GEOLOGIC NOTE AUTHORS Haakon Fossen Center for Integrated Petroleum Research, Department of Earth Sci- Fault linkage and graben ence, University of Bergen, P.O. Box 7800, Bergen 5020, Norway; [email protected] stepovers in the Canyonlands Haakon Fossen received his Candidatus Scien- tiarum degree (M.S. degree equivalent) from (Utah) and the North the University of Bergen (1986) and his Ph.D. in structural geology from the University of Min- Sea Viking Graben, with nesota (1992). He joined Statoil in 1986 and the University of Bergen in 1996. His scientific implications for hydrocarbon interests cover the evolution and collapse of mountain ranges, the structure and evolution of the North Sea rift basins, and petroleum- migration and accumulation related deformation structures at various scales. Haakon Fossen, Richard A. Schultz, Egil Rundhovde, Richard A. Schultz Geomechanics-Rock Atle Rotevatn, and Simon J. Buckley Fracture Group, Department of Geological Sciences and Engineering/172, University of Nevada, Reno, Nevada 89557; [email protected] ABSTRACT Richard Schultz received his B.A. degree in ge- Segmented graben systems develop stepovers that have im- ology from Rutgers University (1979), his M.S. degree in geology from Arizona State University portant implications in the exploration of oil and gas in exten- (1982), and his Ph.D. in geomechanics from sional tectonic basins. We have compared and modeled a rep- Purdue University (1987). He worked at the Lunar resentative stepover between grabens in Canyonlands, Utah, and Planetary Institute, NASA centers, and in and the North Sea Viking Graben and, despite their different precious metals exploration before joining the structural settings, found striking similarities that pertain to University of Nevada, Reno, in 1990. His interests other graben systems. In both cases, the stepovers represent rela- include rock fracture mechanics, growth and statistics of fracture and band populations, and tively high parts within the graben systems that are likely to be planetary structural geology. among the first to be filled with hydrocarbons generated in deeper parts of the grabens. Furthermore, the relay ramps and Egil Rundhovde StatoilHydro, Box 7200, smaller fault offsets in stepovers ease hydrocarbon migration Bergen 5020, Norway; [email protected] and allow stepovers to act as preferred migration routes from Egil Rundhovde received his Candidatus Scien- deep graben kitchens to structurally higher traps in the basin. tiarum degree (M.S. degree equivalent) from the University of Bergen in 1987, studying duc- Graben stepovers and their related structures should be paid tilely deformed rocks of the Norwegian Cale- special attention during exploration because they may repre- donides. In 1992, he received his Ph.D. in struc- sent hydrocarbon accumulations complementary to larger traps tural geology from the University of Trondheim, along the graben flanks. These observations explain the location focusing on precious ore genesis in deformed of the Kvitebjørn, Valemon, and Huldra fields in a stepover ophiolite complexes. He joined Statoil’s research structure of the Viking Graben and encourage increased focus center in 1991 and since 1998 has been involved in the production of the North Sea Gullfaks on similar graben stepovers in the Viking Graben and other field and other assets in the Tampen region of graben systems. the North Sea. He is currently the subsurface manager of the Volve and Glitne fields for StatoilHydro. Copyright ©2010. The American Association of Petroleum Geologists. All rights reserved. Manuscript received May 8, 2009; provisional acceptance June 28, 2009; revised manuscript received September 11, 2009; final acceptance October 13, 2009. DOI:10.1306/10130909088 AAPG Bulletin, v. 94, no. 5 (May 2010), pp. 597–613 597 Atle Rotevatn Center for Integrated Pe- INTRODUCTION troleum Research, University of Bergen, Allegaten 41, 5007 Bergen, Norway; present Large-scale lateral shifts or steps in graben axes (Figure 1) are address: Rocksource ASA, P.O. Box 994 Sentrum, well known from many continental rift systems, such as the East Bergen 5808, Norway; [email protected] African rift system (Rosendahl et al., 1986; Morley et al., 1990; Nelson et al., 1992), the Oslo rift (Sundvoll and Larsen, 1994), Atle Rotevatn received his Candidatus Scien- tiarum degree (M.S. degree equivalent) from the the Rheine rift (Ziegler, 1992), the Suez rift (Bosworth, 1995), University of Oslo in 2004, studying ductilely and the Rio Grande rift (Mack and Seager, 1995), as well as from deformed rocks of the East Greenland Caledonides. mid-ocean rift systems (e.g., Sempéré et al., 1993). In the well- In 2007, he received his Ph.D. in structural ge- explored Late Jurassic North Sea passive rift system, the Viking ology from the University of Bergen, focusing on Graben defines the central rift graben between latitudes 60 reservoir-scale deformation structures and their and 62° (Figure 2). The Viking Graben is segmented in a con- influence on fluid flow in oil and gas reservoirs. In 2006, he joined the Norwegian exploration sistent right-stepping sense (Figures 3, 4). Some of the largest and production company Rocksource, where he North Sea offshore oil fields (e.g., the Statfjord, Gullfaks, Snorre, currently works in international exploration. and Oseberg fields) are located on the flanks of the Viking Gra- ben, and several smaller hydrocarbon-filled structures have re- Simon J. Buckley Center for Integrated Petroleum Research, University of Bergen, cently been discovered in the same area. Although the giant oil Allegaten 41, Bergen 5007, Norway; and gas fields are located on the elevated rift margins, several [email protected] smaller fields and plays are found in areas of overlapping gra- Norway Simon Buckley received his B.S. degree ben segments. The coincidence between graben transfer zones (1999) and Ph.D. (2003) in geomatics from or stepovers and hydrocarbon accumulations is intriguing and Newcastle University, United Kingdom. He has far from coincidental: we suggest that these accumulations are since been a research fellow at the University related to specific structural characteristics of graben step- of Newcastle, Australia, and is currently a re- overs. Although various structural characteristics of graben searcher at the University of Bergen. His research interests include the application and advance- stepover structures and rift accommodation zones have been ment of geomatics techniques, particularly LIDAR discussed in several previous works (e.g., Rosendahl, 1987; and photogrammetry, within the Earth sciences. Morley et al., 1990), we will focus here on structural parallels between graben stepovers in the Viking Graben and a geomet- rically similar graben system in the Canyonlands, Utah. Based ACKNOWLEDGEMENTS on the comparison between the exceptionally well-exposed This manuscript benefited from helpful reviews Canyonlands example and the larger-scale Viking Graben, along by Martin J. Evans, Christopher K. Morley, and with calculations of graben-related topography and boundary- Sandro Serra. The basis for this work developed fault propagation, we discuss the implications of graben seg- during the preparation of Statoil field courses in Utah and later fieldwork, modeling, and mentation to petroleum exploration. analyses. The AAPG Editor thanks the following reviewers for their work on this article: Martin J. Evans, Christopher K. Morley, and Sandro Serra. RELAY STRUCTURES AND GRABEN SEGMENTATION Faults grow from smaller-scale structures by accumulation of slip through seismic activity and aseismic creep (e.g., Cowie et al., 2007). During the evolution of fault populations, indi- vidual faults interact and overlap, forming relay structures (e.g., Larsen, 1988; Childs et al., 1995). A relay structure, also referred to as a stepover, is the site of displacement transfer between two faults. Where displacement is transferred by means of ductile folding of the stratigraphy between the overlapping normal 598 Geologic Note the best exposed examples of laterally stepping or echelongrabensandassociatedrelayrampsisfound in the grabens area of Canyonlands National Park in southeast Utah, an example that can be scaled up to fit the larger scale Viking Graben of the northern North Sea. We refer to areas where grabens shift perpendicular to their strike direction as graben stepovers in this article. SEGMENTED VIKING GRABEN The Viking Graben forms the central graben and the structurally lowest part of the North Sea rift system, extending about 500 km (311 mi) from the triple junction with the Central Graben and Figure 1. Concept of a graben stepover, with associated relay the Moray Firth Basin in the south to the Sogn Gra- ramps and shift in graben axes. ben in the north (Figure 2). The North Sea rift sys- tem has experienced two main phases of rifting; one during the Late Permian to Early Triassic and one fault tips (e.g., Willemse, 1997), the area is defined during the late Middle Jurassic to earliest Cretaceous as a relay ramp (e.g., Peacock and Sanderson, 1994). (e.g., Roberts et al., 1993; Færseth, 1996). The Viking Relay ramps may accumulate strain until the rocks Graben per se is a result of the latter phase; the axis in the ramp fail in shear (Crider and Pollard, 1998). of maximum extension was located farther east dur- At this point, the two fault segments become phys- ing the precursory Permian–Triassic phase (Færseth ically connected by the formation