Fluvial depositional architecture in an unconfined environment is governed by sediment dispersal across the alluvial plain through river-path switching by avulsion. Documented inter-avulsion periodicity from modern rivers ranges from tens to over a thousand years. In this study, a quantitative spatio-temporal reconstruction of avulsion history is presented of the non-vegetated and pristine modern Río Colorado dryland river system in the semi-arid Altiplano Basin (Bolivia), based on the integrated analysis of satellite imagery and absolute age dating using optically stimulated luminescence, complemented with sedimentological and geomorphological ground-truth data. This approach enables us to reconstruct the chronological order of channel belts of the Río Colorado, to determine avulsion recurrence time and inter-avulsion periodicity, to identify mechanisms for flow path changes, and to present a morphodynamic model for the spatio-temporal evolution of fluvial deposits in a semi-arid environment. In a maximum timespan of 12.71 ± 1.5 ka, successive avulsions of the Río Colorado created a sheet of interconnected fluvial deposits, consisting of diverging and juxtaposed alluvial ridges that formed by sediment aggradation in point bars, crevasse splays, levees, and on the channel floor. The ridges show lateral onlap and amalgamation as the result of repeated avulsion and compensational stacking, whereby the river avoided the positive alluvial-ridge relief of its precursors. The resultant morphology is fan-shaped, convex-up with a surface area of approximately 500 km ² and a maximum observed thickness of 3 m. The results show inter-avulsion periods of the river of up to 1.28 ± 0.34 ka. A paucity in fluvial activity around 2 ka BP, and at present, is interpreted as the result of low river discharge related to long-term dry periodicity in the El Niño Southern Oscillation circulation system. Each river path started as a low sinuous, single-thread channel in a narrow belt, and in time increased its width and sinuosity by point-bar expansion and rotation.

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Dryland river fans form by repeated switching (avulsion) of an ephemeral stream as it progrades and accumulates sediment onto a low-gradient alluvial plain. Successive channel belts are organised in a radial pattern through a process of compensational stacking, where each consecutive river path avoids the positive relief left by its predecessors. Several criteria have been proposed for the likelihood of where and when an active channel in such a setting switches its path, including super-elevation, ratio of along-channel slope and cross-floodplain gradient, and hydraulic capacity of crevasses. A key role for the overbank domain is implied in each of these – largely interdependent – parameters for avulsion proneness, but the exact avulsion dynamics remain insufficiently understood. In this study, we combine differential-GPS measurements and a resampled open-source global digital surface model (DSM) to quantify along-channel changes in hydraulic parameters and reconstruct subtle geomorphology across an undisturbed and non-vegetated dryland river fan. This approach allows us to propose a holistic view on autocyclic avulsion dynamics in prograding dryland river systems. The results support the idea that the downstream gradient of an active river decreases over time as result of basinward lengthening (by both progradation and increased sinuosity) mirrored by near-channel deposition. This process gradually decreases the drainage effectiveness of the stream profile and elevates the channel belt relative to the surrounding floodplain. Crevasse splays play a key role in determining whether the river switches or not by effectively testing alternative flow paths. Crevasse channels develop along a local path of steepest descent across the floodplain and build out as long as their stream profile is hydrodynamically more favourable than that of their parent channel, after which they heal up. As the river profile lengthens and rises, crevasse splays extend further onto the floodplain until one reaches base level in a shorter distance than the river itself. At this point, it will receive an increasing proportion of total discharge and the parent river is abandoned. The avulsion process is gradual rather than abrupt and its frequency likely increases downstream, resulting in a dendritic pattern of abandoned river paths. The process proposed here brings together existing criteria for autocyclic avulsion in prograding dryland river systems devoid of vegetation. It incorporates the role of subtle floodplain morphology and the evolution of crevasse splays and explains the resulting depositional architecture. Moreover, our findings enable us to predict when and where a next avulsion will take place, which could help flood risk analysis and mitigation in similar settings. @en

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.

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Accommodation space in the unconfined distal part of low-gradient fluvial fans facilitates abundant floodplain deposition. Here, the development of crevasse splays plays a key role in the aggradation of alluvial ridges and subsequent river avulsion. This study presents an analysis of different stages in the evolution of crevasse splays based on observations made in the modern-day Río Colorado dryland fluvial fan fringing the endorheic Altiplano Basin in Bolivia. A generic life cycle is proposed in which crevasse-splay channels adjust towards a graded equilibrium profile with their lower-lying distal termini acting as a local base level. Initial development is dominantly controlled by the outflow of floodwater, promoting erosion near the crevasse apex and deposition towards the splay fringes. When proximal incision advances to below the maximum level of floodplain inundation, return flow occurs during the waning stage of flooding. This floodwater reflux leads to a temporary repositioning of the local base level to the deeper trunk-channel thalweg at the apex of the crevasse-splay channels. The resultant decrease in the floodplainward gradient of these channels ultimately leads to backfilling and abandonment of the crevasse splay, leaving a subtle local elevation of the floodplain. Consecutive splays form an alluvial ridge through lateral amalgamation and subsequent vertical stacking, which is mirrored by the aggradation of their parent channel floor. As this alluvial ridge becomes increasingly perched above the surrounding floodplain, splay equilibration may cause incision of the levee crevasse down to or below its trunk channel thalweg, leading to an avulsion. The mechanisms proposed in this study are relevant to fluvial settings promoting progradational avulsions. The relatively rapid accumulation rate and high preservation potential of crevasse splays in this setting makes them an important constituent of the resultant fluvial stratigraphy, amongst which are hydrocarbon-bearing successions.

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A generic life cycle applies to crevasse splays in non-degradational fluvial systems, typically ending in healing and abandonment. Crevasse-splay channels adjust to a graded equilibrium profile through proximal erosion and distal deposition, with their distal termini acting as a (prograding) local base level. When proximal incision advances to below the maximum flooding level, a reflux of floodwater occurs during the waning stage of flooding. The resultant decrease in gradient ultimately leads to the backfilling and abandonment of a crevasse splay, provided that the elevation at its distal fringe remains higher than that of the trunk channel floor. Consecutive crevasse splays form an alluvial ridge through lateral amalgamation and subsequent vertical stacking, perching the active river above the surrounding floodplain. Superelevation of the channel thalweg above the distal termini of a prograding crevasse splay leads to avulsion. A high-resolution morphological reconstruction of both the active (and recently abandoned) river(s) and the surrounding floodplain has been established to test the proposed life cycle of crevasse splays and evaluate its role in autogenic avulsion and organisation of the fluvial system. An avulsion can only occur when an overbank path of steepest descent reaches the system base level in a shorter distance (which may partially reuse the existing channel or remnant channel depressions) than the along-channel distance to its terminus. Crevasse splays prograde along this overbank flow path and capture an increasing portion of the total discharge, accelerating their development. When the crevasse apex incises down to or below its trunk channel thalweg, the avulsion is complete. The overbank path of steepest descent (i.e., avulsion path) is governed by floodplain topography, which is largely formed of abandoned alluvial ridges. This leads to compensational stacking of successive prograding channel belts, resulting in fan of amalgamated ridges.@en

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). @en

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 reservoirarchitecture 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. @en

Low-net-to-gross floodplain stratigraphy contains thin-bedded crevasse splays that may have tough gas reservoir potential. Floodplain deposits are abundant in the distal part of dryland fluvial fans in endorheic basins, such as existed in the Permo-Triassic North and Central Atlantic margins. Outcrops of the Huesca fluvial fan (Ebro Basin, Spain) serve as an analogue to intervals of such deposits. Horizontally-laminated clay and fine silt are dominant, whereas low sinuous fluvial channels constitute only a fraction of the stratigraphy. Crevasse splays are common, and frequently occur in stacks of up to two metres thick. This results from aggradation of the active channel belt when sufficient accommodation space is available, forming an heterogeneous elevated fringe around the active channel. Lateral amalgamation, vertical stacking, and interaction with sand-rich channel fill deposits significantly increase the connected reservoir volume of the laterally extensive crevasse splays. The thickness of the stacked intervals can be used as a proxy to determine the channel lag thickness of their feeder channel. This allows to estimate the connectivity in a low-net-to-gross floodplain interval without data from well penetrations. @en

Floodplain shales are abundant in the distal part of dryland fluvial fans fringing endorheic basins, such as the Río 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. @en

Low-net-to-gross floodplain stratigraphy contains thin-bedded crevasse splays that may have tough gas reservoir potential. Floodplain deposits are abundant in the distal part of dryland fluvial fans fringing endorheic basins, such as existed in the Permo-Triassic North and Central Atlantic margins. Outcrops of the Miocene Huesca fluvial fan (Ebro Basin, Spain) serve as an analogue to subsurface floodplain intervals. Horizontally-laminated clays and fine silts are dominant, whereas low-sinuous fluvial channels constitute only a fraction of the total stratigraphy. Crevasse splays are common and frequently occur in stacks of up to two metres thick. This results from aggradation of the active channel belt when sufficient accommodation space is available, forming an heterogeneous elevated fringe around the active channel. Lateral amalgamation, vertical stacking, and interaction with sand-rich channel fill deposits significantly increase the connected reservoir volume of the laterally-extensive crevasse splays. The thickness of the stacked intervals can be used as a proxy to determine the channel lag thickness of their feeder channel. This enables estimations of the connectivity in low-net-to-gross floodplain intervals to be made in absence of any direct well penetrations of the fluvial channels. @en

The Northwest European gas province is a mature area in which the production of gas from conventional reservoirs is declining. Unconventional tough gas reservoirs in low-net-to-gross fluvial stratigraphy may constitute a secondary source of fossil energy to prolong the gas supply in the future. However, as the reservoir potential of floodplain intervals has been under-acknowledged in conventional hydrocarbon exploration, there is limited published research on the accumulation of overbank deposits and their preserved stratigraphy. This research focusses on floodplain aggradation in low-gradient semi-arid fluvial fans on the fringe of endorheic basins, aiming to develop a thorough understanding of overbank deposition within the framework of fluvial system evolution. To this end, field work data from the modern-day Río Colorado fluvial system, Altiplano Basin, Bolivia, and the Miocene Huesca fluvial fan, Ebro Basin, Spain, are combined with subsurface data from the gas-prone Triassic deposits of the West Netherlands Basin. The study aims to establish quantitative spatial relations between preserved facies types, so as to provide characterisation of fluvial plays and the assessment of their reservoir potential. Results show that floodplain deposits constitute the majority of preserved stratigraphy in the distal part of the studied fluvial fans. Floodplain aggradation occurs by levee growth and subsequent crevasse splay breakthrough and sediment redistribution, effectively elevating the floodplain around the active channel. In a low-gradient fluvial system, the resulting lobate geometry causes a significant decrease in channel gradient. The gradual filling of local depressions on the floodplain adjacent to the aggrading channel is mirrored by the evolution of crevasse channels which may develop gradient inversion and backflow phenomena in their proximal reaches. The consequent reduction in transport capacity causes an upstream increase in avulsion proneness, favouring a steeper river profile onto a lower-lying part of the floodplain. The resulting stratigraphy comprises thin but laterally extensive fine-grained beds that may be connected through amalgamation, vertical stacking, and interaction with heterolithic channel fill deposits. Subsurface data show porosities up to 15% and permeability ranging from 0.1 to 10 milliDarcy, qualifying these deposits as potential targets for tough gas production.@en

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.@en

Exploitation of unconventional resources could prolong the gas production in the North Sea. Low-net-togross fluvial intervals have tough-gas reservoir potential in thin-bedded crevasse splays. To assess economic risks associated to the development of these reservoirs, a reliable grain-size distribution model is required. Sparse areal data availability for reservoir models commonly results in the use of stochastic interpolation. Numerical models offer the possibility to support these methods with proven physical concepts.To this end, simulations were conducted with Delft3D process-based modelling software. Input parameters and the validation data sets for these models are derived from outcrop studies in the presentday Río Colorado fluvial system in the Altiplano Basin, Bolivia. The grain-size trends of the simulated surface sediments for a single flood event are consistent with the validation data. These trends were used to populate individual crevasses splays within a static model. This shows that process-based models are able to support sediment trends and depositional mechanisms of a crevasse splay. The combination of numerical models and discrete field data provides a solid case for sediment distribution predications. However, simulations still have a limited accuracy. @en

Exploitation of unconventional resources could prolong the gas production in the North Sea. Low-net-to-gross fluvial intervals may have tough-gas reservoir potential in thin-bedded crevasse splays.To assess economic risks associated to the development of these reservoirs, a numerical model can help to predict the sediment distribution. To this end, simulations were conducted with Delft3D process-based modelling software. Input parameters and the validation data sets for these models are derived from outcrop studies in the present-day Río Colorado fluvial system in the Altiplano Basin, Bolivia. The grain-size trends of the simulated surface sediments for a single flood event show a trend which is consistent with the validation data. For example, grain-size decreases with increasing distance from the channel, which is in line with the physical concept of decreasing sediment size for decreasing flow energy. This shows that numerical models can be used to support sediment trends and depositional mechanisms of a crevasse splay. The combination of numerical models and discrete field data provides a solid case for sediment distribution predications. However, simulations still have a limited accuracy. @en