Quartz Silt Research Articles

Compositional controls on the Lower Cretaceous Rodby Shale pore structure and surface area: a planned CCS top seal caprock for the Acorn storage site

Fine-grained lithologies above CCS reservoirs cannot automatically be assumed to be mineralogically stable, or high quality top-seals to highly pressured CO2 as saline aquifer have previously contained hydrostatically pressured water. We have investigated the mineralogy, pore systems and surface area characteristics of the Lower Cretaceous Rodby Shale, the caprock to the Captain Sandstone at the UK's planned Acorn/Goldeneye CCS site. Rodby Shale core was logged and analysed by XRD, light microscopy, and SEM. Grain size was measured using laser particle size analysis. Mercury intrusion porosimetry and nitrogen adsorption analysis were used to characterise the pore network. The Rodby is smectite-rich and contains abundant calcite as well as quartz silt with small quantities of chlorite and plagioclase. Calcite was sourced from benthic microfossils, locally recrystallised to create a pore-filling cement. The mean pore throat and pore body diameter are about 17 nm putting the Rodby in the mesopore range and suggesting a predominance of slit-like pores. There are three lithofacies in the Rodby Shale: (i) high surface area clay-rich shale, (ii) low surface area calcite-rich shale, (iii) intermediate surface area quartz-rich. if the second lithotype encountered CO2, then the resulting calcite dissolution would lead to increasing surface area of the remaining shale. The Rodby Shale has good potential to be an effective barrier for CO2 escape, based on assessments of diffusion rate and post-breakthrough advection rate as well as stability and sealing assessments.

Impact of silt chemical composition on deflocculation and coagulation of clay-rich paste

This paper investigates the impact of the chemical composition of two silts, quartz and limestone, on the deflocculation/coagulation process for poured earth application. The driving mechanisms behind the evolution of the rheological behavior after the addition of sodium hexametaphosphate are highlighted through zeta potential measurementand the chemical changes with ICP-OES and 31P MAS NMR spectroscopy. The first results show that adding sodium hexametaphosphate into the quartz silt modifies its inter-particle forces through electrostatic repulsion forces induced by the adsorption of phosphate anions onto the quartz structure. For limestone paste, the deflocculation mechanism is affected by the dissolution of the calcite. This dissolution leads to a release of Ca2+ cations and an increase in the paste's pH, reducing the paste deflocculation rate. These dissolved Ca2+ cations in the solution conduct to a partial coagulation making the delayed flocculation targeted initially with the addition of magnesium oxide less effective.

Experimental investigation and modeling on the dissociation kinetics of methane hydrate in clayey silt cores in depressurization

Natural gas hydrate (NGH) in clayey silts might be the majority of its global reserve. A thorough understanding on the dissociation kinetics of NGH in clayey silts is crucial for its safe and efficient exploitation. In this study, the dissociation characteristics of methane hydrate in quartz silts, montmorillonites and South China Sea (SCS) sediments made cores in multi-step depressurization was experimentally studied and modeled. In quartz silt core, methane hydrate thoroughly dissociated once the pressured is lowered to the equilibrium pressure in bulk. In montmorillonite and SCS sediment cores, methane hydrate dissociation initiated at higher pressure than quartz silt. Capillary and clay induced water activity depressions are suggested to be the major causes. Stepwise methane hydrate dissociation was discovered in the montmorillonite and SCS sediment cores in multi-stage depressurization. In one step depressurization, hydrate in the fine grain cores with larger surface area dissociated faster. In montmorillonite and SCS sediment cores, lower water content leads to faster dissociation at the same pressure, which may be attributed to the higher driven force for dissociation (fugacity difference fe-f). A kinetic model for NGH dissociation in clayey silts was established by introducing fitted equivalent diameter and surface area correction factor of hydrate particles to the Kim-Bishnoi model. In the model, the hydrate equilibrium conditions in clayey silts were predicted by the water activity measurement (WAM) technique. With a maximum deviation of 9.81% in gas production, the model prediction and experimental results are in good agreement. The experimental findings and model reported in this work may be helpful to understand the NGH phase transition mechanism in clayey silt sediments.

Local or imported? The origin of the raw material used in manufacturing bricks from castles of the Teutonic knights in north‐central Poland and their significance to our understanding of medieval construction techniques

AbstractThis article presents the results of research on bricks from medieval castles in north‐central Poland. They were characterised based on petrographic analyses of thin section and mutually complementary instrumental methods. The bricks came from the oldest parts of the castles preserved to this day, or from castles not preserved but examined and excavated during archaeological research. The data obtained allowed for the identification of building materials that shared similar material and technical properties and that can be associated with different phases of the castles. Based on the results, it was found that the Pleistocene tills commonly found around the castles were not used in the manufacturing of the bricks. Small local deposits of fatty clays were used—Pleistocene varved clays or Miocene variegated clays. The buildings differed in the composition and texture of their bricks, indicating that the local clay raw material was extracted for each building separately. The share of raw material brought in from larger exposures further afield was small. Analyses of brick samples indicate the use of a fatty clay raw material that was improved by the addition of clastic material (quartz sand or a mixture of sand and quartz silt of various fractions).

Paleoclimate and the origin of two 1000 km Lower Mississippian facies tracts in southeastern Laurentia (USA): Cool-humid Famennian and Kinderhookian – warm-arid Osagean

Two lithologically distinctive Mississippian facies tracts (MFTs) are described from Lower and Middle Mississippian strata (Kinderhookian and Osagean North American Stages) in southeast Laurentia (SEL). The MFTs extend from the central Appalachian Basin westward across the Cincinnati Arch, through the Eastern (Illinois) Basin, onto the Burlington shelf in central Missouri and into the Western Interior (Forest City) Basin, USA. The MFTs developed under radically different climatic regimes. Kinderhookian climate cycles ranged from 3rd order humid to 4th and 5th order humid-subhumid alternation. These climate fluctuations controlled Kinderhookian sediment flux, evidenced by fluvial-deltaic sandstones, coal beds, and pro-deltaic terrestrial-organic-matter-enriched marine black shales. A dramatic climate shift coincided with the Kinderhook-Osage boundary, from 3rd order cool-humid to 3rd order warm-arid conditions, causing cessation of deltaic sedimentation and the onset of eolian sedimentation at that boundary. This abrupt climate reorganization is reflected in 3rd, 4th, and 5th order continental and eolo-marine loessites that replaced fluvial-deltaic facies. Eolianites, evaporites, and calcareous protosols indicate Osagean aridity. Consequently, we reject the deltaic depositional paradigm for Osagean siliciclastic facies. Osagean eolian sediment consisted mainly of quartz silt with significant pristine (unaltered) feldspar silt (∼ ≤ 10%). Pristine feldspars are consistent with an arid sedimentary source that lacked significant chemical weathering under an arid paleoclimate. Dissolution of the chemically reactive disordered lattice of eolian-abraded quartz in subarkosic loess served as the predominant source of silica for massive amounts of biotic and abiotic chert in Osagean eolo-marine sediments. The ≤20 μm fraction of quartz dust is particularly susceptible to dissolution and re-precipitation as microcrystalline quartz (chert). We conclude that tectonic, eustatic, and climatic (allocyclic) processes all exerted some control on deposition; tectonics and eustasy controlled accommodation space, whereas paleoclimate changes (cycles), driven predominantly by orbital forcing, were the principal control on sediment supply and lithostratigraphy.

Clay diagenesis and overpressure development in Upper Cretaceous and Tertiary shales of South Texas

We studied the origins of overpressure, primarily using cuttings and wireline logs, in a nearly 4 km-thick succession of Tertiary and Upper Cretaceous shales and sandstones in the northwestern Onshore Gulf of Mexico. Our goals were: a) to specifically look for the microtextural changes in shale that are often inferred, by pore-pressure specialists, to develop in overpressured shale, and b) to test the utility of cuttings for that purpose. Overpressures begin approximately 2.5 km below the present ground surface and are moderate at the base of our study interval. Much of that section has velocity and density properties that are diagnostic of unloading, excess pore pressures generated after compaction has occurred. Our cuttings-based analyses included XRD and Rock-Eval measurements. Those datasets allowed us to define the changes in mineralogy and organic content (maturity and amount) with depth. We rule out organic maturation as a significant source of overpressure and instead identify clay diagenesis (smectite-to-illite transition) as the major source of excess pore pressure. Finally, we use SEM imaging of ion-milled samples to examine and document the nano-to micron-scale textural and mineralogic changes that occurred during diagenesis. Diagenetic quartz, present as overgrowths on quartz silt grains and microquartz intergrown with illite, is associated with the changes in clay mineralogy. Some nano-to micro-porosity can be preserved locally when detrital silt or rigid diagenetic minerals (mainly quartz and pyrite in our samples) prevent physical compaction. The stratigraphic offset between the onset of overpressure and the onset of clay diagenesis is probably due to the combined effects of the broad-scale stratigraphy (the section becomes sandier at shallower depths), and the temporal changes associated with overpressure generation and dissipation. Our experiment with cuttings demonstrated the utility and limitations of their use. Finally, our results challenge some of the assumptions about shale diagenesis that can be used during pore-pressure studies.

Characteristics of matric suction in unequal sized quartz silt during horizontal freezing

Characteristics of matric suction in unequal sized quartz silt during horizontal freezing

Experimental approaches to assess the effect of composition of abrasives in the cause of dental microwear.

Dental microwear is used to investigate feeding ecology. Animals ingest geological material in addition to food. The full effect of geological abrasives on tooth wear is unknown. To evaluate mineralogical abrasives as tooth wear agents, rats were fed food manufactured with quartz silt, diatomaceous earth, and calcium carbonate. Rats were assigned to treatments and fed for 15 days. Molars were scanned with a Sensofar Plu Neox confocal microscope and evaluated using ISO-25178-2 parameters and traditional microwear variables using light microscopy. Using a pellet-diet as the control, all treatments had influence on microwear and discriminant function analyses indicated that unique surface textures had been produced. ISO variables with high discriminatory values were correlated to scratch and pit frequencies, but more ISO parameters identified changes associated with numbers of scratches than changes associated with pits. The microwear changes associated with the abrasive inclusions were co-dependent on the type of diet that the abrasives had been added to. The abrasives had less effect with pellets but produced more modified and more differentiated microwear when added to the transgenic dough. Although abrasives produce distinctive surface textures, some knowledge of the properties of food with the abrasives is needed to identify the abrasive agent.

The impact of clay fraction on the strength and stress ratio ( K 0 ) in Gulf of Mexico mudrocks and quartz silt mixtures: implications for borehole stability and fracture gradient

Mudrock strength parameters are required to improve the prediction of fracture gradient during drilling in poorly consolidated formations. Understanding the influence that mineral composition and consolidation stress have on the mechanical properties of mudrocks aids safe well design. Small changes in the mudrock clay content are shown to have significant effects on two important engineering parameters: the lateral stress ratio at rest, K 0 , and the critical state effective friction angle, ϕ ′ cs . A laboratory-testing programme using quartz silt and a Gulf of Mexico clay mixture investigates the effects of mudrock clay content on K 0 and ϕ ′ cs at two stress levels: 1 and 10 MPa. Values of K 0 and ϕ ′ cs were obtained by subjecting composition-controlled clay–silt specimens to a drained uniaxial vertical strain ( K 0 consolidation) phase, followed by an undrained compression triaxial shear phase. Triaxial tests illustrate consistent and systematic increases in K 0 and decreases in ϕ ′ cs , with increases in clay content at both stress levels. Stress dependence is observed through increases in K 0 and decreases in ϕ ′ cs with increases in stress level. Stress-dependent behaviour is shown to be more pronounced in mudrocks of high clay content. A predictive model is presented for K 0 in normally consolidated mudrocks, as a function of clay content and effective stress. The model is used to calculate the fracture gradient (least horizontal stress) profile at the International Ocean Drilling Program (IODP) Expedition 308 Site 1324. The estimated fracture gradient illustrates the effects of clay content and consolidation stress on safe mud weight design. Thematic collection: This article is part of the Geopressure collection available at: https://www.lyellcollection.org/cc/geopressure

Ferruginous casts of bromalites in kaolin beds: microbial ferrihydrite‐goethite transformation as early stage taphonomy in lacustrine and riparian sediments

Two spatiotemporally distant deposits in western North America on differing sides of the K-Pg boundary are recognized to have accumulated innumerable ferruginous faecal-like specimens consisting of 3-D casts suspended in kaolin clay fills of lacustrine and riparian areas proximal to fluvial channels. Evidence presented interprets these specimens as organic in origin, consisting of coprolites and other bromalites (cololites), at the Readlyn deposit of the Whitemud Formation (Maastrichtian) in southern Saskatchewan and the Salmon Creek deposit of the Wilkes Formation (Miocene) near Toledo, Washington State. Comparable taphonomic processes at each deposit are suggested by preservation of identical, metre-long, enigmatic casts of the digestive system (cololites) of an unknown animal associated with the innumerable coprolites. Micro-fabrics of specimen interiors suggest that microbial mats entombed faecal and other organic entities in kaolin-rich lacustrine muds. This research proposes that microbial-induced ferrihydrite precipitation rapidly transformed into thin encrustations of goethite, encasing faecal droppings and other organic remains. Ferrihydrite-goethite-hematite biomineral sequencing cemented kaolin and quartz silt grains at the interface with the organic remains, precluding significant decay and thereby preserving external surface morphologies as encrustations. The mm-thick layers of the cemented sediment enveloped the organic droppings as rigid goethite moulds, preventing the collapse of the encasement morphologies as the organic residues were replaced by biominerals. Concentric growth layers of microbial mat-induced ferrihydrite-goethite cement progressed inward, resulting in 3-D casts. Only external morphologies were preserved, but often with finely detailed surface textures. The interiors of the casts preserve evidence of interconnected fabrics of pseudomorphed bacterial cell walls consisting of radially arranged jackets of acicular (Readlyn deposit) or platy (Salmon Creek deposit) goethite crystallites. Concretionary siderite replacement fabrics in some specimens resulted in the obliteration of the earlier microbial induced biomineral fabrics. Previous interpretations that the Wilkes Formation specimens resulted from inorganic processes only, thereby pseudo-coprolites, have been reappraised as faecal droppings comparable to coprolites of the Whitemud Formation.

Geology, geochemistry and depositional history of the Port Campbell Limestone on the eastern flank of the Otway Basin, southeastern Australia

A fully cored sequence of Hesse Clay, Port Campbell Limestone and uppermost Gellibrand Marl in the onshore Otway Basin, southeastern Australia, offers new insight into the evolution of the middle Miocene Port Campbell Limestone. The Port Campbell Limestone comprises grey unconsolidated to semi-consolidated and rarely lithified bioclastic muddy carbonate sands in a stack of thin repetitive cycles within cycles of predominantly shoaling-upward character. A glauconitic band with a distinctive mollusc–echinoderm–bryozoan fauna provides a distinctive marker interval in the sequence. In mineralogy, the Port Campbell Limestone is predominantly calcite with traces of remnant aragonite in muddier low-permeability sands, and with dolomitic zones in permeable intervals. The small non-carbonate component of the Port Campbell Limestone is between 3 and 15 wt% and comprises quartz silt with minor clay, feldspar and mica. Dissolution overprints are prominent throughout the carbonate sequence. Three distinct geochemical signatures of provenance are evident in the Port Campbell Limestone sequence, including possible volcanogenic contributions with felsic sources. Foraminifera are common and generally well preserved. Foraminiferal data suggest a depositional transition from outer shelf conditions in the Gellibrand Marl at ca 15 Ma to middle shelf environments in the lower part of the Port Campbell Limestone during the Middle Miocene Climatic Optimum (MMCO) at ca 14.24 Ma. Shallowing after 14 Ma indicates variable paleodepths of <70 m during and following the end of the Middle Miocene Climatic Transition (MMCT) at ca 13.2 Ma when the sequence was emergent for a brief but undetermined period, corresponding with sharp changes in geochemical ratios. Observed cyclicity in these mid-shelfal, cold-water carbonates is strongly correlated with orbital forcings—eccentricity and obliquity. Sedimentation rates determined from cyclostratigraphic analysis indicate 4–6 cm/kyr at the end of the MMCO, diminishing to 1.5–3 cm/kyr during the MMCT and the subsequent accumulation of the Port Campbell Limestone. KEY POINTS The Port Campbell Limestone is a stack of thin repetitive depositional cycles within cycles. The cyclicity is strongly correlated with orbital forcings—eccentricity and obliquity—and this is reflected in the geochemistry. Foraminiferal data suggest a depositional transition from outer shelf conditions in the Gellibrand Marl at ca 15 Ma to middle shelf environments in the lower part of the Port Campbell Limestone during the Middle Miocene Climatic Optimum (MMCO) at ca 14.24 Ma. The sequence was emergent for a brief but undetermined period at ca 13.2 Ma, corresponding with sharp changes in geochemical ratios. Three distinct geochemical signatures of provenance are evident in the Port Campbell Limestone sequence, including possible volcanogenic contributions with felsic sources.

Effects of particle breakage on the compression behaviour of gap graded carbonate sand–silt mixtures

Isotropic compression tests were conducted on a series of gap-graded mixtures of a carbonate sand (CS) from the South China Sea and a fine crushed quartz silt. The purpose of these tests is to expl...

Implications of thin laminations on pore structure of marine shale reservoir: Goldwyer Formation case study from Western Australia

The pore structure of a shale reservoir is a major control on hydrocarbon potential, yet shale pore systems are complex and affected by various factors. This paper focuses on the differences in pore structure between thinly laminated and massive black shale (MBSh) beds in the Ordovician Goldwyer-III shale, Canning Basin, Western Australia. A multiscale approach included image logs, core descriptions, thin sections, scanning electron microscope and X-ray diffraction analysis with low-pressure nitrogen and carbon dioxide gas adsorption tests. The results indicate that the Goldwyer shale comprises laminated beds of quartz silt and shale with thin beds of organic-rich clay, plus minor interbedded carbonate bands or concretions. The pore types are subjected to rock type, and the thinly laminated shale (LSh) is enriched in intergranular and intragranular pores. In contrast, the MBSh mainly comprises organic matter pores. The LSh is slightly enriched in mesopores but has negligible micropores. The mesopores are wedge-shaped and associated with an inorganic matrix of clay and pyrite. In comparison, the MBSh contains both mesopores and micropores. These pores are slit-like and related to organic matter and clay. The clay content and total organic carbon fluctuations control the development of mesopores and micropores in both the laminated and MBSh beds in the Goldwyer-III shale. The MBSh layers are suggested as the most important rock types for fluid flow via pore systems due to higher total pore volume, specific surface area and gas adsorption capacity.

Quaternary influx of proximal coarse-grained dust altered circum-Mediterranean soil productivity and impacted early human culture

AbstractThe carbonate mountainous landscape around most of the Mediterranean is karstic, is almost barren, and has thin soils. Erosion of preexisting thicker soils is a common hypothesis used to explain this bare terrain. An alternative hypothesis is that in the Mediterranean region, thin soils are attributed to long-distance transport of very fine, silty clay dust, resulting in low mass accumulation rates. Even if accreted over millennia, such dust cannot produce thick, highly productive soils. A pronounced anomaly in the Mediterranean is the thick, more productive soil of the semiarid southern Levant (SL). These soils contain order-of-magnitude coarser grains than the characteristic thin soils in the Mediterranean and a high proportion (&amp;gt;70%) of coarse silt quartz sourced from the nearby Sinai-Negev erg, the primary contributor of the Negev loess. This proximal intense dust supply produced greatly thicker soils. However, influx of coarse silt quartz loess is a geologically recent phenomenon in the SL. Pre-loess (i.e., older than 200 ka, pre-coarse-silt influx) SL soils are much finer and were generated by long-distance dust from the Sahara and Arabia like most other Mediterranean soils. Thus, we hypothesize that the geologically recent Negev Desert loess interval caused a drastic change in mountainous soil properties within the SL, enriching the Levant’s ecology and affecting early human development. The high amounts of coarse silt deposited on the landscape have contributed to the unique sustainable agriculture in the SL, which assisted in transforming the Levant into “the land of milk and honey” and a cradle of civilizations.

Lower Cretaceous Rodby and Palaeocene Lista Shales: Characterisation and Comparison of Top-Seal Mudstones at Two Planned CCS Sites, Offshore UK

Petroleum-rich basins at a mature stage of exploration and production offer many opportunities for large-scale Carbon Capture and Storage (CCS) since oil and gas were demonstrably contained by low-permeability top-sealing rocks, such as shales. For CCS to work, there must be effectively no leakage from the injection site, so the nature of the top-seal is an important aspect for consideration when appraising prospective CCS opportunities. The Lower Cretaceous Rodby Shale and the Palaeocene Lista Shale have acted as seals to oil and gas accumulations (e.g., the Atlantic and Balmoral fields) and may now play a critical role in sealing the Acorn and East Mey subsurface carbon storage sites. The characteristics of these important shales have been little addressed in the hydrocarbon extraction phase, with an understandable focus on reservoir properties and their influence on resource recovery rates. Here, we assess the characteristics of the Rodby and Lista Shales using wireline logs, geomechanical tests, special core analysis (mercury intrusion) and mineralogical and petrographic techniques, with the aim of highlighting key properties that identify them as suitable top-seals. The two shales, defined using the relative gamma log values (or Vshale), have similar mean pore throat radius (approximately 18 nm), splitting tensile strength (approximately 2.5 MPa) and anisotropic values of splitting tensile strength, but they display significant differences in terms of wireline log character, porosity and mineralogy. The Lower Cretaceous Rodby Shale has a mean porosity of approximately 14 %, a mean permeability of 263 nD (2.58 × 10−19 m2), and is calcite rich and has clay minerals that are relatively rich in non-radioactive phases such as kaolinite. The Palaeocene Lista Shale has a mean porosity of approximately 16% a mean permeability of 225 nD (2.21 × 10−19 m2), and is calcite free, but contains abundant quartz silt and is dominated by smectite. The 2% difference in porosity does not seem to equate to a significant difference in permeability. Elastic properties derived from wireline log data show that Young’s modulus, material stiffness, is very low (5 GPa) for the most shale (clay mineral)-rich Rodby intervals, with Young’s modulus increasing as shale content decreases and as cementation (e.g., calcite) increases. Our work has shown that Young’s modulus, which can be used to inform the likeliness of tensile failure, may be predictable based on routine gamma, density and compressive sonic logs in the majority of wells where the less common shear logs were not collected. The predictability of Young’s modulus from routine well log data could form a valuable element of CCS-site top-seal appraisals. This study has shown that the Rodby and Lista Shales represent good top-seals to the Acorn and East Mey CCS sites and they can hold CO2 column heights of approximately 380 m. The calcite-rich Rodby Shale may be susceptible to localised carbonate dissolution and increasing porosity and permeability but decreasing tendency to develop fracture permeability in the presence of injected CO2, as brittle calcite dissolves. In contrast, the calcite-free, locally quartz-rich, Lista Shale will be geochemically inert to injected CO2 but retain its innate tendency to develop fracture permeability (where quartz rich) in the presence of injected CO2.

An additional example of the ichnogenus Zoophycos in Cambrian strata (Gallatin Formation; Wyoming, USA)

The ichnogenus Zoophycos is relatively rare in Cambrian-age sedimentary rocks, being far more commonly encountered in post-Devonian, and particularly post-Paleozoic, strata. A new occurrence of Zoophycos from the Upper Cambrian Gallatin Formation of the Bighorn Mountains, north-central Wyoming, USA, is reported herein. Material consists of a single specimen preserved within quartz silt representing the upper surface of a graded quartz arenite bed. The burrow is a relatively wide, curved, horizontally oriented lobe with a poorly visible marginal tube but well-developed internal lamellae. Discovery of this specimen contributes to the paleobiogeographic distribution of lower Paleozoic Zoophycos, adds to the literature suggesting that most Cambrian Zoophycos were horizontal and shallow-tier structures, and provides further support to the notion that relatively complex deposit-feeding behavior evolved earlier in the Phanerozoic than had long been recognized.

Facies heterogeneity and source potential of carbonate-mudstone-dominated distal ramp deposits, Agrio Formation, Neuquén Basin, Argentina

ABSTRACTThe carbonate-mudstone-dominated Lower Cretaceous Agrio Formation is the youngest marine source rock of the hydrocarbon-prolific Neuquén Basin in Argentina, yet its facies variability and unconventional hydrocarbon potential remains relatively understudied. Detailed studies of mudstone facies variability in thick, carbonate mudstone successions deposited largely below storm wave base (i.e., chalk–marl, black shale, limestone), like the Agrio Formation, are rare and instead commonly focus on biostratigraphy or organic geochemistry alone. A continuous northern section of the Agrio Formation and a southern composite section of the lower Pilmatué and middle Avilé members, totaling ∼ 1,200 m of outcrop, were measured. From these measured sections, programmed pyrolysis (n = 339 samples), X-ray diffraction (XRD; n = 69), and thin sections (n = 69) were used to develop a high-resolution integrated macrofacies and microfacies scheme. The four most volumetrically abundant facies include detrital-quartz-silt-bearing fine mudstone (facies 1), radiolarian-bearing calcareous fine mudstone (facies 2), detrital-quartz-silt- and shell-bearing calcareous fine mudstone (facies 3), and calcareous wackestone (facies 4). All four facies are volumetrically dominated by carbonate mud matrix (i.e., micrite) that represents either 1) original pelagic coccolithophore deposition modified by diagenesis, 2) transported carbonate mud (i.e., bottom currents like contour currents or sediment gravity flows), or 3) a combination of both. Outcrop observations, XRD mineralogic trends, and petrographic variations in grain composition between detrital quartz silt, radiolarian and microfossil to macrofossil content (mainly benthic foraminifera and bivalves) distinguish the four mudstone facies. The facies scheme indicates distinctly more heterogeneous and current-influenced sedimentation in the downdip sub-storm wave base than previously described in the Agrio and in carbonate-dominated basinal settings in general. A depositional model is proposed for further testing that may prove valuable towards re-evaluating basinal carbonate mudstone successions worldwide. Utilizing TOC, S2, and HI value cutoffs, this study defines five discrete stratigraphic packages in the Agrio Formation that have the highest source potential, collectively totaling ∼ 140 m thick. The novel integration of macrofacies and microfacies analysis, stratigraphy, and a geochemical analysis allow both depositional insights and the assessment of a potential source rock. The study adds to a growing body of literature on 1) carbonate ramp (or slope) to basinal processes and 2) facies models for organic-rich, carbonate-dominated mudstone successions that are unconventional hydrocarbon systems.

Coupled Microfracturing and Chemical Weathering of Precambrian Quartzite in the Extremely Humid and Tectonically Active Shillong Plateau, NE India: Implications for In Situ Quartz Weathering and Quartz Silt Production

AbstractThe origin of quartz silt, the most abundant detrital particle, is controversial. Quartz silt is generally considered to be broken quartz sand, attributed to glacial, eolian, and/or long fl...

Detailed facies analysis of Cenomanian–Turonian organic‐rich mudstones: Implications for depositional controls on source rocks

AbstractUnderstanding mudstone depositional processes, including both traction transport and dynamic mechanisms, requires a re‐assessment of how these processes relate to the accumulation and preservation of organic carbon in fine‐grained successions. This relationship was addressed using facies described in detail from a Cenomanian–Turonian aged, organic‐rich, marine mudstone‐dominated succession from the Albertan Western Interior Seaway. All facies exhibit a high degree of heterogeneity at the millimetre‐to‐decimetre scale, contain evidence of bioturbation and display similar ranges of sedimentary structures including wave ripples and current ripples, sharp‐based graded beds and starved ripples. Described facies vary in grain size as well as relative abundances of biogenic versus siliciclastic grains. Siliciclastic grains are dominantly composed of quartz silt as well as silt‐sized to sand‐sized clay mineral–rich aggregates while the biogenic grains are dominantly composed of silt‐sized to sand‐sized calcareous pellets, bivalve fragments and foraminifera. Although the succession is relatively clay mineral‐rich, the majority of clay minerals occur as mud aggregates that were transported by traction processes. The sedimentary structures present are diagnostic of sea floor reworking by waves and currents, indicating that the basin was relatively shallow (i.e. not hundreds of metres deep) with frequent reworking of the sea floor sediment with suspension settling deposits being rare. The abundance of bioturbation and in situ bivalves indicates that there was sufficient oxygen at the sea floor and that widespread bottom water anoxia was not persistent at the time of deposition of the organic‐rich interval of interest. The accumulation of organic‐rich sediment appears to be a preservational phenomenon caused by sediment being buried more rapidly than the organic carbon can oxidize in the surficial layers due to episodic inputs of siliciclastic material during large storm events.

Depositional interpretation and sequence stratigraphic control on reservoir quality and distribution in the Meramecian Sooner trend Anadarko Basin, Canadian, and Kingfisher Counties (STACK) play, Anadarko Basin, Oklahoma, United States

The Mississippian Meramecian play in the greater STACK (Sooner trend Anadarko Basin, Canadian and Kingfisher Counties) play region is an unconventional reservoir in the Anadarko Basin in west central Oklahoma. The play is a fine-grained system composed of quartz silt and sand with varying amounts of carbonate grains and clay. The primary driver of reservoir quality is the quantity of calcite cement. Large volumes of cement are found in coarser grained higher energy facies whereas porosity is preserved in lower energy facies, imparting a strong depositional control on reservoir quality. The play interval comprises a system of deep-water prograding parasequences striking northeast-southwest and prograding to the southeast. Inclinations of clinoforms are less than one degree. Depositional conditions are inferred to be similar along strike, resulting in northeast to southwest facies continuity. The depositional interpretation of this system is a subaqueous delta. These are systems of low-angle, shore-parallel clinoforms fed by fluvial input reworked primarily by longshore currents within or below storm wave base. Core and log analyses indicate the play is a lowstand clastic wedge deposited during early sea-level rise. It is bounded by a sequence boundary and correlative conformity at its base and capped by a transgressive surface of marine erosion. An intermediate order sea-level rise-fall-rise occurs during overall deepening. High order sea-level changes are manifested as stacked parasequences grading upward from argillaceous and quartz siltstones into calcareous siltstones to sandstones. The identified sequence stratigraphic hierarchy imparts a threefold control on reservoir and nonreservoir architecture, as it drives the volume and distribution of porous and nonporous facies at differing scales.