Arsenic contamination in shallow aquifers of Holocene alluvial basins is a serious health risk affecting millions of people [1]. Detection of arsenic hotspots is a slow and tedious process based on the analysis of groundwater samples. This study improves arsenic risk prediction by incorporating geomorphological features such as oxbow lakes and clay plugs into a machine learning (ML) approach. Advances in remote sensing [2], often combined with ML, enable the efficient detection of these and other proxy features, significantly reducing reliance on labour-intensive fieldwork. By combining these features with environmental and demographic data, the approach provides more accurate and cost-effective risk assessments, enabling better-targeted interventions in vulnerable regions and supporting proactive environmental monitoring.

The Main Buntsandstein Subgroup in the Roer Valley Graben in the southern Dutch subsurface is a sand-prone sedimentary interval deposited in a fluvial-aeolian environment, and is currently investigated for its suitability as target for low-entalpy geothermal exploration. The current depositional models in the Roer Valley Graben do not fully address the facies heterogeneities within and between Buntsandstein sedimentary units and their impact on the prediction of reservoir architecture. A detailed analysis of the Main Buntsandstein sedimentary facies heterogeneities to de-risk future sustainable energy operations is therefore crucial. In the present study, the sedimentology and lithostratigraphy of the Buntsandstein are assessed in a multidisciplinary analysis by use of a subsurface dataset composed of well cores, gamma-ray logs, and thin section data. The deposition of the Main Buntsandstein sediments in the Roer Valley Graben is dominated by different fluvial processes, with minor aeolian reworking. River planform style evolved through geological time from highly mobile and ephemeral to more perennial in nature. These changes in river style seem to be dictated by a decrease in climatic aridity along with a decrease in tectonic activity. The depositional processes resulted in the development of six lithofacies associations, developing three different types of reservoir architectures with their own set of heterogeneities at different spatial scales. Amalgamated, stacked sandstones have the highest net-to-gross (N/G) with a high degree of lateral and vertical connectivity, and the highest average porosity and permeability. Compensational-stacked sandstone reservoirs have a lower N/G and are the most heterogeneous due to the frequent occurrence of cemented intervals as well as mud drapes in the sandstone bodies. Marginal isolated sandstones show a well preserved relationship between reservoir properties and depositional facies, while more data are needed to resolve the spatial connectivity and lateral continuity of these sandstone bodies. The results of this study enhance the understanding of Lower Triassic reservoir architecture and sedimentary heterogeneities in the Roer Valley Graben that can be applied well beyond the area and provide a solid basis for future investigation of the relationship between sedimentary facies, diagenesis, and reservoir quality.

In the 40 years since the relation between arsenic (As) toxicity and groundwater extraction was first documented from the Holocene alluvial basin of West Bengal, India, (1) we have become more aware that groundwater contamination with naturally occurring (geogenic) As poses a serious health threat of global proportions. (2) With the aim of implementing effective and sustainable mitigation strategies, research into the occurrence and location of toxic As levels in drinking and irrigation water and in the food chain provided insight into all aspects of the As-contamination issue, including (a) geogenic As provenance in volcanic and metamorphic rocks, hydrothermal additions to groundwater and hot springs, and weathering of rocks in orogenic mountain belts, (b) its accumulation in sedimentary-basin aquifers, (c) the mobilization and transport of the contaminant into the groundwater, and (d) the associated health risks of sustained As ingestion for >200 million people around the world. (3,4) A wide range of potential As-mitigation measures have been proposed over the years, ranging from in situ chemical and biological oxidative processes for immobilizing As to subsequent filtration methods and social awareness programs for the affected population. (5−7)

Groundwater contamination with naturally-occurring arsenic (As) poses a serious health threat of global proportions. Implementation of focused and sustainable As-mitigation measures is hampered by the inability to pinpoint enigmatic As-hotspots in the vast area of continental alluvial basins with elevated toxicity risk. The catalyser role of invasive vegetation in anoxic fluvial oxbow lakes in combination with microbial metabolism processes are key in mobilizing As from its solid state into the shallow aquifer domain and accumulation in porous and permeable fluvial point-bar sediment. This insight opens up the opportunity for a cross-disciplinary approach to construct predictive machine learning-based object-based geospatial models to locate As-hotspots by the analyses of (1) satellite imagery of alluvial geomorphology, (2) oxbow-lake vegetation density, and (3) ground-truth databases of oxbow-lake/aquifer biogeochemistry and fluvial sedimentology.

Sandstones from the Main Buntsandstein Subgroup represent a promising deep geothermal target in the subsurface of the Netherlands considering their widespread distribution and temperatures locally reaching 140-150 °C at depths of ~ 3 to 5 km. The Main Buntsandstein Subgroup is a sand-prone interval, but the reservoir quality of these sandstones is known to be heterogeneous as result of an interplay between depositional and diagenetic processes. This makes the Buntsandstein sediments an uncertain and risky geothermal play.

In this project, we assess the syn- and post-depositional history of these sediments. The aim is to define structural, sedimentary, and diagenetic heterogeneities within the Main Buntsandstein sediments and assess their impact on reservoir quality. This will help reduce uncertainties for geothermal operations in the Triassic in the southern Netherlands and beyond.

The structural analysis of the study area using seismic and well data reveals that the Main Buntsandstein sediments represent an early syn-rift sequence and that their present-day distribution is strongly controlled by faulting. In parallel, the study of the sedimentology and stratigraphy conducted on core and wireline data indicates that the depositional environment evolves through the Buntsandstein stratigraphy, resulting in the development of different reservoir architectures. Diagenesis has largely altered the primary relationship between sedimentary facies and porosity and permeability. Overall cementation seems to have a larger impact on reducing reservoir quality than compaction, with quartz, dolomite, and illite representing the most abundant types of cement. The analysis of fractures using core and image logs suggests that the fracture density is driven by the lithological variability within the Main Buntsandstein and that fracture joints and stylolites locally may contribute to enhancing the system permeability.

The integrated assessment of the results allows the development of prospect play maps for the Buntsandstein in the southern Netherlands, addressing uncertainties and providing future recommendations for further exploration and optimizing geothermal operations in the Triassic.

The Lower Triassic Main Buntsandstein Subgroup represents one of the most promising deep geothermal plays in the Netherlands. Reservoir zones consist of sandstone units deposited in an arid to semi-arid alluvial plain reaching total thicknesses of over 200 m in the Roer Valley Graben. However, the lack of a comprehensive reservoir model integrating stratigraphy, sedimentology, and petrography makes the Buntsandstein a high-risk target. Here, we present a reservoir architecture model of the spatial and temporal variation of sedimentary facies and inherent permeability barriers and baffles, based on the integration of data from 33 wells from different stratigraphic levels and different areas of the Roer Valley Graben.

Core samples and thin sections analysis revealed two major reservoir facies that were interpreted as the products of transport and depositional processes in braided and sinuous river settings. The identified facies were then coupled to wireline logs to assess the spatial and temporal variation in sedimentary architecture. The lower part of the stratigraphy is dominated by braided river reservoir facies with a high degree of connectivity, where regional lacustrine-playa lake sediments represent the main potential permeability barriers. By contrast, the upper part of the stratigraphy is characterized by an increase in the proportion of sinuous river complexes. These latter yield a lower degree of connectivity with different types of baffles such as intercalated fine-grained overbank sediments, abandonment plugs, bar-draping fines, and cemented dolocrete scour fills. These are much more localized compared to the braided complexes-related barriers, making the prediction of the upper stratigraphy architecture uncertain.

The identification of arsenic-contamination hotspots in alluvial aquifers is a global-scale challenge. The collection and inventory of arsenic concentration datasets in the shallow-aquifer domain of affected alluvial basins is a tedious and slow process, given the magnitude of the problem. Recent research demonstrates that oxbow-lake biogeochemistry in alluvial plains, mobilization of geogenic arsenic, and accumulation in geomorphologically well-defined areas are interacting processes that determine arsenic-contamination locations. This awareness provides a tool to identify potential arsenic-hotspots based on geomorphological similarity, and thus contribute to a more robust and targeted arsenic mitigation approach. In the present study, a conceptual predictive geospatial model is proposed for the accumulation of dissolved arsenic as a function of interaction of oxbow-lake biogeochemistry and alluvial geomorphology. A comprehensive sampling campaign in and around two oxbow lakes in the Jamuna River Basin, West Bengal (India) provided water samples of the oxbow-lake water column for analysis of dissolved organic matter (DOM) and microbial communities, and groundwater samples from tube wells in point bars and fluvial levees bordering the oxbow lakes for analysis of the geospatial distribution of arsenic in the aquifer. Results show that abundant natural and anthropogenic (faecal-derived) recalcitrant organic matter like coprostanols and sterols in clay-plug sediment favours microbial (heterotrophs, enteric pathogens) metabolism and arsenic mobilization. Arsenic concentrations in the study area are highest (averaging 505 µg/L) in point-bar aquifers geomorphologically enclosed by partially sediment-filled oxbow lakes, and much lower (averaging 121 µg/L) in wells of levee sands beyond the oxbow-lake confinement. The differences reflect variations in groundwater recharge efficiency as result of the porosity and permeability anisotropy in the alluvial geomorphological elements, where arsenic-rich groundwater is trapped in point-bars enclosed by oxbow-lake clays and, by contrast, levee ridges are not confined on all sides, resulting in a more efficient aquifer flushing and decrease of arsenic concentrations.

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.

An advection–diffusion model of fluvial processes was used to analyze the stratigraphic expression of avulsions in terminal river systems and understand their control on basin-fill architecture. The initial and boundary conditions of the model runs (i.e., catchment area, smoothed initial topographic surface, grain-size distribution and sediment supply rates) were extracted from the modern Rio Colorado dryland terminal river system in the Altiplano Basin (Bolivia). Water-discharge and sediment-load values were derived from global regression curves and the BQART equation, respectively. To evaluate the robustness of the simulations, the model was tested under increasing sediment-load scenarios ranging from 0.003 m³/s to 0.095 m³/s. Data-model comparison provided insights into the role of avulsions in the geomorphological evolution of terminal river systems. The observed stacking of sediments, as captured by geospatial and geochronological data from the Rio Colorado, is consistent with the high sediment-load scenarios, which start with a single-thread fluvial channel that in time radially expands over the floodplain by successive river avulsions on account of alluvial-ridge aggradation and channel-floor elevation above the surrounding floodplain. The model output shows a laterally extensive, convex-upwards lobate topography which is in agreement with the lateral and longitudinal geomorphology in the upper and lower coastal plain of the Rio Colorado. The simulated inter-avulsion period, which is the time period between two successive full (or stabilized) avulsions in the model, varies from 0.18 to 1.2 kyr and is consistent with the OSL-age determination in the Rio Colorado with inter-avulsion periods up to 1.28 ± 0.34 kyr.

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.

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.

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.

The Ganges Delta is a key area where elemental contamination of groundwater constitutes a human catastrophe. The delta plain geomorphology comprises a large number of abandoned meander bends or oxbow lakes (Donselaar et al., 2017; Ghosh et al., 2021) characterized by an anoxic environment in the lower part of the lake water column (hypolimnion). Here we present the critical role of these abandoned-river channels forming oxbow lakes. The geomorphological the juxtaposition of (a) abandoned channels (or: oxbow lakes) where the cocktail of organic matter and sediment leads to the release of various elements, (b) the topographically higher point bars where the released elements accumulate in the aquifer and provide a blueprint to explain the origin and localization of elemental toxicity. Dissolved organic matter (DOM) is implicated in the mobilization of elements via microbial metabolic processes. Organic matter (OM) is preserved in this environment and provides a perfect environment for microbial oxidation and mobilization of Fe-oxides. Additional deposition of human-introduced sewage wastes adds to a rich source of nutrients to the indigenous microbial communities. A multidisciplinary approach was effective in understanding the geomorphology of river meanders, forming abandoned channels, which act as a growth bed for biomass. While acting as an incubator for primary production (lake vegetation dynamics), and subsequent organic debris accumulation (anoxic, hypolimnion water column), where selective preferential preservation of organic carbon compound (anoxic sediment base) occur. We have described how organic compound infiltration, deposition and abundance depends on their hydrophobicity, molecular weights and bioavailability and further, due to diagenetic alteration (microbial metabolic oxidation). Different classes of surface derived organic carbon from vegetation with anthropogenic inputs, can have different effects on the mineral weathering and in controlling the downstream cationic fluxes such as Fe, Mn, As, F etc. and contamination of aquifers in various river plains across the world.

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, based on geomorphological analysis of modern perennial braided rivers, 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 reservoir in the Southern Permian Basin (The Netherlands). The facies models, combined with the analyses of the rock record, provides a comprehensive depositional framework where 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 can be investigated. 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.

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 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.

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.