Multiscale simulation of fluvio-deltaic stratigraphy was used to quantify the elements of the geometry and architectural arrangement of sub-seismic-scale fluvial-to-shelf sedimentary segments. We conducted numerical experiments of fluvio-deltaic system evolution by simulating the accommodation-to-sediment-supply (A/S) cycles of varying wavelength and amplitude with the objective to produce synthetic 3-D stratigraphic records. Post-processing routines were developed in order to investigate delta lobe architecture in relation to channel-network evolution throughout A/S cycles, estimate net sediment accumulation rates in 3-D space, and extract chronostratigraphically constrained lithosomes (or chronosomes) to quantify large-scale connectivity, that is, the spatial distribution of high net-to-gross lithologies. Chronosomes formed under the conditions of channel-belt aggradation are separated by laterally continuous abandonment surfaces associated with major avulsions and delta-lobe switches. Chronosomes corresponding to periods in which sea level drops below the inherited shelf break, that is, the youngest portions of the late falling stage systems tract (FSST), form in the virtual absence of major avulsions, owing to the incision in their upstream parts, and thus display purely degradational architecture. Detailed investigation of chronosomes within the late FSST showed that their spatial continuity may be disrupted by higher-frequency A/S cycles to produce “stranded” sand-rich bodies encased in shales. Chronosomes formed during early and late falling stage (FSST) demonstrate the highest large-scale connectivity in their proximal and distal areas, respectively. Lower-amplitude base level changes, representative of greenhouse periods during which the shelf break is not exposed, increase the magnitude of delta-lobe switching and favour the development of system-wide abandonment surfaces, whose expression in real-world stratigraphy is likely to reflect the intertwined effects of high-frequency allogenic forcing and differential subsidence.

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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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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.@en

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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Routine core analysis (RCA) for Reservoir Quality (RQ) prediction comprises a collection of measurements acquired with different analytical techniques, which does not include operator-bias evaluation, or integration of continuous sedimentological core description with spot measurements on plugs and thin sections. RCA data rarely have verified uncertainty specifications, thus hampering statistically-rigorous extrapolation of spot measurements such as petrographic description, to the entire reservoir volume. Petrographic analysis gives insight into the controls on RQ through unravelling the diagenetic fingerprint that shapes the eventual porosity and permeability in the reservoir. Because thin-section analysis is time consuming and costly, protocols for selection of representative thin sections should aim at maximizing information obtained from small data sets, so as to minimize costs and prevent unnecessary destruction of core material. This paper presents a flexible protocol for representative thin-section selection based on evaluation of RCA data (i.e., poro-perm and grain-density plug measurements), illustrated on a core of a Carboniferous fluvial sandstone reservoir. The results of the petrographic analysis are interpreted in terms of their relation with sedimentological and geochemical signatures, and it is demonstrated that application of the protocol highly increased the RCA data value which to date merely served as petrophysical indicators.@en

In Reservoir Quality (RQ) studies, only cores provide an intact sample of the sedimentary rock, which permits various parameters, such as mineralogical composition, texture, diagenetic alterations and porosity‐permeability distribution to be measured. However, state‐of‐the‐art routine core analysis (RCA) workflow for RQ prediction is still based on a mere collection of such measurements acquired with different analytical techniques and at different resolutions/scales. This protocol does not include operator‐bias evaluation and usually fails at integrating the continuous sedimentological core description with spot measurements on plugs and thin sections. Moreover, RCA data rarely have verified uncertainty specifications, thus hampering statistically‐rigorous extrapolation of spot measurements such as petrographic description, to the entire reservoir volume. Because many sediment properties are interrelated, e.g. porosity is governed by grain size distribution, which in turn is related to mineralogical composition with a specific chemical expression, an integrated analysis of all these data is the best way to understand and predict their behaviour. Non‐destructive, in‐situ hyperspectral X‐ray fluorescence core‐scanning (XRF‐CS) technology provides a spatially continuous, cm‐scale “big‐data” environment in which the relationship between the different variables can be statistically explored. Based on this approach, we have integrated the sedimentological description of a fluvial Carboniferous core with the quantitative petrographic and chemical (i.e. inductively coupled plasma optical emission spectrometry) analysis and plug‐derived poro‐perm measurements, using the XRF-CS output. The calibration (Weltje & Tjallingii, 2008; Weltje et al., 2015; Bloemsma, 2015) and multivariate statistical analysis of these integrated data allowed us to identify the different diagenetic zones affecting reservoir quality, and to predict their occurrence across three different reservoir intervals. This is the first step of the so‐called Integrated Core Analysis (ICA) workflow that we are developing, which intends to overcome some of the main above mentioned issues.@en

Required distance between doublet systems in low enthalpy geothermal heat exploitation is often not fully elucidated. The required distance aims to prevent negative interference influencing the utilisation efficiency of doublet systems. Currently production licence areas are often issued based on the expected extent of the reinjected cold water plume on the moment of thermal breakthrough. The production temperature, however, may not immediately drop to non-economic values after this moment. Consequently, heat production could continue increasing the extent of the cold water plume. Furthermore, the area influenced by pressure because of injection and production spreads beyond the cold water plume extent, influencing not only the productivity of adjacent doublet systems but also the shape of cold water plumes. This affects doublet life time, especially if adjacent doublets have different production rates. In this modelling based study a multi parameter analysis is carried out to derive dimensionless relations between basic doublet design parameters and required doublet distance. These parameters include the spacing between injector and producer of the same doublet, different production rates, aquifer thickness and minimal required production temperature. The results of this study can be used to minimize negative interference or optimise positive interference aiming at improving geothermal doublet deployment efficiency.

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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

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.@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

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

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

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

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

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