Mixed-layer Illite Smectite Research Articles

Natural gamma-ray spectroscopy (NGS) as a proxy for the distribution of clay minerals and bitumen in the Cretaceous McMurray Formation, Alberta, Canada

Detailed examination of the mineralogy of the Cretaceous McMurray Formation within a facies framework is used to assess the use of natural gamma-ray spectroscopy (NGS) and a pulsed neutron generator (PNG) tool in delineating variation in clay mineral and bitumen contents. Characterization of the mixed-layer (interstratified) clay phases in the McMurray Formation provides an improved understanding of clay interaction in bitumen processing and tailings settling behavior, important for mine planning and tailings remediation schemes. Mineral diversity in the McMurray Formation was determined on facies attributed samples using whole rock X-ray diffraction (XRD), cation exchange capacity (CEC) measurements, elemental analysis (XRF), clay size fraction (<2 µm) XRD analysis, reflected light microscopy, and cryogenic-scanning electron microscopy (cryo-SEM). Kaolinite was ubiquitous in the entire McMurray Formation with lower and middle McMurray Formation sediments also containing mixed-layered illite–smectite (I-S) with a low expandability≈ 20–30%. Upper McMurray Formation sediments by contrast had higher expandability (≈ 60–70%). In floodplain sediments of the lower McMurray Formation an additional clay mineral was quantified as a kaolinite-expandable mixed-layer (clay) mineral. The associated CEC values of this mineral are 10 times the baseline for the McMurray Formation. NGS spectra from cores showed that yields of potassium (K), uranium (U), and thorium (Th) had distinct facies associations, correlated with a clay mineral signature. The resultant indicator is capable of highlighting zones within an oil sands ore body that are empirically known, by industry, to process poorly through extraction plants. A bitumen indicator from the carbon yield derived from a PNG logging tool assesses bitumen content. NGS and PNG allow a full assessment of clay mineral (fines) and bitumen profiles, with the future prospect that these techniques could be used to assess ore and tailings behavior in near-real time.

Characterization, thermal behaviour and firing properties of clay materials from Cap Bon Basin, north-east Tunisia, for ceramic applications

AbstractThis study focuses on the characterization, thermal behaviour and firing properties of clays from Cap Bon Basin, Tunisia. Chemical (inductively coupled plasma atomic emission spectroscopy) and mineralogical (X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy) characterization of the clays was carried out. Physical properties were identified using particle-size distribution and Atterberg limits tests. Differential thermal analysis/thermogravimetry, dilatometry, XRD and FTIR analyses were conducted to assess the thermal behaviour, transformations and crystalline phase development during firing. The clays studied show large proportions of silt and clay fractions. They are rich in SiO2, Al2O3 and Fe2O3 and mostly consist of illite, kaolinite, smectite and minor mixed-layer illite-smectite. Associated minerals are mainly quartz, calcite and feldspar. To monitor the firing properties, three clay mixtures referred to as M1, M2 and M3 were prepared according to the physicochemical and mineralogical characteristics of the raw clays. These were sintered at 900°C, 1000°C and 1100°C and tested for firing shrinkage, flexural strength and water absorption. As a result, various mineralogical transformations occurred at 900–1100°C associated with the crystallization of new phases. The main transformations identified at &gt;1000°C resulted in the formation of anorthite and mullite, causing significant densification of the fired ceramic materials. Sintering at 1100°C increased the densification and thus improved the flexural strength. At this temperature, the sintered ceramic tiles exhibited excellent properties, namely water absorption of 2–3% and flexural strength of 30–41 MPa, indicating that these studied clays might be used as raw materials for the production of stoneware tile ceramics.

Middle to late Eocene chemical weathering history in the southeastern Tibetan Plateau and its response to global cooling

Continental weathering is a crucial factor to maintain the balance of the carbon cycle and thus the livability of the Earth. The Tibetan Plateau (TP), known as the Third Pole of the Earth, has undergone strong uplift since the Cenozoic, thus making it an ideal area to study continental weathering processes and associated driving factors. However, early Cenozoic weathering records are rare in this region. Here, we report high quality mineralogical and geochemical records of clay fraction (< 2 μm) of a middle to late Eocene sequence retrieved from the Qujing Basin, southeastern TP, to track the early Cenozoic chemical weathering history in this area. The results show that the main clay mineral assemblages contain illite-smectite mixed layers, illite and palygorskite. The chemical weathering intensity indicators (e.g., clay mineral assemblages and the chemical index of alteration (CIA)) collectively indicate a clear long-term modulation of global cooling on regional chemical weathering processes during middle to late Eocene, as indicated by high correlation between global temperatures and chemical weathering indicators and the similarity in their periodicity characteristics. Moreover, a sudden decrease of the chemical weathering intensity with large fluctuations was identified at 42–39.9 Ma, which may be linked to rapid transport of clay minerals caused by tectonic activity and/or an increased precipitation in the southeastern TP. This study indicates a control of global cooling on chemical weathering of the southeastern TP area during middle to late Eocene, while tectonic activity or other factors might exert catalytic influences at some certain periods.

Inhibition Impact of Amino Acids on Swelling Clays: An Experimental and COSMO-RS Simulation Evaluation

The presence of swelling clays, namely, montmorillonite and illite-smectite mixed layer clays alike, significantly affects well stimulation processes, such as hydraulic fracturing. In this study, t...

Climate Signals in the Late Quaternary Bottom Sediments of Lake Baunt (Northern Transbaikalia)

Abstract —The bottom sediments of lakes with different mineralization located in the basins of inland areas are high-resolution archives of climate and environmental changes. We present results of study of the Late Pleistocene–Holocene sediments of freshwater Lake Baunt, one of the lakes of the Baunt (Tsipa) depression in the permafrost zone in northern Buryatia. The sediments were studied by XRD, IR spectroscopy, laser granulometry, elemental analysis, AMS dating, etc. Mineral analysis of the bottom sediments with an age of ~18 ka has revealed predominant phyllosilicates, quartz, and feldspars. Mathematical modeling of complex XRD spectra made it possible to identify chlorite, muscovite, illite, mixed-layer illite–smectite and chlorite–smectite, and kaolinite among the phyllosilicates. We have determined their structural and crystal-chemical features and quantitative proportions in the section (800 cm long core), which vary in accordance with the climate cycles and lake level fluctuations. The results obtained helped to reconstruct the evolution of the Lake Baunt basin controlled by the regional climate throughout the Late Pleistocene–Holocene. This paper continues a series of our publications concerned with the reconstruction of the Late Pleistocene–Holocene climate in East Siberia by comprehensive studies of the mineral composition of sedimentary sections of small lakes.

Effects of inhibitor KCl on hydration swelling and softening of a smectite-poor mudstone

The swelling and softening of mudstones caused by water often lead to serious geological disasters. In petroleum exploration and development, the hydration expansion of mudstone can also lead to serious wellbore instability hazards. The swelling of smectite clays is the main reason for this hydration expansion in mudstones. It is generally recognized that illite and illite–smectite mixed layer clays are both poor swellable and has a low effect on rock strength of smectite-poor mudstone. Furthermore, weakening law and degree of smectite-poor mudstone in water is not clear. This study conducted swelling tests and rock mechanics experiments on a smectite-poor mudstone from the Sichuan basin. The results show that both water content and swelling capacity tend to remain steady after soaking the samples for 24 h. The mudstone water content reached 5.36%, and the swelling rate was 2% after soaking in water for 72 h, and the rock had a lower unconfined compressive strength (UCS) value of 1.41 MPa. Adding KCl inhibited these processes, and elevated KCl concentrations drastically reduced the mudstone water content and swelling rate. Compared to water, the water content and swelling rate of mudstone soaked in a 6% KCl solution both decreased by 50%. The UCS and the elastic modulus also increased for a half-dry mudstone, although the inhibitor was important. At elevated KCl inhibitor concentrations, the UCS and elastic modulus of the hydrated mudstone were lower than those of the dry mudstone. The findings of this study can help for better understanding of the application of KCl inhibitor in water-based fluid for drilling engineering.

Clay saline diagenesis in lake Plio-Pleistocene sediments rich in organic matter from the Guadix-Baza Basin (Betic Cordillera, SE Spain)

This study presents a mineralogical analysis of the effect of hypersaline lake environments rich in organic matter on the mineral transformations in sediments of the eastern part of the Guadix-Baza Basin (Betic Cordillera). Dark sediments with fragments of organic matter are rich in phyllosilicates (K-white mica, illite-smectite mixed layers, –I/S– minor amounts of paragonite, palygorskite, chlorite and kaolinite) and contain quartz, feldspars, and framboidal pyrite. X-ray diffraction patterns of I/S (R0 type, disordered) revealed a continuous between the positions of pure smectite and 30% of illite layers. Chemical composition of I/S mixed layers indicated a significant beidellitic substitution of Si by AlIV, Fe + Mg > 1 atoms per formula unit (a.p.f.u.) and K content up to 0.42 a.p.f.u. These data suggested the presence of a minor and variable amount of illitic layers among the major smectitic ones. Saline waters of the lake and the reducing environment of the sediment favored Mg and Fe2+ uptake, generating octahedral negative charge, which together with the beidellitic substitution would provide clay layer negative charge enough to admit K incorporation in the interlayer, hence the beginning of a diagenetic low temperature illitization.

Identification of sequence stratigraphy in the Dongying Formation of the Liaoxi low uplift, Bohai Bay Basin, China

A change in sea level (or lake level) causes a change in accommodation space and sediment compensation, which then controls the formation and evolution of sequences. A multidisciplinary approach based on the integration of clay minerals, major and trace elements, paleosalinity, and three-dimensional seismic data were applied to interpret the stratigraphic sequence of the Dongying Formation in the Liaoxi low uplift. The sequence surface indicates that multistage tectonic action, paleoclimate, paleosalinity, and lake level change jointly controlled the sequence stratigraphy. The illite and illite–smectite mixed layers indicate an arid to semi-arid climate, whereas kaolinite implies humid conditions. Based on this principle, it is clear that the stratigraphic sequence is closely related to paleoclimate. Interpretation of the continental or marine character of the sediments via paleosalinity is increasingly based on the concentration of some trace elements. Hence, the sequence surface can be better evaluated through paleosalinity data. Based on the Sr/Ba and boron content, paleosalinity was evaluated and the sequence boundary was identified. Analysis of the concentration of major and trace elements was used to identify the lithology of the sedimentary region and further discern the stratigraphic sequence. These sequences are composed of lowstand, transgressive, and highstand system tracts. The work herein aimed to understand the reasons for the changes in sequences under the paleoclimate. In addition, major and trace elements, studied together with paleosalinity and clay mineral content, result in the determination of implications for sequence stratigraphy in many other basins. This paper can provide a novel method for comprehensively discerning a sequence surface.

Diagenetic and environmental control of the clay mineralogy, organic matter and stable isotopes (C, O) of Jurassic (Pliensbachian-lowermost Toarcian) sediments of the Rodiles section (Asturian Basin, Northern Spain)

Abstract Clay mineralogical and geochemical analyses (δ18Ocarb, and paired carbonate and organic matter carbon isotope), completed by the characterization of organic matter, have been performed on the Pliensbachian sediments exposed on the Rodiles section (Asturian Basin, Northern Spain). The objectives were to precise the Pliensbachian climate evolution at a relatively low paleolatitude of the Tethyan domain by comparison with previously published high-resolution clay mineralogical and geochemical data from the NW Tethyan domain (e.g. Paris Basin and Cardigan Bay Basin, West Wales). The clay mineral assemblages are dominantly composed of illite and illite-smectite mixed-layers (I–S) associated with minor proportions of chlorite and kaolinite. Smectites are not identified what may result either to their absence at the time of sedimentation or to their progressive illitization due to the burial diagenesis. Rock-Eval data show that organic matter reaching more than 5% in black-shale horizons is thermally mature with Tmax comprised between 444 and 454 °C indicating that the section is in the oil window and that the burial temperature reached or exceeded 80–90 °C. The low δ18Obulk values comprised between −3 and −6‰ clearly results from a diagenetic influence (thermal effect due to burial) but their fluctuations compared with δ18O measured on well-preserved belemnites guards show that the overall trend is however preserved. By comparison with higher latitude sedimentary basins, the most striking feature is the very small proportions of kaolinite (generally less than 5% while the proportions of this mineral reach 40% at higher paleolatitudes), except at the Pliensbachian/Toarcian transition, where the proportion of kaolinite increases significantly. Clay minerals likely originated from the erosion of the Iberian Massif located to the South of the Asturian Basin in a semi-arid climatic belt. This may explain the scarcity of kaolinite by comparison with sedimentary basins located at higher latitude influenced by more humid climate. The increasing proportions of kaolinite by the end of the Pliensbachian suggest southward expansion of the humid belt as a consequence of cooler temperatures and the establishment of steeper latitudinal gradients of temperature likely resulting from the formation of restricted polar ice cap. The main δ13C (on both 13Cinorg and 13Corg) Pliensbachian excursions including the negative excursion of the Sinemurian/Pliensbachian Boundary Event (SPBE) and the Late Pliensbachian positive excursion are well recognized.

Hydration behavior by X-ray diffraction profile fitting of smectite-bearing minerals

Clay mineral hydration and dehydration processes are reversible at temperatures &lt;100 °C and strongly affect wellbore stability, fines migration, permeability, and dispersion of pore pressure. The hydration behavior of smectite-rich material as a function of relative humidity (activity of water, aw, controlled by salinity) and temperature was studied using in situ X-ray diffraction on a material retrieved from coring in the Gulf of Mexico. X-ray diffraction profile fitting was used to explore the competition for water between hydratable phases across a range of relative humidity, 2 % to 90 %, and temperature, 25°C to 95°C, conditions. X-ray diffraction profile fitting employed a modified multi-specimen approach in which proportions of minerals were modelled using Ca-exchanged preparations in air-dried and ethylene glycol solvated states. Across the range of hydration states, the mineral proportions and crystallographic parameters remained constant from the multi-specimen approach and only the number of water layers in hydratable phases varied. Quantitative clay mineralogy showed a natural material with a discrete smectite component and a mixed-layered illite-smectite, both capable of hydration/dehydration. Results of this study showed the discrete smectite component and the mixed-layered illite-smectite hydrated at different rates with discrete smectite up-taking more water at lower relative humidity than the mixed-layered illite-smectite. Over geological time this study highlights the non-static nature of smectite hydration with implications of long-term creep and permeability behavior.

Changes in water vapor adsorption and water film thickness in clayey materials as a function of relative humidity

AbstractSoil water, with adsorbed water being an important component, plays an important role in fluid flow and chemical movement in the unsaturated zone. The adsorbed water consists of water film and interlayer water, and its content is related to the relative humidity (RH). This study theoretically and experimentally focuses on the variation in adsorbed water content in clayey materials as the RH changes. Based on a slit‐pore model, three types of water (water film, interlayer water, and capillary water) could be present in the water vapor adsorption process. The variation of water film is obtained from the analysis of interfacial forces, and then the total water content can be quantitatively subdivided into these three types of water for different RH values. The slit‐pore adsorption model was used with experimental measurements (water vapor adsorption and N2 physisorption) to quantify the amounts and interrelationships of these three different types of water to six clayey materials, namely three clay minerals (kaolinite, montmorillonite, and illite–smectite mixed layer) and three clay‐rich sediments from the Jianghan Plain in China. The main results are that (a) the volumes of water vapor adsorption are much greater than those of N2 adsorption except for kaolinite; (b) interlayer water largely dominates the growth of total water content in montmorillonite and results in a concentration of 0.090 ml g−1 at 95% RH; and (c) the minimum thickness of water film is calculated to be 0.23 nm, and the maximum value is one‐third of the pore width by considering the interfacial forces.

Phyllosilicates in bottom sediments of Lake Baunt (northern Transbaikalia) as indicators of paleoclimate

The lacustrine sediments are unique natural archives that store records of environmental and climatic changes. We present results of study of the Late PleistoceneHolocene sediments of freshwater Lake Baunt, located in the permafrost zone in northern Transbaikalia. The sediments were studied by X-ray diffraction (XRD), IR spectroscopy, laser granulometry, elemental analysis, AMS dating, etc. The results of mineral analysis of the bottom sediments with an age of ~18 ka showed that they are mainly represented by phyllosilicates, quartz, and feldspars. By the mathematical modeling of complex XRD patterns chlorite, muscovite, illite, mixed-layer illitesmectite and chloritesmectite, and kaolinite identified among the phyllosilicates. It was established that variations of their structural features as well as quantitative proportions in the section are in high correlation with the climate changes and lake level fluctuations. The results obtained helped to reconstruct the evolution of the Lake Baunt basin controlled by the regional climate throughout the Late PleistoceneHolocene.

Glucose-stimulation of natural microbial activity changes composition, structure and engineering properties of sandy and loamy soils

High inputs of easily available organic matter to the subsurface may quickly activate the native microbial communities, thereby changing soil engineering properties. We studied the effect of glucose addition, an easily available carbon source, on stress-strain properties, mineralogy, and microstructure of several loamy and sandy soils over 30 days in laboratory experiments. During the period of high microbial activity, direct shear tests revealed a reduction of the friction angle of 15–30 % and a raise of cohesion in sands. Unconfined compression tests showed a 20–30 % decrease in the compressive strength of loamy soils. With the decline of microbial activity, the stress-strain properties recovered partially. The alterations of the stress-strain properties with increasing microbial activity were linked to changes in mineralogy and composition. X-ray diffraction showed that the proportion of smectite layers in illite-smectite mixed layer minerals increased, as well as the overall imperfection of clay minerals. Glucose addition resulted in a temporary increase in the content of microaggregates (0.1-0.05 mm). Newly formed linkages of organic matter between solid particles, biofilm formation and direct interaction of cells with mineral surfaces were observed by scanning electron microscopy. Our data show that microbial-mediated processes may adversely influence stress-strain properties of soils and endanger the safety of buildings.

Threshold Pore Pressure Gradients in Water-Bearing Tight Sandstone Gas Reservoirs

Tight gas reservoirs commonly occur in clastic formations having a complex pore structure and a high water saturation, which results in a threshold pressure gradient (TPG) for gas seepage. The micropore characteristics of a tight sandstone gas reservoir (Tuha oilfield, Xinjiang, China) were studied, based on X-ray diffraction, scanning electron microscopy and high pressure mercury testing. The TPG of gas in cores of the tight gas reservoir was investigated under various water saturation conditions, paying special attention to core permeability and water saturation impact on the TPG. A mathematical TPG model applied a multiple linear regression method to evaluate the influence of core permeability and water saturation. The results show that the tight sandstone gas reservoir has a high content of clay minerals, and especially a large proportion of illite–smectite mixed layers. The pore diameter is distributed below 1 micron, comprising mesopores and micropores. With a decrease of reservoir permeability, the number of micropores increases sharply. Saturated water tight cores show an obvious non-linear seepage characteristic, and the TPG of gas increases with a decrease of core permeability or an increase of water saturation. The TPG model has a high prediction accuracy and shows that permeability has a greater impact on TPG at high water saturation, while water saturation has a greater impact on TPG at low permeability.

The cap rocks sealing the reservoir in the Węglówka oil field, sub-Silesian unit, Polish outer Carpathians: Petrographical approach

The Weglowka oil field is located in the outer Carpathians. The outer Carpathians are a region where hydrocarbons were discovered and exploited at the end of the nineteenth century in several dozen oil fields, which are relatively small. The Weglowka oil field is one of the largest in this region. In the 150 yr or so of hydrocarbon exploration in the area, more than 1 million t (>1,237,000 tons [>8,841,000 bbl]) of oil have been produced. Hydrocarbons are concentrated in Lower Cretaceous sandstones (Grodziszcze and Lgota sandstones) that form an anticline sealed by Upper Cretaceous marls called the Weglowka marls. These cap rocks are up to 600 m (2000 ft) thick. Because of the thrust-related exhumation, they were exposed at the surface and represent the youngest deposits in the region. The present work is focused on a detailed petrographic characterization of the Weglowka marls. This study allows petroleum geologists to better understand the evolution of porosity in these cap rocks and can serve as a foundation for the prediction of their sealing properties. The marls appear as a succession of interbedded red and green varieties, which occur in up to 2-m (6-ft)-thick beds. These beds are nonarenaceous, soft, and bioturbated. Grain size corresponds to approximately 80% clay and less than 20% silt fractions. X-ray diffraction (XRD) reveals that the marls contain, on average, 54% clay, 28% calcite, 16% quartz, up to 3% feldspars and, in red marls, 3% hematite. The XRD patterns of clay are typical of mixed-layer illite–smectite ([I–S]; 40% illite in I–S). The clay structures are dioctahedral with similar octahedral Mg and relatively high Fe3+ contents both in the red and green intervals. As revealed by standard petrography combined with high-resolution petrography performed through the use of a field emission scanning electron microscope, the marls have mudstone textures according to Dunham’s (1962) classification and are mostly composed of coccoliths and clay with rare nanoquartz. This rock may be considered an impure chalk. Sealing properties of the Weglowka marls are indicated by the specific surface area, porosity, pore size, and permeability, calculated using N2 gas adsorption, helium, and mercury porosimetry. The sealing potential is postulated to result from a combination of the following: (1) origin of components (i.e., deposition of minute calcareous bioclasts and volcanic material as a source for clay); (2) oxygenated sedimentary environment (as a result of the presence of oxygen in the sediments, burrowing caused the rocks to be homogenized); and (3) tectonic-induced clogging of pore space because of reorganization of clay flakes (the rocks were strongly tectonically deformed, which resulted in reduction of porosity in clay aggregates).

Supercritical CO2 and CH4 Uptake by Illite-Smectite Clay Minerals.

Clay minerals abound in sedimentary formations and the interaction of reservoir gases with their submicron features have direct relevance to many geoenergy applications. The quantification of gas uptake over a broad range of pressures is key toward assessing the significance of these physical interactions on enhancing storage capacity and gas recovery. We report a systematic investigation of the sorption properties of three source clay minerals-Na-rich montmorillonite (SWy-2), illite-smectite mixed layer (ISCz-1), and illite (IMt-2)-using CO2 and CH4 up to 30 MPa at 25-115 °C. The textural characterization of the clays by gas physisorption indicates that micropores are only partly accessible to N2 (77 K) and Ar (87 K), while larger uptakes are measured with CO2 (273 K) in the presence of illite. The supercritical excess sorption experiments confirm these findings while revealing differences in uptake capacities that originate from the clay-specific pore size distribution. The lattice density functional theory model describes accurately the measured sorption isotherms by using a distribution of properly weighted slit pores and clay-specific solid-fluid interaction energies, which agree with isosteric heats of adsorption obtained experimentally. The model indicates that the maximum degree of pore occupancy is universal to the three clays and the two gases, and it depends solely on temperature, reaching values near unity at the critical temperature. These observations greatly support the model's predictive capability for estimating gas adsorption on clay-bearing rocks and sediments.

Division of diagenesis and pore evolution of a Permian Shanxi shale in the Ordos Basin, China

This research analyzed the pore evolution and diagenesis in a Permian Shanxi shale from the Ordos Basin. Thermal simulation method simulated the thermal evolution of organic matter in a sample from mature (Ro = 0.96%) to overmature (Ro = 3.05%). Gas adsorption and mercury intrusion capillary pressure techniques were used to analyze pore-size distribution across a maturation gradient. X-ray diffraction, Focused Ion Beam-Scanning Electron Microscopy and rock pyrolysis techniques were used to divide the stages of diagenesis and pore evolution. Results showed that mesopores and macropores dominate pore volume while micropores and mesopores dominate surface area. Pore volume and surface area initially decreased and then increased with maturity. Kaolinite is converted to illite and illite-smectite mixedlayer in the middle to late diagenesis stage. Pore evolution and diagenesis can be divided into four stages in a sample from mature to overmature. Factors influencing pore-size distribution are different in four stages.

Water-sensitive characterization and its controlling factors in clastic reservoir: A case study of Jurassic Ahe Formation in Northern tectonic zone of Kuqa depression

Water-sensitivity is a factor that must be paid attention to in the reservoir development stage of oil and gas fields. As a clastic reservoir, the water effect of Ahe formation in the northern structural belt of Kuqa is strong as a whole, but the mechanism analysis of medium and strong water-sensitivity effect is restricted by the evaluation method with permeability damage rate as the main parameter. Taking the shape of water-sensitivity test curve as the starting point, combined with the analysis of microscopic pore throat and clay minerals, the difference of permeability change rate measured by samples is characterized. The strong to medium water-sensitivity effects in the study layer are divided into three types: The permeability gradually decreases in the early stage-the rapid decrease in the late stage, the continuous decrease in the permeability, and the rapid decrease in the early stage-the slow decrease in the later stage. As a widely developed reservoir space in the study section, the micropore not only has the characteristics of fine pore size and easy blockage, but also serves as the occurrence space of the main water-sensitive mineral illite-smectite mixed layer. Therefore, the change types of the above different water sensitivity test curves are mainly controlled by the difference of micropore content. When the micropore content is low, the permeability decreases slowly in the early stage and decreases rapidly in the later stage (type I), while when the micropore content is high, the permeability decreases rapidly in the early stage and slowly decreases in the later stage (type II), while when the micropore content is medium, the permeability decreases continuously, and there is no obvious rate change (type III) before and after the micropore content. The results show that the structural characteristics and relative content differences of micropores in the samples are the internal mechanism of the difference of water sensitivity effect types.

Thermodynamic properties of mixed-layer illite-smectite by calorimetric methods: Acquisition of the enthalpies of mixing of illite and smectite layers

The stability of illite-smectite interstratified with respect to discrete illite and smectite minerals was investigated by measuring their thermodynamic properties. The standard thermodynamic properties of the illite-smectite ISCz-1 mineral (G, H, S, Cp, and V) were determined between 298.15 K and 375 K by using calorimetric methods. Moreover, the enthalpies of mixing between the illite and smectite layers were measured at 298.15 K by acid solution calorimetry from a complete series of illite-smectite interstratified minerals (Shinzan area, Japan). The measured values were slightly negative, with a minimum value of −3.7 kJ.mol−1. This contributed to the stability of the mixed-layer with respect to a mechanical mixture of illite and smectite. In addition, the model from Blanc et al. (2015) was implemented to estimate the thermodynamic properties of the interstratified illite-smectite ISCz-1 mineral. The predicted values were consistent with the experimental results. However, the estimates were slightly improved by considering the thermodynamic properties of the mixture of the illite and smectite components and then adding the terms of energies of mixing. This could be confirmed by establishing the stability domains of ISCz-1 and those of the corresponding illite and smectite end-members according to Meunier and Velde’s determination of the smectite to illite reaction pathways (Meunier and Velde, 1989).

Characteristics and distribution of clay minerals and their effects on reservoir quality: Huagang Formation in the Xihu Sag, East China Sea Basin

The characteristics and distribution of clay minerals and their effects on reservoir quality in the Huagang sandstones in the Xihu Sag, East China Sea Basin were studied by using X-ray diffraction, casting thin-sections, scanning electron microscopy, electron microprobe analysis, fluid inclusion analysis, constant-rate mercury injection and nuclear magnetic resonance. Clay minerals consist of kaolinite, chlorite, illite and illite–smectite mixed layer (I/S); kaolinite forms from dissolved feldspars, chlorite occurs as clay coatings that are transformed from clay precursors owing to the flocculation of suspended detrital clays or the crystallisation of pore fluids, and illite forms from the illitisation of detrital smectite, authigenic kaolinite and K-feldspars. Clay distribution is controlled by sedimentary environments, burial history and lithologies. Typical reservoirs in the western sub-sag are thin and developed in braided river facies at relatively shallow burial depths with clays dominated by kaolinite. However, typical reservoirs in the central inversion tectonic zone are thicker and developed in a braided delta front facies at deeper burial depths with clays mainly consisting of chlorite, illite and I/S. High-quality reservoirs are characterised by coarse granularity, high quartz content and low clay content with widespread development of chlorite coatings that inhibit quartz cements at low temperatures. At higher temperatures, the high-quality reservoirs develop more pores providing growth space for quartz cements and result in the coexistence of chlorite coatings and quartz cements. The high-quality reservoirs are controlled by their lithological characteristics rather than chlorite coatings. Illite and I/S clays create severe damage to reservoirs by reducing the size and connectivity of pore-throats.