ABSTRACT The South Viking Graben (SVG) hosts many large oil and gas condensate reservoirs, some within Middle Jurassic and Cenozoic rocks, but most within thick submarine fan sandstone and conglomerate sequences of the Upper Jurassic Brae Formation and their correlative equivalents, collectively termed here the Brae Play. Regional studies carried out over the last few years (based on the extensive well database and a variety of 3-D seismic data) and the recent acquisition of extensive, high-quality, broadband 3-D seismic data across the SVG have led to better definition of the half-graben geometry and the extents of the Upper Jurassic submarine fans that host these hydrocarbon accumulations. A summary structure map, seismic sections that extend across the graben, and a 3-D image of the “Base Cretaceous” are used to illustrate the main structural features. On its western side, the top of an eroded scarp, which grades downdip into the major fault plane, can be used as the lateral limit of the postrift graben fill. The uppermost Kimmeridge Clay Formation (KCF; termed Draupne Formation in Norway), which is the top seal and dominant source rock for Brae Play fields, onlaps this eroded slope and limits the western extent of the synrift section. At depth, the top of the prerift Bathonian Sleipner Formation can be mapped along this fault margin abutting the uneroded footwall fault; this boundary defines the edge of the thickest Upper Jurassic synrift section within the graben. The top of the prerift section becomes progressively shallower to the east, where an approximate minimum limit of the graben can be defined along much of its length by the eastern limit of seismically mappable KCF (Draupne) Formation. Thick sequences of Upper Jurassic conglomerates and sandstones within the KCF (i.e., the Brae Formation) were deposited as submarine fans within the graben. Most sediment was derived from the west (i.e., the Fladen Ground Spur), but some important fan systems were fed from the east (i.e., the Utsira High). The maximum limits of these fan systems are delineated, aided by the use of lithofacies correlation, reservoir pressure, and biostratigraphic data; changes in fan distributions through the Late Jurassic are also shown. An updated palynological zonation scheme that has been widely used throughout the area is also presented. Although the area is in a mature stage of exploitation, further mapping using the most recent high-quality 3-D seismic, available extensive well datasets, and the mapped extents of the fan systems might lead to additional hydrocarbon accumulations being identified.

Abstract: The fossil record (including trace fossils) is remarkably reduced after the end-Permian mass extinction. Faunal recovery increased continuously during the Mesozoic, and a marked restructuring of shallow marine benthic communities during Jurassic and Cretaceous time is known as the Mesozoic Marine Revolution. While middle and late Mesozoic trace-fossil associations are diverse and well studied around the world, those of the Triassic are minimally documented. Especially the bioerosional aspects are poorly understood compared to other periods. An abundant and diverse ichnofauna in the Middle Triassic of the Germanic Muschelkalk Basin provides insights into the ichnologic record at the beginning of the Mesozoic. For more than two centuries, this basin has been the subject of numerous studies, and several ichnogenera were established from the German Triassic (e.g., Rhizocorallium, Trypanites, Balanoglossites, Pholeus). A first overview allows the estimation of about forty invertebrate ichnotaxa, which can be grouped into three categories in terms of their appearance: (1) Burrow trace fossils—besides well-known ichnotaxa such as Bergaueria, Cochlichnus, Conichnus, Curvolithus, Lockeia, Phycodes, Protovirgularia, Rhizocorallium, and Thalassinoides, this group also contains ichnotaxa that are poorly known elsewhere, e.g., Archaeonassa, Arachnostega, and Pholeus. Occurrences of the oldest Mesozoic nearshore Zoophycos from the Muschelkalk are important for the interpretation of the general evolutionary trends of the tracemakers and their behavioral convergence. Of special interest is the interpretation herein of complex trace fossils, such as Mixoteichichnus coniungus and Balanoglossites triadicus. (2) Bioerosion trace fossils— many horizons of the Muschelkalk succession are characterized by omission surfaces and allow the study of bioeroded firmgrounds and hardgrounds with well-established ichnotaxa (e.g., Trypanites weisei). Most bioerosion trace fossils are recognized in the German Triassic for the first time, including ichnospecies of Gastrochaeonolites, Caulostrepsis, Maeandropolydora, and Palaeosabella. (3) Meiobenthic trace fossils—micritic bedding planes exhibit a diverse trace-fossil association with burrows and trails, where in many instances the producer itself is preserved at the end of the trace. The tracemakers, preserved by recrystallization, are mainly in the size range of meiofaunal species and commonly appear as worm-like (nematoid) or arthropod-like organisms. Cochlichnus, Helminthopsis, and Helminthoidichnites are the most common ichnotaxa of this group.