Andy Gardiner - Academia.edu (original) (raw)

Papers by Andy Gardiner

Research paper thumbnail of Characterization of thin-bedded turbidites in the North Brae Field, South Viking Graben, North Sea

Research paper thumbnail of Petroleum Geology of the Georgian Fold and Thrust Belts and Foreland Basins

Regional and Petroleum Geology of the Black Sea and Surrounding Region, 1997

Numerous, mainly small, oil discoveries have been made within the foreland basins and fold and th... more Numerous, mainly small, oil discoveries have been made within the foreland basins and fold and thrust belts of Georgia. The largest field in Georgia (Samgori) contained ~200 MMbbl (million barrels) recoverable reserves reservoired in fractured middle Eocene volcaniclastics--the main proven reservoir in Georgia--trapped in a compressional fold of the Achara-Trialet belt. Where the Achara-Trialet deformation extended into the foreland basins, frontal folds with Miocene-Pliocene clastic reservoirs are known to be oil-bearing (e.g., Supsa field). Within the foreland basins in areas unaffected by Neogene compression, structural closures related to pre-Neogene extensional faulting may include draping lower Miocene and Mesozoic reservoirs: these are largely untested. The frontal folds of the Greater Caucasus have yielded small oil discoveries in lower Miocene fluvial to shallow marine clastic reservoirs. The widespread oil discoveries along the Achara-Trialet frontal folds demonstrate the presence of a working oil sourcing system from the Black Sea through to the Kura Basin. Gas chromatography-mass spectrometry and carbon isotope analyses indicate that a single source rock was responsible for generation of many of the oils in the foreland basins; oil source correlations suggest that this was late Eocene in age, deposited within the Paleogene Achara-Trialet Basin. Further, oil-prone source rocks appear to be present locally within the Greater Caucasus.

Research paper thumbnail of Bed-scale vertical and lateral distribution of massive sandstone in a topographically confined basin (Peïra Cava, SE France): Implications for flow processes

Research paper thumbnail of The Pliocene-Recent Euphrates river system: Sediment facies and architecture as an analogue for subsurface reservoirs

Research paper thumbnail of 3D GPR Surveying with Vertical Radar Profiling of Petroleum Reservoir Outcrop Analogues

62nd EAGE Conference & Exhibition

Research paper thumbnail of Sedimentary and tectonic controls on Lower Carboniferous (Visean) mixed carbonate–siliciclastic deposition in NE England and the Southern North Sea: implications for reservoir architecture

Research paper thumbnail of Value of Geologically Derived Features in Machine Learning Facies Classification

Mathematical Geosciences

The aim of this work is to demonstrate how geologically interpretative features can improve machi... more The aim of this work is to demonstrate how geologically interpretative features can improve machine learning facies classification with uncertainty assessment. Manual interpretation of lithofacies from wireline log data is traditionally performed by an expert, can be subject to biases, and is substantially laborious and time consuming for very large datasets. Characterizing the interpretational uncertainty in facies classification is quite difficult, but it can be very important for reservoir development decisions. Thus, automation of the facies classification process using machine learning is a potentially intuitive and efficient way to facilitate facies interpretation based on large-volume data. It can also enable more adequate quantification of the uncertainty in facies classification by ensuring that possible alternative lithological scenarios are not overlooked. An improvement of the performance of purely data-driven classifiers by integrating geological features and expert kno...

Research paper thumbnail of Outcrop-based stochastic modelling of turbidite amalgamation and its effects on hydrocarbon recovery

Petroleum Geoscience

... Variation in these two factors has a major effect on the primary internal heterogeneity ... t... more ... Variation in these two factors has a major effect on the primary internal heterogeneity ... to the erosive potential of succeeding currents and, hence, is constrained by the depositional environment. ... exposed sections will vary with the orientation of the section relative to palaeocurrent ...

Research paper thumbnail of Turbidity current velocity, grain size and bedforms in turbidite channels and associated depositional systems

Research paper thumbnail of Best practice stochastic fades modeling from a channel-fill turbidite sandstone analog (the Quarry outcrop, Eocene Ainsa basin, northeast Spain)

Research paper thumbnail of The impact of sub‐seismic shale layers on the reservoir's stress sensitivity

SEG Technical Program Expanded Abstracts 2008, 2008

In most clastic reservoirs experiencing pressure depletion due to production, the hydraulically c... more In most clastic reservoirs experiencing pressure depletion due to production, the hydraulically connected sands in the reservoir naturally compact to some degree. As a consequence, the much lower permeability reservoir shales may experience mechanical tension. The effective seismic response of the reservoir interval is thus a mix of both hardening and softening reservoir components. This phenomenon alters the predicted overall stress sensitivity from that anticipated for a homogeneous, fully connected reservoir interval. The time period over which this effect might be observed is influenced by the rate at which the shales reach pressure equilibrium with the surrounding sands. This work indicates that sub-seismic shale layers of approximately 1m thickness take less than 12 months to equilibrate, whilst thicker shale layers of 8m can take over 10 years. It is concluded that the mechanical and dynamic response of sub-seismic reservoir shale must be considered when quantitatively assessing the 4D seismic signature from frequently shot time-lapse surveys with a periodicity of 6 to 12 months, but also perhaps, for conventional 4D seismic surveys shot over 5 to 10 years. These conclusions are strongly affected by the permeability of the shale layers, the stress state, and are also a function of net to gross and depositional environment.

Research paper thumbnail of Reservoir Impact of Small-scale Aeolian Dune Architecture - From Acquisition to Simulation in the Wahiba Sands, Oman

74th EAGE Conference and Exhibition incorporating EUROPEC 2012, 2012

ABSTRACT http://earthdoc.eage.org/publication/publicationdetails/?publication=59265 Summary: Smal... more ABSTRACT http://earthdoc.eage.org/publication/publicationdetails/?publication=59265 Summary: Small-scale dune heterogeneity has a significant impact upon recoverable reserves within aeolian hydrocarbon reservoirs. Complex geometries exist, with bounding surfaces and primary strata types often negatively impacting fluid flow. Incorporating the effects of such architectural elements into reservoir models is essential when accurately determining their effect on development strategies. In order to assess their impact, we acquired a small pseudo-3D dataset from the Wahiba Sands, Sultanate of Oman, using ground-penetrating radar (GPR). In this paper we discuss the acquisition, processing and modelling of this dataset. Data are interpreted to be of a small linear dune. Radar stratigraphic units have been interpreted and mapped in 3D; a small-scale analogue reservoir model has been produced. Simulation studies have been conducted to assess the impact of a range of sensitivities, including the affects of permeability contrast, flow direction and capillary pressure. Results indicate that permeability contrasts have a significant impact on recovery, whilst flow direction is the dominant factor. The resulting models may not be directly transferable to a specific subsurface scenario, but the generic spatial information can be a useful guide.

Research paper thumbnail of Using GPR to extract 3-D turbidite channel architectures from the Upper Carboniferous Ross Formation, County Clare, Westen Ireland. 1: geological setting; 2: outcrop data integration

The Upper Carboniferous (Namurian) Ross Formation represents a thick accumulation of sandstones a... more The Upper Carboniferous (Namurian) Ross Formation represents a thick accumulation of sandstones and shales forming the early fill of the West Irish Namurian Basin (Collinson, 1991). The basin shows a systematic progradational fill from basinal shales (Clare Shales) to deep-water shales and turbidites (Ross Formation) to unstable slope deposits (Gull Island Formation) and shallow-water upward-coarsening deltaic sequences (Clare Group). The Ross Formation is very well exposed in coastal sections around the Loop Head peninsula. Here, the formation is 380m thick and consists of 75% sandstone, but rapidly thins to the North and South. The sandstone depositional units consist predominantly of sheet-like turbidites formed within laterally extensive layered or amalgamated sand-rich units (Clark, 1998). Inter-bedded within these sheet sandstones are erosional, small-scale channels, typically 100 to 500 metres wide and 5 to 15 metres deep.

Research paper thumbnail of New sequence stratigraphic model for fluvial studies

Research paper thumbnail of Assessment of the Eocene fluvial architecture of the Ebro Basin (northeast Spain) using Ground penetrating Radar

Eocene fluvial sediments of the Ebro Basin, northeastern Spain, are spectacularly exposed near th... more Eocene fluvial sediments of the Ebro Basin, northeastern Spain, are spectacularly exposed near the town of Caspe. Stacked sandy channel belt deposits are preserved as sinuous raised ridges up to 10 m higher than inter- channel (i.e. floodplain) areas, the softer mudstones of the latter having preferentially eroded away. These are mapped out in the adjacent figure.

Research paper thumbnail of A 3-D digital model of Peak Cavern, Castleton, Derbyshire, UK, integrating cave survey, geophysics, geology and archaeology

The Lower Carboniferous Castleton Reef, situated in Derbyshire, UK, is riddled with an extensive ... more The Lower Carboniferous Castleton Reef, situated in Derbyshire, UK, is riddled with an extensive network of cave systems. Peak Cavern, situated in Castleton, forms the exit of one of these systems. Ford (1999) recommends further investigations of "promising cave sites, such as Peak Cavern entrance". The entrance (or 'Vestibule') was first recorded in the Domesday Book. The Vestibule may have been a site of human habitation since the Upper Palaeolithic. Hemp rope has been manufactured at Peak Cavern for the last four or five hundred years on terraces sculpted from the cave earth deposits (Hancock, 1999). The depth of the cave fill and its sedimentological and/or archaeological layering were all uncertain. Invasive archaeology of the cave earth deposits is prohibited, so non-destructive, geophysical imaging techniques have been applied.

Research paper thumbnail of Peak Cavern: 3-D CAD integration of Survey and Ground Penetrating Radar (GPR) data

A site of human habitation since the Upper Palaeolithic has been investigated using Ground Penetr... more A site of human habitation since the Upper Palaeolithic has been investigated using Ground Penetrating Radar. Combined with total station survey from Penmap, a 3D CAD model has been created allowing analysis to address geological, speleological and archaeological issues.

Research paper thumbnail of Virt Ocrop

Sedimentary architectures observed in 2-D outcrop sections are commonly used to build petroleum r... more Sedimentary architectures observed in 2-D outcrop sections are commonly used to build petroleum reservoir models. Advances in digital photogrammetry now allow the analysis of outcrop analogue aerial photographs, not only in three dimensions, but also dynamically. Digital photogrammetric techniques have also been adapted to close range photographs of well exposed cliff sections, to derive high resolution, bedding measurements in 3-D. Merging the aerial and terrestrial photogrammetric output with other data, such as shallow cores or Ground Penetrating Radar creates 3-D volumetric, Digital Solid Models (DSM) (see research website). A DSM can be extensively interrogated and the resulting meta-data used to construct 3-D, deterministic, reservoir models. Subsequent fluid flow simulations through these reservoir models can be animated, allowing identification of areas of interest or potential field management problems, such as by-passed hydrocarbons. Animations constructed from digital fly...

Research paper thumbnail of Sand volcanoes of the Carboniferous Ross Formation, County Clare, western Ireland: 3-D internal sedimentary structure and formation

A n exceptionally well preserved sand volcano cluster approximately 7 m (23 ft) in diameter lies ... more A n exceptionally well preserved sand volcano cluster approximately 7 m (23 ft) in diameter lies on top of mass-wasting deposits in the Carboniferous Ross Formation in western Ireland. Sandstone dikes were observed near the volcanoes, emanating from sandstone units beneath the slumped intervals. The volcano cluster was the focus of sedimentary, geophysical (ground-penetrating radar), and surveying (differential global positioning system) methods, with the aim of digitally reconstructing the volcanoes in three dimensions and elucidating their origin and significance. Data analysis determined that single volcanoes were, in fact, composite, with early small cones being overwhelmed by a later dominant cone during deposition. Volcanoes were sourced through feeder dikes that were correlated through slump horizons to underlying in-situ sandstones. A four-stage origin is inferred: (1) initial overpressured sand escaped up through tension gashes in the slump horizons to deposit multiple, small sand cones on the new sea floor; (2) more widely spaced vents allowed multiphase ejection to envelop initial cones and to produce larger volcanoes with a single vent; (3) composite volcanoes loaded and subsided into the underlying substrate and local sand chamber during continued deposition before final cessation; and (4) deposition of overlying pelagic sediments and lithification. Volcanoes may be sited on local seafloor, topographic highs.

Research paper thumbnail of Numerical modeling of the fluid flow impact of thin baffle laminae in cross bedding

Water Resources Research, 2008

Thin layers of less permeable materials can occur in cross-bedded rocks and may function as flow ... more Thin layers of less permeable materials can occur in cross-bedded rocks and may function as flow baffles, thus influencing the bulk fluid flow. However, it can be challenging to model their flow impact using standard grid-based techniques that can only capture the topology and geometry of the layers accurately using a large number of small cells. In this paper, a numerical method, recently developed by the authors for modeling fluid flow in fault damage zones that contain thin low-permeability fault strands, is demonstrated as being applicable to modeling the flow impact of thin baffle laminae in cross beds. This method does not require thin baffles to be discretized explicitly. For a range of permeability contrasts between the thin baffle layers and the rest of the matrix, upscaled permeability values are derived for models that have the same volumetric fraction of baffle material. The results show that when the baffles are completely connected, the upscaled permeability is less than for cases where the baffles do not form a continuous impediment for all levels of permeability contrast and declines more steeply with the increase of the permeability contrast. The flow effect of the layer configuration becomes more apparent for the disconnected situation when the permeability contrast is high. The method is shown to be accurate and efficient for this type of work. These results highlight the importance of capturing the topology and volume of the thin baffle layers in flow modeling of cross beds and the necessity of using appropriate numerical techniques.

Research paper thumbnail of Characterization of thin-bedded turbidites in the North Brae Field, South Viking Graben, North Sea

Research paper thumbnail of Petroleum Geology of the Georgian Fold and Thrust Belts and Foreland Basins

Regional and Petroleum Geology of the Black Sea and Surrounding Region, 1997

Numerous, mainly small, oil discoveries have been made within the foreland basins and fold and th... more Numerous, mainly small, oil discoveries have been made within the foreland basins and fold and thrust belts of Georgia. The largest field in Georgia (Samgori) contained ~200 MMbbl (million barrels) recoverable reserves reservoired in fractured middle Eocene volcaniclastics--the main proven reservoir in Georgia--trapped in a compressional fold of the Achara-Trialet belt. Where the Achara-Trialet deformation extended into the foreland basins, frontal folds with Miocene-Pliocene clastic reservoirs are known to be oil-bearing (e.g., Supsa field). Within the foreland basins in areas unaffected by Neogene compression, structural closures related to pre-Neogene extensional faulting may include draping lower Miocene and Mesozoic reservoirs: these are largely untested. The frontal folds of the Greater Caucasus have yielded small oil discoveries in lower Miocene fluvial to shallow marine clastic reservoirs. The widespread oil discoveries along the Achara-Trialet frontal folds demonstrate the presence of a working oil sourcing system from the Black Sea through to the Kura Basin. Gas chromatography-mass spectrometry and carbon isotope analyses indicate that a single source rock was responsible for generation of many of the oils in the foreland basins; oil source correlations suggest that this was late Eocene in age, deposited within the Paleogene Achara-Trialet Basin. Further, oil-prone source rocks appear to be present locally within the Greater Caucasus.

Research paper thumbnail of Bed-scale vertical and lateral distribution of massive sandstone in a topographically confined basin (Peïra Cava, SE France): Implications for flow processes

Research paper thumbnail of The Pliocene-Recent Euphrates river system: Sediment facies and architecture as an analogue for subsurface reservoirs

Research paper thumbnail of 3D GPR Surveying with Vertical Radar Profiling of Petroleum Reservoir Outcrop Analogues

62nd EAGE Conference & Exhibition

Research paper thumbnail of Sedimentary and tectonic controls on Lower Carboniferous (Visean) mixed carbonate–siliciclastic deposition in NE England and the Southern North Sea: implications for reservoir architecture

Research paper thumbnail of Value of Geologically Derived Features in Machine Learning Facies Classification

Mathematical Geosciences

The aim of this work is to demonstrate how geologically interpretative features can improve machi... more The aim of this work is to demonstrate how geologically interpretative features can improve machine learning facies classification with uncertainty assessment. Manual interpretation of lithofacies from wireline log data is traditionally performed by an expert, can be subject to biases, and is substantially laborious and time consuming for very large datasets. Characterizing the interpretational uncertainty in facies classification is quite difficult, but it can be very important for reservoir development decisions. Thus, automation of the facies classification process using machine learning is a potentially intuitive and efficient way to facilitate facies interpretation based on large-volume data. It can also enable more adequate quantification of the uncertainty in facies classification by ensuring that possible alternative lithological scenarios are not overlooked. An improvement of the performance of purely data-driven classifiers by integrating geological features and expert kno...

Research paper thumbnail of Outcrop-based stochastic modelling of turbidite amalgamation and its effects on hydrocarbon recovery

Petroleum Geoscience

... Variation in these two factors has a major effect on the primary internal heterogeneity ... t... more ... Variation in these two factors has a major effect on the primary internal heterogeneity ... to the erosive potential of succeeding currents and, hence, is constrained by the depositional environment. ... exposed sections will vary with the orientation of the section relative to palaeocurrent ...

Research paper thumbnail of Turbidity current velocity, grain size and bedforms in turbidite channels and associated depositional systems

Research paper thumbnail of Best practice stochastic fades modeling from a channel-fill turbidite sandstone analog (the Quarry outcrop, Eocene Ainsa basin, northeast Spain)

Research paper thumbnail of The impact of sub‐seismic shale layers on the reservoir's stress sensitivity

SEG Technical Program Expanded Abstracts 2008, 2008

In most clastic reservoirs experiencing pressure depletion due to production, the hydraulically c... more In most clastic reservoirs experiencing pressure depletion due to production, the hydraulically connected sands in the reservoir naturally compact to some degree. As a consequence, the much lower permeability reservoir shales may experience mechanical tension. The effective seismic response of the reservoir interval is thus a mix of both hardening and softening reservoir components. This phenomenon alters the predicted overall stress sensitivity from that anticipated for a homogeneous, fully connected reservoir interval. The time period over which this effect might be observed is influenced by the rate at which the shales reach pressure equilibrium with the surrounding sands. This work indicates that sub-seismic shale layers of approximately 1m thickness take less than 12 months to equilibrate, whilst thicker shale layers of 8m can take over 10 years. It is concluded that the mechanical and dynamic response of sub-seismic reservoir shale must be considered when quantitatively assessing the 4D seismic signature from frequently shot time-lapse surveys with a periodicity of 6 to 12 months, but also perhaps, for conventional 4D seismic surveys shot over 5 to 10 years. These conclusions are strongly affected by the permeability of the shale layers, the stress state, and are also a function of net to gross and depositional environment.

Research paper thumbnail of Reservoir Impact of Small-scale Aeolian Dune Architecture - From Acquisition to Simulation in the Wahiba Sands, Oman

74th EAGE Conference and Exhibition incorporating EUROPEC 2012, 2012

ABSTRACT http://earthdoc.eage.org/publication/publicationdetails/?publication=59265 Summary: Smal... more ABSTRACT http://earthdoc.eage.org/publication/publicationdetails/?publication=59265 Summary: Small-scale dune heterogeneity has a significant impact upon recoverable reserves within aeolian hydrocarbon reservoirs. Complex geometries exist, with bounding surfaces and primary strata types often negatively impacting fluid flow. Incorporating the effects of such architectural elements into reservoir models is essential when accurately determining their effect on development strategies. In order to assess their impact, we acquired a small pseudo-3D dataset from the Wahiba Sands, Sultanate of Oman, using ground-penetrating radar (GPR). In this paper we discuss the acquisition, processing and modelling of this dataset. Data are interpreted to be of a small linear dune. Radar stratigraphic units have been interpreted and mapped in 3D; a small-scale analogue reservoir model has been produced. Simulation studies have been conducted to assess the impact of a range of sensitivities, including the affects of permeability contrast, flow direction and capillary pressure. Results indicate that permeability contrasts have a significant impact on recovery, whilst flow direction is the dominant factor. The resulting models may not be directly transferable to a specific subsurface scenario, but the generic spatial information can be a useful guide.

Research paper thumbnail of Using GPR to extract 3-D turbidite channel architectures from the Upper Carboniferous Ross Formation, County Clare, Westen Ireland. 1: geological setting; 2: outcrop data integration

The Upper Carboniferous (Namurian) Ross Formation represents a thick accumulation of sandstones a... more The Upper Carboniferous (Namurian) Ross Formation represents a thick accumulation of sandstones and shales forming the early fill of the West Irish Namurian Basin (Collinson, 1991). The basin shows a systematic progradational fill from basinal shales (Clare Shales) to deep-water shales and turbidites (Ross Formation) to unstable slope deposits (Gull Island Formation) and shallow-water upward-coarsening deltaic sequences (Clare Group). The Ross Formation is very well exposed in coastal sections around the Loop Head peninsula. Here, the formation is 380m thick and consists of 75% sandstone, but rapidly thins to the North and South. The sandstone depositional units consist predominantly of sheet-like turbidites formed within laterally extensive layered or amalgamated sand-rich units (Clark, 1998). Inter-bedded within these sheet sandstones are erosional, small-scale channels, typically 100 to 500 metres wide and 5 to 15 metres deep.

Research paper thumbnail of New sequence stratigraphic model for fluvial studies

Research paper thumbnail of Assessment of the Eocene fluvial architecture of the Ebro Basin (northeast Spain) using Ground penetrating Radar

Eocene fluvial sediments of the Ebro Basin, northeastern Spain, are spectacularly exposed near th... more Eocene fluvial sediments of the Ebro Basin, northeastern Spain, are spectacularly exposed near the town of Caspe. Stacked sandy channel belt deposits are preserved as sinuous raised ridges up to 10 m higher than inter- channel (i.e. floodplain) areas, the softer mudstones of the latter having preferentially eroded away. These are mapped out in the adjacent figure.

Research paper thumbnail of A 3-D digital model of Peak Cavern, Castleton, Derbyshire, UK, integrating cave survey, geophysics, geology and archaeology

The Lower Carboniferous Castleton Reef, situated in Derbyshire, UK, is riddled with an extensive ... more The Lower Carboniferous Castleton Reef, situated in Derbyshire, UK, is riddled with an extensive network of cave systems. Peak Cavern, situated in Castleton, forms the exit of one of these systems. Ford (1999) recommends further investigations of "promising cave sites, such as Peak Cavern entrance". The entrance (or 'Vestibule') was first recorded in the Domesday Book. The Vestibule may have been a site of human habitation since the Upper Palaeolithic. Hemp rope has been manufactured at Peak Cavern for the last four or five hundred years on terraces sculpted from the cave earth deposits (Hancock, 1999). The depth of the cave fill and its sedimentological and/or archaeological layering were all uncertain. Invasive archaeology of the cave earth deposits is prohibited, so non-destructive, geophysical imaging techniques have been applied.

Research paper thumbnail of Peak Cavern: 3-D CAD integration of Survey and Ground Penetrating Radar (GPR) data

A site of human habitation since the Upper Palaeolithic has been investigated using Ground Penetr... more A site of human habitation since the Upper Palaeolithic has been investigated using Ground Penetrating Radar. Combined with total station survey from Penmap, a 3D CAD model has been created allowing analysis to address geological, speleological and archaeological issues.

Research paper thumbnail of Virt Ocrop

Sedimentary architectures observed in 2-D outcrop sections are commonly used to build petroleum r... more Sedimentary architectures observed in 2-D outcrop sections are commonly used to build petroleum reservoir models. Advances in digital photogrammetry now allow the analysis of outcrop analogue aerial photographs, not only in three dimensions, but also dynamically. Digital photogrammetric techniques have also been adapted to close range photographs of well exposed cliff sections, to derive high resolution, bedding measurements in 3-D. Merging the aerial and terrestrial photogrammetric output with other data, such as shallow cores or Ground Penetrating Radar creates 3-D volumetric, Digital Solid Models (DSM) (see research website). A DSM can be extensively interrogated and the resulting meta-data used to construct 3-D, deterministic, reservoir models. Subsequent fluid flow simulations through these reservoir models can be animated, allowing identification of areas of interest or potential field management problems, such as by-passed hydrocarbons. Animations constructed from digital fly...

Research paper thumbnail of Sand volcanoes of the Carboniferous Ross Formation, County Clare, western Ireland: 3-D internal sedimentary structure and formation

A n exceptionally well preserved sand volcano cluster approximately 7 m (23 ft) in diameter lies ... more A n exceptionally well preserved sand volcano cluster approximately 7 m (23 ft) in diameter lies on top of mass-wasting deposits in the Carboniferous Ross Formation in western Ireland. Sandstone dikes were observed near the volcanoes, emanating from sandstone units beneath the slumped intervals. The volcano cluster was the focus of sedimentary, geophysical (ground-penetrating radar), and surveying (differential global positioning system) methods, with the aim of digitally reconstructing the volcanoes in three dimensions and elucidating their origin and significance. Data analysis determined that single volcanoes were, in fact, composite, with early small cones being overwhelmed by a later dominant cone during deposition. Volcanoes were sourced through feeder dikes that were correlated through slump horizons to underlying in-situ sandstones. A four-stage origin is inferred: (1) initial overpressured sand escaped up through tension gashes in the slump horizons to deposit multiple, small sand cones on the new sea floor; (2) more widely spaced vents allowed multiphase ejection to envelop initial cones and to produce larger volcanoes with a single vent; (3) composite volcanoes loaded and subsided into the underlying substrate and local sand chamber during continued deposition before final cessation; and (4) deposition of overlying pelagic sediments and lithification. Volcanoes may be sited on local seafloor, topographic highs.

Research paper thumbnail of Numerical modeling of the fluid flow impact of thin baffle laminae in cross bedding

Water Resources Research, 2008

Thin layers of less permeable materials can occur in cross-bedded rocks and may function as flow ... more Thin layers of less permeable materials can occur in cross-bedded rocks and may function as flow baffles, thus influencing the bulk fluid flow. However, it can be challenging to model their flow impact using standard grid-based techniques that can only capture the topology and geometry of the layers accurately using a large number of small cells. In this paper, a numerical method, recently developed by the authors for modeling fluid flow in fault damage zones that contain thin low-permeability fault strands, is demonstrated as being applicable to modeling the flow impact of thin baffle laminae in cross beds. This method does not require thin baffles to be discretized explicitly. For a range of permeability contrasts between the thin baffle layers and the rest of the matrix, upscaled permeability values are derived for models that have the same volumetric fraction of baffle material. The results show that when the baffles are completely connected, the upscaled permeability is less than for cases where the baffles do not form a continuous impediment for all levels of permeability contrast and declines more steeply with the increase of the permeability contrast. The flow effect of the layer configuration becomes more apparent for the disconnected situation when the permeability contrast is high. The method is shown to be accurate and efficient for this type of work. These results highlight the importance of capturing the topology and volume of the thin baffle layers in flow modeling of cross beds and the necessity of using appropriate numerical techniques.