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TMS-01 04 Butler (to_d) 14/10/05 10:42 am Page 43 The Ravenscar Group: a coeval analogue for the Middle Jurassic reservoirs of the North Sea and offshore Mid-Norway N. BUTLER1, M. A. CHARNOCK2, K. O. HAGER2,3 & C. A. WATKINS1 1Robertson Research International Ltd., Llanrhos, Llandudno LL30 1SA, UK (e-mail [email protected]) 2Norsk Hydro AS, Research Centre, N-5020, Bergen, Norway 3Norsk Hydro AS, N-0246, Oslo, Norway Abstract: A palynostratigraphical study of the Middle Jurassic Ravenscar Group, Cleveland Basin, northern England involving the integration of miospore and microplank- ton data with sedimentary facies data has resulted in improvements in the stratigraphical resolution of offshore hydrocarbon-bearing strata in the North Viking Graben and Mid- Norway. The Dogger Formation is of ‘earliest’ Aalenian age and is correlative with the uppermost Dunlin Group, Drake Formation and Båt Group, Ror Formation. The overly- ing Aalenian Saltwick and Eller Beck formations correlate with the Brent Group, Broom–Rannoch–Etive–Ness and the Fangst Group, Ile–Not–‘lower’ Garn genetic packages. The Cloughton Formation is either unrepresented or condensed in the Brent Province and Mid-Norway due to a regional unconformity, which truncates lower Bajocian sediments. The Scarborough Formation is of ‘latest’ early to ‘earliest’ late Bajocian age and correlative with the older part of the Tarbert–Heather and the ‘upper’ Garn–Melke genetic packages. These interpretations contrast markedly with the majority of those published. Despite their maturity of exploration and Methodology and techniques development, Middle Jurassic sandstones of the North Sea and offshore Mid-Norway still repre- The analyses were concentrated on three key sent some of the most prolific hydrocarbon sections near Whitby, Ravenscar and Scar- reservoirs in northwest Europe. Of these, the borough (Fig. 1). Additionally, some of these Brent and Fangst Groups (Deegan & Scull 1977; sections were logged sedimentologically to Dalland et al. 1988) are particularly important place the samples in a sedimentary facies and, despite the vast number of wells, the context. Samples were selected from mudstones, published chronostratigraphic schemes are siltstones and argillaceous sandstones as poorly calibrated with confidently dated, previous experience has demonstrated these onshore sections. The Ravenscar Group of the lithologies to be most productive for palynology. Cleveland Basin, Yorkshire (Fig. 1) consists of Palynological preparations were made using a five formations (Rawson & Wright 1995; Fig. 2). standard acid maceration and oxidation process This group comprises similar facies to those with the residue strewn onto a permanent glass encountered in the Brent Province and Mid- slide and analysed under the light microscope. Norway with sandstones, siltstones, mudstones For each sample, a split count technique was and coals of Aalenian to Bathonian age undertaken where 200 specimens were counted deposited in a coastal/delta plain to shelfal in the 10 µm fraction. An additional count was environment. A palynostratigraphical scheme made where 100 dinoflagellate cysts were has been developed from quantitative palyno- counted, wherever practical, concurrent to the logical analyses of 114 field samples taken from first count, providing a high-resolution, quanti- Ravenscar Group outcrops. This has allowed a tative database. Additionally, each slide was chronostratigraphical subdivision of the Raven- thoroughly scanned to ensure that all minor scar Group and enabled regional correlation of occurrences were recorded and that the abun- the constituent formations with coeval sedi- dance data were representative of the entire ments of similar facies of the North Viking slide. A combination of numerical abundance Graben (Brent Group) and offshore Mid- changes in conjunction with evolutionary tops Norway (Fangst Group). and bases of key palynomorph taxa (mainly dinoflagellate cysts; Williams et al. 1998 provide a comprehensive index for all dinoflagellate cyst From: POWELL,A.J.& RIDING, J. B. (eds) 2005. Recent Developments in Applied Biostratigraphy. The Micropalaeontological Society, Special Publications, 43–53. 1747-602X/$15.00 © The Micropalaeontological Society 2005. TMS-01 04 Butler (to_d) 14/10/05 10:42 am Page 44 44 N. BUTLER ET AL. 00°30’W WHITBY Fault at surface EAST CLIFF Fault beneath base of Chalk REDCAR Fault located at base of Jurassic Onshore Offshore Cretaceous P MIDDLESBROUGH E Jurassic A K N Triassic 10° 5° 0° 5° 10° T R 65° Mid O Norway U G BLEA WYKE POINT Brent WHITBY H Province Robin Hoodʼs Bay CLOUGHTON WYKE 60° CLEVELAND Ravenscar BASIN PEAK FAULT STUDY 55° AREA SCARBOROUGH UK PICKERING RED CLIFF FAULT Cayton Bay 50° THIRSK 0 200km FILEY 54°10’N MALTON HO ZONE WARDIAN - FLA FAULT MBOROUGH 0 20 km Fig. 1. Location map for the Cleveland Basin and its geographical relationship with the Brent Province and Mid-Norway (after Rawson & Wright 1992). taxa discussed herein) have been used to (1958). However, some stratigraphical data are constrain the gross stratigraphy. This approach of interest, particularly regarding the distri- has proven instrumental in many studies bution of Classopollis spp. that broadly concurs particularly of the Lower–Middle Jurassic of the with the distribution recorded herein. Hancock Mid-Norway Heidrun Field (Pedersen et al. & Fisher (1981) documented the palynofacies of 1989) and in the Viking Graben to the south selected parts of the Saltwick, Eller Beck, (Mitchener et al. 1992; Whitaker et al. 1992; Bray Cloughton and ‘lower’ Scarborough formations. et al. 1995). The abundance events may be mani- Although this information is useful, it provides fested as top increases, acmes or base increases little foundation for stratigraphical interpret- of a specific taxon or group of taxa. A number ation and correlation. Woollam & Riding (1983) of significant events are established herein and published a palynostratigraphical account on a summarized in Figure 3. These are regionally limited number of field samples. Riding (1984) correlative with the Brent and Fangst groups, documented dinoflagellate cyst assemblages which has facilitated a refined stratigraphy for from the mid-Toarcian to Aalenian of Blea these areas. Wyke Point, Ravenscar. Palynofloral composi- tion compares closely with that recorded herein. Previous palynostratigraphical work The Scarborough Formation was documented by Gowland & Riding (1991) and although a Little has been published on the palynology of useful sedimentary log was produced (and the Middle Jurassic of Yorkshire. The earliest utilized herein), only eight palynological palynological work was a comprehensive evalu- analyses were undertaken from the entire ation of miospores from the entire Ravenscar formation. Hogg (1993) produced a comprehen- Group concentrating on taxonomy by Couper sive account of the largely non-marine Scalby TMS-01 04 Butler (to_d) 14/10/05 10:42 am Page 45 MIDDLE JURASSIC NORTH SEA RESERVOIRS 45 SYSTEM STAGE LITHOSTRATIGRAPHY 164.4 Ma BATHONIAN Long Nab Member Scalby Formation 60m BATHONIAN- LATE BAJOCIAN Moor Grit Member ? 172.87 Ma Scarborough Formation 30m C I S Gristhorpe Member S EARLY Ravenscar A R BAJOCIAN U Group Cloughton 85m Lebberston J Formation Member 9m E L D D Sycarham Member I 176.5 Ma M ? Eller Beck Formation 8m AALENIAN Saltwick Formation 57m Lias Dogger Formation 12m 180.1 Ma Group Fig. 2. Subdivision of the Middle Jurassic (Aalenian–Bathonian) succession (modified after Rawson & Wright 1992). Formation. It is apparent that very few palyno- (at Whitby East Cliff). These sediments yield stratigraphical data have been produced for the high numbers and a high diversity of dinoflagel- more proximal facies of the Saltwick Formation, late cysts assigned to the Family Phallocystaceae Sycarham Member and Gristhorpe Member or (including Parvocysta nasuta, Susadinium scro- the marine Eller Beck Formation. foides, Susadinium faustum, Ovalicysta hiata, There is considerably more published palyno- Moesiodinium raileanui, Phallocysta eumekes, logical information regarding the Brent Group. and P. elongata). It is possible that these records Authors including Graue et al. (1987), Mitch- in the Dogger Formation at Whitby East Cliff ener et al. (1992), Whitaker et al. (1992), may represent reworked occurrences since Helland-Hansen et al. (1992) and Husmo et al. reworked mud-clasts derived from the Whitby (2002) have published stratigraphical schemes, Mudstone Formation are present. However, a the main conclusion being that the Broom diverse and abundant suite of Phallocystaceae Formation is Aalenian in age, the Rannoch, (dominated by Susadinium scrofoides and Oval- Etive and Ness formations are Bajocian and the icysta hiata with low but consistent numbers of Tarbert Formation is ‘latest’ Bajocian or Phallocysta and Parvocysta spp.) are present in Bathonian in age. This interpretation contrasts the much thicker Dogger sequence at Blea markedly with the new data presented herein. Wyke Point. The abundance and diversity of this group however decreases sharply in the upper- most part of the formation, a feature that has Palynostratigraphy of the Ravenscar also been identified in the uppermost Drake Group and correlation with the Brent and Formation (Dunlin Group) in the North Viking Fangst Groups Graben (Bray et al. 1995) and uppermost Ror Formation (Båt Group) in Mid-Norway (Butler, The Lias Group and Dogger Formation were personal observation). It is possible that these sampled at two locations, namely at Blea