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- Volume 22, Issue 4, 2010
Basin Research - 4, 2010
4, 2010
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Subsurface sediment remobilization and fluid flow in sedimentary basins: an overview
ABSTRACTSubsurface sediment remobilization and fluid flow processes and their products are increasingly being recognized as significant dynamic components of sedimentary basins. The geological structures formed by these processes have traditionally been grouped into mud volcano systems, fluid flow pipes and sandstone intrusion complexes. But the boundaries between these groups are not always distinct because there can be similarities in their geometries and the causal geological processes. For instance, the process model for both mud and sand remobilization and injection involves a source of fluid that can be separate from the source of sediment, and diapirism is now largely discarded as a deformation mechanism for both lithologies. Both mud and sand form dykes and sills in the subsurface and extrusive edifices when intersecting the sediment surface, although the relative proportions of intrusive and extrusive components are very different, with mud volcano systems being largely extrusive and sand injectite systems being mainly intrusive. Focused fluid flow pipes may transfer fluids over hundreds of metres of vertical section for millions of years and may develop into mud volcano feeder systems under conditions of sufficiently voluminous and rapid fluid ascent associated with deeper focus points and overpressured aquifers. Both mud and sand remobilization is facilitated by overpressure and generally will be activated by an external trigger such as an earthquake, although some mud volcano systems may be driven by the re‐charge dynamics of their fluid source. Future research should aim to provide spatio‐temporal ‘injectite’ stratigraphies to help constrain sediment remobilization processes in their basinal context and identify and study outcrop analogues of mud volcano feeders and pipes, which are virtually unknown at present. Further data‐driven research would be significantly boosted by numerical and analogue process modelling to constrain the mechanics of deep subsurface sediment remobilization as these processes can not be readily observed, unlike many conventional sediment transport phenomena.
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Fluid dynamics and subsurface sediment mobilization processes: an overview from Southeast Caribbean
More LessABSTRACTThis paper discusses the origin and the dynamics of subsurface sediment mobilization processes in tectonically mobile regions and shale‐rich environment. This is illustrated by the example of Trinidad and the south of the Barbados prism. In this area of the southeast Caribbean, geophysical acquisitions have spectacularly shown the widespread development of sediment mobilization features in the interference area between the southern part of the Barbados prism and the active turbidite system of the Orinoco. Numerous mud volcanoes are especially developed along ramp anticline crests through hydraulic fracture systems. The area also exhibits trends of structures that correspond to massive uplifts of well‐preserved turbidite and hemipelagic sediments that cut up the surrounding sediments. Some of these structures are complicated by the development of collapse structures, calderas and superimposed mud volcanoes. The mobilized sediments expelled by the mud volcanoes are not only liquefied argillaceous but also fine sandy material from deep horizons, and various shallower formations pierced by the mud conduits. Both in the Barbados prism and in Trinidad, the expelled mud is rich in thin, angular and mechanically damaged quartz grains, which are probably cataclastic flows issued from sheared and collapsed deep sandy reservoirs. The exotic clasts and breccias result mostly from hydraulic fracturing. In Trinidad, the gas phase is mainly deep thermogenic methane associated with hydrocarbon generation at depth. Subsurface sediment mobilization notably differs from salt mobilization by the role taken by the fluid dynamics that control overpressured shale mobilization and induce sediment liquefaction. A reaction chain of several deformation processes develops around the conduits. Massive sedimentary uplift corresponds to large movements of stratified solid levels, possibly due to the tectonic inversion of pre‐existing mud volcano systems. All these phenomena are controlled by the development of overpressure at depth. No evidence for piercing shale diapirs has been observed in the area studied.
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Deep pore pressures and seafloor venting in the Auger Basin, Gulf of Mexico
Authors Matthew J. Reilly and Peter B. FlemingsABSTRACTPore fluid overpressures in four reservoir sandstones in the Auger Basin, deepwater Gulf of Mexico, are similar across the basin, suggesting that these sandstones are hydraulically connected over distances >20 km. Small overpressure gradients within them suggest upward flow rates between 1 and 20 mm year−1. At the crest of these sandstones, pore pressure equals or exceeds the least principal stress, and we interpret that high fluid pressure is fracturing the caprock and driving flow vertically. A well drilled into the crest of the Auger sandstones confirmed the presence of extreme overpressures that converge on both the least principal stress and the overburden stress. Above these zones, spectacular mud volcanoes are venting fluids today. Overpressured aquifers with significant structural relief may drive fluid vents and mud volcanoes around the world.
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Cenozoic mud volcano activity along the Indus Fan: offshore Pakistan
Authors G. Calvès, A. M. Schwab, M. Huuse, P. Van Rensbergen, P. D. Clift, A. R. Tabrez and A. InamABSTRACTThis study documents the tectono‐stratigraphic setting and expulsion history of a major, previously undescribed mud volcano (MV) province in the Indus Submarine Fan, offshore Pakistan. A buried MV field of nine composite MVs has been recognized using two‐dimensional (2D) and 3D seismic reflection data in a confined area of 50 × 65 km2. Conduits are recognized on each of these MVs connecting the pre‐Eocene parent beds to the stacked mud cones. The buried MVs are up to 8.4 km wide (4.5 km average) with a central conduit of 1.23 km average diameter and an average mud cone thickness of 0.33 km. Three major phases of fluid and mud remobilization occurred in the Early to Middle Miocene, intra‐Middle Miocene and in the Late Miocene to Plio‐Pleistocene transition. Most of the mud source (parent beds) seems to be of pre‐Eocene origin. Geometrical information from 21 mud cones allows an estimate of the volume required to build these fluid escape features. The calculated volume of remobilized sediments is 71.5±9 km3. The location of the MV field is limited to the pre‐Eocene main depocentre, with major tectonic deformation occurring along the wrench system of the Indo–Arabian plate boundary, i.e. the southern edge of the Murray Ridge. The Indus MV field is, to our knowledge, the longest lived (∼22 Myr) remobilized, Cenozoic sedimentary system observed worldwide. No evidence of present‐day mud flow activity is seen on the seabed seismic reflection in the study area.
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Combustion metamorphism in the Nabi Musa dome: new implications for a mud volcanic origin of the Mottled Zone, Dead Sea area
Authors E. Sokol, I. Novikov, S. Zateeva, Ye. Vapnik, R. Shagam and O. KozmenkoABSTRACTNumerous high‐temperature combustion metamorphic foci within brecciated mainly calcareous sedimentary rocks in the Dead Sea area (the so‐called ‘Mottled Zone’ complexes) have been interpreted as resulting from in situ oxidation and ignition of dispersed organic matter in bituminous chalks. Geological, chemical and petrological data for the Nabi Musa dome, one of 15 Mottled Zone complexes data presented in this paper, suggest an alternative interpretation relating the Mottled Zone complexes to Pliocene–Pleistocene mud volcanism and the associated methane combustion. The geochemistry and mineralogy of sedimentary, combustion metamorphic, localized hydrothermally, altered rocks, and ignition foci marked by ultrahigh temperature (up to 1500 °C) pseudowollastonite–rankinite–nagelschmidtite‐bearing paralava, indicate that Nabi Musa is a fossil mud volcano, comprising a large diatreme edifice with brecciated sedimentary rocks in its main feeder. The mud volcanic event mobilized sediments of underlying Cretaceous strata from depths of at least 0.8 km, and the eruption was driven by hydrocarbon gases (predominantly methane), with gas flaming causing local combustion metamorphism. Besides ultrahigh temperature combustion metamorphic processes, ejected sedimentary rocks were subsequently altered by low‐temperature hydrothermal fluids from various sources, which produced specific rock compositions with local enrichments in Mg, Na, Cl and B. Later, carbonation almost completely replaced the original smectite‐bearing parent mud and preserved the edifice from erosion. The proposed mud‐volcanic origin of the Mottled Zone complexes may have implications for gas prospecting in the Levantine basin.
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Structure of exhumed mud volcano feeder complexes, Azerbaijan
Authors K. S. Roberts, R. J. Davies and S. A. StewartABSTRACTWe report the first structural field mapping of exhumed mud volcano feeder complexes. Three mud volcanoes outcropping onshore in Azerbaijan were selected on the basis of outcrop quality and scale. These examples are all located within 1 km of the axes of NW–SE‐trending folds associated with the southern margin of the Greater Caucasus mountain belt. The mapping shows that the intrusive complexes are 200–800 m wide and roughly circular. These feeder complexes consist of a megabreccia of country rock blocks at a scale of tens of metres, enclosed in a matrix of intrusive mud. Minor structures include grid like fractures sets, sinuous fractures, mud plugs and breccia pipes. The country rock blocks are deformed and rotated relative to surrounding sedimentary strata. Alternative mechanisms to explain the strain history of these large blocks in the feeder complexes are: a. stoping, b. flow rotation and c. caldera collapse. Our mapping indicates that the most likely mechanism involves stoping processes, similar to those identified in igneous systems. This study provides a basis for reservoir distribution in commercial geological models that contain the feeder complexes of mud volcano systems, and also constrains conduit geometry for modelling studies of evolution and flow dynamics.
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Mud diapirs, mud volcanoes and fluid flow in the rear of the Calabrian Arc Orogenic Wedge (southeastern Tyrrhenian sea)
Authors Fabiano Gamberi and Marzia RovereABSTRACTIn the northern Calabrian margin offshore, the Paola Ridge, seaward from the 700 m deep Paola intraslope basin, tops at a depth of around 600 m. Multibeam bathymetry, seafloor reflectivity and seismic data are available. The Paola Ridge consists of circular or elongated ridges cored by a transparent seismic facies that are interpreted as mud diapirs. The diapirs have radius in the order of 5 km and elevate on average 200 m from the adjacent seafloor. The elongated shape of the diapirs is an evidence of a tectonic influence on the pathways exploited by the rising structures. The most recent seismic unit drapes the topographic relief associated with the diapirs showing that diapir rise is at present mainly quiescent. Pockmarks fields and evidence of gas charged sediments are due to degassing from the inactive diapirs. Two mud volcanoes, shown by high backscatter mud flows fed from circular high backscatter areas centred by a collapse feature, are also present on top of one of the dormant diapirs. The only diapir that is actively rising and deforming the seafloor is not associated with pockmarks. Thus, a relationship between fluid expulsion from the diapiric mass and the arrest of the diapir rise is apparent. The increased seafloor steepness due to diapir rise and the presence of gas within the sedimentary succession promotes sediment instability as shown by a thick slump deposit and numerous mass‐wasting scars. Sometimes, the diapirs rise in coincidence with extensional faults that offset the Messinian evaporites. The mud remobilization structures are located along a NW–SE trending belt characterized by active extensional faulting. Hence, genetic processes similar to many of the mud diapir and volcano provinces of the Mediterranean, consisting of pre‐Messinian source rocks mobilized along discrete belts of active tectonic deformation, is advanced as controlling the setting of the study area.
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Three‐dimensional seismic analysis of the morphology and spatial distribution of chimneys beneath the Nyegga pockmark field, offshore mid‐Norway
Authors Steinar Hustoft, Stefan Bünz and Jürgen MienertABSTRACTThree‐dimensional (3D) seismic data analysis has identified 415 seismic chimneys in the Nyegga area, on the mid‐Norwegian continental margin. The majority of the seismic chimneys connect with pockmarks at the modern seafloor, suggesting that the seismic chimneys represent the acoustic imprint of highly focused, cross‐stratal fluid flow. A semi‐quantitative spatial analysis of the seismic chimney complex, measuring parameters describing the dimension, geometry and root zone, provides constraints on the geological controls on chimney formation. Most of the seismic chimneys in the Nyegga region descriptively fall into the categories of ‘blowout chimneys’, because they diagnostically link to topography‐controlled leakage locations within strata showing acoustic indications of free gas. We find that approximately 60% of the fluid‐escape chimneys emanate from free gas layers below the gas hydrate stability zone. The timing of highly focused fluid expulsion through chimneys and the formation of the Nyegga pockmark field postdate the last glacial maximum at 25 000 years before present.
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3D seismic expression of km‐scale fluid escape pipes from offshore Namibia
Authors J. L. Moss and J. CartwrightABSTRACTThis paper documents a large number of large km‐scale fluid escape pipes with complex seismic expression and a diatreme‐like geometry from the mapping of a 3D seismic survey, offshore Namibia. These pipes are crudely cylindrical, but occasionally have steep conical geometry either narrowing upwards or downwards. They are generally ovoid in planform and their ellipticity varies with pipe height. Vertical dimensions range from ca. 100 to >1000 m and diameters range between 50 and 600 m. The lower part of the typical pipe is characterised by a sag‐like or collapse type of structure, but this is only imaged well in the wider pipes. The upper part of the typical pipe is characterised by gently concave upwards reflections, with a negative relief of tens of metres. There is some evidence (pipe cross‐section geometrical variations and amplitude anomalies) that these concave upwards reflections are vertically stacked palaeo‐pockmarks. A conceptual model for pipe formation is proposed that involves hydraulic fracturing and localisation of focused vertical fluid escape with volume loss at the base of the pipe inducing collapse within the pipe. Continuing episodic fluid migration through the pipe produces further collapsing of the core of the pipe and pockmark structures at the top of the pipe. Longer term seepage through pipes is manifested in zones of amplification of reflections above the top of the pipe.
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Regionally extensive emplacement of sandstone intrusions: a brief review
More LessABSTRACTLarge sandstone intrusion provinces (LSIPs) are described from three basins with contrasting tectonic settings, the North Sea Basin, the Faeroe‐Shetland Basin and the San Joaquin Basin. Hundreds of reservoir‐scale sandstone intrusions have been identified in each of these basins over areal extents of >500 km2, and with vertical extents of >1 km from their underlying source units. It is likely that these large intrusion networks were emplaced over geologically short time periods i.e. as a single hydrodynamic event, because of their highly interconnected geometry. Large sills are extensively intruded into the host claystones in each basin, and the presence and distribution of these intrusions demonstrates that pore fluid pressures in the source units may have been in excess of lithostatic at the time of remobilization and intrusion. Mechanisms that could bring source sand bodies of large volume and areal extent to a condition of supralithostatic pore fluid pressure are not well understood in the context of sandstone intrusion. Of the possible candidate mechanisms, shear‐induced liquefaction resulting from a large earthquake or bolide impact and geologically rapid pressure transfer from higher pressure cells are the most likely. The relative rarity of LSIPs worldwide is either a result of undersampling at outcrop or perhaps signifies that a rare combination of factors is required to form them. Once formed, they disrupt the sealing properties of the sequences they intrude, and this has major implications for fluid migration in these areas.
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Three‐dimensional seismic characterisation of large‐scale sandstone intrusions in the lower Palaeogene of the North Sea: completely injected vs. in situ remobilised sandbodies
Authors Ewa Szarawarska, Mads Huuse, Andrew Hurst, Wytze De Boer, Liwei Lu, Steven Molyneux and Peter RawlinsonABSTRACTA large number of km‐scale, saucer‐shaped sandstone bodies of enigmatic origin have recently been documented in the North Sea and the Faroe Shetland Basin. This study utilises three‐dimensional seismic data, calibrated by well data, to examine two such bodies that exhibit very similar saucer‐shaped geometries in cross‐section. The Volund and Danica structures, located 250 km apart are interpreted as end members of a spectrum of large‐scale remobilised and injected sandstones present in the North Sea Palaeogene. Both are characterised by a central 1–2 km‐wide low area surrounded by a discordant, 2–300 m tall inclined dyke complex, that tips out into a bedding concordant body interpreted as a shallow‐level sill and/or partly extruded sandstone. The origin of the central concordant sandstone body as either injected (laccolith) or depositional is of key importance to a complete understanding of the origin and prospectivity of these structures. The key criteria for recognising an injected vs. depositional origin for the central concordant sandbody are: (1) a flat, nonerosional base; (2) ‘jack‐up’ of the overburden equal to the underlying sand thickness; (3) equally thick layers of encasing mudstones; and (4) paleogeographic context. This study suggests that the Danica structure was deposited as a channel sandstone and remobilised in situ; this led to the formation of wing‐like intrusions along the channel margins. In contrast, the Volund structure overburden displays a forced‐fold geometry, arguably a diagnostic feature of an intrusive origin. The ability to recognise and differentiate completely injected vs. in situ remobilised sandbodies is important both from a basin analysis, hydrocarbon exploration and rock mechanics points of view. An improved understanding of these aspects will lead to a reduction of risks associated with the exploration and development of such a sandbody and an enhanced understanding of sediment remobilisation and fluid flow on a basin scale.
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Structural and stratigraphic relationships of the Palaeocene mounds of the Utsira High
Authors John Wild and Nowell BriedisABSTRACTThe Balder and Ringhorne Tertiary oilfields of the Utsira High are a cluster of prominent Palaeocene mounds, whose presence has confounded geologists since they were first observed on 2D seismic data back in the 1960s and 1970s. Until recently, the Balder Field geologic model consisted of distal, deep‐water Heimdal, Hermod and Balder Formation sandy‐debrite and turbidite sands, compensationally stacked along the flanks of the Utsira palaeo‐high, with intervening layers of hemipelagic shale. Remobilization of the sands by large‐scale fluidization accentuated the high‐relief mounds and sand injections linked reservoirs that were originally isolated. Reprocessed seismic data show strong primary reflectors that cannot be reconciled with this model; continuous sand bodies are observed to cross‐cut normal, biostratigraphically constrained sequence‐stratigraphic reflectors. The implication is that many, if not most, of the Hermod and Balder sands are not depositional, but were emplaced by injection. Furthermore, most of the Palaeocene mounds are associated with major ‘tears’ in the normally smooth Top Cretaceous chalk seismic reflector, and overlying chalk ‘rafts’. The chalk rafts were formerly thought to be detached glide‐blocks, shed from distant scarps to the east, but the improved data show that they are always positioned above matching depressions in the main Chalk, implying a local origin. Some of the rafts are immense, exceeding half a kilometre in width and millions of tonnes in mass. We believe the association of the chalk features with the mounds is more than a coincidence and suggest that they are genetically related. A number of driving mechanisms have been proposed for the formation of the Palaeocene mounds of the Utsira High, including normal post‐depositional compaction, fluidization by earthquakes, overpressuring due to gas migration and gravity‐sliding back into the Viking Graben. Our observations are, however, more consistent with the mounds resulting from episodic supra‐lithostatic pressure escape from beneath the Chalk.
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Sand remobilization and injection above an active salt diapir: the Tyr sand of the Nini Field, Eastern North Sea
More LessABSTRACTThe Selandien [58 Ma (PP3c‐d)] Tyr sand in the Nini area, Siri Fairway, Danish North Sea, is severely influenced by the syn‐depositional movement of the Nini Salt Diapir. The sand is faulted and remobilized into a degree where no original depositional signature can be recognized. The Tyr sand is drilled (and cored) by a number of wells, and the sand is never found in the same stratigraphic position. In some wells, the Tyr sand is injected down into the chalk of the Danian Ekofisk Formation, whereas other wells show the Tyr sand embedded in the Selandien Vile Member claystone, with varying degree of remobilization. The Tyr sand is thin (2–7 m) and is therefore below seismic resolution and close to seismic detection. Standard reflection seismic data has proven problematic in determining the actual thickness and spatial distribution of the thin Tyr sand located within or immediately above the chalk. A simultaneous AVO (amplitude vs. offset) inversion using time‐aligned angle stack seismic data, has improved resolution of the thin and complex reservoir, allowing a better understanding of the remobilization processes occurring above the rising salt diapir, and thereby an improved understanding of the reservoir and its performance. Three different remobilization features are described: injection into the chalk, injection up along fault planes and compactional driven injection. The force for the remobilization spans in orders of magnitude from metre scale phenomena, to injections of 100s of metres, moving millions of tons of material and fluids.
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Subsurface sediment remobilization as an indicator of regional‐scale defluidization within the upper Tortonian Marnoso‐arenacea formation (Apenninic foredeep, northern Italy)
More LessABSTRACTThe upper Tortonian Marnoso‐arenacea Formation displays 10–30 m‐thick sand bodies encased within up to 100 m‐thick packages of thin bedded turbidites and hemipelagites. In situ soft‐sediment deformation resulting in load casts, flames, ball‐and‐pillow and water escape structures often affected the sand body sedimentation units. It is interpreted as being due to a combination of overloading through the rapid deposition of thick massive sandstones and of uneven loading due to the topography created by erosional processes within the sand bodies. Post‐depositional deformation structures are also present. A variety of small‐scale sediment remobilization structures, such as pillars and plastically deformed consolidation laminae are associated with mound‐shaped deformed sandstones. Frequently, the deformed and intruding sand caused the erosion and break up of the overlying fine‐grained beds resulting in mudclast layers and networks of dykes and sills. Thus, the deformation assemblage resembles that associated with larger scale sandstone intrusions. Moreover, an intrusion complex occurs just below the base of one of the sand bodies. It comprises dykes that intrude along synsedimentary extensional faults formed adjacent to a slide scar. The sand intrusion occurred at a shallow burial depth suggesting a close relationship between seafloor instability, sliding and sediment injection along the faults. The suite of post‐depositionally deformed and injected sands is interpreted to result from overpressure build‐up within the sand bodies. In the study area, fluid seepage at the seafloor is marked by ‘calcari a Lucine’ chemiosynthetic carbonates. Thus, it is hypothesized here that the post‐depositional deformation of the sand bodies and injection structures was favoured by an upward flow of deep, hydrocarbon‐rich fluids caused by the ensuing tectonic compressive regime.
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Diverse products of near‐surface sediment mobilization in an ancient eolianite: outcrop features of the early Jurassic Navajo Sandstone
Authors Gerald Bryant and Andrew MiallABSTRACTThe Navajo Sandstone of the Colorado Plateau, USA, displays a wide range of soft‐sediment deformation (SSD) features, including decametre‐scale features that have not been found in any modern desert environment. Laboratory simulations and partial analogues from other depositional environments suggest that these features derived from episodic liquefaction and fluidization of unconsolidated dune deposits. Outcrop details at many locations preserve the effects of fluid‐escape dynamics through porous, permeable, well‐sorted sand, which was partitioned by subtle textural changes at depositional boundaries between successive dune deposits and, less commonly, by distinct lithofacies changes marking the interface between wet and dry depositional environments. Extreme deformation and turbulent sediment flow have effaced primary structures in some zones of deformation; but other sites preserve the ductile modification of primary structures. Some outcrops preserve evidence of dramatic alterations in topography and sedimentation patterns due to localized compaction and large, subsurface, sediment displacements. Particularly notable among these extraordinary features are those representing the foundering of active dunes, sediment eruptions, and the subsidence of interdune surfaces in the ancient erg. The distinctive patterns of deformation in the Navajo Sandstone, interpreted in the context of its primary sedimentary architecture, provide unique insights into environmental conditions in the region during the Early Jurassic. Widespread ductile deformation in the Navajo indicates extraordinarily wet conditions for an active dune environment. The truncation of successive deformation features by deflation surfaces establishes the episodic nature of deformation in the unit and also suggests climatic variability. The widespread, episodic occurrence of SSD features, viewed within the seismic context provided by palaeotectonic reconstructions, strongly suggests that earthquake triggering of localized liquefaction controlled the distribution of deformation in this unit.
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Liquid–solid suspension theory with reference to possible applications in geology
More LessABSTRACTThe first part of this paper provides an overview of the state of the art of the hydrodynamics of liquid suspensions of solid particles. It is shown that knowledge of the suspension Archimedes number is sufficient to estimate the voidage–velocity parameters for suspensions of solids possessing homogenous characteristics, thereby completely defining the system from a fluid dynamic point of view. A general relation is presented which will indicate if a solid–fluid system will be in the fixed or in the suspended state depending on the relative velocity between the two phases. Modifications of the previous approach when the solid particles are not spherical have been also indicated. The basic features of the pseudo‐fluid approach are then presented. This approach is useful when the solids making up the suspension have different sizes: by the introduction of the pseudo‐fluid apparent characteristics (density and viscosity) it is possible to make basic estimations of the suspension behaviour. The second part of the paper outlines possible application of suspension theory to geological phenomena, such as the vertical transport of water–sand suspensions. Aspects of interest are the minimum pressure required for the transport of the suspension, the estimation of suspension flow rate through an overall pressure balance, the vertical transport of large ‘breccia’ blocks and the behaviour of the solid particles present in the rising conduit once the overpressure falls below the minimum value needed to sustain the flow. Finally, a warning is given on the limitations of the presented relationships (e.g. the assumption of homogeneous solid dispersion in the suspension) and on situations such as inclined conduits which require a different approach.
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Sand‐rich injectites in the context of short‐lived and long‐lived fluid flow
By Rene JonkAbstractSand‐rich injectites are a common attribute of clastic sedimentary successions, and they have received increased attention in the last decade by geoscientists and engineers dealing with shallow subsurface flow and retention of aqueous and hydrocarbon fluids. Injectites form due to fluidized flow of sediment‐entraining fluids through high‐permeability strata. Given that sediment entrainment is confined to high‐permeability sediments with low horizontal effective stress and negligible cementation, the formation of sand‐rich injectites is restricted to the first kilometre of burial, where sand‐rich sediments are prone to fluidization and clay‐rich sediments are generally more cohesive and may hydraulically fracture to allow the creation of injectites. When reviewing conditions that may lead to aqueous and hydrocarbon flow within this shallow section at velocities that may cause sand fluidization and injection, we can rule out geologic time‐scale processes (disequilibrium compaction, hydrocarbon migration, and lateral pressure transfer) as plausible causes. This emphasizes the need for high‐strain‐rate processes to cause rapid fluid overpressuring and flow associated with sand injection. Earthquake‐induced shaking, instantaneous loading and release of overpressured fluids along moving fault planes are the most likely causes of sand remobilization and injection and these processes may enforce each other during tectonic stress‐release events. An additional mechanism that may trigger sand‐rich injections is the mechanical failure of shallow oil accumulations, particularly as the fluidization velocity of sand entrained in oil can be several orders of magnitude lower than for sand entrained in aqueous fluids, particularly if the oil is biodegraded (and thus has a high dynamic viscosity). Flow of hydrocarbon gas is unlikely to cause sand injection, although gas dissolved in upward‐flowing aqueous fluids may evolve at or near the surface as pressures in the aqueous phase drop.
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Volumes & issues
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Volume 36 (2024)
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Volume 35 (2023)
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Volume 34 (2022)
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Volume 33 (2021)
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Volume 32 (2020)
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Volume 31 (2019)
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Volume 30 (2018)
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Volume 29 (2017)
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Volume 28 (2016)
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Volume 27 (2015)
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Volume 26 (2014)
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Volume 25 (2013)
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Volume 24 (2012)
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Volume 23 (2011)
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Volume 22 (2010)
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Volume 21 (2009)
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Volume 20 (2008)
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Volume 19 (2007)
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Volume 18 (2006)
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Volume 17 (2005)
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Volume 16 (2004)
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Volume 15 (2003)
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Volume 14 (2002)
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Volume 13 (2001)
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Volume 12 (2000)
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Volume 11 (1999)
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Volume 10 (1998)
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Volume 9 (1997)
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Volume 8 (1996)
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Volume 7 (1994)
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Volume 6 (1994)
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Volume 5 (1993)
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Volume 4 (1992)
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Volume 3 (1991)
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