Abstract Three-dimensional seismic data from the Levant Basin, eastern Mediterranean, was used to quantify longitudinal strains in thick, multilayered Messinian evaporites at an early stage of salt tectonics. Gravity-spreading is driven by basin subsidence and tilting of the Levant margin and by progradation of the Nile Cone. Similar styles of shortening in two separate 3D survey areas comprise detachment folds, thrust-ramp folds and conjugate arrays of strike-slip faults. These Pleistocene structures can be explained with a single deformation phase with a tectonic transport direction of NE to ENE, obliquely opposed to the extension updip, which began in the Late Pliocene. Four major detachments within the Messinian are probably halite-rich intervals in the multilayer. Shortening of competent interlayers varies from 1–2% near the base to c. 7% near the top of the Messinian, with a sharp reduction in shortening at the top Messinian and roof to 1–2%. This shortening profile is attributed to asymmetric Poiseuille flow, indicating that salt is flowing downdip faster than the overburden is translating. Physical modelling supports the inferred flow profile, showing that each mobile layer flows faster than adjoining competent layers and that strains in evaporites can be far greater than in the overburden. This is the first published use of seismic data to demonstrate the flow regime within salt on a regional scale.

Abstract Postdepositional remobilization and injection of sand are important processes in deep-water clastic systems. Features resulting from these processes are particularly well documented in the Paleogene of the central and northern North Sea, where large-scale sandstone intrusions significantly affect reservoir geometries and fluid-flow properties of sand and mudstone intervals throughout large areas. Large-scale sandstone intrusions seen in seismic data from the Paleogene of the North Sea can be grouped into three main categories based on their size, morphology, and relation to their parent sand body: Type 1: Winglike sandstone intrusions are seen as discordant seismic anomalies that emanate from the sides and sometimes from the crests of steep-sided concordant sand bodies, which may be of depositional or intrusive origin. The intrusions may be as much as 50 m (164 ft) thick, and crosscut some 100–250 m (330–820 ft) of compacted mudstone section at angles between 10 and 35°. Winglike intrusions may form regardless of preexisting structures, but commonly exploit polygonal fault systems in the encasing mudstones. Type 2: Conical sandstone intrusions are seen as conical amplitude anomalies that emanate some 50–300 m (164–1000 ft) upward from distinct apexes located a few meters to more than 1 km (0.6 mi) above the likely parent sand body. The intrusions may be as much as 60 m (196 ft) thick and are discordant to bedding along most of their extent, with dips ranging from 15 to 40°. The nature of the feeder system is conjectural, but may comprise subvertical zones of weakness such as blowout pipes or polygonal fault planes, whereas the intrusions themselves do not appear to be controlled by preexisting fault systems. Type 3: Crestal intrusion complexes comprise networks of intrusions above more massive parent sand bodies. These intrusions are either too thin or too geometrically complex to be well imaged by seismic data. Despite the small scale of their component intrusions, crestal intrusion complexes may be volumetrically important. Large-scale sandstone intrusions commonly terminate at unconformities such as base Balder (uppermost Paleocene), top Frigg (lower Eocene), or base Oligocene, where they may have extruded onto the paleo-sea-floor. Because sandstone intrusions are commonly highly porous and permeable, they are important as reservoirs and as efficient plumbing systems in thick mudstone sequences. Because the intrusions occur in unusual stratigraphic positions not predicted by standard sedimentary facies models, they may constitute drilling hazards by hosting shallow gas accumulations or by acting as sinks to dense and overpressured drilling fluids. Predrill prediction of the occurrence of large-scale sandstone intrusions based on seismic data and predictive models is thus vital to successful exploration of deep-water clastic plays.

Abstract Three-dimensional seismic data from the continental margin offshore Israel (eastern Mediterranean) show several large-scale mounded structures interpreted to be clastic intrusions. The structures are confined to the Zanclean (early Pliocene) and lower Gelasian (late Pliocene) intervals and restricted to an area of 40 × 20 km (24 × 12 mi) along the Afiq submarine canyon, a former depositional fairway of Oligocene age. Most of the features are circular to oval in plan view, range from 0.5 to 2 km (0.3 to 1.2 mi) in diameter at their base, and are flanked by kilometer-scale depressions interpreted as regions of sediment depletion. In cross section, the mounds are as much as 400 m (1300 ft) in height and have flank dips of as much as 20–25°. The largest structures may reach as much as approximately 0.75 km 3 (0.17 mi 3 ) in volume and represent economic hydrocarbon reservoirs. Well data and direct hydrocarbon indicators show that the mounds are predominantly composed of gas-saturated sandstones along their flanks and crests, whereas their center is heterolithic. Petrophysical interpretation indicates the presence of chaotic and remobilized sediments in the core of the structures. The relationships of the mounds to the overburden exhibit both depositional and deformational geometries (e.g., onlap, forced folding). The proposed model for their formation is hydraulic jacking up of the overburden by forceful vertical and lateral intrusion of clastic sediments during shallow burial. Several episodes of intrusion alternated with the deposition of fine-grained clastic sediment during the Zanclean and early Gelasian to create the complex structures presented in this chapter. The suggested model has implications for the understanding of the trapping mechanism and reservoir properties of the mounded structures and needs to be incorporated in exploration and production strategies.

Abstract Subsurface and outcrop data are used to describe sand injectites, a group of genetically related features that includes sandstone dykes and sills, but also structures within depositional sand bodies. Fluidization is identified as the process by which sand is injected but we draw attention to the lack of constraints regarding fluidization velocity and fluid viscosity. Injectites are shown to develop between < 10 m and 500 m below the seafloor. No relationship between depth of generation and injection geometry is found. Liquefaction of sand may produce sufficient excess pore fluid to create small sand injections during shallow burial. Large injectite bodies are identified on seismic data that may exceed 4 × 10 7 m 3 are unlikely to be related to sand liquefaction. The general validity of hydraulic fracture as the mechanism for seal failure and propagation of injections is questioned. The association between the formation of polygonal faults and sand injection provides one of several alternative mechanisms for seal failure. Multi-phase intrusion is proposed as a likely mechanism for the formation of large sand intrusions, both because of the cyclical nature of most of the process invoked in their formation, and the author's own observations. Many of the processes of sand injection remain poorly constrained.

Abstract Polygonal fault systems are widely developed in fine-grained sedimentary successions and have been recognized in over 50 basins worldwide. They are normal faults with modest throw values (typically 10–100m), organized with a characteristic plan form pattern that is crudely polygonal, but with considerable variation in specific planform patterns. They have been attributed to four genetic mechanisms: gravity collapse, density inversion, syneresis and compactional loading. Their strain characteristics allow them to be distinguished from tectonic normal faults. The strengths and weaknesses of the four genetic mechanisms are considered in the light of these strain characteristics. It is argued that syneresis offers the likeliest mode of genesis and best explains the local and global features of these extraordinary structures. The detailed physical mechanism driving syneresis remains poorly understood.

Abstract 3D seismic and well data from the Ormen Lange Field, Mid Norway have been used to analyse the development of a system of polygonal faults affecting the Late Cretaceous-early Paleocene reservoir. These faults have the typical properties of polygonal fault systems recognized elsewhere in mainly fine-grained successions. They grew by upward propagation from the thick, shale-prone interval of the Late Cretaceous in the Møre Basin and were reactivated during the deposition of the Balder Formation. They have throws ranging from a few metres to 80 m, are typically 1–3 km in length and have highly irregular throw distributions along strike, mainly as a result of complex fault intersection geometries. The Ormen Lange Field is the first described example of polygonal faults that completely transect a major sandstone reservoir interval. The presence of these faults has important implications for the likely production behaviour of the field. Fault seal analysis shows that they are unlikely to form juxtaposition seals, except locally, but that they may have a significant risk for clay smear seals, particularly in the lower reservoir unit.