Jakobbsen et al. (1980), the Gos and oil shows distribution of wells, and the lines of -0 Oil show seismic and stratigraphic sections. Note 0 km 100 that the most westerly fault depicted is 0 Other a southern extension of the East Shetland Fault. trending faults that lie immediately east of the Median Line as feature through much of the Mesozoic. In addition, basement shown in Fig. 1. rocks are closer to the present-day surface here than elsewhere. In all published structural analyses of the northern North It may be possible, therefore, to infer a transfer fault as the Sea, the Beryl Basin (or ‘Embayment’ as it has been loosely reason for the southern ‘closure’ of the ‘embayment’. called) is depicted as a westward kink in the East Shetland Fault. The precise nature and cause of this deviation from a Triassic sediments of the Beryl Basin generalized north-south alignmentis the subject of speculation (Donato & Tully 1982; Swallow1986). The presence of a The Triassic sediments of the Beryl Basin have been largely Caledonian intrusion to the northwest (Threlfall 1981) may ignored in previous studies. Regional analyses of the Triassic have caused a curvature of the fault around its margin, so give only a general treatment of the sequence across an area producing the westward embayment. However, the reason for covering the whole of the Viking Graben (e.g. Brennand 1975; ‘closure’ ofthe basin in the southis less obvious. The boundary Lervik et al. 1989) and none describes the sediments in detail. fault is undoubtedly offset eastward in this area,while regional There are 93 published well-logs for the Beryl region of seismic sections reveal that there wasa positive topographical which 54 contain information about Triassic rocks. However, Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/149/1/13/4891751/gsjgs.149.1.0013.pdf by guest on 01 October 2021 TRIASSICSEDIMENTATION, NORTH SEA 15 of these, only 10 penetrate the whole of the succession into either very thin or absent in manyof the wells in theUK sector older rocks. This means that there are 14305m of well in- of the basin especiallytowards the south, althoughit does form formation in total, but within this framework there is only part of the oil reservoirin the Beryl field and reaches a respect- 148m of publicly available core. The distribution of wells is able thickness in the Norwegian Sectorwells (e.g. 251 m in the inevitably patchy, tending to concentrate close to structural type-sequence well N33112-2). For this reason, and because of ‘highs’(Fig. 1). Thismakzs basinwide interpretation of its equivocal status on the Triassic-Jurassic boundary, it has palaeoenvironments rather speculative. In addition, the Lower not been included in this study. Triassic strata arevery poorly represented in the record. These The contrasting characterof the three Triassicformations is shortcomings of the available database are not unique to the evident froman analysis of percent shale volume ineach bed as Triassic Beryl Basin. However, they serve to highlight the im- defined by the Automatic Bedding Disciminator (Fig. 3). From portance of maximizing the extraction of useful information. this it can beseen that beds of the LomviFm. are dominatedby Well-log interpretation is traditionallylaborious and highlyindividualistic. In this study, data-handling has been facilitated by the use of an Automatic Bedding Descriminator (Reid et al. 1989). In addition, regional seismic sections were deployed not only in the analysis of basin structure but also as a way of augmenting the patchywell data through an interpre- H Lunde Fm. tation of broad features of the sedimentary sequence such as Lomvi Fm.