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Meandering-river geomorphology, forming abandoned channels/lakes with organic carbon-burial and microbial reductive dissolution, play many crucial roles in controlling arsenic (As) fluxes in sinks such as contaminated aquifers of riverine alluvial plains across the world. Suhiya oxbow-lake in the middle alluvial plain of the River Ganga, was selected as the natural laboratory. A top-down multidisciplinary approach was chosen employing satellite imagery to analyse the annual oxbow-lake surface vegetation dynamics (Eichhornia and Hydrilla). Side-scan sonar profiles across two oxbow lakes along with River Ganga core data and vintage topographical maps, estimated the lake-sedimentation rate of 9.6 cm/yr. Organic carbon [amino acids, aromatics, lingo-phenols and lipids hydrocarbons] infiltration-based on hydrophobicity and molecular-mass was detected at different depths along the water and sedimentary column. Elemental analysis showed lake surface to groundwater the As conc. varied from (0.37 to 185 μg/l). A microbial diversity based study showed that large sized photoautotrophs Nostoc, Anabaena are replaced by Fe-oxido-reducing As-metabolizing bacteria e.g. Acidovorax, Dechloromonas and enteric organisms e.g. Enterobacter, Salmonella at bottom of water column. Based on these inferences, a conceptual organic carbon transport model was constructed to understand the preferential preservation and microbial diagenesis resulting in mobilization of As and other geogenic elements.

Sandy braided dryland rivers, deposited under arid to semi-arid conditions, constitute important clastic reservoirs worldwide whose reservoir quality reflects the effect of primary sediment properties on near-surface diagenetic processes. A literature review is presented of existing facies models, mainly based on geomorphological analysis of modern perennial braided rivers, which is further illustrated with own outcrop and subsurface examples of dryland river deposits in: (a) Argana Basin (Triassic, Morocco), (b) Iberian Meseta (Triassic, Central SE Spain), (c) Huesca Fluvial Fan (Miocene, Spain) and (d) Rotliegend hydrocarbon reservoirs in the Southern Permian Basin (The Netherlands). The facies models combined with the analyses of the rock record provide a comprehensive depositional framework where investigated for the influence of depositional characteristics, such as grain-size and sorting variations, interstitial clay distribution and composition and amount of intrabasinal components, on the extent and pathway of near-surface mechanical and chemical processes. The particular dryland setting is characterized by prolonged periods of inactivity and short, episodic periods of peak discharge which lead to the transport and deposition of poorly-sorted, clay-rich extra- and intraformational conglomeratic sediments upstream. Downstream, channels, mid-channel and bank attached bars develop with conspicuous anisotropic trough and tabular cross-stratified sandstones. The most common near-surface diagenetic processes that alter the textural properties of these deposits are the rapid decrease of porosity and permeability due to grain-rearrangements and mechanical compaction of mud intraclasts along with pervasive early carbonate cementation. The results of this study will help to better configure the boundary conditions (e.g., external versus local supply for carbonates cements) in state-of-the-art reservoir quality forward models for ancient dryland river sandstones, which are especially prominent in Devonian, Permian, Triassic and Jurassic reservoir settings. ispartof: Earth-Science Reviews vol:208 status: published

In the past 10 years the mature hydrocarbon province the West Netherlands Basin has hosted rapidly expanding geothermal development. Upper Jurassic to Lower Cretaceous strata from which gas and oil had been produced since the 1950s became targets for geothermal exploitation. The extensive publicly available subsurface data including seismic surveys, several cores and logs from hundreds of hydrocarbon wells, combined with understanding of the geology after decades of hydrocarbon exploitation, facilitated the offtake of geothermal exploitation. Whilst the first geothermal projects proved the suitability of the permeable Upper Jurassic to Lower Cretaceous sandstones for geothermal heat production, they also made clear that much detail of the aquifer geology is not yet fully understood. The aquifer architecture varies significantly across the basin because of the syn-tectonic sedimentation. The graben fault blocks that contain the geothermal targets experienced a different tectonic history compared to the horst and pop-up structures that host the hydrocarbon fields from which most subsurface data are derived. Accurate prediction of the continuity and thickness of aquifers is a prerequisite for efficient geothermal well deployment that aims at increasing heat recovery while avoiding the risk of early cold-water breakthrough. The potential recoverable heat and the current challenges to enhance further expansion of heat exploitation from this basin are evident. This paper presents an overview of the current understanding and uncertainties of the aquifer geology of the Upper Jurassic to Lower Cretaceous strata and discusses new sequence-stratigraphic updates of the regional sedimentary aquifer architecture.

The sand-prone fluvial Delft Sandstone Member of the Nieuwerkerk Formation is the main target for geothermal exploitation in the West Netherlands Basin. Uncertainty reduction in the modelling of permeability heterogeneity and connectivity of the sandstone reservoir units is of prime importance in the low-profit geothermal energy market. Lithostratigraphic correlation of the Delft Sandstone Member dates back to the 1970s, and considers the fluvial deposits as Valanginian (140–134 Ma) in age and synchronously deposited throughout the basin. Analysis of newly-acquired palynological data, in combination with well-log correlation and seismic interpretation, enabled the construction of a sequence-stratigraphic framework for the Nieuwerkerk Formation, in which the fluvial sandstones were deposited in a much wider age range than previously believed, from Late Ryazanian to Valanginian (142–134 Ma). The depocentre of fluvial deposition shifted in time from NE to SW across the width of the basin. This diachronous development resulted in a discontinuous spatial arrangement of fluvial sandstones. The new reservoir architecture model will aid in the well placement of geothermal doublets and the assessment of interference risk of adjacent geothermal projects.

Shallow aquifers in many Holocene alluvial basins around the world have in the last three decades been identified as arsenic pollution hotspots, in which the spatial variation of natural (or: geogenic) arsenic concentration is conditioned by the meandering-river geomorphology and the fluvial lithofacies distribution. Despite the large amount of publications on the specifics of the pollution, still many uncertainties remain as to the provenance and processes that lead to arsenic enrichment in aquifers. In this paper, arsenic in abandoned and sediment-filled meandering-river bends (or: clay-plugs) is highlighted as a primary source of aquifer pollution. The combination of high organic-carbon deposition rates and the presence of chemically-bound natural arsenic in sediment of this specific geomorphological setting creates the potential for microbially-steered reductive dissolution of arsenic in an anoxic environment, and subsequent migration of the desorbed arsenic to, and stratigraphic entrapment in, adjacent sandy point-bar aquifers. To assess the magnitude of the arsenic source in clay-plug, bulk sediment volume calculations were made of twenty clay plugs on the Middle Ganges Plain of Bihar (India), by combining clay-plug surface area analysis of Sentinel-2 satellite data, side-scan sonar depth profiling of oxbow lakes and the Ganges River, and sedimentological data from five cored shallow wells. ICP-MS based elemental analysis of 36 core sub-samples, complemented with published concentration data in a similar geomorphological setting in West Bengal, India, yielded an average arsenic content of 28.75 mg/kg sediment in the 12-m-thick clay plugs, which amounts to a total arsenic volume of 0.07 – 3.13 . 106 kg per clay plug. A scenario is presented for the release of arsenic from the clay-plug sediment by microbial metabolism, followed by migration of the desorbed arsenic to the bordering point-bar sands. ispartof: Catena vol:202 status: Published online

DELTAP is s multi-disciplinary research project, where geologists, water treatment scientists and industrial design engineers join forces to develop an integrated approach towards small-scale piped water supply (SPWS) systems in the arsenic-affected Ganges-Brahmaputra-Meghna Delta. The project has started in 2016 with a field study in Bihar (India) with a focus on water quality mapping with mobile crowd participation. The coming years the research will continue with 3 PhD candidates, both in India and Bangladesh, with the ultimate aim to develop blueprints for end-user inclusive SPWS systems.

In light of the huge investments needed to acquire sediment cores and the growing need of energy-providing companies to predict reservoir quality, it is remarkable that the standard workflow in core analysis has not been optimized to extract as much information from cores as possible. The Integrated Core Analysis (ICA) protocol presented in this study provides a statistical framework for multivariate calibration and prediction of a wide range of properties measured in core, based on integration of high-resolution X-ray fluorescence core-scanning (XRF-CS) proxy records with records of sparsely sampled petrophysical data and sedimentological core descriptions. The downscaling of numerical data involves the application of invertible transformations to remove range constraints on data, followed by calibration with Partial Least Squares regression through cross validation. Categorical data (i.e. lithofacies) are downscaled by associating each class with a statistical model based on the XRF-CS data. All data points are reassigned to their most likely class by applying Quadratic Discriminant Analysis. The result of the ICA is a multivariate data set in which all properties are specified at the same high resolution along the core with their prediction uncertainties. Downscaling of all variables to 1-cm vertical resolution permits investigation of the variability among petrophysical properties, geochemical proxies, and lithofacies memberships. Petrographic analysis is fundamental for interpretation of the XRF-CS records (element-mineral affinity) and for understanding the sedimentological controls on predicted petrophysical properties. Application of the ICA protocol to a 32 m thick, heterogeneous, Upper Carboniferous fluvial sandstone interval resulted in a near 30-fold increase of the petrophysical data base, which allowed identification of the main depositional and diagenetic controls on the spatial distribution of reservoir quality. Successful implementation of the novel ICA protocol will greatly increase the economic value of legacy core data in studies that aim to re-use depleted hydrocarbon reservoirs.

Floodplain deposits are abundant in low-gradient dryland river systems, but their contribution to connected reservoir volumes has not yet been fully acknowledged due to their poor detectability with typical wireline log suites and relatively-lower reservoir quality. This study presents an analysis of stacked crevasse splays in the distal part of the Miocene Huesca fluvial fan (Ebro Basin, Spain). Vertical stacking of crevasse splays implies local aggradation of the active channel belt. Lateral amalgamation of crevasse splays created an elevated rim around their feeder channel, raising its bankfull height. Subsequent crevasse splays were deposited on top of their predecessors, creating sand-on-sand contact through incision and further raising the active channel belt. This process of channel-belt super-elevation repeated until an upstream avulsion occurred. Amalgamated crevasse splays constitute connected reservoir volumes up to ~ 107 m3. Despite their lower reservoir quality, they effectively connect channel deposits in low net-to-gross fluvial stratigraphy, and hence, their contribution to producible volumes should be considered. Unswept intervals of amalgamated crevasse splays may constitute a secondary source of natural gas. Their interval thickness can serve as a proxy for feeder-channel dimensions, which can in turn be used to estimate the degree of stratigraphic connectivity.

The Rotliegend feather-edge area in the central part of the endorheic Southern Permian Basin in the Dutch offshore is characterized by a predominance of mud-prone, evaporite-bearing playa and lake deposits with a subordinate amount of interbedded, thin, fluvial sheet sandstones. The distribution and lateral facies changes of the sandstone bodies have been analyzed by generating a long-range, high-resolution chronostratigraphic correlation framework. The correlation technique of pattern matching of GR logs was applied, supported by calculating spectral trend curves. Flooding events are the primary near-synchronous correlation surfaces, which can be traced up to and over 100 km. The basin setting of the Southern Permian Basin, the studied sandstone depositional architecture (logs) and sedimentary characteristics (core) are analogous to the depositional setting of laterally-amalgamated terminal lobes of dryland-river systems in an endorheic basin, such as the Holocene Altiplano Basin in Bolivia, present-day Lake Eyre (Australia) and the Miocene Ebro Basin (Spain). The integrated approach has yielded a stratigraphic reservoir-architecture framework in which the reservoir sandstones, with net sand up to 10 m, have been identified as amalgamated terminal-splay sandstone sheets formed at the end of dryland-river pathways, alternating with lacustrine mudstone layers deposited during short-duration, high-magnitude flooding in intermittent wet climate periods.

© 2018 The Author(s) A new solution for harvesting energy simultaneously from two different sources of energy by combining geothermal energy production and thermal enhanced heavy oil recovery is introduced. Numerical simulations are employed to evaluate the feasibility of generating energy from geothermal resources, both for thermally enhanced oil recovery from a heavy oil reservoir and for direct heating purposes. A single phase non-isothermal fluid flow modeling for geothermal doublet system and a two-phase non-isothermal fluid flow modelling for water flooding in an oil reservoir are utilised. Sensitivity and feasibility analyses of the synergy potential of thermally-enhanced oil recovery and geothermal energy production are performed. A series of simulations are carried out to examine the effects of reservoir properties on energy consumption and oil recovery for different injection rates and injection temperature. Our results show that total oil production strongly depends on the shape of heat plume which can be affected by porosity, permeability, injection temperature, well spacing and injection rate in the oil reservoir. The favourable oil recovery obtains at high amount of (a) injection rate, (b) injection temperature, (c) porosity and (d) low amount of oil reservoir permeability respectively. Furthermore, our study indicates the wellbore spacing plays an important role in oil recovery and an optimum wellbore spacing can be established. The analyses suggest that the extra amount of oil produced by utilising the geothermal energy could make the geothermal business case independent and may be a viable option to reduce the overall project cost. Furthermore, the results display that the enhance oil productions are able to reduce the required subsidy for a single doublet geothermal project up to 50%. ispartof: APPLIED ENERGY vol:212 pages:1433-1447 status: published

The dryland anabranching river Rio Capilla is characterized by nonvegetated and vegetated reaches with prominent channel morphology. To identify the morphodynamics of such dryland anabranching systems and their controls, we investigated the Rio Capilla of the southern Altiplano Plateau using high-resolution satellite imagery and field measurements. Comparison of high-resolution satellite data reveals that erosion exceeds deposition for the main channel, accompanied by changes in channel planform, such as meander and channel morphology. On-site surveys combined with high-precision GPS and high-resolution satellite imagery show that channels are characterized by shallowness and poor development of levees. The study area of the Rio Capilla is divided into two zones of different slopes: zone 1 with a high slope and zone 2 with a low slope. Zone 1 has a relatively straight main channel with few anabranches and grass-covered banks that are stable despite the high gradient; whereas zone 2 is typified by more anabranches with nonvegetated banks, and the main channel experiences prominent bank accretion and erosion. Excavations show that point-bar deposits are fine-sand-dominated in two reaches and that river banks primarily consist of silt and clay. The limited vegetation cover and abundance of desiccation cracks and macropores make the river bank more erodible leading to pronounced lateral migration in this low-gradient dryland river system. Shallow channels and poor development of levees in combination with in-channel accretionary benches result in frequent overbank flooding, which results in a high density of crevasse splays over unconsolidated river banks and accretionary benches. Connection of headcuts and crevasse channels together with lateral migration and chute channels and reactivation of partially abandoned meanders produces an anabranching pattern in such dryland river systems.

Summary Rotliegend aeolian and fluvial sandstones are prolific gas reservoirs throughout the Southern Permian Basin (SPB). Exploration and research efforts concentrated on the sand-dominated southern flank of the SPB, where the major gas fields are located. The reservoir potential of the Rotliegend feather edge in the central part of the SPB, i.e. the area where the south-derived sands pinch out towards the Silver Pit salt lake, has long been underestimated. The sand-starved, claystone-dominated and evaporite-bearing lithology in the central part of the basin hampered the detectability of thin-bedded, potential reservoir sandstones with conventional well logs. The present study uses high-resolution well correlations in a sequence-stratigraphic framework. The employed methodology is a combination of pattern analysis (trends and trend changes) in gamma-ray (GR) logs, and maximum entropy-based (MEM-based) spectral trend curves of GR to graphically evaluate the validity of the pattern analysis. This allowed for the correlation of at least twelve individual thin-bedded sheets (1–2m thick) a 20–25m thick reservoir interval over distances larger than 20km. Core analysis has identified the sheets as unconfined fluvial terminal-splay sandstone. The facies characteristics and sedimentary architecture were corroborated in an outcrop analogue study of a present-day river system in the Altiplano Basin (Bolivia).

The primary challenge for efficient geothermal doublet design and deployment is the adequate prediction of the size, shape, lateral extent and thickness (or aquifer architecture) of aquifers. In the West Netherlands Basin, fluvial Lower Cretaceous sandstone-rich successions form the main aquifers for geothermal heat exploitation. Large variations in the thickness of these successions are recognised in currently active doublet systems that cannot be explained. This creates an uncertainty in aquifer thickness prediction, which increases the uncertainty in doublet lifetime prediction as it has an impact on net aquifer volume. The goal of this study was to improve our understanding of the thickness variations and regional aquifer architecture of the Nieuwerkerk Formation geothermal aquifers. For this purpose, new palynological data were evaluated to correlate aquifers in currently active doublet systems based on their chronostratigraphic position and regional Maximum Flooding Surfaces. Based on the palynological cuttings analysis, the fluvial interval of the Nieuwerkerk Formation was subdivided into two successions: a Late Ryazanian to Early Valanginian succession and a Valanginian succession. Within these successions trends were identified in sandstone content. In combination with seismic interpretation, maps were constructed that predict aquifer thickness and their lateral extent in the basin. The study emphasises the value of palynological analyses to reduce the uncertainty of fluvial hot sedimentary aquifer exploitation. Copyright © 2018 Netherlands Journal of Geosciences Foundation.

© 2016 Elsevier Ltd This study finds that the geothermal doublet layout with respect to the paleo flow direction in fluvial sedimentary reservoirs could significantly affect pump energy losses. These losses can be reduced by up to 10% if a doublet well pair is oriented parallel to the paleo flow trend compared to perpendicular. The chance that flow paths are formed perpendicular to this trend strongly depends on the net sandstone volume in the reservoir. Detailed fluvial facies architecture realisations which are used in this study, are generated with a process-based approach utilizing geological data from the Lower Cretaceous Nieuwerkerk Formation in the West Netherlands Basin. Finally, this study emphasizes the importance of detailed facies architecture modelling for the assessment of both risks and production strategies in Hot Sedimentary Aquifers. publisher: Elsevier articletitle: On the connectivity anisotropy in fluvial Hot Sedimentary Aquifers and its influence on geothermal doublet performance journaltitle: Geothermics articlelink: http://dx.doi.org/10.1016/j.geothermics.2016.10.002 content_type: article copyright: © 2017 The Authors. Published by Elsevier Ltd. ispartof: Geothermics vol:65 pages:222-233 status: published

Sedimentation at the terminus of low-gradient river systems in a semi-arid climate setting is characterized by thin- but laterally extensive amalgamated sand sheets. Analysis of absolute age dating with Optically Stimulated Luminescence measurements of fluvial sands in the Holocene Rio Colorado (Altiplano Basin, Bolivia) aimed to (1) analyze the processes that created this sedimentary architecture, and (2) provide an analogue for thin but laterally-extensive sandstone reservoirs in an overall low net-to-gross setting such as the Rotliegend feather edge. The Rio Colorado created a network of laterally-amalgamated fluvial sands by successive river-channel switching as the result of avulsion in a time period of only 4000y. The network formed by successive river avulsions with frequencies from 60 to 910y. The total area covered with these deposits approximates 500 km2; maximum thickness of the deposits is 2m. Fluvial sediment accumulated by vertical aggradation of sand in levees, vertical stacking and lateral amalgamation of crevasse splays, and vertical aggradation of channel-floor sand. This resulted in alluvial ridges with a positive relief on the floodplain. Subsequent river positions avoided the positive relief, and juxtaposition of successive river positions resulted in compensational stacking and amalgamation of fluvial sand deposits to a laterally connected extensive sand sheet.

In this remote sensing-based study, we present the analysis of the geomorphological development at the low-gradient terminus of the modern Rio Colorado dryland river system in the endorheic Altiplano Basin (Bolivia). Changes in the river morphology occur after short periods of catastrophic peak discharge which result in the expansion of existing crevasse splays, formation of new crevasse splays and in river path avulsion. Episodic peak discharge events in the study area were pinpointed and quantified by combining daily precipitation records from gauging stations in the vicinity with catchment area analysis from ASTER global DEM (GDEM) remote sensing data. A time series of Landsat imagery for the period 1975–2001 was then used to analyze the river morphology changes after major peak discharge events. Compensational stacking of crevasse splays in combination with river avulsion produced a thin but aerially extensive connected sand sheet at the terminus of the fluvial system.

The Dacian Basin formed an important link between the central and eastern parts of the Paratethys, a chain of late Tertiary inland seas and lakes. This study presents constraints on Miocene sea and lake level fluctuations in the Dacian Basin and on the connectivity between it and other Paratethys basins, based on the interpretation of seismic lines, a micropalaeontological study and lithofacies analysis of a large number of outcrops. It is shown that relative sea level fluctuations in the western part of the Dacian Basin during the Middle Miocene were primarily driven by tectonic activity in the nearby Carpathian Mountains. From the Maeotian (Late Miocene) onwards, however, tectonic activity was minor and relative sea level fluctuations were primarily driven by changes in basin connectivity and climate. The connection between the Central and Eastern Paratethys was broken at the end of the Middle Miocene, leading to the development of an endemic fauna in the former, but new data presented here suggest that isolation was not sustained completely as Central Paratethys species appeared in the Dacian Basin during the Maeotian (Late Miocene). Besides the isolation two falls in water level occurred in the basin during the latest Miocene: Of these, the intra-Pontian sea-level drop is the best known. We show, however, that this drop was preceded by a larger sea or lake-level drop in the late Sarmatian/Maeotian. This latter event may have affected much larger parts of the Paratethys, and we recommend more study of the bordering basins. The hypothesis that the connection between the Dacian and Central Paratethys basins was located in the region where the Danube River presently crosses the Carpathians was tested, but no supporting evidence was found.

The Rio Colorado dryland river system in the southeast of the endorheic Altiplano Basin (Bolivia) terminates on a very flat coastal plain at the edge of the Salar de Uyuni, the world's largest salt pan with an area of ca. 12,500 km 2 . Since the Pleistocene the basin has experienced several lake expansion and contraction cycles in response to wetter and drier climate periods, respectively. At present the basin is in a dry climate period which results in a lake level lowstand and progradation of fluvial systems such as the Rio Colorado onto the former lake bottom. The present field study of the terminus of the Rio Colorado shows that the river experiences a gradual downstream decrease of bankfull width and depth. This bankfull decrease is caused by the combined effects of: (1) extremely low gradient of the lake bottom and, hence, loss of flow energy, and (2) downstream transmission losses due to high evaporation potential and river water percolation through the channel floor. Peak water discharge in seasonal, short-duration rain periods causes massive overbank flooding and floodplain inundation. On satellite images the morphology of the river terminus has a divergent pattern and resembles a network of coeval sinuous distributary channels. However, field observations show that only one channel is active at low flow stage, and at high-flow stage an abandoned, partially infilled channel may be active as well. The active channel at its termination splits into narrow and shallow anastomosing streams before its demise on the lacustrine coastal plain. The rest of the channels which form the divergent network are older sediment-filled abandoned sinuous river courses with multiple random avulsion points. These channel deposits, together with extensive amalgamated crevasse-splay deposits, form an intricate network of fluvial sand deposits. Successive stages of progressively deeper crevasse-channel incision into the floodplain are the result of waning-stage return flow of floodwaters towards the main channel. This results in increased crevasse-channel width, depth and length, and redeposition of eroded lacustrine coastal-plain sediment at the junction of the crevasse channel with the present-day river. A sedimentary model is proposed in which deep river bank incision by return flow in crevasse-channels creates the preferential locus for river avulsion. In a Lowstand Systems Tract with little vertical accommodation increase, the recurring avulsed-river paths result in a thin but laterally extensive network of amalgamated channel-fill, point-bar, crevasse-channel and crevasse-splay deposits. The resulting sedimentary architecture could easily be mistaken as produced by a distributary system of simultaneously active fluvial channels.

Floodplain shales are abundant in the distal part of dryland fluvial fans fringing endorheic basins, such as the Rio Colorado fluvial system in Bolivia. In a low-gradient coastal plain, lowstand progradation of a fluvial system creates accommodation space. Long periods of low-flow stage only support a single channel, decreasing in capacity downstream. Flooding during short episodes of peak runoff results in massive overbank deposition and aggradation of the floodplain around the active channel. Assuming a constant flow capacity, this is mirrored by an elevation of the channel floor, combining to form an elongated lobe of up to several kilometres in width. Distributary channels within crevasse splays develop reflux phenomena, effectively decreasing the flow capacity of their parent channel. This, combined with the decreased gradient of the active channel profile, causes an upstream increase in avulsion proneness and is a dominant control on the autocyclic switching of channel belts in a process of large-scale compensational stacking. The resulting stratigraphy is heterogeneous and may have tough gas reservoir potential.

Pollution of groundwater with natural (geogenic) arsenic occurs on an enormous, world-wide scale, and causes wide-spread, serious health risks for an estimated more than hundred million people who depend on the use of shallow aquifers for drinking and irrigation water. A literature review of key studies on arsenic concentration levels yields that Holocene fluvial and deltaic flood basins are the hotspots of arsenic pollution, and that the dominant geomorphological setting of the arsenic-polluted areas consists of shallow-depth meandering-river deposits with sand-prone fluvial point-bar deposits surrounded by clay-filled (clay plug) abandoned meander bends (oxbow lakes). Analysis of the lithofacies distribution and related permeability contrasts of the geomorphological elements in two cored wells in a point bar and adjacent clay plug along the Ganges River, in combination with data of arsenic concentrations and organic matter content reveals that the low-permeable clay-plug deposits have a high organic matter content and the adjacent permeable point-bar sands show high but spatially very variable arsenic concentrations. On the basis of the geomorphological juxtaposition, the analysis of fluvial depositional processes and lithofacies characteristics, inherent permeability distribution and the omnipresence of the two geomorphological elements in Holocene flood basins around the world, a generic model is presented for the wide-spread arsenic occurrence. The anoxic deeper part (hypolimnion) of the oxbow lake, and the clay plugs are identified as the loci of reactive organic carbon and microbial respiration in an anoxic environment that triggers the reductive dissolution of iron oxy-hydroxides and the release of arsenic on the scale of entire fluvial floodplains and deltaic basins. The adjacent permeable point-bar sands are identified as the effective trap for the dissolved arsenic, and the internal permeability heterogeneity is the cause for aquifer compartmentalization, with large arsenic concentration differences between neighboring compartments.

Thin-bedded crevasse splays in low-net-to-gross fluvial stratigraphy were previously not considered as potential reservoir targets. This paper focuses on the construction of a high-resolution static reservoir-architecture model of crevasse splay deposits associated with meandering rivers on the low-gradient coastal plain of endorheic basins. Outcrops of the Miocene Huesca fluvial fan (Ebro Basin, Spain) display low-net-to-gross fluvial stratigraphy, bounded by two large sand-prone channel belts. A static model of the study area is constructed following a sequential macro-to-micro approach. The model shows connectivity between the channel belts through crevasse splays. Connections between separate crevasse splays are present through incisions of younger crevasse splays and channels. Without the connectivity between the channel belts, connectivity would still be present in vertical wells through the extensive crevasse splays originating from the channel belts. This makes the model suitable for reservoirs with less connections as well. Ongoing research on process-based modelling of crevasse splays will yield an improved understanding of the grain-size distribution and can be used to populate the static model. The model will be upscaled to allow for fluid flow simulations in which several production mechanisms will be evaluated in order to assess the economic potential of these secondary tough gas reservoirs.

The location, shape and stacking pattern of deep‐marine clastic sediments on drifting stage passive continental margins are strongly influenced by the slope and basin floor topography. The tectonic control on sediment routes and dispersal patterns, however, is less understood on rift margins, particularly the impact of subaqueous transfer zones or relay ramps. In this study, an area of the Palaeocene marine syn‐rift succession in the Voring Basin is mapped in detail to unravel the relationship between fault geometries and sedimentary infill patterns. Using root‐mean‐square (RMS) amplitudes and deposit thicknesses interpreted from seismic data, sedimentary elements in the Fenris Graben and the Gjallar Ridge are related to the fault patterns and the overall basin geometry. Older deposits are found to be aligned parallel to the basin axis, with the greatest sediment thicknesses on the hanging walls and adjacent to rotated faults. The main sediment supply is interpreted to be sourced from the Voring Marginal High and Greenland, presumably containing a significant proportion of coarser grained material and comprising numerous local depocentres. With continued rifting and decreased fault activity, finer grained deposition draped the previous basin infill and smoothed the basin floor topography. Deposits close to the foot of relay ramps along the Gjallar Ridge, however, suggest that the high may have acted as a local sediment source leading to local depocentres. Transfer zones played a significant role in sediment transport during the early rifting phase, and were able to maintain some influence into the late rifting and early drifting stage. Identification of early‐ and late‐stage transfer zones may therefore help in locating coarser grained depocentres and potential hydrocarbon reservoirs.

Unlike for subaerial settings, the impact of subaqueous relay ramps on sediment dispersal is still poorly understood. A combination of analogue laboratory experiments in a sandbox with numerical flow calculations is used to simulate relay ramp topographies on rifting continental margins and to analyse the resulting turbidity current pathways and their deposits. Various scenarios are investigated, including inflow perpendicular and oblique to the relay ramp axis as well as flow constrained by an incised channel on the ramp and by a landward-directed tilt of the ramp. Without channelling, most sedimentation takes place on the basin floor because the bulk of the flow follows the steepest gradient down the fault and into the rift basin. With a channel along the relay ramp, significant flow occurs initially down the ramp axis, but channel spillover and basinward ramp tilting combine to redirect much of the sediment down the fault slope into the basin. When the relay ramp has a landward-oriented tilt, most of the current flows down the ramp and deposits its sediment load there and at the foot of the ramp. However, also here a considerable amount of the flow is shed over the hanging wall fault and into the basin, forming a secondary depocentre, while ponding redistributes thin deposits over a wider area of the basin. The quantitative dependence of these results on the specific ramp geometries remains to be investigated further but may bear great importance for refined sedimentary models in subaqueous rifted settings as well as for hydrocarbon exploration therein. © 2009 The Authors. Journal compilation © 2009 International Association of Sedimentologists.

The Huesca fluvial fan (Miocene, Ebro Basin, Spain) contains a low-gradient, mixed-load fluvial system. A detailed outcrop study of its meandering river deposits shows that the preservation of elongate channel-floor sandstone ribbons is common and that these deposits create a continuous along-stream sand-to-sand connectivity between successive crescent-shaped sandy point-bar accumulations on both sides of the channel. The combined appearance of the sandstone resembles a string of beads consisting of a thin, sinuous ribbon with thick and wide protuberances on either side. The studied meandering river sandstone bodies are laterally amalgamated and vertically stacked with a net-to-gross (N/G) ratio of about 40%. They occur in 1–1.5-km (0.62–1-mi)-wide, northeast–southwest-oriented elongate meander belts occupying paleochannels. Beyond these belts, the sandstone is limited to isolated bodies with a very low N/G ratio. A generic model of the string-of-beads geometry, based on the outcrop data analysis, showed a significant increase of bulk rock volume for the connected string of beads compared with the model of isolated point bars. The outcrop results demonstrate the potential for channel-floor sandstone bodies to be preserved in a low-gradient, mixed-load fluvial system and their importance in connecting point-bar units in an along-stream direction. We recommend that fluvial reservoir architecture modeling programs include a function that allows the connectivity between channel-floor and point-bar architectural elements. This may greatly impact the estimated reservoir volumes and recovery factors in primary and secondary production as well as influence the sweep efficiency of enhanced recovery technologies. Marinus E. Donselaar received his M.Sc. degree in geology from the University of Utrecht and his Doctor of Science degree in reservoir geology from Delft University of Technology, both in the Netherlands. He worked at the Comparative Sedimentology Division at the University of Utrecht. Since 1987, he has been a lecturer in sedimentology at Delft University of Technology. His research interests are in the modeling of fluvial and barrier island reservoir architecture. Irina Overeem has a Ph.D. from the Faculty of Civil Engineering and Geosciences, Delft University of Technology, Netherlands, where she was an assistant professor in geological modeling from 2005 to 2007. Her research focuses on the numerical modeling of fluviodeltaic processes. Over the last six years she has held a position at Institute of Arctic and Alpine Research (INSTAAR), and she is presently working at the Community Surface Dynamics Modeling Facility at the University of Colorado.

Unconventional tough gas reservoirs in low-net-to-gross fluvial stratigraphic intervals may constitute a secondary source of fossil energy to prolong the gas supply in the future. To date, however, production from these thin-bedded, fine-grained reservoirs has been hampered by the economic risks associated with the uncertainties in their geometry, spatial distribution and reservoir properties. This study aims to provide a better insight into the reservoir architecture and tough gas reservoir potential of stacked and interconnected crevasse splays in a fluvial floodplain stratigraphy. Despite their thickness not exceeding decimetre scale, the surface area of individual crevasse splays may be up to several square kilometres and intervals of stacked and interconnected crevasse splays range up to several metres in thickness. Through incision of consecutive crevasse splays into underlying lobes and the channel fill of cut-through avulsed channels, sand-on-sand contact is established, effectively connecting individual splays. The net reservoir volume of single crevasse splays ranges up to several hundred thousand cubic metres, yielding GIIP estimations in the order of several million cubic metres. For intervals of stacked and interconnected crevasse splays, these figures are up to one order of magnitude higher, making them suitable as a secondary reservoir capacity.

Electrical borehole image logs yield high-resolution information about variations in micro-resistivity along the borehole wall. To interpret these variations in terms of sedimentary structures and lithofacies types, calibration with real rock is needed. Normally, the only real rock available is core, and this only provides one-dimensional information. In this paper, the interpretation of fluvial facies types from borehole image logs was established by direct comparison with outcrops. Four fluvial facies associations were established in an outcrop study of a low net-to-gross fluvial succession: (i) meandering rivers, (ii) braided rivers, (iii) crevasse deltas, and (iv) crevasse splays. The lithofacies characteristics and palaeocurrent distributions of each fluvial facies association were established. Two 200 m deep wells were drilled behind the cliff face outcrops. One well was cored to a depth of 150 m and borehole image logs were recorded in both wells. The wells were correlated with the outcrop. The borehole image logs were analysed by their vertical colour succession and the dipmeter pattern. Image log facies were established, and these were interpreted in terms of the fluvial facies associations encountered in the corresponding outcrops. The study of borehole image logs yields a set of diagnostic criteria for a detailed fluvial facies interpretation and the establishment of depositional trends, and thus provides a powerful tool for the direct interpretation of fluvial facies in a reservoir setting.