Andrea Moscariello

Tunnel valleys: current knowledge and future perspectives

Abstract Tunnel valleys are elongated incisions that are commonly interpreted as being the result of erosional processes by subglacial meltwater occurring under continental ice sheets. The abundance, size and the primarily coarse-grained infill of these features have made tunnel valleys important hydrocarbon and groundwater reservoirs. Although numerous tunnel valleys have been described over the last century, their formation and infill remain poorly understood. This review summarizes and discusses the current knowledge of tunnel valleys, providing an overview of the observations around the world. Morphological aspects that separate tunnel valleys from other landforms are discussed, as well as the wide variety of sedimentary environments found to contribute to the infilling of these features. The depth of the incision and the character of ice retreat significantly determine the final infill architecture. The formational hypotheses proposed in the literature are assessed to test their wider applicability to all other tunnel valleys in order to find a generic model that helps in the prediction of the morphology and infilling of both Pleistocene and pre-Pleistocene age. A quasi-steady-state model, with small meltwater outbursts that erode tunnel valleys near the ice margin, seems compatible with most of the known valleys. Other proposed models require specific geographical or climatic conditions.

Glaciogenic Reservoirs and Hydrocarbon Systems

Glaciogenic reservoirs and hydrocarbon systems occur intermittently throughout the stratigraphic record, with particular prominence in Neoproterozoic, Late Ordovician, Permo-Carboniferous and Late Cenozoic strata. Recent interest in glaciogenic successions has been fuelled by hydrocarbon discoveries in ancient glaciogenic reservoirs in North Africa, the Middle East, Australia and South America. Glaciogenic deposits of Pleistocene age are noteworthy for their content of groundwater onshore and potentially prospective and/or hazardous gas accumulations offshore. The abundant imprints of Pleistocene glaciations in both hemispheres can be used to reconstruct complex histories of repeated ice cover and retreat, and glacier-bed interactions, thus informing our view on the dynamics of older ice caps and predictions of future glaciations. This volume aims to provide a better understanding of glaciogenic processes, their stratigraphic record and reservoir characteristics of glaciogenic deposits. The book comprises 3 overview papers and 16 original case studies of Neoproterozoic to Pleistocene successions on 6 continents and will be of interest to sedimentologists, glaciologists, geophysicists, hydrologists and petroleum geologists alike.

The glaciogenic unconformity of the southern North Sea

Abstract The southern North Sea is a shallow epicontinental sea that was glaciated several times during the Quaternary. The area is known for its remarkable record of tunnel valleys, the age and origin of which are debated. The recent availability of continuous three-dimensional seismic data between the coasts of Britain and the Netherlands provides the opportunity to establish a new seismic interpretation workflow adapted to the intracratonic glaciogenic successions. By analysing the geomorphology of the buried basal glaciogenic unconformity, four distinct major ice fronts are identified and correlated onshore. The ice fronts provide robust relative timelines, and the analysis of tunnel-valley orientations and their merging points indicates that the number of glacial phases has been underestimated. By comparing the erosion capacities of sand and chalk substrates, it is suggested that mechanical abrasion processes are also involved during tunnel-valley genesis. The methods and observations used in this study are applicable to the ancient glaciogenic record in general and constitute a basis for the sedimentological analysis of tunnel valleys.

Late glacial and early Holocene palaeoenvironmental changes in Geneva Bay (Lake Geneva, Switzerland)

Abstract Detailed interdisciplinary investigations demonstrate that Geneva Bay (Lake Geneva) sediments clearly record important palaeoenvironmental and palaeoclimatic changes occurred during the Late glacial and early Holocene. Sediments are in fact differentiated by changes in texture, mineralogical and geochemical composition. Distal turbidite and glacial rhythmite deposition associated with wind-transported sediment supply dominate during the Oldest Dryas. These were replaced during the Allerod by detrital settling of sediment from turbid water and by endogenic calcite precipitation. The Younger Dryas climate reversal (for the first time in Lake Geneva well documented by a pollen record) was characterized by an increase in detrital supply owing to increased run-off from the bay slopes surrounding and within the catchment area, caused by thinning of vegetation cover. A brief pause in endogenic precipitation related to decreased productivity also occurs at this time. Endogenic carbonate sedimentation abruptly resumed at the start of the Preboreal biozone in response to the rapid global climatic warming. In the middle Preboreal, renewed detrital sedimentation is interpreted as the sedimentological response to increased erosion in high-altitude regions of the catchment area, caused by a minor early Holocene cooling phase (Preboreal oscillation), and interrupts the trend towards increasing endogenic calcite precipitation. Favourable limnic conditions are reached during the late Preboreal, when diatom–calcite rhythmites begin to form. Stable isotope analysis ( δ 18 O, δ 13 C) in bulk carbonate highlight the transition from clastic-dominated to endogenic-dominated sedimentation. Endogenic calcite deposition continues during the Boreal biozone, occasionally interrupted by local high-energy sedimentary processes (wave-induced erosion and reworking of littoral deposits) enhanced by progressive lake-level lowering. During the Older Atlantic biozone, sedimentation was mainly dominated by low-energy deposition, creating enhanced conditions for the development of benthic fauna. We will discuss the factors that make Geneva Bay an important site for recording the Late glacial–Holocene climate and environmental changes. The comparison with other Swiss Plateau lacustrine systems allowed us to emphasize the role played by the particular geographical and morphological setting of the site investigated and links with climate-sensitive regions in the catchment area.

Glaciogenic reservoirs and hydrocarbon systems: an introduction

Abstract Glaciogenic reservoirs host important hydrocarbon and groundwater resources across the globe. Their complexity and importance for exploration and palaeoclimate reconstruction have made glaciogenic successions popular subjects for study. In this paper we provide an overview of the palaeoclimatic and tectonic setting for Earth glaciation and a chronological account of glaciogenic deposits since c. 750 Ma, with particular emphasis on their reservoir potential and associated hydrocarbon systems. Hydrocarbon accumulations within glaciogenic reservoirs occur principally in Palaeozoic (Late Ordovician and Permo-Carboniferous) sandstones in South America, Australia, North Africa and the Middle East, with relatively minor occurrences of shallow gas hosted in Pleistocene deposits in the North Sea and Canada. Groundwater reserves occur within glaciogenic sandstones across the northern European lowland and in North America. The main glaciogenic environments range from subglacial to glacier front to proglacial and deglacial. Rapidly changing environments, hydrodynamic regimes and glacier-front and subglacial deformation often result in very complex glaciogenic sequences with significant challenges for reconstruction of their origin and resource importance, which this volume seeks to address.

ILLITE-SMECTITE-RICH CLAY PARAGENESES FROM QUATERNARY TUNNEL VALLEY SEDIMENTS OF THE DUTCH SOUTHERN NORTH SEA – MINERAL ORIGIN AND PALEOENVIRONMENT IMPLICATIONS

The Pleistocene sediment infill of elongated glacial incisions of the southern North Sea (SNS) is often referred to as tunnel valleys (TVs). The depositional environment is not yet fully understood and the present study addresses this challenge from the perspective of clay-mineral transformation (illite to illite-smectite) reported from the largest Elsterian TV of the SNS. Material acquired from the K14-12 borehole in the Dutch offshore was analyzed by X-ray diffraction (XRD), electron microscopy, electron microprobe, and laser particle-size analysis. Illite and illite-smectite (I-S) appeared as dominant clays along with minor amounts of kaolinite, kaolinite-smectite, and chlorite. The largest amount of I-S is recognized in the main TV portion, while in pre-glacial and uppermost deposits, I-S is less abundant. The XRD peak fitting and deconvolution suggest that I-S consists of several intermediates — ordered (well crystallized illite + R3 I-S) and disordered (R0 I-S + R0 I-SS). Given the average particle sizes (>2 µm) and Kübler index values (0.415–0.341°Δ2θ), illite as well as chlorite and kaolinite were interpreted as detrital. On the basis of the distinctive distribution, grain sizes, and compositional variations of I-S, formation by means of early diagenetic in situ smectitization of illite under a cold climate is proposed. The process operated via a series of mixed-layer intermediates derived from an illite component being converted progressively to low-charged smectite. The reaction is marked by a significant net loss of K and Al with replacement by Si in a tetrahedral coordination. Layer-charge imbalance is accommodated by Fe(III) and Mg entering an octahedral sheet, whereas Ca partly fills the interlayer sites. Smectitization rates were controlled by illite grain sizes. The results of the present study support strongly the existence of an ice-marginal freshwater depositional environment at the glacial maximum in the SNS in which early diagenesis at low temperatures resulted in incomplete conversion of illite to smectite.

The Glaciogenic Reservoir Analogue Studies Project (GRASP): An integrated approach to unravel genesis, infill and architecture of tunnel valleys reservoirs

Tunnel galleys are common features in Palaeozoic glacigenic succession in North Afrcica and Middle East and they are amongst the most challenging target for hydrocarbon exploration and developing drilling in these regions. Similarly, these buried valleys form important groundwater reservoirs in Quaternary glaciated areas and their nature and sediment composition is critical to drive a sustainable production strategy and assess their vulnerability. Seismic resolution however, often limits the understanding of channel valleys morphology, 3D geometry and internal reservoir distribution, thus increasing the risk associated with developing effectively these reservoirs. Therefore a analogue-based predictive stratigraphical and sedimentological model can help to steer drilling strategy and reduce uncertainties and associated risks. For this purpose the GRASP joint industry programme was established four years ago focusing on an integrated study of tunnel valley infill and architecture from the southern North Sea (UK and NL). The thorough examination of existing large subsurface and outcrop data set regarding the last 3 Pleistocene glaciations in NW Europe resulted in an improved understanding of tunnel valley genesis, sedimentary process and internal architecture. This learning can be used ultimately to improve our predictive capability when exploring and developing natural resources from these geological systems.

Dolomitization of the Upper Jurassic carbonate rocks in the Geneva Basin, Switzerland and France

The Upper Jurassic carbonates represent important potential targeted reservoirs for geothermal energy in the Geneva Basin (Switzerland and France). Horizons affected by dolomitization, the focus of the present study, are of particular interest because they proved to be productive in time-equivalent deposits currently exploited in Southern Germany. The study is based on sub-surface samples and outcrops in the Geneva Basin. Petrographic analyses allowed to constrain the paragenesis of the Upper Jurassic units prior to discussing the cause(s) and effect(s) of dolomitization. Data reveal that the facies are affected by early and late diagenesis. All samples show at least two stages of burial blocky calcite cementation with the exception of those from the sub-surface, which display an incomplete burial blocky cementation preserving primary intercrystalline porosity. Dolomitization affected all units. The results point to an early dolomitization event, under the form of replacement dolomite. Dedolomitization, through calcitization and/or dissolution, is an important process, creating secondary pore space. Results of the present study favor a reflux model for dolomitization rather than the mixing-zone model suggested in earlier work. However, considering the geodynamic context, other dolomitization models cannot be excluded for the subsurface. The presence of secondary pore space might contribute to the connectivity of the porous network providing enhanced reservoir properties. These results are a first step towards a better understanding of the diagenetic history of the Upper Jurassic in the Geneva Basin. Moreover, it provides a reasonable framework for further geochemical analyses to constrain the nature and timing of fluid migration. The paragenesis and the dolomitization model hold the potential to help in ongoing exploration for geothermal energy beyond the Geneva Basin.

Alluvial fans and fluvial fans at the margins of continental sedimentary basins: geomorphic and sedimentological distinction for geo-energy exploration and development

Abstract Alluvial fans and fluvial fans are the most common depositional landforms along the margins of continental sedimentary basins. Their occurrence is determined by the area, relief and hydrology of the catchment, which ultimately control the relative ratios of sediment v. flow discharge and runoff regimes. Fundamental morphological and process distinctions exist between these two kinds of deposystem, which are seen as essentially different facies associations and internal architectures for the corresponding deposits. Alluvial fans commonly develop over short radial distances (hundreds of metres to a few kilometres) and are constructed by ephemeral, flash flow events that lead to poor organization of the sedimentary facies and overall architecture of the corresponding deposits. By contrast, fluvial (mega)fans are fed by proper river systems, which aggrade much larger volumes of clastic sediment over distances of up to a few hundred kilometres. Distinct channel belt and overbank domains are developed with a marked heterogeneity in the distribution of sedimentary facies, represented by hierarchically well-organized fluvial deposits. The general properties of alluvial and fluvial fans reflect the different potentials for the corresponding successions to host economically attractive oil and gas resources and the different approaches required in exploration and prediction.

Seismic Geometry and Facies Analysis of a Quaternary Tunnel Glacial Valley Infill in the Dutch North Sea: Preliminary Results

We present a preliminary seismic interpretation of internal and external geometries for the terminal sector of the largest tunnel valley (TV) of the southern North Sea. Well logs and sediment cuttings are used to constrain the interpretation of different seismostratigraphic units. The valley floor undulations and grooves suggest that subglacial overpressured meltwater was the dominant valley genetic process, most likely associated with several phases of ice-sheet retreat and readvance. The geometries of the TV infill could be associated either with synglacial processes (e.g., backsets) or with postglacial fluvial sediments related to the drainage of a proglacial lake that developed during the Elsterian glacial maximum to the southwest of the ice-sheet termination.