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Erosional scars, slumps, slides and debrites (mass-transport deposits) on submarine slopes form relief that influences turbidity current behaviour. However, the interaction of mass-transport deposit emplacement kinematics (i.e. rapid emplacement versus creep), the morphology of the evolving seafloor topography and subsequent flow types is complicated. This study describes two outcrop examples of deep-water, predominantly turbiditic, deposits overlying mass-transport deposits, from the Eocene slope succession of the Aínsa Basin (Spanish Pyrenees). In both examples, the mass-transport deposit substrate continued to creep contemporaneously with turbidity current deposition and bypass. In the first case study, structures in the mass-transport deposits are extensional and oriented parallel to flow. In the second, structures are compressional and oriented perpendicular to flow. Mudstones dominate the slope succession, but deposits overlying mass-transport deposits form sandstone-prone accumulations. Lateral confinement by flow-parallel extensional faults enhanced channel-formation. Channel incision occurred close to the exhumed fault plane on the hangingwall. Incision ceased as the fault gradually locked-up, and channels avulsed to the hangingwall of a newly active normal fault, while the abandoned channel was filled by a thinning and fining-upward succession. Barriers formed where the long axes of compressional anticlines developed in mass-transport deposits formed perpendicular to flow. Here, turbidites filled the bathymetric lows in the axes of synclines. Continued tightening of synclinal depocentres led to repeated stoss side trapping and upslope accretion of coarse-grained sediment accumulation. This generated thickening and coarsening-up, followed by thinning and fining-upward successions, tracking the decreasing, then increasing flow bypass associated with topographic expression on the mass-transport deposit. This study shows how post-emplacement creep, and orientation of topographic features on mass-transport deposits, influence the routing and deposition of contemporaneous turbidity currents, and illustrates examples of facies successions that could be misinterpreted as the product of other autogenic submarine slope processes, such as lateral migration of sinuous channels, or compensational lobe stacking.

Determination of mineral texture and diagenetic features in mudstones is crucial to reveal the history of their pore systems and provides key information to predict their future sealing ability, reactivity and storage capacity for sequestered CO 2, hydrogen storage or nuclear waste disposal. To understand the spatial transport and storage of fluids, it is necessary to map the distribution of minerals and fractures in three dimensions (3D). This study proposes a novel, multi-scale three-dimensional (3D) imaging method, i.e., a combination of synchrotron- sourced micro- x-ray tomography and lab- sourced nano-tomography, to investigate the sedimentology and diagenetic features of the Bowland Shale, one of the most volumetrically important mudstone-dominated systems in the UK. Diagenetic minerals have been identified and characterised, including pyrite, calcite, kaolinite, illite, chlorite, dolomite, ankerite and authigenic quartz (micro-sized quartz and quartz overgrowths). Multi-scale 3D images provide detailed information about dolomite-ankerite zonation and carbonate dissolution pores. These features cannot be observed or quantified by conventional 2D methods, and they have not been reported in this subject area before. Using these results, potential reactions during carbon storage and other subsurface storage applications are predicted.

Mud dominates volumetrically the fraction of sediment delivered and deposited in deep-water environments, and mudstone is a major component of basin-floor successions. However, studies of basin-floor deposits have mainly focused on their proximal sandstone-prone part. A consequent bias therefore remains in the understanding of depositional processes and stratigraphic architecture in mudstone-prone distal settings beyond the sandstone pinchouts of basin-floor fans. This study uses macroscopic and microscopic descriptions of over 500 m of continuous cores from research boreholes from the Permian Skoorsteenberg Formation of the Karoo Basin, South Africa, to document the sedimentology, stratigraphy, and ichnology of a distal mudstone-prone basin-floor succession. Very thin- to thin-bedded mudstones, deposited by low-density turbidity currents, stack to form bedsets bounded by thin packages (< 0.7 m thick) of background mudstones. Genetically related bedsets stack to form bedset packages, which are bounded by thicker (> 0.7 m thick) background mudstones. Stratigraphic correlation between cores suggests that bedsets represent the distal fringes of submarine fan lobe elements and/or lobes, and bedset packages represent the distal fringes of lobe complexes and/or lobe complex sets. The internal stacking pattern of bedsets and bedset packages is highly variable vertically and laterally, which records dominantly autogenic processes (e.g., compensational stacking, avulsion of feeder channels). The background mudstones are characterized by remnant tractional structures and outsize particles, and are interpreted as deposited from low-density turbidity currents and debris flows before intense biogenic reworking. These observations challenge the idea that mud accumulates only from hemipelagic suspension fallout in distal basin-floor environments. Thin background mudstones separating bedsets (< 0.7 m thick) are interpreted to mainly represent autogenically driven lobe abandonment due to up-dip channel avulsion. The thicker background mudstones separating bedset packages (> 0.7 m thick) are interpreted to dominantly mark allogenically driven regional decrease of sand supply to the basin floor. The recognition of sandstone-prone basin-floor fans passing into genetically linked distal fringe mudstones suggests that submarine lobes are at least ∼ 20 km longer than previously estimated. This study provides sedimentological, stratigraphic, and ichnological criteria to differentiate mudstones deposited in different sub-environments in distal deep-water basin-floor settings, with implications for the accurate characterization of basin-floor fan architecture, and their use as archives of paleoenvironmental change.

Mineral authigenesis in mudstones responds differently across the compositional spectrum of fine-grained rocks, resulting in a complex array of possible rock fabrics that affect the mechanical capacity of mudstones. Within carbonate-rich mudstones, geochemical controls on silicate and carbonate mineral reactions are variable and poorly understood. Here we analyse the diagenetic minerals of the carbonate-rich Hodder Mudstone from the Carboniferous Bowland Basin, UK, using SEM petrography combined with X-ray-based inorganic geochemical data. Our findings show that up to 90% of quartz cements in the samples are diagenetic, originating mainly from biogenic silica dissolution and clay mineral reactions. It is also evident that due to the varying diagenetic silica-yielding reactions, authigenic silica overgrowths and isolated authigenic quartz crystals are localized in argillaceous samples, while calcareous samples are characterized by silica replacement textures and quartz/calcite intergrowths. Moreover, euhedral dolomite crystals are concentrated within argillaceous units, whereas calcareous units are characterized by anhedral dolomite precipitation and replacement textures. These finding presents a case for facies-selective cementation, as both early and burial diagenetic alterations were observed to be controlled by primary depositional components of biogenic debris and extrabasinal silicate detritus that resulted in complex and variable precipitation of authigenic minerals.

A novel type of burial dolomitization related to alteration of intermediate volcaniclastic rocks is discussed based on mineralogical, petrological and geochemical analyses. Lacustrine, fine-grained (

Microstructures and pore systems in shales are key to understanding the role of shale in many energy applications. This study proposes a novel multi-stage upscaling procedure to comprehensively investigate the heterogeneous and complex microstructures and pore systems in a laminated and microfractured shale, utilizing 3D multi-scale imaging data. Five imaging techniques were used for characterisation from sub-nanoscale to macroscale (core-scale), spanning four orders of magnitude. Image data collected using X-ray tomography, Focused Ion Beam, and Electron Tomography techniques range in voxel size from 0.6 nm to 13 μm.Prior to upscaling, a novel two-step analysis was performed to ensure sub-samples were representative. Following this, a three-step procedure, based on homogenising descriptors and computed volume coefficients, was used to upscale the quantified microstructure and pore system. At the highest resolution (nanoscale), four distinct pore types were identified. At the sub-micron scale equations were derived for three pore-associated phases. At the microscale, the volume coefficients were recalculated to upscale the pore system to the macroscale (millimetre). The accuracy of the upscaling methodology was verified, predicting the total porosity within 7.2% discrepancy. The results provide a unique perspective to understand heterogeneous rock types, breaking though prior scale limitations in the pore system.

Carbonate cementation of sandstones at channel bases is commonly interpreted to be a result of solutes generated from surrounding rocks in a burial-diagenetic regime. However, in rapidly buried and under-compacted conglomerates presented in this study, we interpret carbonate cements formed during early-diagenesis (1000–1600 m) to be precipitated directly from Ca- and CO2-rich saline formation water, derived from hypersaline lake water and driven by gravity and compaction. In the Bayindulan Sag of the Er'lian Basin, which is a continental rift basin, the sandstones and conglomerates of the A'ershan Formation (K1ba) were deposited in fan delta front settings during the syn-rift phase. Within these, tightly non-ferroan calcite-cemented conglomerates are present at the bases of distributary channels. In addition, some Fe-dolomite cements are present in sandstones with high present-day porosity, which are located in the upper parts of distributary channels. A petrographic analysis and fluid-inclusion microthermometry study show that the calcite cement predates the Fe-dolomite. Stable isotopic values indicate that both calcite and Fe-dolomite are characterized by enrichment of 13C and depletion of 18O (δ18OV-PDB: −15‰ to −8‰), and the value of δ13C V-PDB in calcite (+1.6‰ to +3.8‰) is less positive than that in Fe-dolomite (+3.1‰ to +8.4‰). Integrated analyses imply that the studied carbonate cements were probably formed within an open diagenetic system and were influenced by multiple diagenetic fluids. During deposition of the A'ershan Formation (K1ba), the lake water was brackish to hypersaline (Sr/Ba>1 and Sr content > 300 ppm). During deposition of the Tenggeer Formation (K1bt), the K1ba progressed rapidly into a deep burial phase (90 m/Ma), diagenetic fluids were controlled by hypersaline lake water, incursion of meteoric water occurred along faults, and methanogenic fermentation. Calcium was probably derived from the saline formation water and the transformation of smectite to illite in associated mudstones, leading to the precipitation of calcite (54–78 °C). In the Saihantala period, the K1ba experienced a slight uplift, the fault activity ceased, and the diagenetic fluids were derived from the saline formation water and were influenced by methanogenic fermentation, and Fe-dolomite began to precipitate. In deeper burial detrital feldspar grains and some calcite cement were probably dissolved by organic acid discharged from mudstones. In conclusion, the early diagenetic characteristics in the continental rift basins were controlled by the composition of the depositional waters and tectonics that controlled the opening and closing the diagenetic system thus influencing fluid composition.

Deep-water mudstones are often considered as background sediments, deposited by vertical suspension fallout, and the range of transport and depositional processes are poorly understood compared to their shallow-marine counterparts. Here, we present a dataset from a 538.50 m-thick coredsuccession through the Permian muddy lower Ecca Group of the Tanqua depocentre (southwest Karoo Basin, South Africa). This study aims to characterize the range of mudstone facies, transport and depositional processes, and stacking patterns recorded in deep-water environments prior to deposition of the Tanqua Karoo sandy basin-floor fans. A combination of macroscopic and microscopic description techniques and ichnological analysis has defined nine sedimentary facies that stack in a repeated pattern to produce 2–26-m-thick depositional units. The lower part of each unit is characterized by bedded mudstone deposited by dilute, low-density turbidity currents with evidence for hyperpycnal-flowprocesses and sediment remobilization. The upper part of each unit is dominated by more organic-rich bedded mudstone with common mudstone intraclasts, deposited by debris flows and transitional flows, with scarce indicators of suspension fallout. The intensity of bioturbation and burrow size increases upward through each depositional unit, consistent with a decrease in physicochemically stressed conditions, linked to a lower sediment accumulation rate. This vertical facies transition in the single well dataset can be interpreted to represent relative sea level variations, where the hyperpycnal stressed conditions in the lower part of the units were driven by sea level fall and the more bioturbated upper part of the unit represents backstepping, related to sea level rise. Alternatively, this facies transition may represent autogenic compensational stacking. The prevalence of sediment density flow deposits, even in positions distal or lateral to the sediment entry point, challenges the idea that deep-water mudstones are primarily the deposits of passive rainout along continental margins.

The Cenomanian-Turonian Oceanic Anoxic Event (OAE2) is a significant global event that has been linked to major source rock development in marine environments. The aim of this study is to integrate biostratigraphic, sedimentological and geochemical data (including stable carbon isotopes and trace elements) to document the timing and controls of organic matter enrichment across the Cenomanian-Turonian boundary.Morocco has excellent exposures of Tethyan carbonate platform and basinal facies and this paper details a high-resolution analysis of organic-rich Late Cretaceous intervals exposed in the Errachidia-Goulmima Basin, to investigate their relationship to the OAE2. Integrating planktic foraminiferal and ammonite biostratigraphy with carbon isotope correlations indicate that organic-rich black mudstones identified were deposited from latest Cenomanian to early Turonian, suggesting a late-OAE2 to post-OAE2 age.The sedimentology, redox water conditions and palaeoproductivity of the palaeoenvironments were studied to characterise the lithofacies and geochemical signatures, and to analyse the controlling factors for organic–enrichment. In the study area during the lower OAE2 interval, a shallow water mid-ramp environment prevailed, with predominantly bivalve-rich limestone deposition. Subsequent organic-rich mudstone development can be correlated with the latest Cenomanian (late OAE2 interval) and early Turonian (post-OAE interval) transgression, which allowed development of anoxic to euxinic conditions in the outer-ramp environment, characterized by high TOC values, from 2.7 to 17 wt%. Shallow platform locations continued to experience limestone deposition with no organic enrichment.Trace and major element data from the C/T black mudstones show an extremely high concentration in palaeoproductivity-sensitive (P, Zn, Ni and Cu) and redox-sensitive elements (V, Mo, and U). The redox environments evolved from dysoxia to anoxia and sulfidic euxinia, which is interpreted to have provided optimum conditions for organic carbon preservation in the Errachidia area.The results highlight the important interplay of local paleogeography and eustatic sea-level (the late Cenomanian/early Turonian transgressions) as the main controls on organic mudstone deposition, rather than simply global OAE2 control. The development of organic-rich mudstones reflects both increased productivity at this time and enhanced organic matter preservation due to the change in redox bottom water conditions.

Incorporation of reduced inorganic sulfur (S) into organic matter (OM) in euxinic environments is considered a vital route through which OM is preserved for millions of years. However, precipitation of iron sulfides is believed to compete with OM preservation via sulfurisation. High resolution geochemical, petrographic, and electron microprobe (EMPA) analyses were employed to better understand S controls on OM preservation with particular focus on iron sulfides precipitation in organic S-rich mudstones from the Kimmeridge Clay (KCF), Monterey (MF), and Whitby Mudstone [Grey Shale (GS) and Jet Rock (JR) Members] Formations. Analyses indicated that S in the KCF and MF is mostly bound to OM as shown by the relatively high abundance of organic sulfur compounds (OSCs) in their pyrolysates. The relatively low amounts of iron (Fe) in these mudstones permitted extensive reaction of S and OM to occur efficiently, promoting preservation of high amounts OM through sulfurisation. State-of-the-art EMPA imaging of total S and Fe indicated that the S was mainly bound to organics, supporting the geochemical analyses. In contrast, S in the GS and JR was predominantly sequestered as pyrite (FeS2) rather than organically bound, as indicated by the relatively low quantity/absence of OSCs in their pyrolysates, hence, preservation through sulfurisation was less significant in these settings. EMPA imaging of S in these mudstones shows discrete shapes ranging in geometry and size and in most cases correlated with Fe in the corresponding Fe EMPA images, confirming that the S exists in discrete authigenic FeS2. Overall, in line with previous research, this study indicates that preservation of OM through sulfurisation is an important pathway for sequestration of OM. However, an abundance of reactive Fe could hinder this process by preferentially reacting with reduced S, to form iron sulfides (mainly FeS2), thus impeding reactions between OM and S to occur efficiently.

The Bell Island and Wabana groups together comprise a ca. 150-m-thick succession of interbedded clastic andchemical sedimentary rocks composed of eight distinct lithofacies that accumulated along the northern marginof the Rheic Ocean. Lithofacies stacking patterns indicate that deposition occurred during a marine transgressionwith superimposed small-scale sea level fluctuations producing at least six parasequences. Parasequences containingironstone are 10 to 20-m-thick and composed of hummocky cross-stratified sandstone interbeddedwith organic-rich mudstone and phosphatic Fe-silicate-bearing siltstone,which is overlain by hematitic granularironstone capped by an erosive flooding surface.This lithofacies association is interpreted to record the deposition of upwelling-related ironstone on a stormdominatedshelf. The close relationship between Fe-silicates and phosphorite typical of upwelling systemssuggests that Fe was delivered from deep, anoxic, nutrient-rich seawater that also stimulated high surface productivities.The result was the precipitation of authigenic sedimentary apatite in anoxic organic-rich sedimentsthat accumulated near the upwelling front. The gradual advection of Fe-rich waters away from the upwellingfront, initiated precipitation of Fe-silicate coated grains and cements in suboxic pore-waters. Iron pumpedinto shallower environments through advection and Fe-redox cycling is interpreted to have precipitated Fe-(oxyhydr)oxide grains in sediment of the oxygenated middle shelf. These coated grains were subsequentlyconcentrated by fairweather and storm currents on the shoreface to create granular economic Fe deposits.We challenge conventional models of Paleozoic ironstone deposition that rely on a continental source of Fe byproposing a hydrothermal source that supplied Fe2+ to the shelf through upwelling. It also highlights the potentialconnection between the delivery of anoxic, ferruginous seawater to the margins of the Rheic Ocean and theEarly Ordovician extinctions that punctuated the beginning of the Great Ordovician Biodiversification Event.

An appreciation of the diagenetic processes impacting basinal mudstones is important for understanding the properties of mudstones and the nature of fluids which might have been expelled onto adjacent carbonate platforms. This study utilizes petrographic and mineralogical data from mudstones from the Serpukhovian Morridge Formation in the Widmerpool Gulf, UK to investigate the paragenetic succession and associated mineral precipitation during diagenesis of mudstones. Authigenic minerals in the studied samples contain kaolinite, calcite, non-ferroan and ferroan dolomite, pyrite, ankerite, and quartz. The Si/Al ratios in most samples are above 2.5, indicating that most of the studied samples contain excess silica. The widespread presence of authigenic kaolinite and quartz provides important information on the sources for dissolved silica in pore water. The presence of pyritized radiolarians indicates that the dissolution of siliceous radiolarian skeletons is the most likely principle silica source within the system. The illitization of kaolinite also provided a possible silica source for quartz overgrowth during burial diagenesis. It has been long-proposed that the supply of base metals (i.e. Ba, Zn and Mg) for mineralization on the adjacent Derbyshire Platform came from juxtaposed basins, but evidence for this has been limited. The results of this study show that the diagenetic evolution of mudstones in the late Carboniferous links closely to the mineralization observed on the Derbyshire Platform. In the Widmerpool Gulf, the source of magnesium incorporated into dolomite is likely from basinal brine that also provided a source of barium for the barite mineralization. Based on the burial history model, the studied samples were buried to 1.3 km with a temperature of 82 °C in the late Carboniferous. Hydrocarbon was generated during maturation of organic matter and migrated with basinal brines through factures into surrounding formations during basin inversion. This mixed fluid likely contributed to the mineralization on the Derbyshire Platform.

Atomic force microscopy (AFM) has been widely used in materials science and chemistry research since it was invented in the 1980s. It is a relatively new tool for geosciences and is very powerful for micro- and nano-scale surface characterization with a resolution down to atomic scale. One of the major advantages of this microscopy technique is that it can perform continuous non-destructive measurements on a sample surface. Surface studies provide important structural information for geological materials, especially in energy geosciences. In this review, recent applications of AFM in energy geosciences are critically reviewed, including surface topography, mechanical properties, surface dynamics, and state-of-the-art AFM-based infrared spectroscopy (AFM-IR) and the AFM-Raman technique. AFM imaging enables the mapping and recording of surface information and enhances the understanding of morphology and mechanical properties of geological materials, and is especially useful for those displaying a strong heterogeneity. This review aims to demonstrate the accessibility, versatility, limitations, and promising developments of AFM techniques in energy geoscience. We highlight the major advantages and disadvantages of AFM analysis of geological materials and highlight where factors can cause potential deviations between different experimental results. We further highlight where we believe are the most promising potential uses of AFM. It is foreseen that with the improvement of experimental processes and hardware, AFM will provide indispensable information on surface characterization in a wide range of geological studies.

The development of pore and fracture networks at the nano-scale as a response to heating can reveal coupled physical relationships relevant to several energy applications. A combination of time-lapse 3D imaging and finite-element modelling was performed on two typical thermally immature shale samples, Kimmeridge Clay and Akrabou shale, to investigate thermal response at the nm-scale for the first time. Samples were imaged using Transmission X-ray Microscopy (TXM) with a voxel resolution of 34 nm at the I13–2 beamline at Diamond Light source, UK. Images were taken after heating to temperatures of 20 °C, 300 °C, 350 °C and 400 °C. The initiation of nano-pores within individual minerals and organic matter particles were observed and quantified alongside the evolution from nano-pores to micro-fractures. The major expansion of pore-volume occurred between 300 and 350 °C in both samples, with the pores elongating rapidly along the organic-rich bedding. The internal pressures induced by organic matter transformation influenced the development of microfractures. Mechanical properties and strain distributions within these two samples were modelled under a range of axial stresses using Finite-element modelling (FEM). The results show that the overall stiffness of the shale reduced during heating, despite organic matter becoming stiffer. The varied roles of ductile (e.g., clay minerals, organic matter) and brittle materials (e.g., calcite, pyrite) within the rock matrix are also modelled and discussed. The configurations of organic matter, mineral components, porosity and connectivity impact elastic deformation during shale pyrolysis. This work extends our understanding of dynamic coupled processes of microstructure and elastic deformation in shales to the nm-scale, which also has applications to other subsurface energy systems such as carbon sequestration, geothermal and nuclear waste disposal.

Analyzing the development of fracture networks in shale is important to understand both hydrocarbon migration pathways within and from source rocks and the effectiveness of hydraulic stimulation upon shale reservoirs. Here we use time-resolved synchrotron X-ray tomography to quantify in four dimensions (3-D plus time) the development of fractures during the accelerated maturation of an organic-rich mudstone (the UK Kimmeridge Clay), with the aim of determining the nature and timing of crack initiation. Electron microscopy (EM, both scanning backscattered and energy dispersive) was used to correlatively characterize the microstructure of the sample preheating and postheating. The tomographic data were analyzed by using digital volume correlation (DVC) to measure the three-dimensional displacements between subsequent time/heating steps allowing the strain fields surrounding each crack to be calculated, enabling crack opening modes to be determined. Quantification of the strain eigenvectors just before crack propagation suggests that the main mode driving crack initiation is the opening displacement perpendicular to the bedding, mode I. Further, detailed investigation of the DVC measured strain evolution revealed the complex interaction of the laminar clay matrix and the maximum principal strain on incipient crack nucleation. Full field DVC also allowed accurate calculation of the coefficients of thermal expansion (8 × 10−5/°C perpendicular and 6.2 × 10−5/°C parallel to the bedding plane). These results demonstrate how correlative imaging (using synchrotron tomography, DVC, and EM) can be used to elucidate the influence of shale microstructure on its anisotropic mechanical behavior.

Silica diagenesis has the potential to drastically change the physical and fluid flow properties of its host strata and therefore plays a key role in the development of sedimentary basins. The specific processes involved in silica diagenesis are, however, still poorly explained by existing models. This knowledge gap is addressed by investigating the effect of silica diagenesis on the porosity of Cenozoic mudstones of the North Viking Graben, northern North Sea through a multiple linear regression analysis. First, we identify and quantify the mineralogy of these rocks by scanning electron microscopy and X-ray diffraction, respectively. Mineral contents and host rock porosity data inferred from wireline data of two exploration wells are then analyzed by multiple linear regressions. This robust statistical analysis reveals that biogenic opal-A is a significant control and authigenic opal-CT is a minor influence on the porosity of these rocks. These results suggest that the initial porosity of siliceous mudstones increases with biogenic opal-A production during deposition and that the porosity reduction during opal-A/CT transformation results from opal-A dissolution. These findings advance our understanding of compaction, dewatering, and lithification of siliceous sediments and rocks. Moreover, this study provides a recipe for the derivation of the key controls (e.g., composition) on a rock property (e.g., porosity) that can be applied to a variety of problems in rock physics.

Organofacies analysis, a fundamental component within source rock appraisal based on the study of kerogen within a source rock, is typically produced from microscopy (palynological) and geochemical (kerogen kinetic) data, both of which are costly to acquire. One-Run-Fixed-Arrhenius (ORFA) kerogen kinetic analysis based on Rock–Eval pyrolysis offers a substantially cheaper kinetic dataset. Here, ORFA and palynological analyses are compared in organofacies characterization of a prospective Mississippian source rock reservoir (Bowland Shale, UK). Two-end-member organofacies were determined based on the abundance of the 56 kcal/mol activation energy peak derived from ORFA data: absence (< 5%) indicating ‘organofacies A’ containing the highest proportion of algal material (Type I kerogen); and presence (> 15%) indicating ‘organofacies B’ containing the highest proportion of sporomorphs (Type II kerogen). A mud-dominated slope setting for the rock reservoir was also used to test the accuracy of organofacies analysis in determining depositional environment. Organofacies A found within lithofacies deposited from dilute waning density flows and hemipelagic suspension settling occurred between shelf edge, slope and basin. Organofacies B found within lithofacies deposited from dilute waning density flows, and low-strength cohesive debrites occurred only within the lower slope. This study demonstrates that ORFA kerogen kinetic analysis provides comparable net results to palynological analysis, enabling cheaper and faster organic characterization during initial source rock appraisal. However, caution must be exercised in drawing interpretations as to biological source(s), organic matter mixing and preservation state(s) without additional investigation using data from detailed palynological analysis.

Here we describe the first automated fully integrated in-microscope broad ion beam (BIB) system. Ar+-BIB has several advantages over Ga+ focused ion beam (FIB) and Xe+ plasma-FIB (PFIB) methods inducing less beam damage, especially for ion beam sensitive materials. It can mill areas several orders of magnitude larger (up to millimetre scale), and is not confined to the edge of the sample with associated curtaining issues. BIB is shown to have sputter rates up to five times higher than comparable FIB techniques. This new coupled BIB–SEM system (commercial name ‘iPrep™II’) enables in-microscope surface polishing to remove contaminants or damage for two dimensional (2D) imaging, as well as automated serial section tomography (SST) by milling and imaging hundreds of slices, cost and time efficiently. The milled slice thickness can be controlled from a few nanometers up to a micrometre. A novel sample transfer, handling and interlock system allows automated and sequential BIB polishing, scanning electron microscopy (SEM) and analysis by secondary electron (SE) imaging, electron back scatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) for 3D microstructure analysis. Furthermore, insulating surfaces can be sputter coated after milling each slice to reduce charging during SEM analysis. The performance of the instrument is demonstrated through a series of case studies across the materials, earth and life sciences exploiting the imaging, crystallographic and chemical mapping capabilities. These include the study of butterfly defects in bearing steels, meta-stable intermetallic phases in bronze bearings, shale gas rock, aluminium plasma electrolytic oxide (PEO) coatings as well as liver and mouse brain tissues.

Despite recent advances, diagenetic processes in fine-grained sediments are still relatively poorly understood. Key questions still to be resolved include the types of diagenetic minerals present in mudstones and the extent of element mobility in these low permeability systems. This study utilizes data from the Haynesville-Bossier Shale, USA, to analyze lithologies, discriminate authigenic phase, and identify mobile elements during diagenesis. It has implications for understanding how authigenic minerals develop and the sources of those authigenic minerals in fine-grained sediments.On the basis of grain-size and mineralogy five lithologies are designated: (i) silica-rich argillaceous mudstones; (ii) argillaceous siliceous mudstones; (iii) mixed siliceous mudstones; (iv) mixed mudstones; and (v) authigenically-dominated mudstones. The diagenetic development of the Haynesville-Bossier Shale can be divided in to early and late diagenesis. Ferroan and non-ferroan dolomite, framboidal pyrite and bioclast pore-filling kaolinite and calcite grain replacements and cements all formed during early diagenesis. Late diagenetic minerals phases include illite formed by the illitisation of smectite, replacive and displacive chlorite, calcite-replacive albite, quartz-replacive calcite and replacive and/or displacive quartz.The presence of extensive late diagenetic mineral precipitates indicates that there was a degree of element mobility on at least the local scale. Aluminium present in albite is most likely to have resulted from the illitization of smectite. Quartz overgrowths probably resulted from illitization and the pressure dissolution at quartz silt grain boundaries. Externally, hydrothermal fluids resulting from regional-scale igneous activity appear to have played a role in the formation of chlorite and possibly albite. The work indicates that extensive mineral development and element mobility occurred during late diagenesis.

As the fastest growing energy sector globally, shale and shale reservoirs have attracted the attention of both industry and scholars. However, the strong heterogeneity at different scales and the extremely fine-grained nature of shales makes macroscopic and microscopic characterization highly challenging. Recent advances in imaging techniques have provided many novel characterization opportunities of shale components and microstructures at multiple scales. Correlative imaging, where multiple techniques are combined, is playing an increasingly important role in the imaging and quantification of shale microstructures (e.g. one can combine optical microscopy, scanning electron microscopy/transmission electron microscopy and X-ray radiography in 2D, or X-ray computed tomography and electron microscopy in 3D). Combined utilization of these techniques can characterize the heterogeneity of shale microstructures over a large range of scales, from macroscale to nanoscale (c. 100–10−9 m). Other chemical and physical measurements can be correlated to imaging techniques to provide complementary information for minerals, organic matter and pores. These imaging techniques and subsequent quantification methods are critically reviewed to provide an overview of the correlative imaging workflow. Applications of the above techniques for imaging particular features in different shales are demonstrated, and key limitations and benefits summarized. Current challenges and future perspectives in shale imaging techniques and their applications are discussed.

Biogenic CH4 generation has been observed in many shallow, low temperature shale gas basins and coal seams. The depletion of conventional resources and the increasing demand of natural gas for human consumption have spurred the development of so-called unconventional gas resources such as shale gas (SG) and coal-bed methane (CBM). Such unconventional systems represent the opportunity for the stimulation of biogenic CH4 generation. Biogenic CH4 in shale and coal is produced by anaerobic biodegradation of organic matter (OM): methanogenic Archaea represent only the final step of biogenic CH4 generation. Several communities of microorganisms are involved in the initial breakdown of complex geopolymers and the production of intermediate compounds used by methanogens. There are several key knowledge gaps on biogenic CH4 production in unconventional gas systems, such as the exact fraction of bioavailable OM, the microbial communities involved and how they can be stimulated to enhance microbial methanogenesis. Progress on biodegradation studies, isotopic signatures, as well as DNA analyses and proteomics could help unravel interactions within the syntrophic community involved in the methanogenic biodegradation of OM. Questions also remain regarding the environmental impact of unconventional gas production, such as water quality and the mobility of toxic metals and radionuclides. The answers to these questions might have implications for both recovery practices and a sustainable development of unconventional resources. This review summarises the current knowledge regarding biogenic CH4 in SG and CBM: from the nature of the rocks to the producing microbial community and the indicators of biogenic CH4, illustrating how these two environments show remarkably similar opportunities for the stimulation of biogenic CH4 generation.

Silica cementation exerts a key control on the compaction and geotechnical properties of mudstones, and byextension, the style of hydrocarbon and/or mineral systems present in a given sedimentary basin. Integratedmicroscopic and bulk geochemical observations demonstrate that siliceous mudstones in the Bowland ShaleFormation, a target for UK shale gas extraction, exhibit abundant dispersed, discrete, μm-scale quartz cements,and exhibit silica enrichment (‘excess’) above a local detrital Si/Al threshold of 2.5. Dissolution of siliceousradiolarian tests during early diagenesis is identified as the main source of silica (opal A) required for quartzprecipitation, either via opal CT or directly to quartz, and potentially generated as a product of anoxic marine‘weathering’ (dissolution) of reactive silicates during early diagenesis. Excess silica correlates with free hydrocarbons(S1) normalised to total organic carbon (oil saturation index; OSI); we propose early diagenetic quartzprecipitation suppressed pore collapse (‘buttress effect’), retaining the pore space capacity to host oil. Quartzprecipitation was likely catalysed, for example via low porewater pH, elevated Al and/or Fe oxide content, and/or abundant labile organic matter. Juxtaposition of siliceous mudstones and mudstones lacking quartz cementindicates silica was immobile beyond the bed scale. Thus metre-scale siliceous packages likely represent moreprospective units within the Bowland Shale (in terms of unconventional hydrocarbons), on the basis of earlydiagenetic biogenic-derived quartz cementation leading to improved hydrocarbon storage capacity coupled toenhanced brittleness. These findings are relevant for shale oil and shale gas systems, specifically where oilretained in pores subsequently cracks to generate gas. These findings also suggest the Bowland Shale is a subclassof black shale, defined by the potential to host a relatively large volume of early diagenetic fluids,derived from anoxic bottom waters, which were potentially S- and/or metal-bearing. This is potentially relevantfor understanding the genesis of adjacent and related Pb-Zn mineral deposits.

Summary Despite recent advances, diagenetic processes in fine-grained sediments remain relatively poorly understood. Key questions still to be resolved include how mineral cements develop in low porosity and permeability systems and the extent of element mobility during the diagenesis of fine-grained sediments. This study uses petrographic, mineralogical and geochemical data from the Haynesville-Bossier Shale identify early and late diagenetic mineral development. On the basis of grainsize and mineralogy five facies are designated: (i) silica-rich argillaceous mudstones, (ii) argillaceous siliceous mudstones, (iii) mixed siliceous mudstones, (iv) mixed mudstones and (v) cemented mudstones. The diagenetic development of the Haynesville-Bossier Shale can be divided in to early and late diagenesis. Ferroan and non-ferroan dolomite, framboidal pyrite, and microfossil pore-filling kaolinite cements all formed during early diagenesis. Late diagenetic cements include microcrystalline calcite, euhedral pyrite, replacive and displacive chlorite, calcite-replacive albite, and replacive and/or displacive quartz. Late diagenetic cements would have required a wide range of geochemically mobile elements, (bed scale); illitisation of smectite during late diagenesis is a potential source for these elements. The work impacts porosity and permeability evolution and fracture susceptibility in unconventional shales.

The Lower Permian Collyhurst Sandstone Formation is an important gas reservoir both offshore and onshore in the East Irish Sea Basin of north-western UK, and is considered to be a tight gas reservoir. Diagenetic processes have significantly influenced the reservoir quality of the sandstone. This study employs petrographic, mineralogical, and stable isotopic analyses on two different onshore core successions to investigate the composition and texture, the diagenetic history, and the impact of lithology and diagenesis on reservoir quality. The Collyhurst Sandstone Formation consists of fine- to very coarse-grained, poorly to well-sorted, sub-arkosic to lithic arenite sands. The reservoir properties of the sandstone are relatively poor, with average porosity and permeability of 6.6% and 0.387 mD respectively. On the basis of petrographic analyses, five sandstone lithofacies were identified: fine-grained, conglomeratic, carbonate-cemented, quartz-cemented and matrix-free. Contrasting characteristics between the reservoir units suggest the development of divergent diagenetic pathways. Features observed during early diagenesis include compaction, growth of pore-filling early carbonate cements (calcite I, dolomite and siderite I) and vermicular kaolinite. Late diagenetic features include: further carbonate cementations (ferroan dolomite/ankerite, calcite II and siderite II-III), clay mineral cementation (illite and chlorite) with both grain-coating and pore-filling textures. Also common in late diagenesis are syntaxial quartz overgrowths. Albite, barite and anhydrite are minor late diagenetic cement phases. Compaction processes rather than cementation accounts for the major destruction of the initial sandstone porosity. But cementation still accounts for up to 32% of porosity loss. Carbonate cements mostly fill intergranular pores, thus block pore throat and prevent migration of fluid. Clay minerals bridge and occlude pores, resulting in further destruction of the reservoir quality. The study provides insights into how lithofacies (primary composition, texture and sorting) and diagenesis can influence reservoir qualities of tight gas sandstones.

The microstructure of a highly laminated Lower Bowland Shale sample is characterized at the micron-to millimeter scale, to investigate how such characterization can be utilized for microstructure-based modelling of the shale's geomechanical behavior. A mosaic of scanning electron microscope (SEM) back-scattered electron (BSE) images was studied. Mineral and organic content and their anisotropy vary between laminae, with a high variability in fracturing and multi-micrometer aggregates of feldspars, carbonates, quartz and organics. The different microstructural interface types and heterogeneities were located and quantified, demonstrating the microstructural complexity of the Bowland Shale, and defining possible pathways for fracture propagation. A combination of counting-box, dispersion, covariance and 2D mapping approaches were used to determine that the total surface of each lamina is 3 to 11 times larger than the scale of heterogeneities relative to mineral proportion and size. The dispersion approach seems to be the preferential technique for determining the representative elementary area (REA) of phase area fraction for these highly heterogeneous large samples, supported by 2D quantitative mapping of the same parameter. Representative microstructural models were developed using Voronoï tessellation using these characteristic scales. These models encapsulate the microstructural features required to simulate fluid flow through these porous Bowland Shales at the mesoscale.

An understanding of the nature and scales of diagenetic variability within organic-rich mudstones is critical to the accurate assessment of shale-gas reservoir properties, as well as for elucidating chemical evolution pathways within mudstones. Here we integrate field observations with thin section descriptions (optical and electron optical techniques) and mineralogical data for the Blackhawk Member time-equivalent Mancos Shale in Book Cliffs, Utah, to determine the impacts of early and burial diagenesis on this mudstone succession.The detrital assemblage in the Mancos Shale comprises quartz-silt, feldspar, clay minerals, dolomite and organic matter (TOC of 1 to 2.5%). Biogenic silica is negligible. Field mapping reveals laterally continuous (km scale), ferroan dolomite cemented units up to 0.3 m thick, are present. These cemented units cap both coarsening-upward units (1 to 3 m thick), and stacked successions of coarsening-upward units (5 to 15 m thick). These upward-coarsening sediment packages, capped by dolomite cemented strata, correlate to bedsets and parasequences in updip settings. Pervasive cementation in these dolomite-cemented units is likely to have occurred prior to compaction as a result of bacterially mediated respiratory processes. Cementation at these levels is particularly evident because cement precipitation occurred during breaks in sediment accumulation below marine flooding surfaces. The abundance of dolomite cements highlights the importance of macroscopic-scale diagenetic carbonate mobility in these mudstones.In addition to carbonate-cements, diagenetic alteration and precipitation of quartz and alumina-silicate minerals are also important in these mudstones. Kaolinite is present both in uncompacted test of organisms and as vein fills in septarian concretions. Kaolinite precipitation is interpreted to have occurred prior to significant compaction and indicates that both silicon and aluminium were mobile during early diagenesis. We interpret the abundance of early diagenetic kaolinite cement to be the result of Al-mobilization by organic acids generated during organic matter oxidation reactions, with the Al sourced from poorly crystalline detrital aluminium oxides and clay minerals. There is also indirect evidence for burial diagenetic kaolinitization of feldspar grains. Quartz cement takes the form of quartz overgrowths and microcrystalline quartz crystals. Textures and CL spectra for the quartz microcrystalline cement suggests that recrystallization of biogenic silica (opal-A) was likely to have been an important source for quartz cements, although smectite-to-illite transformation may have contributed some. These mineral phases highlight that microscopic-scale diagenetic mobility of silica is important, even within mudstones lacking obvious sources of biogenic silica and is likely to be an important processes in a wide range of mudstones.

Diagenesis significantly impacts mudstone lithofacies. Processes operating to control diagenetic pathways in mudstones are poorly known compared to analogous processes occurring in other sedimentary rocks. Selected organic-carbon-rich mudstones, from the Kimmeridge Clay and Monterey Formations, have been investigated to determine how varying starting compositions influence diagenesis. The sampled Kimmeridge Clay Formation mudstones are organized into thin homogenous beds, composed mainly of siliciclastic detritus, with some constituents derived from water-column production (e.g., coccoliths, S-depleted type-II kerogen, as much as 52.6% total organic carbon [TOC]) and others from diagenesis (e.g., pyrite, carbonate, and kaolinite). The sampled Monterey Formation mudstones are organized into thin beds that exhibit pelleted wavy lamination, and are predominantly composed of production-derived components including diatoms, coccoliths, and foraminifera, in addition to type-IIS kerogen (as much as 16.5% TOC), and apatite and silica cements. During early burial of the studied Kimmeridge Clay Formation mudstones, the availability of detrital Fe(III) and reactive clay minerals caused carbonate- and silicate-buffering reactions to operate effectively and the pore waters to be Fe(II) rich. These conditions led to pyrite, iron-poor carbonates, and kaolinite cements precipitating, preserved organic carbon being S-depleted, and sweet hydrocarbons being generated. In contrast, during the diagenesis of the sampled Monterey Formation mudstones, sulfide oxidation, coupled with opal dissolution and the reduced availability of both Fe(III) and reactive siliciclastic detritus, meant that the pore waters were poorly buffered and locally acidic. These conditions resulted in local carbonate dissolution, apatite and silica cements precipitation, natural kerogen sulfurization, and sour hydrocarbons generation. Differences in mud composition at deposition significantly influence subsequent diagenesis. These differences impact their source rock attributes and mechanical properties.

Pore systems of tight-gas reservoirs control important parameters such as flow capacity, producible pore volumes, hydrocarbon flow rates and storage which are complex to characterise and to quantify. Detailed understanding of 3D pore structure is vital in predicting tight-gas reservoir productivity. This study investigates the pore architecture of a sandstone sample from Collyhurst Sandstone Formation, northern England, a tight gas reservoir. X-ray Computed Tomography (XCT) was employed to quantify the pore system (pore size, shape, geometry, and connectivity). Three types of pores were identified: inter-grain pores which are at the boundary of detrital grains and/or authigenic minerals, intra-grains pores which are pores within the detrital mineral grains, and intra-cement pores which are pores within the cement phase (authigenic minerals). The first pore type is primary in origin, whereas the latter two are secondary pores. A centerline tree and a pore network model (PNM) algorithm were applied for pore network analysis and a porosity value was estimated. The image-based porosity value was validated with laboratory experimental measurement. 3D image estimates 3.1% volume porosity, while 4.6% was recorded from the laboratory experiment. Spatial quantification of pore types and analysis of pore connectivity of the studied sandstone improves understanding on gas flow behaviour and gas storage. Furthermore, the results are useful in predicting permeability and multi-flow phase properties in a tight gas reservoir.

Source-rock reservoirs are fine-grained petroleum source rocks (“shales” or “mudstones”) having geomechanical properties that allow those rocks to produce hydrocarbons at economic rates after stimulation by hydraulic fracturing. Many of the assumptions commonly adopted by geophysicists to characterize shales cannot be applied to source-rock reservoirs. For example, the mineralogies of many source-rock reservoirs are not dominated by clay minerals and so mathematical and/or conceptual models developed for clay-dominated mudstones are not appropriate and cannot be applied to them. Instead, mudstones of shale plays are generally dominated by biogenic calcite and/or quartz. We use terminology of sedimentary geology to show that anisotropy is scale-dependent in source-rock reservoirs, and we discuss the depositional and diagenetic processes that control these and other geophysical properties of interest. The mudstones of source-rock reservoirs may or may not be anisotropic at the lamination scale (i.e., millimeters), the scale commonly used to measure anisotropic parameters via core plugs, but they are nearly always anisotropic at the bedset (centimeters to several meters) and member (tens of meters) scales. Because of the anisotropic nature of mudstones, elastic properties are not scalars at the length/thickness scales that can be defined using seismic methods. Properties of interest are likely to be different parallel to bedding compared to perpendicular to bedding. Because of the subseismic scale of much of this variability, thin-bed effects are likely to influence the AVO behavior of source-rock reservoirs.

In sandstone reservoirs, despite grain rearrangement during compaction, significant pore volumes may be retained prior to the onset of late diagenetic quartz cementation. In mudstone reservoirs, grain rearrangement during compaction results in significant pore volume reduction prior to late diagenesis. Where quartz overgrowths have been previously reported in post-compaction mudstones they have been in volumetrically low concentrations and interpreted as anomalous occurrences. Quartz cementation alters rock brittleness resulting in changes to mechanical fracture properties. Quartz overgrowths reduce reservoir porosity and permeability. We present petrographic evidence of two phases of quartz cement in the Haynesville-Bossier Shale: (i) grain replacive and (ii) quartz overgrowths. Carbonate grain replacement is volumetrically low (< 1%). Quartz overgrowths identified from SEM-CL imaging are volumetrically more significant (8–13%). Quartz overgrowths were most commonly observed in the coarse mudstone microfacies, but are present in both medium and fine mudstone microfacies. Petrographic evidence indicates three processes in the development of quartz overgrowths. Mica and pyrite are (i) engulfed and (ii) displaced by quartz overgrowth cement. The absence of a supportive, primary granular framework surrounding engulfed detrital and early authigenic minerals would indicate that quartz overgrowths are also replacive. Pressure dissolution of quartz silt grains and smectite-to-illite transformation are likely sources of silica for quartz cement. This study is the first to document large-scale, replacive, authigenic quartz overgrowth development within a producing mudstone.

The Lambda-Mu-Rho (LMR) approach of characterizing reservoir intervals in seismic data is a useful tool for efficient well placement and completion. The successful exploitation of unconventional hydrocarbons in the UK could play a fundamental role in helping to satisfy the growing energy demand. To achieve this, it has been proved necessary to differentiate the more brittle from ductile rocks to maximize reservoir recovery. Economic gas production in unconventional plays requires hydraulic fracture stimulation. This study utilizes the impedance, density and the Lame parameters (Lambda, λ and Mu, μ) logs derived from the Amplitude Variation with Offset (AVO) inversion to compute the more seismically intuitive LMR logs and the elastic properties Poisson’s Ratio and Young’s Modulus. The optimal Carboniferous tight sandstone reservoir intervals at Kirby Misperton in the Cleveland Basin were successfully characterized using the LMR crossplot method. Tight gas-filled sandstones are characterized by a low λρ and a high μρ value as a result of their low incompressibility, high rigidity and intrinsic brittleness. The analysis confirmed that the Lower Bowland sequence is prospective from an unconventional perspective due to its high quartz content and therefore is likely to respond positively to hydraulic fracturing. Unconventional resources currently account for 43% of US gas production with gas consumption expected to increase between 2010 and 2030 (Rogers, 2011). Tight gas sandstones represent approximately 70% of all global unconventional gas production and reserves that are yet to be developed (Khlaifat et al., 2011). Although commercial exploration and production of unconventional gas in the UK is in its infancy, it offers significant potential for future gas production growth and will become increasingly important as the easily exploitable conventional gas reserves are exhausted (Law and Curtis, 2002). Most tight gas reservoirs are often characterized by a multi-layered reservoir system that are often hydraulically fractured to enhance recovery rates from the reservoir intervals (Meckel and Thomasson, 2008).

Extensive, large-scale pervasive cementation in the form of cement bodies within fluvial strata has rarely been documented although fluvial strata commonly act as important hydrocarbon reservoirs, as well as groundwater aquifers. Here, we present outcrop, petrographic and geochemical data for pervasive ferroan dolomite cement bodies up to 250 m in size from Upper Cretaceous Desert Member and Castlegate Sandstone fluvial strata exposed in the Book Cliffs in Utah. These cement bodies are present with coastal plain fluvial strata within both the Desert and Castlegate lowstand sandstones and are most abundant in the thin, distal fluvial strata. Cement bodies are almost entirely absent in updip, thicker, fluvial strata. Petrographic observations suggest a predominantly early diagenetic timing to the mildly ferroan dolomite, with a component of later burial origin. δ 13 C values for the cement (+4.8 to −5.7‰ V-PDB) suggest a marine-derived source for the earliest phase with a burial organic matter source for later cement. δ 18 O data (−6.3 to −11.8‰ V-PDB) suggest precipitation from freshwater dominated fluids. It is proposed here that dolomite was derived from leaching of detrital dolomite under lowstand coals and cementation took place in coastal aquifers experiencing mixed meteoric-marine fluids as a result of base-level fluctuations. This data presented here shows that large cement bodies can be an important component within fluvial sandstones with a potentially significant impact upon both reservoir quality and fluid flow within reservoirs, especially at the marine-non-marine interface.

As an essential nutrient and energy source for the growth of microbial organisms, iron is metabolically cycled between reduced and oxidized chemical forms. The resulting flow of electrons is invariably tied to reactions with other redox-sensitive elements, including oxygen, carbon, nitrogen, and sulfur. Therefore, iron is intimately involved in the geochemistry, mineralogy, and petrology of modern aquatic systems and their associated sediments, particulates, and porewaters. In the geological past, iron played an even greater role in marine geochemistry, as evidenced by the vast deposits of Precambrian iron-rich sediments, the “banded iron formations.” These deposits are now being used as proxies for understanding the chemical composition of the ancient oceans and atmosphere.

Whilst the relationship between stratigraphic development and carbonate cementation within siliciclastic succession has been documented through a number case studies, these studies have been generally restricted to observations upon individual sequences and/or limited sub-surface data. In this paper, long-term (5 million years), large-scale (>200 km) stratigraphic controls on carbonate cementation patterns are documented from the Upper Cretaceous Panther Tongue Member, Blackhawk Formation and Castlegate Sandstone exposed in the Book Cliffs in Utah and Colorado, USA. Together, these comprise eight progradational wedges of sandstones, which interfinger with the Mancos Shale, deposited within the Western Interior Seaway foreland basin. Petrographic analyses of ferroan dolomite cement bodies within these sandstone wedges show that the ferroan dolomite cements are all early, relative to burial diagenesis within the host sandstones. Stable isotope analyses indicates that a significant meteoric component was present in precipitating fluids and this is consistent with the observation that cements, are always present down-dip of sequence boundaries and/or leached whitecaps beneath coals. In addition, the lateral distribution of cement bodies increases consistently up-succession from less than 5 km in extent in the older sequences, to 30 km in extent in the youngest sequences. These changes in distribution are in response to the increased progradation and increased and more aerially extensive sequence-boundary development in younger sequences. The implications of these data are that whilst localized spatial patterns of diagenesis, and in particular carbonate cementation, are predictable and controlled by the nature and presence of individual stratal surfaces, systematic diagenetic alteration patterns are also present at the sedimentary basin scale and controlled by the nature of larger-scale stratigraphic development and basin evolution. This evolution may be driven by eustatic shifts, or through tectonic or climatic driven base-level shifts. These observations allow an improved insight into the basin-scale processes that control the macroscopic diagenetic properties of sedimentary successions and sub-surface hydrocarbon reservoirs.

Discovery Bay, a carbonate-dominated embayment in north Jamaica, has been subject to inputs for 40 years of iron-rich bauxite sediment associated with the local mining and transport of processed bauxite. As such, this site is an ideal natural laboratory to study the records and impacts of iron oxide inputs upon geochemical, diagenetic, and microbial processes in tropical carbonate sediments. Total Fe contents in sites in the bay not receiving bauxite inputs are negligible and porewater Ca 2+ , SO 4 2− and Cl − indicate that bacterial sulphate reduction is an important process. In contrast, surface sediments receiving bauxite inputs contain significant total Fe, from 44 μmol/g in shallow (5 m water depth) sites to 110 μmol/g in deeper (20 m water depth) sites. Up-core increases in total Fe record increased temporal inputs into the bay. Within these Fe-rich sediments porewater data shows the presence of Fe II released by bacterial Fe III reduction. There is no direct evidence for significant bacterial sulphate reduction in these sediments. Iron oxides within all bauxite-impacted sediments display a high potential reducibility, from 40% to 80% of the total Fe present as dithionite-extractable Fe III . Experimental analysis of the potential susceptibility to, and rates of, bacterial Fe III reduction, utilising Discovery Bay sediment and Shewanella putrefaciens CN32 (a known Fe III -reducer) has confirmed the high bacterial reducibility of iron oxides within the sediment. Up to 75% of initial dithionite-extractable Fe III in the sediments was reduced over 15 days. The presence of iron oxides within the Discovery Bay shallow marine carbonate systems has markedly altered the chemical diagenetic processes taking place, with a shift from apparent dominance of bacterial sulphate reduction at non-impacted (Fe-poor) sites, to highly significant bacterial Fe III reduction in Fe-rich bauxite-impacted sediments. Given the perceived global increases in terrigenoclastic sediment inputs into tropical carbonate systems as a result of land-use and climate changes, coupled with the documented role that iron oxide reduction plays in nutrient and contaminant cycling in sediment systems, more research into the perturbation of early diagenesis by iron oxide inputs is required.

The extent of lithofacies variability in fine-grained sedimentary rocks is very poorly known in comparison to that present in coarser grained clastic and carbonate successions. The absence of this information means that sediments present on continental shelves are rarely considered as integrated systems because fine-grained facies (such as shales) are mostly excluded from sophisticated, regional facies models. To shed light upon lithofacies and grain size in shale-dominated successions, so that they can be incorporated into shelf-wide depositional models, the Blackhawk Formation of the Mancos Shale (Campanian age), exposed in the Book Cliffs, Utah was investigated using combined field, whole-rock geochemical, optical, and electron optical methods. These sediments show systematic grain-size variations, are intensely bioturbated, and composed predominantly of detrital clays (mainly dioctahedral micas), quartz, and feldspar with minor pyrite and organic matter. They are organized into very thin (< 10 mm), upward-fining, genetic beds; they exhibit both systematic lateral (103 m scale), and vertical (10−2 to 100 m scales) lithofacies variability. Preferentially cemented units occur close to sequence boundaries (unconformities). These cemented units typically contain a very different detrital assemblage (including significant quantities of chlorite) from the rest of the succession and are located close to levels where there are marked stacking-pattern discontinuities. Overall, lithofacies variability is interpreted in terms of deposition occurring on an oxic continental shelf, with the coarser-grained facies being deposited in proximal settings (offshore-transition zone), in contrast to the finer-grained units that were deposited in more distal environments (offshore zone). Storms are interpreted to have been the dominant mechanism dispersing the sediment. Once deposited, the surface layers of the sediment were intensely reworked by burrowing organisms. Preferential preservation of ichnogenera from mid- and lower tiers suggest that the depositional environment was energetic and erosion commonly removed the surface sediment layers, prior to deposition of thin (10−2 m) storm beds. The larger scale (100 m) upward-coarsening units that are composed of stacked beds, are interpreted to be parasequences. Preferential cementation, at levels where large-scale stacking patterns change, is interpreted to occur at horizons where there were breaks in sediment accumulation, and bacterial metabolic processes were able to supply sufficient solutes to fill uncompacted pore space with cement. The presence of a very different detrital assemblage in these units suggests that winnowing occurred. Despite their relatively innocuous appearance in the field, these surfaces are likely to be significant bypass surfaces, over which sediment was delivered to the deeper parts of the basin during times of relative lowstand of sea level.

The early diagenetic chemical dissolution of skeletal carbonates has previously been documented as taking place within bioturbated, shallow water, tropical carbonate sediments. The diagenetic reactions operating within carbonate sediments that fall under the influence of iron-rich (terrigenous) sediment input are less clearly understood. Such inputs should modify carbonate diagenetic reactions both by minimizing bacterial sulphate reduction in favour of bacterial iron reduction, and by the reaction of any pore-water sulphide with iron oxides, thereby minimizing sulphide oxidation and associated acidity. To test this hypothesis sediment cores were taken from sites within Discovery Bay (north Jamaica), which exhibit varying levels of Fe-rich bauxite sediment contamination. At non-impacted sites sediments are dominated by CaCO3 (up to 99% by weight). Pore waters from the upper few centimetres of cores show evidence for active sulphate reduction (reduced SO4/Cl− ratios) and minor CaCO3 dissolution (increased Ca2+/Cl− ratios). Petrographic observations of carbonate grains (specifically Halimeda and Amphiroa) show clear morphological evidence for dissolution throughout the sediment column. In contrast, at bauxite-impacted sites, the sediment is composed of up to 15% non-carbonate and contains up to 6000 μg g−1 Fe. Pore waters show no evidence for sulphate reduction, but marked levels of Fe(II), suggesting that bacterial Fe(III) reduction is active. Carbonate grains show little evidence for dissolution, often exhibiting pristine surface morphologies. Samples from the deeper sections of these cores, which pre-date bauxite influence, commonly exhibit morphological evidence for dissolution implying that this was a significant process prior to bauxite input. Previous studies have suggested that dissolution, driven by sulphate reduction and sulphide oxidation, can account for the loss of as much as 50% of primary carbonate production in localized platform environments. The finding that chemical dissolution is minor in a terrigenous-impacted carbonate environment, therefore, has significant implications for carbonate budgets and cycling, and the preservation of carbonate grains in such sediment systems.

Discovery Bay, north Jamaica, forms a large (1.5 km wide), deep (up to 56 m) embayment that acts as a sink for reef-derived and lagoonal carbonate sediments. Since the mid-1960s, the bay has also provided a sink for inputs of bauxite sediment that are spilled during loading at a boat terminal constructed within Discovery Bay. Bauxite has accumulated across much of the southern section of the bay with surficial sediments presently composed of up to 35 weight% non-carbonate. Cores recovered from sites on the western side of the bay provide a stratigraphic record of this history of bauxite contamination across water depths from 5 to 25 m. The bauxite-influenced upper sediment horizons are clearly visible in each core from the distinctive redebrown colouration of the sediment. These sediments are composed of approximately 10% noncarbonate (bauxite) and have Fe contents of around 2e3000 mg/g (up to 7000 mg/g). The thickness of this upper bauxite-contaminated sequence increases down transect (approximately 18 cm in the shallowest core, to around 47 cm in the deepest core), and in each core overlies a sequence of ‘clean’ lagoon carbonates. These typically are poorly sorted carbonate sands with variable amounts of coral rubble. Down-core data on CaCO3 and Fe content provide a chemical record of decreasing sediment contamination with depth, with the lower ‘clean’ carbonates composed of only around 2% non-carbonate and

Leached zones (‘whitecaps’) and dolomite cement bodies are volumetrically and areally significant features in shoreface sandstones of the Upper Cretaceous Spring Canyon and Aberdeen Members, Blackhawk Formation, Book Cliffs, Utah. Quantitative spatial and stratigraphic data of their distribution, together with petrography and geochemistry, allow development of a model for their formation within a sequence stratigraphic framework. Leached zones, up to 5 m thick and 15 km in lateral extent, are present beneath coals capping parasequences. Concretionary dolomite cement is present in shoreface sandstones downdip of parasequence capping coals and associated “whitecaps”. These concretions are up to 2 m thick and 6 m in length and are present for up to 7 km downdip of these coals. Petrographic and field data indicate an early diagenetic timing for “whitecap” and dolomite cement formation. δ13C data suggest carbonate derivation from detrital dolomite, whereas δ18O data suggest that cement precipitated from porewater with a significant meteoric component, although later diagenetic re-equilibration and recyrstallization cannot be rejected. We propose that organic-acid generation during early diagenesis of coals led to the leaching of detrital dolomite from underlying sandstones, and the formation of “whitecaps”. Meteoric water flowed basinward in response to base-level lowering and sequence boundary development in the upper part of the Aberdeen Member, remobilizing dolomite into shoreface sandstones. Dolomite precipitation resulted from the mixing of these meteoric fluids with downdip marine pore fluids, and the presence of detrital dolomite in the shoreface sandstones that acted as nucleation sites for dolomite precipitation. The geometry and extent of dolomite cement in the Spring Canyon and Aberdeen Members is in marked contrast to the much greater spatial extent of such cement in low accommodation space settings, such as the stratigraphically younger Desert Member, Blackhawk Formation. The relatively simple aggradational to progradational evolution of the Spring Canyon and Aberdeen Member parasequences, with only minor sequence boundary development in its upper part, resulted in spatially limited meteoric fluid input into shoreface sandstones. This resulted in dolomite cementation of shoreface sandstones up to a maximum of 7 km downdip of leached zones. In the Desert Member, extensive meteoric fluid migration as a result of major sequence boundary development resulted in dolomite cementation of shoreface sandstones for over 25 km downdip of leached zones. This highlights that the degree and extent of early fluid diagenesis can be significantly controlled by the nature of sequence development, having clear implications for developing predictive models for porosity–permeability evolution in sandstone reservoirs.

Sub-tidal, reef-lagoon surficial sediment samples from Discovery Bay, north Jamaica were analysed in order to quantify the impacts of anthropogenic bauxite inputs on trace metal (Fe, Zn, Mn) levels and sediment characteristics. Contaminant levels exhibit marked spatial variations, but are highest across central and southern parts of the bay and primarily reflect wind blown inputs, modified by subsequent suspended sediment transport. Highest contaminant levels (Fe—13,701 ppm, Mn—237 ppm, Zn—74 ppm) occur immediately adjacent to, and just north-east of, a bauxite loading terminal in the south-west of the bay. Sites of high sediment contamination are negatively correlated with per cent CaCO3 content, indicating the diluting effect of bauxite on the carbonate substrate. At high contamination sites CaCO3 levels are reduced to 63.7% and are in the range of 75–80% across much of the central and southern area of the bay. Magnetic susceptibility level measurements are positively correlated with iron and n...

Remobilisation of contaminant metals from sediments can occur by chemical, biological or physical changes. This in turn can lead to contaminant fluxes to the porewaters and ultimately the water column. The aim of the research presented here is to document post-depositional controls on metal mobility and fluxes in a heavily metal-contaminated estuary. This will allow for an improved understanding of the impact of contaminated sediments on water quality from both a short-term and long-term perspective. Dulas Bay is situated on the east coast of Anglesey, North Wales, and receives polluted waters from Parys Mountain. Metals within Dulas Bay sediments show surface enrichment and variations in mineralogical form. Diagenesis clearly plays a role in post-depositional behaviour of the metals, forming sulphides and potentially carbonates. The presence of a dominant exchangeable/carbonate fraction, and elevated porewater metals, in this sulphidic system is significant and could indicate the presence of freshwater diagenesis, or, reflect the high levels of metals in the sediment.

Contaminated sediments deposited within urban water bodies commonly exert a significant negative effect on overlying water quality. However, our understanding of the processes operating within such anthropogenic sediments is currently poor. This paper describes the nature of the sediment and early diagenetic reactions in a highly polluted major urban water body (the Salford Quays of the Manchester Ship Canal) that has undergone remediation focused on the water column. The style of sedimentation within Salford Quays has been significantly changed as a result of remediation of the water column. Pre-remediation sediments are composed of a range of natural detrital grains, predominantly quartz and clay, and anthropogenic detrital material dominated by industrial furnace-derived metal-rich slag grains. Post-remediation sediments are composed of predominantly autochthonous material, including siliceous algal remains and clays. At the top of the pre-remediation sediments and immediately beneath the post-remediation sediments is a layer significantly enriched in furnace-derived slag grains, input into the basin as a result of site clearance prior to water-column remediation. These grains contain a high level of metals, resulting in a significantly enhanced metal concentration in the sediments at this depth. Porewater analysis reveals the importance of both bacterial organic matter oxidation reactions and the dissolution of industrial grains upon the mobility of nutrient and chemical species within Salford Quays. Minor release of iron and manganese at shallow depths is likely to be taking place as a result of bacterial Fe(III) and Mn(IV) reduction. Petrographic analysis reveals that the abundant authigenic mineral within the sediment is manganese-rich vivianite, and thus Fe(II) and Mn(II) released by bacterial reactions may be being taken up through the precipitation of this mineral. Significant porewater peaks in iron, manganese and silicon deeper in the sediment column are most probably the result of dissolution of furnace-derived grains in the sediments. These species have subsequently diffused into porewater above and below the metal-enriched layer. This study illustrates that the remediation of water quality in anthropogenic water bodies can significantly impact upon the physical and chemical nature of sedimentation. Additionally, it also highlights how diagenetic processes in sediments derived from anthropogenic grains can be markedly different from those in sediments derived from natural detrital material. Copyright © 2003 John Wiley & Sons, Ltd.

The Eagle Ford Fm in southwest Texas is a self-sourced oil and gas reservoir currently stimulated through hydraulic fracturing to produce economic quantities of hydrocarbons. Both early and burial diagenesis has had a major impact upon the rocks and their resulting reservoir quality. In this presentation we infer mineral reactions and precipitation that took place during diagenesis through a combination of mineralogical and petrographic analysis using XRD, optical and electron microscopy. We highlight how such an understanding can improve prediction of rock properties and reservoir quality. Along with compaction and de-watering, we infer that bacterial sulphate reduction had a major impact during early diagenesis as it resulted in significant calcite cement precipitation. Calcite cements infill bioclasts and foraminifera chambers, thereby significantly reducing intra-granular porosity. Fine grained calcite cements the matrix and coccolith fragments that resulted in reduction in inter-granular porosity and its interlocking texture will likely lead to an increase in rock brittleness. Optical microscopy and cathodoluminescence (CL) highlight the extensive and invasive calcite precipitation that occurs within concretional features in the Eagle Ford Fm. Zonation within the calcite cements suggests evolution in pore water chemistry and this is interpreted to be caused by changes in microbial organic matter oxidation. Foraminifera chambers are commonly infilled with kaolinite, as well as or instead of calcite. There is no clear petrographic evidence to suggest which came first, but based on the fact the foraminifera are not compacted we infer early diagenetic origin. Unlike the calcite infills, kaolinite infills preserve significant inter-crystalline porosity. Authigenic kaolinite is also present as multiple crystal grains within the matrix, and replacing 30-60µm detrital grains- which we infer to be feldspars. During late burial, authigenic quartz cement commonly precipitated around detrital quartz grains and calcite cements, further reducing inter-particle and inter-crystalline porosity. This source of this silica may have been clay mineral reactions or biogenic silica dissolution. Chlorite is present in the form of 5-15µm wispy flakes in the most thermally mature samples. The precipitation of clay minerals during deeper burial leads likely to a decrease in rock brittleness and a further reduction in micro-porosity in the matrix.

This study investigates the application of high sensitivity electron spin resonance (ESR) to environmental magnetism in conjunction with the more conventional techniques of magnetic susceptibility, vibrating sample magnetometry (VSM) and chemical compositional analysis. Using these techniques we have studied carbonate sediment samples from Discovery Bay, Jamaica, which has been impacted to varying degrees by a bauxite loading facility. The carbonate sediment samples contain magnetic minerals ranging from moderate to low concentrations. The ESR spectra for all sites essentially contain three components. First, a six-line spectra centred around g= 2 resulting from Mn2+ ions within a carbonate matrix; second a g= 4.3 signal from isolated Fe3+ ions incorporated as impurities within minerals such as gibbsite, kaolinite or quartz; third a ferrimagnetic resonance with a maxima at 230 mT resulting from the ferrimagnetic minerals present within the bauxite contamination. Depending upon the location of the sites within the embayment these signals vary in their relative amplitude in a systematic manner related to the degree of bauxite input. Analysis of the ESR spectral components reveals linear relationships between the amplitude of the Mn2+ and ferrimagnetic signals and total Mn and Fe concentrations. To assist in determining the origin of the ESR signals coral and bauxite reference samples were employed. Coral representative of the matrix of the sediment was taken remote from the bauxite loading facility whilst pure bauxite was collected from nearby mining facilities. We find ESR to be a very sensitive technique particularly appropriate to magnetic analysis of ferri- and para-magnetic components within environmental samples otherwise dominated by diamagnetic (carbonate) minerals. When employing typical sample masses of 200 mg the practical detection limit of ESR to ferri- and para-magnetic minerals within a diamagnetic carbonate matrix is of the order of 1 ppm and 1 ppb respectively, approximately 102 and 105 times the sensitivity achievable employing the VSM in our laboratory.

Ooidal ironstones form prominent units, and an appreciation of their sedimentological and stratigraphic significance is important for the interpretation of sedimentary successions. Ooidal ironstones from the Albian Paddy Member of the Peace River Formation, Alberta, Canada, and the Campanian Castlegate Sandstone of the Mesaverde Group in Colorado have been analyzed. The ooidal ironstones are associated with a major sequence boundary and subsequent transgressive surface. Both are interpreted to have been deposited under conditions of low net sediment accumulation in well-oxygenated bottom-water conditions, with episodic storm events reworking the sediments. The Paddy Member ironstone is interpreted to have been deposited in an estuarine environment, and the Castlegate ironstone is interpreted to have been deposited in a shallow-marine environment. Both ironstones are dominated by berthierine ooids and grain-rimming and pore-filling siderite, with later ferroan dolomite and calcite in the Castlegate ooidal ironstone. Petrographic, geochemical, and isotopic evidence, coupled with thermodynamic considerations, indicates that the ooidal ironstone mineralogy formed during suboxic diagenesis. The conditions required for suboxic diagenesis were extensive sediment reworking and slow net sediment accumulation rates. We propose that these conditions arose as a result of marine transgression during the initial relative sea-level rise following the development of a sequence boundary. Ooidal ironstone formation ceased once sedimentation rates increased and transgression deepened water depths considerably. These results illustrate the association between ooidal ironstones and major stratal surfaces in sedimentary successions, and highlights the complexity that early diagenesis at such surfaces can take.

The floodplain deposits of the Wealden Group (Lower Cretaceous) of the Isle of Wight, southern England, were formed in a seasonal wetland setting, a type of environment widespread today along higher-order tropical and subtropical river systems but rarely identified in the geological record. The unit consists of four main lithofacies: sheet sandstones with dinosaur footprint casts; green-gray mudstones with vertebrate remains, abundant lignite, pyrite, and siderite; spectacularly color-mottled mudstones with goethite and locally pseudo-anticlines; and red mudstones with pseudo-anticlines, hematite, and carbonate nodules. The sheet sandstones are interpreted as crevasse deposits; the green-gray mudstones were deposited in shallow ponds on the floodplain, which acted as sinks for debris released by local floods following wildfires; the mottled mudstones represent surface-water gley soils formed in seasonally waterlogged areas; and the red mudstones resemble present-day Vertisols that formed on topographically elevated areas only intermittently flooded. These mudstones show vertical transitions from one to another, and although they could be interpreted as components of simple catenas, the absence of associated facies changes implies that topographic differences were not the only control. It is proposed that these three mudstone types formed as seasonal wetland catenas, in which differences in soil drainage conditions resulted from variations in the flooding hydroperiod affecting areas with minor relief differences, rather than drainage variability simply reflecting static topographic differences. Such seasonal wetland systems are rarely documented in the stratigraphic record despite being a widespread environment in present-day tropical regions, and the Wealden deposits are used to identify criteria for the recognition of this important environment in the rock record. These southern English wetlands are compared with other Lower Cretaceous wetlands from northern Spain, enabling hydrological factors which controlled deposition to be recognized.

Three macroscopic diagenetic features can be recognized in the sandstones of the Upper Cretaceous Desert Member of the Blackhawk Formation and Castlegate Sandstone of the Mesaverde Group exposed in the Book Cliffs, Utah, each of which have distinctive form, geometry, and stratigraphic distribution. Diagenetic alterations are: (1) leached zones ("whitecaps"), up to 10 m thick, beneath coal beds; (2) large (up to 8 m) concretionary carbonate-cemented bodies in amalgamated shoreface and thin fluvial sandstones; and (3) thin (up to 2 m), laterally extensive carbonate-cemented horizons beneath major marine flooding surfaces. Each feature has distinct petrographic and geochemical signatures, and formed through discrete diagenetic processes. Large isolated carbonate-cemented bodies are composed of ferroan dolomite, most of which precipitated during early diagenesis. Field and petrographic data, coupled with stable-isotope data (early cements, 13C = -2.5 to +3.4o/oo VPDB; 18O = -7.8 to -12.0o/oo VPDB; 87Sr/86Sr = 0.7078; later cements, 13C = -3.1 to -5.7o/oo VPDB; 18O = -12.0 to -15.1o/oo VPDB; 87Sr/86Sr = 0.7093) suggest precipitation from meteoric fluids, input into sediments during times of relative sea-level fall. The source of carbonate for the dolomite cement was dissolution of detrital dolomite from beneath coals by organic acids and subsequent mobilization by meteoric fluids. Carbonate precipitation in laterally extensive cement horizons appears to have started as a result of hiatus in sediment accumulation during marine flooding events (relative sea-level rise). Cement precipitation in these horizons continued through sediment burial as a result of organic-matter oxidation reactions in overlying organic-rich mudstones. The results of this study show a link between sedimentation (related to changes in relative sea level) and diagenesis, leading to the potential for the development of process-based, predictive models of early diagenesis in depositional successions.