M.E. Donselaar

Connectivity of fluvial point-bar deposits: An example from the Miocene Huesca fluvial fan, Ebro Basin, Spain

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.

On the relation between fluvio-deltaic flood basin geomorphology and the wide-spread occurrence of arsenic pollution in shallow aquifers

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.

Facies architecture of heterolithic tidal deposits : The Holocene Holland Tidal Basin

The size, shape and spatial position of lithofacies types (or facies architecture) in a tidal estuarine basin are complex and therefore difficult to model. The tidal currents in the basin concentrate sand-sized sediment in a branching pattern of tidal channels and fringing tidal flats. Away from the sandy tidal flats the sediment gradually changes to mud-dominated heterolithic deposits and clay. In this paper the facies analysis of a tidal estuarine basin, the Holocene Holland Tidal Basin (HHTB) is presented based on core data and Cone Penetration Tests (CPT). Four lithofacies associations are recognized: (1) tidal channel sand, (2) sand-dominated heterolithic inter-tidal flat, (3) mud-dominated heterolithic inter-channel, and (4) fresh-water peat. The high data density allowed for the construction of a detailed facies architecture model in which the size, shape and spatial position of the tidal estuarine facies elements were established. The results can be used to improve the reservoir modelling in highly heterogeneous estuarine reservoir settings

The Application of Borehole Image Logs to Fluvial Facies Interpretation

Electrical borehole image logs have the potential for the direct interpretation of lithofacies types. The challenge is to create a set of diagnostic criteria with which microresistivity variations can be translated to lithofacies characteristics such as bedding sequences, sedimentary structures, and vertical grain-size successions. Behind-outcrop logging is a method to directly validate the borehole images to real rock. In this chapter, sediments of the Huesca fluvial fan (Miocene, Ebro Basin, Spain) are used for the validation of behind-outcrop borehole image logs. The logs were recorded in two 200-m (656-ft)-deep wells behind cliff-face outcrops. In addition to the outcrop control, one well was cored. Borehole image log facies were defined from the vertical color succession and the dipmeter pattern and were correlated with the fluvial facies associations in outcrop. Four fluvial facies associations and corresponding borehole image facies were interpreted: (1) meandering rivers, (2) braided rivers, (3) crevasse deltas, and (4) crevasse splays. Vertical dipmeter successions were analyzed and yielded directional trends of the fluvial channels.

The life cycle of crevasse splays as a key mechanism in the aggradation of alluvial ridges and river avulsion: Life cycle of crevasse splays in aggrading and avulsion

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 crevassesplay 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. Copyright

Application of synthetic aperture radar methods for morphological analysis of the Salar De Uyuni distal fluvial system

Knowledge of the present-day activity of river channels in distal fluvial systems strongly contributes to the reconstruction of past branching and avulsion processes. Established remote sensing techniques can be applied to monitor the formation of flooding planes (crevasse splays) and channel activity. In this research variations in the amplitude in Synthetic Aperture Radar images are interpreted as soil moisture changes. Interferometric SAR showed minor phase changes during dry season and loss of coherence after peak run-off. After peak discharge during the dry season in 2009 reactivation of multiple avulsed river paths and crevasse channels was detected. These results show that analysis of SAR images can contribute to the monitoring of fluvial systems. It is expected that these initial results will be confirmed by field data and analysis of alternative remote sensing data sources.

High-Resolution Reservoir Architecture Modelling of Thin-bedded Fluvial Terminal-splay Sandstone in the Rotliegend Feather-edge Area

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).

Avulsion History of a Holocene Semi-arid River System - Outcrop Analogue for Thin-bedded Fluvial Reservoirs in the Rotliegend Feather Edge

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.

Process-based Modelling of Sediment Distribution in Fluvial Crevasse Splays Validated by Outcrop Data

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 Rio 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.

Stacked Crevasse Splays in the Semi-arid Huesca Fluvial Fan (Ebro Basin, Spain) - Implications for Connectivity

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.