Transformation Of Smectite Research Articles

Surface morphologic changes and mineral transformation mechanisms after CO2-brine-rock reaction in organic matter-rich ferroan shale

Sequestrating CO2 in shale reservoirs can not only reduce CO2 emission, and enhance oil and gas recovery. Nevertheless, shale reservoirs were normally formed in reducing environments, and are abundant in Fe2+ and organic matter. Fe2+ influence the dissolution and precipitation behavior of minerals within the reservoir. Simultaneously, organic matter occupies the pore spaces within the reservoir, thereby affecting the reservoir physical properties. The CO2-brine-rock reaction experiment was carried out using organic matter-rich ferroan shale in this study. The changes in surface morphology of minerals and organic matter utilizing X-ray Diffraction localization monitoring techniques. Based on the TOUGHREACT, a quantitative analysis was conducted on the dissolution and precipitation of individual components. The results suggest that the carbonates preferentially dissolved at the early stage, resulting in a rougher surface morphology andthe formation ofcracks. During the early stage, ankerite transformed into pyrite and chlorite. As the reaction progresses, an elevation in the solution’s acidity facilitates converts chlorite to pyrite and kaolinite. After the dissolution of dolomite, calcite is the primary secondary mineral. The formation of clay minerals consumes Mg2+, resulting in minimal magnesite. The dissolution of feldspar was weaker than carbonate minerals. Dissolution of plagioclase results in the formation of dawsonite in solution. The dissolution of K-feldspar induces the transformation of smectite to illite. In addition, as the reaction solution acid escalated, the soluble mineral components in the large-particle polymer dissolved and dispersed into small-particle organic matter. The small-particle organic matter absorbs secondary minerals generated to form a cake-like polymer.

Induced polarization of clay-rich materials — Part 5: Influence of hyperalkalinity in bentonites

A total of 54 new core samples were built by mixing smectite-rich MX80-bentonite and Fontainebleau (silica) sand to study the effect of a hyperalkaline pore solution (including the presence of potassium) on the induced polarization properties of these mixtures. Two types of experiments were performed. In the first set of experiments, we performed induced polarization measurements 10 minutes after the preparation of the core samples (static experiments) for a broad variety of water contents and saturations. The second set of experiments was performed for over 1000 hours to study the potential effect of illitization upon the induced polarization properties (time-lapse experiments). Indeed, in the presence of potassium in the hyperalkaline pore solution favors the transformation of smectite into illite. Furthermore, we expect cation replacement (sodium replaced by calcium and potassium) in the smectite interfoliar pore network. For the static experiments, the relationships developed in the previous papers of this series regarding the dependence of the in-phase and quadrature conductivities on the water content and saturation remain valid. For the time-lapse experiments, we observed a decrease of the in-phase and quadrature conductivities over time. These trends can be fitted by a decaying exponential function. This behaviour is explained by the kinetics of the illitization process and its effect on the cation exchange capacity of the material. Induced polarization appears therefore as a promising technique to monitor the effect of hyperalkaline solutions associated with the leaching of concrete and permeating a bentonite seal in Deep Geologic Disposal facilities that could be potentially used for high-level and long-lived radioactive wastes.

The evolution of clay mineral and its indication of hydrocarbons under overpressure: An example from the shale of the Qingshankou Formation in the Gulong Sag

The enrichment and development of shale oil are significantly influenced by the evolution of clay minerals. In this paper, the mineralogy and clay mineral crystallinity of shale samples from Wells X1, X2 and X3 in the Gulong Sag are characterized by X-ray diffraction analysis (XRD) and field emission scanning electron microscopy (FE-SEM). Geochemical parameters, including total organic carbon (TOC) and rock-eval pyrolysis, were also evaluated. The results reveal that illite in the shale primarily exists in the matrix, originating mainly from the transformation of smectite and I/S mixed layer. Chlorite in pores is predominantly formed through fluid precipitation and crystallization. The study area exhibits abnormal evolution of illite and I/S mixed layers, as well as the phenomenon of rapid chlorite growth under overpressure condition. The abnormal evolution of illite and I/S mixed layer may attribute to the inhibition of the conversion reaction from I/S mixed layer to illite. Chlorite's rapid growth occurs through the nucleation mechanism. Furthermore, through the analysis of clay and organic matter correlation, coupled with overpressure and hydrocarbon-rich section considerations, it is observed that chlorite may play a significant role in the storage and generation of S1. This study contributes to a better understanding of the relationship between clay mineral evolution and shale reservoir overpressure, offering valuable insights for the accurate assessment of shale oil.

Diagenetic controls on the quality of shallow marine sandstones: An example from the Cambro-Ordovician Saq Formation, central Saudi Arabia

The Saq Formation is the most important Paleozoic aquifer in central and northern Saudi Arabia. However, owing to the continued depletion of the aquifers and the great depths to which the aquifers are buried in undrilled/underexplored regions, the aquifers porosity and permeability are increasingly becoming an issue of concern. Here, we employed thin-section petrography, petrophysical measurements, X-ray diffraction and scanning electron microscopic analyses, and fluid inclusion microthermometry to investigate the composition and diagenetic evolution of the Saq sandstones. The results of this study indicate that the sandstones are mainly quartz arenite in composition, mostly well sorted, and ranged in grain size from silt to coarse-grained sandstones. We observed three diagenetic stages (early, middle, and late) that impacted the porosity evolution of the Saq sandstones. Mechanical compaction, formation of authigenic kaolinite, and infiltration of smectitic clays occurred during early shallow burial diagenesis. The middle (deep-burial) diagenesis was dominated by mechanical and chemical compaction, formation of authigenic quartz overgrowths, transformation of kaolinite and smectite into illite, and precipitation of late calcite. The late (uplift-related) diagenesis resulted in the formation of authigenic kaolinite, dissolution of the late calcite, and the formation of predominantly pore-filling Fe-oxide (goethite) cement. Fluid inclusion results of this study show that quartz cement mostly formed at temperatures ranging between 151 °C and 175 °C and reached up to 272 °C, with, at least, two phases of overgrowth development. The overall high amounts of rigid grain contents (>65 %), low detrital matrix (<10 %), and coarser grain size have maintained the Saq sandstone quality (average permeability = 2100 mD) despite having been buried to great depths. The findings of this study provide useful insights into the diagenetic evolution of the Saq Formation, which has provided important input data for forward diagenetic modelling of their subsurface equivalent in Saudi Arabia and sandstones of similar depositional settings elsewhere.

Mineral transformation, element transport and hydrological impact in weathering at the Bingling Temple Grottoes: Implications for weathering in alkaline environments in NW China

Alkaline environments are common in arid and semi-arid regions in northwestern China and have preserved quantities of rock heritages along the Silk Road. In contrast to the frequently reported weathering mechanism of stone heritages in acidic conditions, mineral weathering mechanism (especially the transformation mechanism of smectite) in natural alkaline environments still remains to be evaluated. To deepen the understanding of weathering in such an environment, research idea from critical zone science was applied to investigate the element transport and mineral transformation from protolith (interbedded sandstone and conglomerate) to regolith in a world cultural heritage, Bingling Temple Grottoes (BTG), in northwestern China. From geochemical and mineralogical characterizations on boreholes, excavated profiles, fractures and cliff surface, we found that dissolution of calcite, plagioclase (anorthite, albite) and K-feldspar (orthoclase) caused major elemental loss of Na, K and Ca (to aqueous solution) in the BTG, and elements of Mg and Fe mostly retained in solid materials. Characterizations on the cliff surface demonstrated higher extents of element depletion and mineral weathering and lower ultrasonic P-wave velocities in sandstone than in conglomerate, and in the bottom sandstone layers (located in the capillary zone and exposure to rain scouring) than in upper sandstone layers (located in the cave and free of precipitation). The transformation of smectite from protolith to regolith follows the sequential reactions: dehydrated smectie → bihydrated smectite → randomly interstratified illite/smectite → illite. Such transformation pathway is favored by alkaline pH and wetting–drying cycles frequently occurred at the BTG. Typical weathering patterns such as clay swelling damage, efflorescence, salt precipitation, etc., are highly-linked to seasonal changes in the hydrological process influenced by both reservoir construction and groundwater system. Our study provides more evidence for the illitization of smectite in natural alkaline environments and highlights the hydrological impacts on weathering patterns of rock grottoes.

Clay mineral transformation mechanism modelling of shale reservoir in Da’anzhai Member, Sichuan Basin, Southern China

Shale reservoirs often undergo intense clay mineral transformation, which plays a crucial role in the formation and evolution of pores. The reservoir lithofacies types of Da’anzhai Member in the Sichuan Basin are complex, the heterogeneity is strong, and the transformation mechanism of clay minerals is unclear, limiting the understanding of reservoir diagenesis and reservoir formation mechanism. In this study, we selected the typical shale reservoir in the Da’anzhai Member of the eastern Sichuan Basin and innovatively introduced the multiphase fluid-chemical-thermal multi-field coupled numerical simulation technique to focus on the dissolution, precipitation and transformation laws of diagenetic minerals in the shale reservoir. We calculated the transformation of diagenetic minerals and their physical response under different temperatures, pressure and fluid conditions and identified the main controlling factors of mineral transformation in shale reservoirs in the study area. The results show that the transformation of smectite to illite in the Da’anzhai Member is a complex physicochemical process influenced by various factors such as temperature, pressure, fluid, and lithology. The increase in temperature can promote illitization until the critical temperature of 110°C–115°C, below which the conversion rate of smectite to illite increases as the temperature increases. However, when it is higher than the critical temperature, the degree of illitization decreases. In specific K-rich fluids, organic acids significantly affect the conversion of clay minerals in the Da’anzhai Member of the formation. The acidic fluid promotes the dissolution of minerals such as K-feldspar and releases K+, thus provides the material basis for illitization. The research results provide theoretical support for the diagenetic and formation mechanism of the shale reservoir in the Da’anzhai Member of the Sichuan Basin and even for the efficient exploration and development of shale gas.

Clay mineralogical evidence of near-equatorial Palaeocene–Eocene Thermal Maximum in Barmer Basin, India

Abstract The Palaeocene–Eocene Thermal Maximum (PETM) was a global extreme climatic event, but it is relatively unknown from lower latitudes or equatorial regions in comparison to mid- and high latitudes. The present study provides the first clay mineralogical evidence of the PETM and subsequent hyperthermal events in a near-equatorial region represented by the Akli Formation in the Barmer Basin, India. The 32 m-thick succession of the Akli Formation shows abrupt changes in smectite and kaolin abundances preceding, during and succeeding the PETM event. Within the studied section, the kaolin content increases from 5–8% pre-PETM to 30–35% during the PETM, and then again decreases to 5–6% during the post-PETM period. The smectite, however, is marked by a corresponding decrease and its transformation into kaolin in acid weathering conditions. The transformation of the smectite is first marked by hydroxy interlayering and then transformation into kaolin during the PETM. The transformation of smectite into kaolin also resulted in extensive precipitation of iron oxide in sediments. The clay mineralogical changes in the Palaeocene–Eocene transition sediments of the Akli Formation were caused by 3–5°C warming and a 25–50% increase in rainfall during the hyperthermal events. Unusually high charcoal (~20%) fragments during the Palaeocene–Eocene transition also suggest warming and widespread biomass burning during the PETM in the lower latitudes.

Characterization of Compacted Ca- and Na-Bentonite with Copper Corrosion Products in the KAERI Underground Research Tunnel

In a deep geological disposal system, bentonite buffer material is an important barrier used to protect the disposal canister from the inflow of groundwater and prevent the outflow of radionuclides. This study aimed to characterize the mineralogical and chemical reactions of bentonite caused by copper corrosion of the canister in a radioactive waste repository. We investigated the d-spacings of montmorillonite in Gyeongju bentonite (Ca-type, KJ-I) under groundwater-saturated conditions over 10 years and compared their characteristics with those of Wyoming bentonite (Na-type, MX-80) in the Korea Atomic Energy Research Institute Underground Research Tunnel. Mineralogical investigations using X-ray diffraction and focused ion beam energy-dispersive spectroscopy indicated that no transformation of smectite or neo-formed clay phases occurred. In the Ca-type bentonite (KJ-I), the swelling was observed when it was in contact with rolled plate (RP) and cold-spray-coated (CSC) copper, with d-spacing expansions of 2.9% and 3.8%, respectively. In contrast, the Na-type bentonite (MX-80) showed d-spacing expansions of 17.6% and 19.6% when it was in contact with the RP and CSC Cu, respectively. The Cu concentration and distribution indicated that the corrosion products dissolved and then diffused into the surrounding bentonite, with maximum penetration depths of 2.0 and 0.5 mm over 10 years, respectively.

Mechanism of microorganisms to relieve reservoir water sensitivity damage

Reservoir water sensitivity damage means that the contact between the external fluid and reservoir rock lead to the hydration, expansion, dispersion, and migration of clay minerals, thereby reducing the formation permeability and causing damage to the reservoir. In this study, the transformation of smectite to illite (S-I) is promoted by endogenous microorganisms to achieve anti-swelling of the clay minerals and reduce the water sensitivity damage of the reservoir. The microbial community reducing Fe(III) was enriched from the samples of Karamay Oilfield, Xinjiang, and microbial community diversity was analyzed by the 16S rRNA gene sequencing. The Fe(II) ion concentration in the interaction medium was monitored, and the maximum iron reduction rates reached 60.2%. The structural changes of smectite before and after microbial treatment were observed by Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. The result demonstrated that the reduction of Fe(III) was accompanied by the transformation of S-I. There may be some endogenous microorganisms that transform S-I in low permeability reservoirs. This study is of great significance for oilfields to realize clay mineral anti-swelling and eliminate reservoir water sensitivity damage by endogenous microorganisms.

The origin of two distinct diagenetic depth zones in hydrocarbon-bearing sediments of Huanghekou Sag, offshore Bohai Bay Basin, China

Formation waters from the Huanghekou Sag, offshore Bohai Bay Basin, exhibited two depth zones with elevated total dissolved solid (TDS) composition. The shallow diagenetic zone at a depth of ∼1200–2000 m (∼45–70 °C) and the deep diagenetic zone at a depth of ∼3000–4000 m (∼100–130 °C) have TDS concentrations exceeding 70,000 mg/L, while the depth interval in between the two zones has TDS concentrations lower than 35,000 mg/L. Isometric log-ratio interpretations of the reconstructed formation water compositions, crude oil compositions, stable hydrogen and oxygen isotopes, light microscopy and clay mineral analysis were used to interpret the origins of the two diagenetic depth zones. Formation waters within each stratigraphic unit were dominantly composed of Na–K–Cl type. Processes affecting the deep diagenetic zone were attributed to (1) the dissolution of halite, albite and albitization, which predominantly caused the TDS enrichment, and (2) the dilution of TDS caused by the inter-layer H2O released from the transformation of smectite to illite (salinity reversal at depths > ∼3500 m). In comparison, the enrichment in TDS in the shallow diagenetic zone was attributed to hydrocarbon biodegradation, which consumed free water. The crude oil composition indicates severe hydrocarbon biodegradation. Biodegradation of low-molecular-weight organic carbons such as hexadecane (through methanogenesis) and benzene (through microbial sulfate reduction) could consume sufficient H2O to enrich TDS to the observed levels. At the same time, relatively constant Na/Cl and Ca/Cl ratios versus TDS can be explained by water consumption as well. This work could serve as a reference for the interpretation of two diagenetic zones characterised by distinct TDS enrichments in other hydrocarbon-bearing sedimentary basins.

Genesis of authigenic clay minerals and their impacts on reservoir quality in tight conglomerate reservoirs of the Triassic Baikouquan formation in the Mahu Sag, Junggar Basin, Western China

Authigenic clay minerals are widely distributed and considerably affect the reservoir quality of the conglomerate reservoirs in the Triassic Baikouquan Formation (BF), Mahu Sag, Junggar Basin, Western China. Their occurrences, origins, and impacts on reservoir quality were investigated utilizing petrographical analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), Mercury intrusion capillary pressure (MICP), and fractal analysis. Pore-lining chlorite coatings (Ch1) and pore-filling chlorite aggregates (Ch2), respectively, originate from smectite precursors and directly precipitate from pore water. The first-stage kaolinite (K1) formed by plagioclase dissolution appears as flake-shaped single crystals, whereas the second-stage kaolinite (K2) formed by K-feldspar particle dissolution appears as complete false-hexagonal single crystals and vermiform-shaped aggregates. Smectite transformation produces first-stage illite (I1) with honeycomb-like shaped aggregates. The second-stage illite (I2) displaying a flake-shaped occurrence with a directional arrangement is attributed to the transformation of K-feldspar. Based on the burial history, contact relationship of diagenetic minerals, and hydrocarbon charging period, the paragenetic sequence of the conglomerate reservoir is summarized.Afterward, Neimark's fractal model is employed to calculate the fractal dimensions (Fd) of the reservoir with different dominant clay minerals. It is revealed that pore-lining chlorites have the smallest Fd values (<2.5) and their pore size reached 0.6 μm, whereas pore-filling chlorites possessed the greatest heterogeneity with the narrowest pore size distribution. Illite and kaolinite affect reservoir quality in both favorable and detrimental ways. Eventually, the pore evolution within different lithofacies was analyzed. The first promising reservoirs are well-sorted fine-grained conglomerates that have experienced two phases of oil charging, wherein well-developed primary intergranular apertures, secondary dissolution apertures, and kaolinite are commonly observed. Medium-grained conglomerate reservoirs with high clay amounts and poor sorting have been subjected to rapid and intense compaction, resulting in the worst reservoir quality while no dissolution occurred. This work may provide a useful reference for hydrocarbon exploitation of reservoirs which formed under similar geological conditions.

Role of diagenetic alterations on porosity evolution in the cretaceous (Albian-Aptian) Bima Sandstone, a case study from the Northern Benue Trough, NE Nigeria

The Cretaceous Bima Sandstone is a potential hydrocarbon reservoir that is currently being explored in the Northern Benue Trough, NE Nigeria. The sedimentological characteristics and tectonic evolution of the formation have been extensively studied by previous researchers. However, the impact of diagenetic alterations on porosity evolution of the sandstone is poorly understood. This study employed a multi-technique approach, including thin-section petrography, SEM, XRD, and cathodoluminescence analyses to investigate the role of framework composition, primary depositional characteristics, and diagenetic alterations on porosity evolution of the sandstones. The results of the study indicate that the Bima Sandstone has a present-day intergranular porosity ranging from 0 to 25.9%, averaging 5.8%. Diagenetic alterations, including mechanical and chemical compaction and quartz cementation, have significantly modified the porosity of the sandstones more than the primary depositional features (grain size and sorting). Porosity reduction was mainly due to mechanical compaction (av. 32.1%) than cementation (av. 7.4%). Quartz overgrowths (up to 15%) are the most abundant cement in the sandstones, formed primarily due to lack of grain-coating clays. The main eogenetic alterations related to shallow burial include dissolution and kaolinitization of feldspars and, to some extent, mechanical infiltration of smectitic clays, which formed largely discontinuous clay coatings and pore-filling clays. Mesogenetic alterations related to deep-burial include the formation of quartz overgrowths, transformation of kaolinite and smectite into illite, and pervasive albitization of both plagioclase and K-feldspar, which is exclusively associated with the deepest, Lower Bima Sandstone Member, and has significantly reduced the secondary porosity in the sandstones. Telogenetic alterations related to uplift, mainly include dissolution and kaolinitization of unstable feldspars, which has significantly increased the present-day, secondary porosity of the sandstones to up to 15%. Pore-filling clays (chlorite, illite, and kaolinite) have had a small impact on the intergranular porosity of the sandstones, mainly because of their low volume. The study provides insights into how diagenesis can significantly affect the reservoir porosity of the sandstones and could also assist in reducing the risk of exploration of the fluvial reservoir in the basin and elsewhere.

Palaeoclimatic and diagenetic controls based on clay mineralogy and organic matter distribution: The continental rift Cuyo Basin (Triassic), west‐central Argentina

AbstractThe Triassic Cuyo rift Basin is one of the renowned petroleum systems in South America that records distinctive alluvial, lacustrine and fluvial settings with volcanic input. The present study focuses on the clay mineral assemblages that represent the syn‐rift to post‐rift fills. Four clay mineral assemblages are defined based on X‐ray diffraction, optical and electron microscopy analyses. The illite/mica‐dominated assemblage is documented in the syn‐rift I (Cerro de las Cabras Formation). Such mineral phases are inherited and derived from the erosion of pre‐rift units, suggesting that physical weathering associated with a warm and semi‐arid climate played a leading role in their occurrence. Authigenic smectite clays are diagnostic in organic‐lean mudstones of the syn‐rift II (lower Potrerillos Formation) which point to alteration of pyroclastic material under a warm and semi‐arid to humid climate. A warm and humid climate prevailed during the deposition of lacustrine hydrocarbon source rocks (upper Potrerillos and Cacheuta formations). Organic‐rich mudstones show the illite/mica–kaolinite–smectite assemblage in conjunction with amorphous organic matter and restricted terrigenous elements. Diagenetic kaolinite booklets and bipyramidal quartz are attributed to the acidification of pore‐water related to kerogen transformation during burial diagenesis. Vitrinite reflectance values (ca 0.63%) confirm that organic‐rich sediments reached the earliest oil window; however, smectite is devoid of illite layers throughout the Triassic succession, probably related to the absence of potassium in the system and the high crystallinity of the smectite, which delayed the transformation of smectite into mixed layers illite–smectite. Red mudstones of the post‐rift II (the fluvial‐lacustrine Rio Blanco Formation) show the kaolinite‐dominated assemblage where the authigenic kaolinite is interpreted to be derived from the alteration of volcaniclastic detritus and indicative of warm climate with markedly wet periods.

Deformation-enhanced diagenesis and bacterial proliferation in the Nankai accretionary prism

Abstract. Understanding diagenetic reactions in accreted sediments is critical for establishing the balance of fluid sources and sinks in accretionary prisms, which is in turn important for assessing the fluid pressure field and the ability for faults to host seismic slip. For this reason, we studied diagenetic reactions in deformation bands (shear zones and veins) within deep mud sediments from the Nankai accretionary prism (SW Japan) drilled at site C0001 during IODP Expedition 315, by means of microscopic observation, X-ray diffraction, and major- and trace-element analyses. Deformation bands are not only more compacted than the host sediment but are also enriched in framboidal pyrite, as observed under microscopy and confirmed by chalcophile-element enrichments (Fe, S, Cu, As, Sb, Pb). In tandem, one shear zone sample displays a destabilization of smectite or illite–smectite mixed layers and a slight crystallization of illite relative to its sediment matrix, and another sample shows correlated increases in B and Li in shear zones and veins compared to the host sediment, both effects suggesting a transformation of smectite into illite in deformation bands. The two diagenetic reactions of sulfide precipitation and smectite-to-illite transformation are explained by a combined action of sulfate-reducing and methanogen bacteria, which strongly suggests an increased activity of anaerobic microbial communities localized in deformation bands. This local bacterial proliferation was possibly enhanced by the liberation of hydrogen from strained phyllosilicates. We suggest that the proliferation of anoxic bacteria, boosted by deformation, may contribute to the pore water freshening observed at depth in accretionary prisms. Deformation-enhanced metabolic reactions may also explain the illitization observed in major faults of accretionary prisms. Care is therefore needed before interpreting illitization, and other diagenetic reactions as well, as evidence of shear heating, as these might be biogenic instead of thermogenic.

Methanogenesis Potentials: Insights from Mineralogical Diagenesis, SEM and FTIR Features of the Permian Mikambeni Shale of the Tuli Basin, Limpopo Province of South Africa

Carbonaceous shale is more topical than ever before due to the associated unconventional resources of methane. The use of FTIR, SEM-EDX, and mineralogical analyses has demonstrated a promising approach to assess methanogenesis potentials in a more rapid and reliable manner for preliminary prospecting. Representative core samples from the borehole that penetrated the carbonaceous Mikambeni shale Formations were investigated for methanogenesis potentials. The absorption band stretches from 1650 cm−1 to 1220 cm−1 in wavenumber, corresponding to C-O stretching and OH deformation of acetic and phenolic groups in all studied samples, thereby suggesting biogenic methanogenesis. The CO2 was produced by decarboxylation of organic matter around 2000 cm−1 and 2300 cm−1 and served as a source of the carboxylic acid that dissolved the feldspar. This dissolution process tended to release K+ ions, which facilitated the illitization of the smectite minerals. The SEM-EDX spectroscopy depicted a polyframboidal pyrite structure, which indicated a sulfate reduction of pyrite minerals resulting from microbial activities in an anoxic milieu and causes an increase in alkalinity medium that favors precipitation of dolomite in the presence of Ca and Mg as burial depth increases. The contact diagenesis from the proximity of Sagole geothermal spring via Tshipise fault is suggested to have enhanced the transformation of smectite to chlorite via a mixed layer corrensite in a solid-state gradual replacement reaction pathway. The presence of diagenetic chlorite mineral is characteristic of low-grade metamorphism or high diagenetic zone at a temperature around 200 °C to 230 °C and corresponds to thermal breakdown of kerogen to methane at strong absorption band around 2850 cm−1 and 3000 cm−1, indicating thermal methanogenesis.

Diagenesis of the Malmian Mikulov Formation source rock, Vienna Basin: Focus on matrix and pores

Diagenetic processes and pore development in the matrix of the 1000 m thick main source rock for oil and gas in the Vienna Basin, the autochthonous Malmian mudstones of the Mikulov Formation have been studied. Core samples from wells over a true vertical depth range of 1400 m–8551 m were available. The bulk samples contain quartz, minor amounts of plagioclase, pyrite and a large but variable proportion of calcite; the clay mineral content ranges from 14 to 47%. The clay fraction contains a prominent illite-smectite (I–S) mixed-layer phase, illite, chlorite and kaolinite. The quantities of I–S and kaolinite decrease with depth, whereas illite and chlorite increase with depth. Diagenesis has involved a gradual transformation of smectite to illite through mixed-layer I–S intermediates. The ordering of the mixed layer I–S changes with increasing depth from R0 to R1 and R3. The R1 transformation of the mixed-layer I–S occurs at approximately 3000 m and vitrinite reflectance values of 0.4%–0.6%. Based on petrographic evidence, the cations resulting from the illitization of smectite were the source of a variety of late diagenetic mineral cements, such as Fe and Mg for chlorite formation and for ferroan dolomite precipitation. Illitization also provided Si for local quartz cementation. During diagenesis nanometer to micrometer size pores developed because of specific mineral frameworks and dissolution processes. Organic matter pores developed in deeper, thermally mature samples. Phyllosilicate framework pores between brittle grains are commonly observed. Pores caused by partial dissolution of carbonate grains also occur. In places diagenetic cements, such as quartz overgrowths or carbonate cements keep pores propped open. The connectivity of the pores cannot be established unequivocally from SEM photomicrographs, but they likely contribute to the creation and preservation of effective porosity and gas storage capacity of these rocks.

Petrographic features and diagenetic alteration in the shale strata of the Permian Lucaogou Formation, Jimusar sag, Junggar Basin

Reservoir quality is the key parameter controlling shale oil enrichment. However, the complicated lithofacies, which control diagenesis, result in reservoir heterogeneity in the shale strata, creating challenges for reservoir prediction. Combining with the detail analysis of inorganic and organic geochemistry, characteristics of the main lithofacies and diagenetic alteration were investigated in Permian Lucaogou Formation, Jimusar sag, Junggar Basin. Laminated rocks with laminae combinations containing pyroclastic and terrigenous clastic laminae, terrigenous clastic and dolomite laminae, and pyroclastic and carbonate laminae were identified. Massive rocks mainly includes dolomicrite, dolomitic tuff, silt dolostone, dolomitic siltstone, and tuffaceous siltstone. Feldspar dissolution and carbonate cementation were the two dominant diagenetic types, further causing heterogeneity of the shale oil reservoirs. Feldspar dissolution generally occurred in laminated rocks containing pyroclastic laminae with low content of carbonate minerals (PL1), as well as in their adjacent interbedded siltstones. Carbonate cementation generally occurred in laminated rocks containing carbonate laminae, as well as in their adjacent interbedded siltstones. Whether in laminated and massive rocks, sparry carbonate crystals were also precipitated adjacent to organic matter and in microfractures. In the burial processes, enriched transition metal elements of V and Ti in pyroclastic laminae promote catalytic pyrolysis of organic matter. Abundant CO 2 produced during the catalytic pyrolysis promoted feldspar dissolution. Dissolution of sedimentary carbonate minerals by organic acids and CO 2 provided the material sources for secondary carbonate cements. It promoted the recrystallization and re-precipitation of sparry carbonate crystals adjacent to organic matter and in the terrigenous clastic laminae or interbedded siltstones. Ca 2+ , Mg 2+ Sr 2+ , Fe 2+ , and Mn 2+ produced by the transformation of smectite to illite in the pyroclastic laminae also provided Ca source for secondary carbonate cements. External diagenetic fluids providing materials along microfractures is another genesis of carbonate cements in the Permian Lucaogou Formation. • Petrographic features in the shale strata in Permian Lucaogou Formation were identified based on in situ analytical method. • Differential feldspar dissolution and carbonate precipitation generally occurred in different lithofacies. • Catalytic pyrolysis of organic matter in presence of transition metal elements in pyroclastic laminae promoted dissolution. • Development of carbonate cements in the shale strata were mainly provided by three material sources.

Effect of potassium saturation on the mineralogical composition and ceramic properties of smectitic clays from the Eastern Desert, Egypt

Dry saturation of clays from the Maastrichtian-Danian Dakhla Formation, comprising high smectite clays, using 3% potassium bromide (KBr) at 300oc resulted in a partial transformation of the original Ca-smectite to mixed-layer illite-smectite like structure. The clays' wet saturation mentioned above with 0.5, 1.0, 2.0, and 9.0% potassium chloride (KCl) at room temperature causes a full transformation of smectite layers to a mixed clay-like structure. The rate of transformation is directly proportional to K+ ion concentration. The K+ ions are fixed in smectite by filling vacancies left by Ca++ ions. The change of temperature from room temperature to 80 °C slightly reduces the original smectite's transformation rate, while the increase of maturation time shows no effect. The transformation of smectite to mixed clay layer like structure is accompanied by a decrease in the linear shrinkage and drying sensitivity coefficient of the original clays. The Dakhla clays' wet saturation with 1.0% KCl at room temperature is the most suitable treatment to enhance the studied clays' ceramic properties as raw material for brickmaking. The linear shrinkage and drying sensitivity coefficient of the treated clay samples under these conditions results in new properties similar to those exhibited by Nile River-silts, which are the traditional raw material for brickmaking in Egypt.

Diagenetic alteration and geochemical evolution during sandstones bleaching of deep red-bed induced by methane migration in petroliferous basins

Bleaching of red-bed sandstones is an important process related to the organic-inorganic interactions and diagenetic alteration, playing an important role in reservoir heterogeneity and carbon cycle in petroliferous basins. Distinguishing from the aeolian red-bed in the outcrop, a case study of deep red-bed of fluvial facies was conducted on the Permian Xiashihezi Formation, NE Ordos Basin, China. Four diagenetic facies can be identified as red, brown, green and white sandstones (RS, BS, GS, WS) with typical diagenetic alteration and geochemical characteristics. Thereinto, diagenetic alterations are dominated by clay minerals and carbonate cements including calcite and siderite. Carbon and oxygen isotopes showed a synergistic decreasing trend from RS, BS to WS with the increase of altered degree. The unaltered RS retained abundant early hematite, smectite and calcite, while strongly bleached WS had lowest feldspar but extensive kaolinite and illite, and manganese-rich calcite. Medium-altered BS and GS are characterized by late magnesium-rich siderite and chlorite coating respectively. The isotopic data, fluid inclusions and modeling indicated that CH4 is the important causal fluid for sandstones bleaching. Diagenetic alteration and geochemical evolution in deep red-bed of fluvial facies was influenced by upward-fining sedimentary sequence, open/closed diagenetic system and natural gas migration. The geochemical behaviors of Fe are responsible for the diversity of diagenetic facies. Fe2+ from redox between Fe3+ and CH4, integrated Mg2+ released from the transformation of smectite to illite into magnesium-rich siderite, or into chlorite in coarser sandstones in relatively open system. Extensive CH4-rich reducing fluids were oxidized to acidic fluids in the more open diagenetic system with coarser grained sandstones, resulting in the intense dissolution and the high-quality reservoirs distributed in the lower sequence of fluvial facies. Furthermore, CH4 oxidation by Fe(III) in deep red-bed can be extremely important CH4 sink and expected to potential sites of geological carbon sequestration.

Chemical compaction of deep buried mudstone and its influence on pressure prediction

The chemical compaction of mudstones which is dominated by the transformation of clay minerals leads to significant changes in the mineral composition and microstructure of mudstone during process of deep burial. In particular, the transformation of smectite to illite in mudstones results in noticeable impact on the pore pressure formation and the overpressure logging responses. At present, the study about the pressurization mechanism of chemical compaction and the impact on overpressure logging responses is really weak, which made it hard to pore pressure identification and pressure prediction for deep buried formations. Taking the Paleogene Shahejie Formation in the Dongying depression of the Bohai Bay Basin in eastern China as typical case, this paper analyses the characteristics of clay mineral transformation of the Shahejie Formation in the Dongying depression, the logging responses of overpressures, and the influence of chemical compaction on the prediction of pore pressure. The results showed that the chemical compaction of mudstones changes the relationship between the petrophysical properties of mudstone and vertical effective stress and the logging responses of overpressure. The typical characteristic of chemical compaction manifested as density increase continuous with the depth. The normal compaction trends of the different compaction stages are the basis for overpressure mechanisms identification and pore pressure prediction. The depth of the rapid transformation of clay minerals has a good consistency with the top of overpressure zone (2000–2800 m) in Dongying depression, which indicates that the overpressure and its logging responses may be related to the chemical compaction of mudstones. The measured pressure in intervals deeper than 3000 m is closer to the predicted pressure based on the normal compaction trend of chemical compaction.