We C2 06 Pre-Cretaceous Structural Development of the Leirdjupet Fault Complex and its Impact on Prospectivity, Southwestern Barents Sea, Norway

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1 We C2 06 Pre-Cretaceous Structural Development of the Leirdjupet Fault Complex and its Impact on Prospectivity, Southwestern Barents Sea, Norway K.H. Dittmers* (DEA Norge As), I. Kjørsvik (Wintershall Norge AS), F. Hatton (Aker BP) Summary DEA Norge as operator of PL 721 has spent several years exploring this area, maturing it from 2D seismic prelicense round into a fully 3D interpreted, drill ready project. The understanding of the structural development of this area has increased markedly as ideas and concepts have been firmed up using new data. The Leirdjupet Fault Complex (LFC) generally trends in a N-S direction, while NE-SW directed faults splay out from the main fault and give rise to the development of rotated fault blocks in the vicinity of the fault complex. It separates the deep Bjørnøya Basin in the west from the shallower Fingerdjupet Sub-basin to the east. By Late Paleozoic times, thinning above the LFC is observed associated with movement along the N-S and NE- SW fault trends due to E-W extension. This may have lead to exposure and karstification of Paleozoic carbonates. During Mesozoic times, dominant NE-SW and subsidiary WNW-ESE fault trends were active, indicating NW-SE extension with a sinistral wrench component. This influenced the deposition of the siliciclastic strata of the Realgrunnen Sub-group during the Mid Jurassic, as extensional tectonics created accommodation space allowing for thicker sand development in the LFC than that observed in the wells in Q7321.

2 Introduction The western Barents Sea is part of the continental shelf of northwestern Eurasia, located north of Fennoscandia and bordered by the Norwegian-Greenland Sea in the west and the Svalbard Archipelago to the north (Figure 1A). The Barents Sea tectonic setting is the result of multiple tectonic processes including three orogenic phases (Timanian, Caledonian and Uralian) and widespread Late Paleozoic and Mesozoic/Paleocene rifting phases prior to the final North Atlantic breakup in the earliest Eocene. The entire western Barents Sea rift system reflects the grain of the Caledonide orogeny (Dorè, 1991; Ritzmann and Faleide, 2007) which is characterized by a predominant NE-SW strike direction (Gudlaugsson et al., 1998; Figure 1B). The fanshaped geometry of the Caledonian collision zone seems to have influenced younger, extensional movements. However, these earlier rifts never reached a continental breakup stage and seafloor generation until the North Atlantic opening in the Earliest Paleocene (see Figure 2 for an overview of major rift events). The Leirdjupet Fault Complex (LFC, Figure 1B) generally trends in a N-S direction, while NE-SW directed faults splay out from the main fault and give rise to the development of rotated fault blocks in the vicinity of the fault complex. The LFC separates the deep Bjørnøya Basin in the west from the Fingerdjupet Subbasin in the east. As prolongation of the Castberg Trend at the edge of the Bjørnøya Basin this area has been the focal point for exploration in the recent years. In terms of petroleum system all ingredients are present, as the area appears to be above the cementation threshold (7321/-1!) and is a focal point for (re-) migration. Figure 1: A) Bathymetric map of the Norwegian continental margin (modified after Faleide et al., 2008); B) Depth to Base Cretaceous Unconformity (BCU) in km and main structural elements (NPD 2015). Major fault complexes are: LFC Leirdjupet Fault Complex, BFZ Bjørnøya Fault Zone, and HFC Hoop Fault Complex, area of interest in red frame. Note both the N-S oriented lineaments in the Loppa High and LFC, whereas in most of the prominent faults display a NE-SW caledonian structural grain. Data and Methods Interpretation is based on 3D seismic data and various vintages of 2D data: Block 7321 wells have been used for regional sections and well ties (Figure 3). We used Midland valley to carry out a 2D back stripping/restoration pilot study on selected key lines, followed by a 3D structural reconstruction focusing on the Mesozoic section. This helped to constrain the style and kinematics of faulting in the region (continuation of 2D analysis). For a detailed presentation see Reilly et al., th EAGE Conference & Exhibition 2017

3 Results and Interpretation Devonian Carboniferous (Rift I), formed NE-SW to NNE-SSW striking grabens and horsts, defined by steeply dipping normal faults often extending into the basement. Typically, throw is in the order of hundred meters and a complete syn-to post- rift depositional cycle is contained in the half-grabens. It appears that the rift episodes in the graben are associated with footwall uplift, but this observation is based purely on seismic stratigraphy. Late Permian/Early Triassic extension (Rift II) Following the Devonian Carboniferous Rift I extension was renewed in the Late Permian while carbonate deposition on platform areas prevailed. E-W extension along N-S trending faults is documented by Isochoremaps. The Caledonian inherited NE-SW trending faults were also activated, as strata is thickneng across those lineaments as well. In basinal areas an upper Permian growth section can be documented, which is also confirmed by data from the nearest offset well 7321/8-1. The central crest of the Leirdjupet FCP shows thinning during this time, indicating it could have been subaerillay exposed; at least a condensed section or hiatus is expected similar to a recently described section of the Wordiekammen Fm in Svalbard (Ahlborn and Stemmerik 2015). Figure 2: Lithostratigraphic chart for the South West and South East Barents Sea with the chronological time scale (Norlex, 2010), Major rift episodes after Clark et al., Coloured boxes indicate major tectonic phases as identified on seismic (see Figure 3). Re-occurring number indicates that the Faults have been re-activated.

Late Triassic to Early Cretaceous extension (Rift III) Isochore maps indicate that NE-SW trending faults created accommodation space (subtle) from the Late Triassic onwards, documenting mild

Similar features are also reported by Ostanin et al., 2012 from the Hammerfest basin and Safrononva et al., 2015 from the Northern Bjørnøyabasin.

4 Late Triassic to Early Cretaceous extension (Rift III) Isochore maps indicate that NE-SW trending faults created accommodation space (subtle) from the Late Triassic onwards, documenting mild extension. 3D data interpretation reveals a set of transtensional WNW ENE small faults that seem to postdate the former NE-SW faults and are ubiquitously distributed over the whole area. Similar features are also reported by Ostanin et al., 2012 from the Hammerfest basin and Safrononva et al., 2015 from the Northern Bjørnøyabasin. The accommodation space created by the fault activity allowed for a thickening of the Realgrunnen section in the order of 3-4 times with respect to the offset 7321 wells. Mesozoic deformation dynamics (Rift IV) Throw profiles obtained from 3D reconstruction could document how deformation was shifting between different major faults. Along-strike throw profiles indicate that the nature of faulting was predominantly normal on the faults analyzed. Displacement attributed to a minor shear component may have occurred during the Cretaceous. As the LFC consists of several major, conjugated faults (especially the eastern flank), one observation was that the fault activity stepped out progressively from the fault core complex and migrated to the east. The newly (re)activated fault would then take up most of the displacement, while movement along older faults ceased During the Cretaceous deformation accelerates and N-S fault splays into en echelon normal faults during transtension (after Fossen 2010; Figure 3: where the number 3 box is located) Figure 3 Top BCU map of the investigated area. Inlet displays well tie and N-S section; Major Fault episodes as in Figure 2: 0= Caledonian, 1= Upper Permian, 2 = Jurassic. Note the more subtle movements that re-activated 1 and O at Uppermost Triassic and Lower Jurassic, most of the deformation was extensional, with only mild inversion in 3. This could be wrench tectonics rather that pure compression! Data courtesy of TGS

5 Conclusions Detailed structural analysis reveals that prospectivity was controlled by both re-activation of the Caledonian grain and initiation of new faults. The Upper Permian is characterized by extension and fault movement in a N-S and NE-SW orientation. The core area of the LFC was potentially sub aerially exposed; at least thinning over the structure could be documented. The next major extension phase begins already in the upper most Triassic continuing into the Early Cretaceous where tectonic movements accelerate markedly. The initial subtle movements concentrated on the NE-SW directed faults eventually lead to the formation of abundant WNW-ESE faults. This extension event created accommodation space for the Realgrunnen Group. Acknowledgements All partners in PL 721 are thanked for their fruitful cooperation. Hugh Anderson from Midland valley is thanked for numerous discussions around the structural history of this area. We thank TGS for permission to publish key seismic lines. References Ahlborn, M. and Stemmerik, L. (2015). Depositional evolution of the Upper Carboniferous Lower Permian Wordiekammen carbonate platform, Nordfjorden High, central Spitsbergen, Arctic Norway NORWEGIAN JOURNAL OF GEOLOGY NORWEGIAN JOURNAL OF GEOLOGY Vol 95 Nr Dore, A. G. (1991) The structural foundation and evolution of Mesozoic seaways betweeneurope and the Arctic, Palaeogeogr. Palaeoclimatol. Palaeoecol., 87, pp Faleide, J. I., Tsikalas, F., Breivik, A. J., Mjelde, R., Ritzmann, O., Engen, Ø., Wilson, J. and Eldholm, O. (2008). Structure and evolution of the continental margin off Norway and the Barents Sea, Episodes, 31(1), Gabrielsen, R. H. (1984). Long-lived fault zones and their influence on the tectonic development of the southwestern Barents Sea, J. Geol. Glørstad-Clark, E., Faleide, J. I., Lundschien,,B. A. and Nystuen, J. P. (2010). Triassic seismic sequence stratigraphy and paleogeography of the western Barents Sea area, Mar. Pet. Geol., 27(7), Gudlaugsson, S. T., Faleide, J. I., Johansen, S. E. and Breivik, A. J. (1998). Late Palaeozoic structural development of the south-western Barents Sea, Mar. Pet. Geol., 15(1), Ostanin, I., Anka, Z., di Primio, R. and Bernal, A., (2012). Identification of a large Upper Cretaceous polygonal fault network in the Hammerfest Basin: Implications on the reactivation of regional faulting and gas leakage dynamics, SW Barents Sea. Mar. Geol. 332, Reilly, C, Anderson, H., Dittmers,.K., Karlsen and F., Kjørsvik, I. (2017). The Leirdjupet Fault Zone, Barents Sea the nature and timing of deformation in a complex fault zone Polar Petroleum Potential (3P) - Geodynamics and Evolution of Arctic Basins. Ritzmann, O., and Faleide, J. I. (2007). Caledonian basement of the western Barents Sea, Tectonics, 26, TC5014. Safronova, P. A., et al. (2015). Structural style and tectonic evolution of the northern part of the Bjørnøya Basin, South-Western Barents Sea, Norway. 77th EAGE Conference and Exhibition

We A Multi-Measurement Integration Case Study from West Loppa Area in the Barents Sea

We A Multi-Measurement Integration Case Study from West Loppa Area in the Barents Sea We-16-12 A Multi-Measurement ntegration Case Study from West Loppa Area in the Barents Sea. Guerra* (WesternGeco), F. Ceci (WesternGeco), A. Lovatini (WesternGeco), F. Miotti (WesternGeco), G. Milne (WesternGeco),