Summary The kinematic understanding of the relationship between relative plate motion and the structure of orogenic belts depends upon a knowledge of relative plate motion across the plate boundary system, the relative motion of small blocks and flakes within the system, an evaluation of orogenic body forces, and an understanding of the thermomechanical evolution of the upper part of the orogenic lithosphere in determining strength and detachment levels. We have built a preliminary model for the Cenozoic kinematic evolution of the western Mediterranean oceanic basins and their peripheral orogens that integrates (1) the motion of Africa relative to Europe based upon a new study of Atlantic fracture zones using SEASAT data and the Lamont-Doherty magnetic anomaly database, (2) a new interpretation of the rotation of Corsica/Sardinia and the opening of the Balearic and Tyrrhenian oceanic basins, (3) sedimentary facies sequences in the Apennines, Calabria, and Sicily, and (4) Apennine/Calabrian structure and structural sequence.
An integrated approach using plate tectonic analyses and detailed comparative stratigraphy of the North Atlantic has placed new constraints on the Mesozoic to Cenozoic geological history of the Atlantic margin of NW Europe. Key reconstructions from Mesozoic time to the present day have been plotted to show the evolution of the North Atlantic, and in particular the Rockall Trough. The reconstructions show Rockall Plateau attached to Greenland from Late Paleozoic time (380 Ma) to Late Cretaceous time (83 Ma) since when Rockall remained attached to Eurasia. The Rockall Trough probably initiated during end-Carboniferous to Early Permian time and underwent further stretching episodes in the Early Triassic, Early Jurassic, Middle Jurassic, Late Jurassic, Early Cretaceous, mid-Cretaceous and Late Cretaceous to give the present-day Rockall Trough configuration. The Permo-Triassic rift was dominated by oblique opening with a left-lateral component of strike-slip. Jurassic through Early Cretaceous extension was characterized by predominantly left-lateral strike-slip with a minor dip-slip component in the Faeroe basin and north Rockall Trough, and mainly dip-slip extension in central and south Rockall Trough. In Early Cretaceous time (mid-Aptian) the majority of the United Kingdom Continental Shelf (UKCS) Atlantic margin underwent orthogonal opening followed by continued extension in Late Cretaceous to Paleocene time, culminating in the opening of the North Atlantic west Rockall Plateau. The main Late Jurassic and Early Cretaceous rift episodes conveniently divide the stratigraphy into pre-, syn- and post-rift megasequences which form gross play fairways along the North Atlantic margin. Analysis of these fairways permits integration of data from both mature (e.g. North Sea) and immature (e.g. North Atlantic margin) exploration provinces and helps provide a consistent, predictive approach to the assessment of future hydrocarbon potential of the frontier basins lying along the North Atlantic margin.
The majority of North Sea structural traps requires that at least one fault be a sealing fault. Over 400 faults from 101 exploration targets and 25 oil and gas fields were analyzed in a regional study of the North Sea. The faults cut clastic successions from a variety of depositional environments (marine, paralic, and nonmarine). The emphasis of the study was on fault-related seals that act as pressure or migration barriers over geologic time. Parameters such as fault strike and throw, reservoir thickness, depth, net-to-gross ratio, porosity, and net sand connectivity were plotted against seal performance to define trends and correlations to predict fault seal characteristics. A correlation appears to exist between fault orientation and sealing, although this is not stati tically significant. Sealing is proportional to fault throw normalized as a fraction of the reservoir thickness. The great majority of faults with throw greater than the thickness of the reservoir interval were sealing faults. The most useful parameters in fault seal prediction are fault displacement, net-to-gross ratio, and net sand connectivity. The conclusions of this study have general applicability to fault seal prediction in exploration, development, and production of hydrocarbons in clastic successions in the North Sea and perhaps other areas as well.
Fault zone displacement and thickness data (230 measurements) plus orientation and slip vectors from 55 faults were obtained from outcrops of Permo-Triassic sandstones. The data indicate that, generally, as fault displacement increases, the width of a fault zone also increases. In detail, the relationship between displacement and thickness is stepped with thickness increasing sharply above certain values (thresholds) of displacement. This indicates a discontinuous growth of the fault zone. There are at least two thresholds in the data presented. Previous well- and seismic-based studies in the North Sea have shown a clear positive correlation between the displacement, normalized as a fraction of the reservoir thickness, and the probability of a fault sealing. Displacement, therefore, can be used to give a rough estimate of the probability of a fault acting as a pressure barrier to fluids in sandstone-dominated successions. Porosity and permeability of the reservoir rock are reduced within a fault zone, therefore the wider the zone the more likely the fault will seal hydrocarbons. Rapid jumps in fault zone thickness at displacements of roughly 300 mm and 5000 mm indicate that the relationship between fault displacement and fault seal probability may not be linear, but to a fair approximation (±15%) may be taken as such.
Abstract A regional fault seal analysis study of SE Asian basins was carried out in order to define trends for predicting sealing faults. Map and well data were analysed and relationships were obtained between fault and reservoir parameters and fault seal. The parameters found to be most useful for discriminating between sealing and non-sealing faults in the region were fault strike, fault mode (oblique slip faults are most likely to be sealing), fault throw (faults with high throws are most likely to be sealing), depth (shallow faults are most likely to be sealing) and thickness-throw. Other parameters measured, some of which are useful for fault seal prediction in some basins, are: fault type (bounding faults, within-field faults, across-field faults), net-to-gross ratio, average reservoir porosity, average reservoir permeability, differences in hydrocarbon contact elevation across faults, hydrocarbon column height held back by sealing faults, and reservoir pressure and temperature. Data from over 150 faults from fields within West Java are presented to illustrate the main results of the analysis.
Current views for explaining the present structure of the Calabrian arc emphasize bending or buckling of an initially straight zone by rigid indentation. Although bending has played an important role, bending itself cannot explain all structural features now seen in the arc for the following reasons: (1) across-arc extension is inconsistent with buckling, (2) north-south compression predicted by a bending mechanism to occur in the internal part of a curved mountain belt is not present in the Calabrian arc, and (3) lateral shear occurs throughout the arc, not just along the northern and southern boundaries. The model presented here is based on lateral bending of mantle and lower crust (demonstrated by variation in extension in the Tyrrhenian basin) and semibrittle faulting and block rotation in the upper crust. These two styles of deformation are confined to the upper plate of the Calabrian subduction system. This deformation is considered to have been active from the beginning of extension in the Tyrrhenian basin (late Tortonian) and is still active today (based on Holocene seismicity). Block rotations are a consequence of lateral heterogeneous shear during extension. Therefore, some of the observed rotation of paleo-magnetic declinations may have occurred in areas undergoing extension andmore » not just during thrusting. Inversion of sedimentary basins by block rotation is predicted by the model. The model will be a useful aid in interpreting reflection seismic data and exploring and developing offshore and onshore sedimentary basins in southern Italy.« less
Abstract The Gulf of Suez (Clysmic) rift is a classic and well documented extensional province and has many similarities to the North Sea rift to which it is often compared. New structural mapping and fault analysis in western Sinai have shown that post-rift crustal shortening and inversion occurred across the Gulf of Suez, demonstrated by the presence of reverse faults formed as a result of overturning of normal faults, widespread sub-horizontal hydraulic fractures due to vertical stretching, low-angle thrusting and associated folding in the hangingwalls of normal faults, and steepening of bedding dips adjacent to basement stress risers, in some cases overturning stratigraphy. These features show the effect that superposition of inversion on extensional structures has on the geometry of the bedding in footwalls and hangingwalls. Observations and data from outcrop can be used to assist in the interpretation of similar structural relations in the North Sea, where post-rift contraction is also seen. Recognition of inversion-related structures is important in exploration, appraisal and development of North Sea reservoirs, for example, in estimating palaeobathymetry of Late Jurassic depocentres, in constructing cross-sections for reservoir zonation, and in evaluating fault seal and reservoir compartmentalization. Shortening in the Gulf of Suez occurred immediately after rift failure and was probably a gravity-driven process. Similar processes may have occurred in other failed rifts.
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