1,721,071 research outputs found
Sedimentary flow heterogeneities in the Triassic U.K. Sherwood Sandstone Group. Insights for hydrocarbon exploration
No full textFluvial and aeolian sedimentary successions host important hydrocarbon resources as well as major groundwater aquifers. This review of the lithological characteristics of Triassic fluvio‐aeolian successions of the Sherwood Sandstone Group (United Kingdom) demonstrates how distance from a fluvial sediment source and rate of rift‐related tectonic subsidence play important roles in governing reservoir quality in continental successions. Increasing distance from the fluvial sediment source area results in increased porosity and permeability in deposits of mixed fluvial and aeolian reservoir successions that accumulated in arid and semiarid settings. Indeed, succes- sions of the U.K. Sherwood Sandstone Group reveal an increase in the proportion of highly permeable deposits of aeolian origin with increasing distance from the principal uplands, represented by the Armorican Massif in northern France, which formed the main source for delivery of fluvial sediment to a series of rift basins. A pro- gressive reduction in the discharge of fluvial systems entering and passing through a series of interlinked rift basins encouraged aeolian accumulation in more distal basins.
Extensional tectonics enabled preservation of highly permeable aeolian facies in basins subject to high rates (≳100 m/Myr) of tectonic subsidence by rapidly placing such deposits below the water table. However, successions exclusively characterized
by fluvial lithofacies record decreases in both porosity and permeability with increas- ing distance (~250–750 km) from the sediment source due to the coupling of porosity reduction and increasing clay content
Characterizing flow pathways in a sandstone aquifer. Tectonic vs sedimentary heterogeneities
No full textSandstone aquifers are commonly assumed to represent porous media characterized by a permeable matrix. However, such aquifers may be heavy fractured when rock properties and timing of deformation favour brittle failure and crack opening. In many aquifer types, fractures associated with faults, bedding planes and stratabound joints represent preferential pathways for fluids and contaminants. In this paper, well test and outcrop-scale studies reveal how strongly lithified siliciclastic rocks may be entirely dominated by fracture flow at shallow depths (≤ 180 m), similar to limestone and crystalline aquifers. However, sedimentary heterogeneities can
primarily control fluid flow where fracture apertures are reduced by overburden pressures or mineral infills at
greater depths.
The Triassic St Bees Sandstone Formation (UK) of the East Irish Sea Basin represents an optimum example for study of the influence of both sedimentary and tectonic aquifer heterogeneities in a strongly lithified sandstone aquifer-type. This fluvial sedimentary succession accumulated in rapidly subsiding basins, which typically fa- vours preservation of complete depositional cycles including fine grained layers (mudstone and silty sandstone) interbedded in sandstone fluvial channels. Additionally, vertical joints in the St Bees Sandstone Formation form a pervasive stratabound system whereby joints terminate at bedding discontinuities. Additionally, normal faults are present through the succession showing particular development of open-fractures. Here, the shallow aquifer
(depth ≤ 180 m) was characterized using hydro-geophysics. Fluid temperature, conductivity and flow-velocity logs record inflows and outflows from normal faults, as well as from pervasive bed-parallel fractures. Quantitative flow logging analyses in boreholes that cut fault planes indicate that zones of fault-related open fractures characterize ~50% of water flow. The remaining flow component is dominated by bed-parallel fractures. Howev- er, such sub-horizontal fissures become the principal flow conduits in wells that penetrate the exterior parts of fault damage zones, as well as in non-faulted areas.
The findings of this study have been compared with those of an earlier investigation of the deeper St Bees Sand- stone aquifer (180 to 400 m subsurface depth) undertaken as part of an investigation for a proposed nuclear waste repository. The deeper aquifer is characterized by significantly lower transmissivities. High overburden pressure and the presence of mineral infillings, have reduced the relative impact of tectonic heterogeneities on transmissivity here, thereby allowing matrix flow in the deeper part of the aquifer. The St Bees Sandstone aquifer contrasts the hydraulic behaviour of low-mechanically resistant sandstone rock-types. In fact, the UK Triassic Sandstone of the Cheshire Basin is low-mechanically resistant and flow is supported both by matrix and fracture. Additionally, faults in such weak-rocks are dominated by granulation seams representing flow-barriers which strongly compartmentalize the UK Triassic Sandstone in the Cheshire Basin
Characterization of a fluvial aquifer at a range of depths and scales. The Triassic St Bees Sandstone Formation, Cumbria, UK
Full text linkFluvial sedimentary successions represent porous media that host groundwater and geothermal resources. Additionally, they overlie crystalline rocks hosting nuclear waste repositories in rift settings. The permeability characteristics of an arenaceous fluvial succession, the Triassic St Bees Sandstone Formation in England (UK), are described, from core-plug to well-test scale up to ~1 km depth. Within such lithified successions, dissolution associated with the circula- tion of meteoric water results in increased permeability
(K~10−1–100 m/day) to depths of at least 150 m below ground level (BGL) in aquifer systems that are subject to rapid
groundwater circulation. Thus, contaminant transport is likely to occur at relatively high rates. In a deeper investigation (> 150 m depth), where the aquifer has not been subjected to rapid groundwater circulation, well-test-scale hydraulic con-
ductivity is lower, decreasing from K~10−2 m/day at 150– 400 m BGL to 10−3 m/day down-dip at ~1 km BGL, where the pore fluid is hypersaline. Here, pore-scale permeability
becomes progressively dominant with increasing lithostatic load. Notably, this work investigates a sandstone aquifer of fluvial origin at investigation depths consistent with highly enthalpy geothermal reservoirs (~0.7–1.1 km). At such depths, intergranular flow dominates in unfaulted areas with only minor contribution by bedding plane fractures. However, extensional faults represent preferential flow pathways, due to presence of high connective open fractures. Therefore, such faults may (1) drive nuclear waste contaminants towards the highly permeable shallow (< 150 m BGL) zone of the aquifer, and (2) influence fluid recovery in geothermal fields
Influence of fluvial crevasse-splay deposits on sandbody connectivity: Lessons from geological analogues and stochastic modelling
Full text linkTo investigate the importance of crevasse-splay elements for the connectivity of fluvial sandbodies, an integrated study has been undertaken that combines data from outcropping and subsurface fluvial successions, and modern rivers, with outputs from forward stratigraphic models and object-based modelling. Literature-derived analogue data on over 2100 crevasse-splay elements from many fluvial systems have been synthesized to obtain quantifications of the proportion and geometry of crevasse-splay units, and of their topological relationships with channel deposits. Some of the analogue data are used to inform inputs to stochastic modelling that is applied to determine how the presence of crevasse-splay deposits affects the connectivity of successions in which channel sandstones are preserved as channelized bodies or compartmentalized point-bar units.
Architectural configurations are documented that are variably characterized by (i) crevasse-splay sandbody compartmentalization, (ii) channel-sandbody connections that increase sandbody size and the likelihood of borehole connectivity, and (iii) intercommunication between different channel sandbodies. Analyses of analogue data demonstrate scaling relationships between the morphometry of crevasse-splay elements and associated channel elements and river catchments. Results also demonstrate how the proportions of channel and crevasse-splay deposits tend to covary in stratigraphic volumes. This information, when ported to stochastic models, yields results showing how the influence of crevasse-splay elements on static connectivity varies – in relative terms – as a function of architectural style, net-to-gross ratio and channel-sandbody geometry. Channel-body connections may be more important for smaller and relatively sand-poor fluvial systems, whereas mud-plug compartmentalization of splay elements may be more important for successions with larger channel-body width-to-thickness aspect ratios and lower net-to-gross ratios.
The results of this work can be applied to facilitate and improve the characterization and modelling of the static connectivity of subsurface successions that include crevasse-splay deposits. The results also reveal the possible influence of certain geological controls (channel avulsion, river-system scale, overbank cannibalization) on static connectivity
Quantitative Characterization of the Sedimentary Architecture of Shallow-Marine and Paralic Reservoir Analogs: A Database Approach
No full textShallow-marine and paralic clastic depositional systems are described by a large volume of sedimentological, architectural and geomorphological data. A new method that enables the convergence of these datasets into a common descriptive scheme facilitates the identification and application of potential outcrop and modern hydrocarbon-reservoir analogs. To this end, a database has been developed for the collation of data in standardized format, in a way that allows significant comparisons between different depositional systems, and the derivation of consistently defined attributes that can be applied in subsurface studies. The Shallow Marine Architectural Knowledge Store is a relational database devised to include data on the sedimentary architecture of shallow marine and paralic ancient depositional systems, and on the geomorphic organization of corresponding modern environments. The database incorporates data on sedimentary bodies and surfaces and geomorphic units, which are classified on descriptive (e.g. grain size) and interpretive (e.g. sub-environment) categories, and characterized on a variety of attributes (e.g. geometries, spatial relationships, hierarchical relationships). Depositional systems, and stratigraphic intervals or planform segments thereof, are classified on descriptive parameters (e.g. shelf gradient) and controlling factors (e.g. tidal regime) to allow the selection of relevant outcrop or modern analogs. The database can be queried to return a quantified characterization of multiple analogs, and data can be synthesized in models that incorporate uncertainty related to variability in sedimentary heterogeneity. To illustrate the range of genetic units types, depositional systems, associated data and potential applications, example database output is showcased relating to: - the hierarchical arrangement and scaling relationships of architectural elements that form constructional units in Quaternary deltas of different types; - the facies organization of nearshore sandstone belts and the geometry of associated parasequences, from the Upper Cretaceous of the Western Interior Seaway in Utah (USA); - the geometry of modern geomorphic features, and their relations with the geometry of architectural elements interpreted as the preserved product of the morphodynamic evolution of corresponding landforms. Particular attention is paid to how the database output can be applied to the construction of accurate, quantitative 3D geological models
Permeability of rock discontinuities and faults in the Triassic Sherwood Sandstone Group (UK). Insights for management of fluvio-aeolian aquifers worldwide
No full textFluvio-aeolian sedimentary successions host groundwater aquifers at shallow depths (<~0.15 km), which overlie geothermal and shale-gas reservoirs, and nuclear waste repositories at intermediate depths (~0.15–2.0 km). Additionally, such deposits represent petroleum reservoirs at greater depths (~2.0–4.0 km). The need to improve conceptual understanding of the hydraulic behaviour of fluvial-aeolian sandstone successions over a large depth interval (~0–4 km) is important for socio-economic reasons. Thus, the hydraulic properties of the Triassic Sherwood Sandstone aquifer in the UK have been reviewed and compared to similar fluvio- aeolian successions. The ratio between well-scale and core-plug-scale permeability (Kwell-test/Kcore-plug) acts as a proxy for the relative importance of fracture versus intergranular flow. This ratio (which typically varies from ~2 to 100) indicates significant contribution of fractures to flow at relatively shallow depths (<~0.15 km). Here, permeability development is controlled by dissolution of calcite-dolomite in correspondence of fractures. The observed ratio (Kwell-test/Kcore-plug) decreases with depth, approaching unity, indicating that intergranular flow dominates at ~1 km depth. At depths ≥ ~1 km, dissolution of carbonate cement by rock alteration due to groundwater flow is absent and fractures are closed. Aeolian and fluvial deposits behave differently in proximity to normal faults in the Sherwood Sandstone aquifer. Deformation bands in aeolian dune deposits strongly compartmentalize this aquifer. The hydro-structural properties of fluvio-aeolian deposits are also controlled by mineralogy in fault zones. A relative abundance of quartz vs. feldspar and clays in aeolian sandstones favours development of low-permeability deformation bands
Tectonic Influence on the Geomorphology of Submarine Canyons: Implications for Deep-Water Sedimentary Systems
Full text linkA database-informed metastudy of 294 globally distributed submarine canyons has been conducted with the aim of elucidating the role of tectonic setting on submarine-canyon geomorphology. To achieve this, data from seafloor and subsurface studies derived from 136 peer-reviewed publications and from open-source worldwide bathymetry datasets have been statistically analyzed. In particular, relationships between margin type (active vs. passive) or plate-boundary type (convergent vs. transform vs. complex) have been assessed for key morphometric parameters of submarine canyons, including: streamwise length, maximum and average width and depth, canyon sinuosity, average canyon thalweg gradient, and maximum canyon sidewall steepness. In addition, possible scaling relationships between canyon morphometric parameters and characteristics of the associated terrestrial catchment, continental shelf and slope, and of the broader physiographic setting for canyons along both active and passive margins have been evaluated. The following principal findings arise: 1) overall canyon geomorphology is not markedly different across tectonic settings; 2) slope failure might be more important in passive-margin canyons compared to active ones, possibly due to seismic strengthening in the latter; 3) some aspects of canyon geomorphology scale with attributes of the source-to-sink system and environmental setting, but the strength and sign in scaling might differ between active and passive margins, suggesting that the extent to which canyon geomorphology can be predicted depends on the tectonic setting. Insights from our analysis augment and improve conceptual, experimental and numerical models of slope systems at the scale of individual canyons and source-to-sink systems, and increase our understanding of the complex role played by tectonic setting in shaping deep-water systems
Palaeoenvironment of braided fluvial systems in different tectonic realms of the Triassic Sherwood Sandstone Group, UK
No full textFluvial successions comprising the fills of sedimentary basins occur in a variety of tectonic realms related to extensional, compressional and strike-slip settings, as well as on slowly subsiding, passive basin margins. A major rifting phase affected NW Europe during the Triassic and resulted in the generation of numerous sedimentary basins. In the UK, much of the fill of these basins is represented by fluvial and aeolian successions of the Sherwood Sandstone Group. Additionally, regions that experienced slow rates of Mesozoic subsidence unrelated to Triassic rifting also acted as sites of accumulation of the Sherwood Sandstone Group, one well-exposed example being the eastern England Shelf. The fluvial depositional architecture of deposits of the Sherwood Sandstone Group of the eastern England Shelf (a shelf-edge basin) is compared with similar fluvial deposits of the St Bees Sandstone Formation, eastern Irish Sea Basin (a half-graben). The two studied successions represent the preserved deposits of braided fluvial systems that were influenced by common allogenic factors (climate, sediment source, delivery style); differences in preserved sedimentary style principally reflect their different tectonics settings. Analysis of lithofacies and architectural elements demonstrates that both studied successions are characterized by amalgamated channel-fill elements that are recorded predominantly by downstream-accreting sandy barforms. The different tectonic settings in which the two braided-fluvial systems accumulated exerted a dominant control on preserved sedimentary style and long-term preservation potential. On the eastern England Shelf, the vertical stacking of pebbly units and the general absence of fine-grained units reflect a slow rate of sediment accommodation generation (18–19.4 m/Myr). In this shelf-edge basin, successive fluvial cycles repeatedly reworked the uppermost parts of earlier fluvial deposits such that only the lowermost channel lags tend to be preserved. By contrast, in the eastern Irish Sea Basin of west Cumbria, the rate of sediment accommodation generation was substantially greater (119 m/Myr) such that space was available to preserve complete fluvial cycles, including silty drape units that cap the channelized deposits
Controls on the depositional architecture of fluvial point‐bar elements in a coastal‐plain succession
No full textThe architecture and lithofacies organisation of fluvial point‐bar elements record the spatio‐temporal evolution of river channels. This study discusses the factors that control facies distributions and geometries of point‐bar elements present in a fluvial succession that accumulated on a low‐gradient coastal plain on the western margin of the Western Interior Seaway (Campanian Neslen Formation, eastern Utah, USA). Forty outcropping point‐bar elements located within an established sequence stratigraphic framework have been examined through facies, architectural and palaeocurrent analyses. Point‐bar elements increase in width‐to‐thickness aspect ratio vertically through the succession. Four point‐bar element types are identified based upon their lithofacies assemblages and geometry. Two point‐bar types conform to those depicted in traditional facies models; they are dominated by cross‐bedded sandstone, with subordinate amounts of ripple‐laminated and horizontally laminated sandstone. In contrast, the other two point‐bar types exhibit unusually low proportions of cross‐bedded sandstone and higher proportions of massive sandstone, horizontally laminated sandstone and ripple‐laminated sandstone. The occurrence of these atypical point‐bar assemblages is restricted to the marine‐influenced lower and middle parts of the Neslen Formation. An up‐succession increase in aspect ratio and degree of amalgamation of point‐bar elements through the Neslen Formation may reflect a decrease in the rate of accommodation generation, an increase in the rate of sediment supply, or autogenic processes that operated on an overall prograding coastal plain. The accumulation of point‐bar elements with lower proportions of cross‐bedded sandstone in the lower Neslen Formation can be attributed to decreased stream power. Database‐assisted analysis has been undertaken to compare the lithofacies and architecture of point‐bar elements from the Neslen Formation to those in other humid‐climate, coastal‐plain successions. This comparison reveals that the geometry and facies observed in point‐bar elements of the Neslen Formation might record an unusual set of combined allogenic (accommodation generation and fluvial discharge variations) and autogenic (backwater processes and presence of peat mires) process interactions
Seismic-driven geocellular modeling of fluvial meander-belt reservoirs using a rule-based method
Full text linkA novel workflow is presented for building static models of fluvial reservoirs composed of large point-bar architectural elements, based on the application of a specialized forward stratigraphic model, named ‘Point-Bar Sedimentary Architecture Numerical Deduction’ (PB-SAND).
The approach uses interpreted horizontal slices from 3D seismic datasets to reconstruct the planform evolution of meander loops, on which basis the geometry of point-bar deposits and associated accretion units can be simulated deterministically. The resulting meander-belt geometry is then populated with different types of facies, through a rule-based algorithm that generates facies architectures that reflect geologic understanding, enabling users to establish linkages between styles of meander evolution (e.g., meander growth via expansion, translation, rotation) and facies distributions. Input parameters define the proportions, geometries and distributions of types of point-bar deposits, as captured from subsurface data and/or from geologic analogs. Multiple stochastic realizations of facies architecture can be generated.
To demonstrate the application of this modeling approach, the workflow has been applied to a meander-belt reservoir where large point-bar and channel-fill elements are imaged in seismic. A detailed example is used to illustrate workflows that can be used to build high-resolution sector models in pre-drill contexts, suitable for guiding development plans. An additional example is used to show how to achieve well match for densely drilled sectors, by means of a hybrid approach that combines the new algorithm with traditional geostatistical techniques.
It is shown how the workflow allows consideration of point-bar growth styles, as inferred from seismic data, on distribution and geometry of heterogeneities, and how this facilitates the reproduction of geologic features that are important controls on the static connectivity of point-bar reservoirs (e.g., distribution and characteristics of bar-front mud drapes, and of mud-prone packages arising from progressive meander-bend tightening or from downstream fining of deposits beyond the apex of a meander bend). A comparison with traditional variogram-based methods is undertaken to compare metrics that describe intra-point-bar static connectivity and that represent proxies for the degree of compartmentalization of upper-bar sands by mud drapes
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