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Article

Coupled Evolution of Clay Minerals and Organic Matter During Diagenesis: Mechanisms of Smectite Illitization in Organic-Rich Shale

by
Kun Ling
1,2,
Ziyi Wang
1,2,
Changhu Zhang
1,2 and
Lin Dong
1,2,*
1
School of Earth and Space Sciences, Peking University, Beijing 100871, China
2
Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, Beijing 100871, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(9), 2966; https://doi.org/10.3390/pr13092966
Submission received: 29 July 2025 / Revised: 15 September 2025 / Accepted: 15 September 2025 / Published: 17 September 2025
(This article belongs to the Topic Reservoir Characteristics and Evolution Mechanisms of the Shale)
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Abstract

The transformation of smectite to illite documents multi-scale water–rock–hydrocarbon interaction dynamics. Current studies predominantly emphasize the influence of inorganic systems on this process, while overlooking the dynamic regulation by organic matter and the synergistic effects of multiple controlling factors under actual geological conditions. In this study, we conducted integrated semi-open pyrolysis experiments on natural samples from the Chang-7 Member and hydrothermal experiments using synthetic analogs. The illitization process of smectite was characterized through XRD analysis and SEM observations, while organic geochemical testing was employed to track the corresponding thermal evolution of organic matter. The semi-open pyrolysis results reveal that significant changes in illite–smectite (I/S) mixed layer minerals and illite content/morphology occur above 320 °C, which coincides with the critical threshold for extensive organic matter evolution. Thermal degradation of organic matter generates pore space, thereby enhancing water–rock interactions involving clay minerals. This demonstrates the co-evolution of organic matter and smectite, and indicates that temperature indirectly influences illitization by regulating organic matter thermal evolution. The hydrothermal simulation experiments demonstrate the early-stage characteristics of illitization. Unlike long-term geological evolution, K+ under experimental conditions primarily originates from the aqueous medium due to kinetic constraints on feldspar dissolution. Notably, organic matter regulates K+ partitioning dynamics—increased organic matter content hinders K+ incorporation into smectite interlayers, thereby suppressing the illitization process. Cross-system experimental analysis reveals that organic matter exhibits temporally dependent dual functionality, serving both mediating and modulating roles within inorganic diagenetic systems. This study delineates diagnostic-stage-dependent mechanisms governing smectite illitization through multifactorial synergistic interplay, establishing a predictive framework applicable to organic-rich systems exemplified by the Chang-7 Shale.

1. Introduction

Clay minerals are ubiquitous and essential components of rocks in petroliferous basins, documenting multiscale water–rock–hydrocarbon interactions throughout their formation and evolutionary processes. This renders them of broad scientific significance [1]. Among the various mineralogical transformations, the smectite-to-illite conversion has received particular attention due to its strong links to key geological processes, including thermal maturity reconstruction, hydrocarbon generation and reservoir quality evolution [2,3,4,5,6,7,8].
Illitization is a diagenetically-driven transformation in which precursor minerals (particularly smectite) gradually convert to illite, typically initiating under low-temperature conditions during early burial and continuing with increasing temperature and pressure [9]. Regardless of illitization mechanism (dissolution-precipitation vs. solid-state transformation), the rate and extent of illitization are fundamentally affected by time, temperature, and diagenetic fluid chemistry, particularly the concentrations and availability of potassium [10]. Previous studies have conducted extensive kinetic investigations of smectite illitization using both natural samples and laboratory-controlled experiments. These works have established various reaction models based on first-order and second-order kinetic equations, consistently highlighting the dominant role of temperature in the transformation process [11,12,13,14]. Furthermore, hydrothermal experiments have systematically examined the influence of other key parameters, including liquid/solid ratio, and reaction duration [15,16,17,18].
However, most existing research has focused on illitization in purely inorganic systems, overlooking the co-existence and interactions of clay minerals with organic matter in natural shale systems. In fact, in organic-rich shales (e.g., Chang-7 Shale), clay minerals and organic matter are two principal components, representing the inorganic and organic fractions, respectively. These constituents have been closely associated since deposition, forming widely documented clay-organic nanocomposites that subsequently undergo coupled burial and diagenetic evolution [6,19,20]. During diagenesis, clay minerals and organic matter exert mutual influence, evidenced by the parallel progression of smectite illitization and hydrocarbon generation—demonstrating the intrinsic connection between these two critical geological processes [18,21]. Experimental pyrolysis studies involving smectite and organic matter have further revealed stage-specific correlations between organic decarboxylation/cracking processes and smectite-to-illite transformation [22]. A growing body of evidence indicates that smectite illitization can effectively catalyze hydrocarbon generation [23,24,25]. Yet, the reverse relationship—how the thermal evolution of organic matter affects the smectite illitization process—remains poorly understood. Even the limited number of studies has typically introduced only a single type of organic compound into experimental setups [26], failing to capture the complex coexisting organic-inorganic conditions characteristic of actual geological settings.
The development of rapid-heating pyrolysis techniques, driven by the temperature-time compensation concept in hydrocarbon kinetics, has enabled simulation of organic matter evolution under controlled thermal conditions [27,28,29]. This approach allows the reconstruction of geological processes that typically occur over millions of years within a significantly shortened experimental timeframe by employing temperatures significantly exceeding those encountered in natural diagenesis. These pyrolysis systems, classified as open, semi-open, and closed, serve different purposes. Open systems, although widely applied as a classical method for source rock evaluation, exhibit significant discrepancies compared to actual geological conditions [30,31]. Semi-open systems effectively simulate episodic hydrocarbon expulsion, thereby more accurately reproducing the diagenetic processes accompanying hydrocarbon generation [32,33]. Within such systems, it is possible to conduct experimental simulations using intact core samples, which significantly enhances the consistency between thermal simulation experiments and actual geological conditions. By better preserving the natural rock fabric and geochemical characteristics, this approach enables more realistic interactions between organic and inorganic phases, thereby providing a distinct advantage for reconstructing diagenetic evolution processes. In contrast, closed systems prevent material exchange with the external environment. Among the various experimental approaches utilized in such systems, gold-tube pyrolysis is the most extensively applied method [34,35]. A key advantage of closed systems is their ability to facilitate detailed studies of various water–rock reactions in the presence of aqueous media, rendering them particularly suitable for research on the evolution of clay minerals. Salisu et al. successfully reproduced smectite authigenesis and subsequent illitization, replicating a mineral transformation sequence analogous to that observed in natural settings [36]. Moreover, by modulating experimental parameters and performing structural modification of smectite, recent studies have increasingly explored the mechanisms of illitization and identified distinct factors influencing this process [9,37]. With increasingly diverse application scenarios and more sophisticated experimental apparatus, pyrolysis techniques have also become crucial for comprehensively simulating the evolution of organic matter, inorganic minerals, pore structures, and fluids [38,39,40,41,42].
In this study, we aim to bridge the gap between organic and inorganic perspectives by investigating smectite-to-illite transformation under integrated organic-inorganic systems. In contrast to prior studies that examined illitization in isolation, this work is designed to simulate complex geological conditions, with a specific focus on the illitization in the presence of organic matter, addressing the synergistic effects of temperature, potassium ion concentration, K-feldspar and organic components. These factors are inherently interdependent and vary in influence across different diagenetic stages. Through a series of integrated thermal simulation experiments using both natural samples and synthetic powders, we seek to reconstruct the illitization process and unravel the complex, multivariate controls that govern smectite transformation in realistic geological settings.

2. Samples and Methods

2.1. Natural Sample: Semi-Open Pyrolysis Experiments

In this study, natural core samples were used for semi-open pyrolysis experiments. The samples were obtained from the Chang-7 Member of Well Z233, located in the southern Ordos Basin (Figure 1). The shale exhibits high total organic carbon content (TOC = 11.7%) and Type-II kerogen composition (Table 1), with low thermal maturity evidenced by Ro = 0.51%. Six cylindrical samples (25 mm in diameter and 60 mm in height) were wire-cut from the core, with all samples oriented vertically to maintain bedding consistency and ensure experimental reproducibility.
The pyrolysis experiments were conducted at the Research Institute of Petroleum Exploration & Development (RIPED) using a semi-open pyrolysis system (Nantong Huaxing Petroleum Devices, Jiangsu, China). The device comprises four subsystems: (1) pyrolysis reaction apparatus, (2) hydrocarbon expulsion system, (3) product separation/collection apparatus, and (4) computerized control apparatus.
The experimental procedure was as follows: first, cylindrical samples were placed in the reaction vessel, with both ends sealed using graphite rings, and the vessel was then mounted in the base of the heating furnace. Subsequently, vertical pressure (35 MPa) was applied via a bidirectional hydraulic apparatus to simulate lithostatic pressure, while a water injection pump introduced fluid to achieve the target formation pressure (20 MPa). A pre-programmed heating process was initiated to control the pyrolysis temperature. The system’s openness was regulated by a backpressure valve and pressure sensor, with the hydrocarbon discharge valve automatically releasing products into a collection container when pressure exceeded the set limit. Once the target pyrolysis temperature was reached, the discharge valve was manually opened to facilitate product collection.
Six parallel pyrolysis experiments were conducted in this study, employing a sequential heating protocol with final target temperatures ranging from 280 °C to 420 °C (Figure 2). Specifically, all six samples underwent unified pretreatment: initial 1-h preheating from 20 °C to 120 °C with subsequent 1 h of isothermal holding. The system was then heated to 240 °C at a rate of 20 °C/h and maintained for 2 h. During the staged heating phase, all six samples were heated to 280 °C at 20 °C/h and held isothermally, followed by product collection from the 280 °C-targeted specimen. The remaining five replicates were subjected to programmed pyrolysis at 320 °C, maintaining identical thermal parameters, to enable temperature-stage-specific product collection. This sequence was iterated through increasing temperature tiers (350 °C → 380 °C → 400 °C → 420 °C), where each subsequent stage incorporated thermal histories from previous phases. The cumulative thermal progression enabled reconstruction of continuous clay mineral evolution pathways.
Upon completion of the pyrolysis protocol, the reactor underwent rapid cooling. Solid residues were collected for subsequent characterization of clay minerals and organic matter. Clay mineral analysis comprised X-ray diffraction (XRD) and scanning electron microscopy (SEM), while organic geochemical evaluation included vitrinite reflectance analysis, Rock-Eval pyrolysis, and total organic carbon (TOC) quantification.

2.2. Synthetic Samples: Hydrothermal Experiments

The synthetic samples in this study were fabricated through proportioned mechanical blending of pristine mineral components with isolated kerogen concentrates. Mineralogical constituents comprised Na-smectite (Hohhot, Inner Mongolia), chlorite (Chifeng, Inner Mongolia), kaolinite (Chengdu, Sichuan), K-feldspar (Lingshou, Hebei), and AR Grade quartz (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), with cumulative purity levels ranging from 95% to 99.5%. K-smectite was also produced via cation exchange treatment of Na-smectite using 1 M KCl solution to study the effect of interlayer cations on smectite-to-illite conversion [25]. Fresh shale samples for kerogen isolation were collected from the Yishicun section, exhibiting high organic carbon content (TOC = 16.8%) with negligible thermal maturity (Ro = 0.41%). The kerogen was isolated through a standardized protocol involving: (1) Soxhlet extraction with an azeotropic dichloromethane/methanol mixture (93:7 v/v) of 200-mesh (75 μm) powdered samples for 48 h, followed by (2) sequential acid treatments using 6 M hydrochloric acid for carbonate dissolution and 40% hydrofluoric acid for silicate removal [44].
One reference sample was prepared based on the TOC and primary mineral composition of Chang-7 shale for experimental control (Table 2), along with two supplementary samples configured at identical proportions to investigate aqueous medium effects. Six additional samples were configured through systematic adjustments in kerogen content, K-feldspar proportion, and the smectite type. Hydrothermal experiments were conducted at 250 °C and 300 °C, respectively, therefore duplicate sets of samples were prepared accordingly. To ensure sample homogeneity, all powdered mixtures were thoroughly homogenized through grinding (>30 min) in corundum mortars prior to experimental runs, achieving both consistent particle size distribution and intimate inter-mineral contact.
Hydrothermal experiments were conducted at China University of Petroleum (Beijing) and RIPED using flexible gold tubes (8 mm outer diameter, 1mm thickness and 60 mm length) contained within steel pressure vessels (Nantong Huaan Scientific Research Devices Co., Ltd., Jiangsu, China) [45]. The 3 g ground powder sample and 3 mL KCl solution were loaded into a gold tube through six successive additions to ensure thorough wetting of the powder, while maintaining slight fluid mobility within the system. Upon completion of loading, the gold tubes were purged with argon to remove oxygen, sealed, and tested for airtightness.
The hermetically sealed gold tubes were precisely positioned within the high-pressure reactor chamber. Formation pressure conditions (20 MPa) were established through controlled water injection employing a hydraulic pump system. The thermal regime was then initiated with a heating rate of 2 °C/min until reaching predetermined isothermal conditions, which were maintained for 72 h to ensure complete reaction equilibrium. Following simulation experiments, the reactors were allowed to cool naturally to ambient temperature. The pyrolysis products were thoroughly oven-dried at 60 °C under controlled conditions. The dried solids were subsequently reground and homogenized using an agate mortar to achieve particle size uniformity prior to XRD characterization for mineralogical analysis.

2.3. Organic-Inorganic Composition Analyses

2.3.1. Mineralogical Characterization

Both bulk rock and clay mineral X-ray diffraction (XRD) analysis were performed using a D8 Advance X-ray diffractometer (Bruker Corporation, Karlsruhe, Germany) with CuKα radiation (λ = 1.5418 Å), voltage at 30 kV, current at 20 mA. Whole-rock samples were ground to <100-mesh size (150 μm), and the <2 μm clay fraction was subsequently isolated from homogenized bulk rock samples following the suspension-centrifugation methodology prescribed by China Petroleum Standard SY/T 5163-2018 [46]. When necessary, pretreatments with H2O2 and dilute HCl were applied to remove organic matter and carbonate minerals prior to clay separation. The extracted clay-sized fractions were prepared as oriented mounts. All samples underwent XRD analysis under three standardized treatment conditions: air-dried (N) at room temperature; ethylene glycol solvated (EG) after being saturated at 60 °C for 8 h; and heat-treated state (HT) after heating at 550 °C for 3 h. The XRD patterns were subjected to phase identification and semi-quantitative analysis using MDI Jade 9 software.
Integrated scanning electron microscopy (SEM) analysis was performed on naturally fractured surfaces, obtained by controlled rock cleavage and polished specimens, prepared through mechanical polishing and argon ion milling [47]. Prior to SEM characterization using a FEI Quanta 650 FEG field-emission scanning electron microscope (Field Electron and Ion Company, Hillsboro, OR, USA), samples were sputter-coated with a 5 nm chromium layer. Mineral crystal morphology was characterized through observation of fresh fracture surfaces, while clay mineral and organic matter fabric features were analyzed using secondary electron (SE)/backscattered electron (BSE) imaging combined with energy-dispersive spectroscopy (EDS).

2.3.2. Organic Geochemical Analyses

Total organic carbon (TOC) analyses were performed on a LECO CS230 carbon/sulfur analyzer (LECO Corporation, St. Joseph, MI, USA) [48]. Samples were pulverized to powders finer than 100 mesh and decarbonated by hydrochloric acid treatment. Processed materials were combusted at 550 °C under oxygen-rich conditions for TOC quantification.
Vitrinite reflectance (Ro) analysis was conducted on whole-rock samples through a standardized preparation protocol involving crushing to <10-mesh size (2 mm), homogenization with agar in a 1:1 mass ratio, and consolidation prior to thin-section preparation. The resulting polished sections were analyzed under oil immersion using a Zeiss Scope A1 incident-light microscope equipped with a photomultiplier tube and 546 nm monochromatic illumination (ZEISS Group, Oberkochen, Germany). Between 20 and 55 valid measurement points were systematically acquired per sample, with exclusion of weathered or altered domains to ensure data integrity.
Rock-Eval pyrolysis analysis was conducted on a Rock-Eval 6 instrument (Vinci Technologies, Nanterre, France) employing programmed heating from 300 °C to 650 °C at a rate of 25 °C/min under a nitrogen atmosphere. Approximately 100 mg of homogenized samples with particle sizes between 80 and 100 mesh was subjected to thermal decomposition to quantify free hydrocarbons (S1), pyrolyzable hydrocarbons (S2), and maximum pyrolysis temperature (Tmax). Hydrogen index (HI) was calculated as S2/TOC × 100. Analytical accuracy was verified through concurrent measurement of certified reference materials.

3. Results

3.1. Mineralogical Characteristics of Semi-Open Pyrolysis Products

XRD analysis reveals that the total clay mineral content in pyrolysis products gradually increased with rising temperature. Specifically, the illite–smectite mixed-layer (I/S) underwent progressive illitization, characterized by a systematic decline in I/S content and a corresponding increase in discrete illite. This mineralogical transformation became geologically significant above the 320 °C threshold. Furthermore, chlorite content follows a trend consistent with bulk clay fraction, suggesting that neoformation of chlorite contributed to the observed compositional changes. These quantitative mineralogical trends are systematically presented in Figure 3 and Table S1.
The SEM analysis of natural fracture surfaces further characterizes the morphological transitions of I/S mixed-layer minerals and illite during thermal evolution. In the sample subjected to 280 °C thermal maturation, clay minerals exhibit matrix-like textural characteristics (Figure 4a). Illite displays a characteristic platy morphology, while I/S mixed-layer minerals appear as thin flakes with slightly curled edges. The absence of the typical honeycomb-like I/S structures suggests an advanced stage of illitization. At this stage, no identifiable chlorite monomers or aggregates were observed. When the pyrolysis temperature was increased to 320 °C, clay minerals developed distinct morphological features (Figure 4b). Illite and I/S mixed-layer minerals maintained their intergrown associations but demonstrated enhanced crystallinity, reflected in more well-defined platy morphologies. Notably, authigenic carbonate minerals were observed precipitating along mineral edges, potentially associated with the co-evolution of clay minerals and organic matter. In addition, sparse acicular chlorite crystals began to appear, although their abundance remained low at this stage (Figure 4c).
At the terminal temperature of 380 °C, clay minerals demonstrated enhanced orientation, reflecting the advancement of illitization and alignment under simulated lithostatic pressure. Authigenic quartz observed within mineral interstices further corroborates active diagenetic fluid-rock interactions (Figure 4d). During this phase, chlorite crystallized rapidly, manifesting two distinct morphological types: acicular microcrystals (~1 μm) and rosette-like aggregates (~10 μm), suggesting temperature-dependent crystallization kinetics (Figure 4e). With continued heating, pyrobitumen derived from organic coking partially filled the pore spaces, progressively occluding pore networks and ultimately obscuring diagnostic crystal morphology along fracture surfaces (Figure 4f). Comparative analysis suggests that the clay mineral transformations observed during pyrolysis experiments resemble those in natural geological samples, particularly in the development of chlorite textures [49].
The argon-ion-polished SEM analysis comprehensively documented the diagenetic fabric evolution of clay minerals and organic matter in semi-open pyrolysis products. The unheated samples exhibited well-preserved interlaminated organic-rich and clay-rich laminae (Figure 5a,a′), containing detrital grains that lack evidence of dissolution. Organic matter was extensively distributed within the interlayers of clay minerals—an association commonly observed in the Chang-7 shale [47]. At 280 °C, the sample retained its microstructural fabric, with only minor organic matter loss (Figure 5b,b′). When the pyrolysis temperature was increased to 350 °C, the organic content decreased significantly, although clay-organic associations persist in certain enriched domains (Figure 5c,c′). By 420 °C, organic laminae are largely replaced by bedding-parallel banded pores, indicating substantial organic degradation and hydrocarbon generation (Figure 5d,d′).

3.2. Organic Geochemical Analyses of Semi-Open Pyrolysis Products

Organic geochemical indices systematically document the thermal maturation of organic matter throughout the pyrolysis experiments, exhibiting marked evolutionary trends in response to increasing thermal severity (Figure 3 and Table S2). TOC content gradually decreased, with a notably accelerated reduction occurring within the critical temperature window of 350–380 °C. A slight rebound in TOC observed above 400 °C is likely attributed to carbonaceous residue from coking during late-stage pyrolysis, a phenomenon also supported by SEM observation (Figure 4f). Comparative analysis reveals an initial S1 value reduction in the 280 °C product versus the raw sample, suggesting the expulsion of free hydrocarbons under thermobaric conditions, compounded by limited hydrocarbon generation at this stage. Following this phase, S1 values remained essentially stable until reaching the 380 °C thermal threshold, where a transient peak in S1 manifested, indicating significant enhancement in hydrocarbon-generating capacity during this critical stage. Progressive thermal elevation induced expulsion of hydrocarbons formed during earlier maturation stages, leading to concomitant depletion in S1 values through hydrocarbon migration processes. The S2 value and Hydrogen Index (HI) remained relatively constant during low-temperature pyrolysis (<320 °C) but underwent rapid attenuation above this threshold. These values approached zero at 420 °C, indicating the effective exhaustion of hydrocarbon generation potential.

3.3. XRD Spectral Characteristics of Hydrothermal Experiment Products

3.3.1. Reference Samples

The XRD analysis of hydrothermal experimental products from control samples exhibits diagnostic mineralogical features of the smectite-to-illite transformation. Serving as baseline controls, these samples were also systematically compared with other reaction products to elucidate component-specific controls on illitization dynamics within the system. Figure 6 depicts the XRD patterns of solid residues generated through hydrothermal reactions of Na- and K-smectite with 1 M KCl at 250 °C and 300 °C. Comparative XRD patterns of reaction products relative to pristine Na-smectite reveal significant structural reorganization. A significant decrease in interlayer expandability of clay minerals is demonstrated by upward shifts in the S(001) reflection toward higher °2θ in both oriented air-dried (N) and glycolated (EG) mounts. For instance, when employing Na-smectite as the starting material, the air-dried d(001)-spacing decreased from 12.47 Å for untreated material, to 11.82 Å at 250 °C and 11.31 Å at 300 °C, while d-spacing for EG samples shifted from 16.88 Å to 15.35 Å and 13.92 Å correspondingly.
Given that these peaks are systematically positioned between the characteristic reflections of expandable (smectite) and non-expandable (illite) clay minerals, the ubiquitous presence of I/S mixed-layer phases in the reaction products is conclusively demonstrated. Despite these transformations, discrete illite (d(001)-spacing = 10 Å) remained absent in all products except for minor fluctuations observed in 300 °C K-smectite derivatives.
Notably, the K-smectite reaction products demonstrated greater d-spacing reduction in EG mounts compared with Na-smectite simulation residues, particularly under lower temperature. The d(001)-spacing shifted from 16.88 Å to 15.35 Å at 250 °C and 13.92 Å at 300 °C. A plausible explanation may reside in the incomplete interlayer cation exchange at lower temperatures. In experiments using K-smectite, the higher density of interlayer K+ ions enhanced attraction between layers, effectively inhibiting ethylene glycol intercalation [9]. With increasing simulated temperature, enhanced cation exchange efficiency and progressive illitization mitigate this initial disparity.

3.3.2. Effects of KCl Solution and K-Feldspar

Considering K+ ions required for the smectite-to-illite transformation under geological conditions may originate either from formation brines or K-feldspar dissolution [50,51], the K+ concentration in the aqueous medium and the K-feldspar content were systematically modulated in the experimental setup.
In Figure 7, we report the XRD results of simulation products derived from Na-smectite treated with 0.1 M KCl solution at 250 °C and 0.01 M KCl solution at 300 °C, demonstrating distinct structural modifications compared to the reference samples shown in Figure 6. Samples treated at both 250 °C and 300 °C exhibited minimal structural alterations, as evidenced by XRD patterns showing negligible shifts in the EG S(001) reflection and d(001)-spacing consistently around 16.85 Å. Comparative analysis of K-feldspar-bearing systems revealed consistent I-S d-spacing values (11.83–11.88 Å), indicating minimal influence of initial feldspar content on transformation progress. Similarly, no apparent shifts in characteristic peaks were detected in air-dried scans, demonstrating inhibited illitization in K+-depleted aqueous systems. Figure 8 compiles XRD patterns of solid products generated through experiments with K-feldspar content modulation. In all air-dried mounts, the d(001)-spacing of I/S mixed-layer minerals consistently ranged from 11.83 Å to 11.88 Å, showing negligible inter-sample variations.
Under natural geological conditions, when paleosalinity fell below 15‰, such as in the Chang-7 shale, the K+ required for smectite-to-illite transformation was primarily sourced from the dissolution of potassium-bearing minerals (mainly K-feldspar) [51], with negligible contributions from aqueous media. However, the current experimental results demonstrate that variations in the initial K-feldspar content of samples did not yield significant mineralogical differences. This observation is likely attributable to kinetic constraints on feldspar dissolution under experimental conditions and the masking effect of relatively elevated background K+ concentrations. Consequently, in low-K+ concentration experiments, the illitization process was restricted due to insufficient supplementary K+ supply.

3.3.3. Effects of Organic Matter

The extensive association between organic matter and clay minerals in geological settings indicates their interactive evolution during diagenetic processes. Systematic modulation of kerogen content (5–15 wt%) in precursor assemblages allows effective evaluation of organic controls on smectite illitization. Figure 9 demonstrates corresponding air-dried oriented XRD results under two experimental temperature conditions. Under 250 °C hydrothermal conditions, increasing kerogen content in starting assemblages from 5% to 10% induced expansion of the d(001)-spacing in I/S interstratified minerals from 11.63 Å to 11.85 Å. At 300 °C, the d-spacing expanded from 11.3 Å to 11.5 Å corresponding to kerogen content elevation from 10% to 15%. Notably, increased d-spacing correlates with higher smectite layer proportion in I/S, suggesting that organic enrichment inhibits smectite-to-illite transformation. This finding differs from the organic effects observed in semi-open systems, demonstrating the existence of distinct underlying mechanisms under different geological conditions, such as variations in diagenetic stages. The promotion of organic acid release on K-feldspar dissolution remains unobserved due to restricted thermal evolution degree.

4. Discussion

4.1. Thermally Driven but Organically Mediated Smectite Illitization

Smectite becomes unstable during burial diagenesis due to changes in the chemical environment, and it progressively transforms into illite through mixed-layer intermediates. This transformation is conventionally attributed to thermally activated mechanisms [4,52]. As such, the composition and ordering of illite–smectite minerals serve as critical mineralogical indicators for reconstructing thermal histories and constraining low-grade metamorphism in sedimentary basins. While temperature fundamentally governs reaction kinetics through Arrhenius-type relationships, the transformation of smectite to illite in natural systems is also influenced by complex organic-water–rock interactions. These interactions impose multiple, interrelated controls on the illitization process under geological conditions.
Hydrothermal experiments at 250 °C and 300 °C reveal structural reorganization patterns during the incipient stages of smectite evolution. Elevated temperatures significantly reduce the expandability of clay minerals, as evidenced by corresponding reductions in d(001)-spacing values. However, no fundamental reorganization is observed in the crystallographic ordering of I/S mixed-layer minerals under these thermal regimes. A distinctive feature is that the hydrothermal simulation products obtained at 300 °C exhibit significant peak broadening of I(001)/S(001) in EG XRD scans, revealing pronounced interlayer heterogeneity. In I/S mixed-layer minerals, the rigid illite layers constrain the expansion of adjacent smectite layers during ethylene glycol solvation [53]. Increasing illite layers enhance constraints on the expansion of smectite layers. However, disordered stacking with heterogeneous illite distribution will lead to differential expansion suppression across smectite layers, ultimately resulting in non-uniform swelling of the I/S mixed-layer mineral. This indicates that the progression of illitization does not fundamentally alter the disordered nature of mixed-layer minerals.
Considering the initial I/S mixed-layer minerals with ~80% illite layers, semi-closed pyrolysis experiments reflect an advanced stage of illitization. At 280 °C and 320 °C, the proportions of illite and I/S mixed-layer remain stable. Consistent with the XRD analysis, SEM observations of the corresponding products reveal negligible modifications in both crystal morphologies and textural characteristics throughout this stage. The only notable change is the disappearance of matrix-like clay minerals seen at 280 °C, which were replaced by minor acicular chlorite at higher temperatures. This suggests the neoformation of chlorite through recrystallization. Notably, a similar ‘calming stage’ persists during smectite illitization in Chang-7 shale samples, where illite-layer proportions remain essentially static despite substantial thermal increase indicated by organic maturation indices [54]. A significant shift occurs at 320 °C, marking a critical turning point, beyond which clear mineralogical and textural transformations are observed. Illitization intensifies, with an increasing number of clay minerals exhibiting a well-defined platy morphology. Under simulated vertical lithostatic pressure, these clay minerals also display a preferred orientation. The concurrent increase in authigenic quartz further supports the occurrence of illitization.
Notably, this stage-wise transformation closely parallels the thermal maturation of organic matter. Beyond 320 °C, S2 and HI indices demonstrate progressive decrease, ultimately approaching zero, indicative of rapid organic matter consumption under thermal stress. SEM observations confirm that organic laminae become obliterated at 420 °C, accompanied by enhanced bedding-parallel pore networks. This implies that organic degradation actively facilitates illitization by creating porosity and releasing interlayer constraints. The interdependence fundamentally stems from clay-organic associations: under organic-rich conditions, constrained effective specific surface areas (SSA) limit the water–rock interactions [55,56]. Only upon progressive thermal degradation of organic matter are intracrystalline spaces liberated and additional porosity pathways established, enabling sustained illitization.

4.2. Organic Regulation of K+ Availability and Cation Exchange Dynamics

Clay minerals and organic matter exhibit intimate associations commencing from initial deposition, as evidenced by microscopic observations revealing that organic matter is intercalated with clay minerals as a nanocomposite [19,57]. Within sedimentary basins, the smectite-to-illite transformation demonstrates precise temporal coupling with hydrocarbon generation during progressive burial; this relationship provides critical evidence for coevolutionary feedback mechanisms between clay mineral diagenesis and organic matter maturation pathways [58]. Our semi-open pyrolysis experiments further corroborate this documented geological phenomenon. In particular, the catalytic role of clay minerals in organic matter hydrocarbon generation has been extensively documented [25], particularly regarding the enhanced hydrocarbon generation potential of clay-bound organic fractions attributed to selective adsorption mechanisms.
Conversely, the impact of organic matter on the illitization of smectite is equally noteworthy. In our hydrothermal experiments, variations in organic matter were simulated by adjusting the kerogen content. XRD analysis of I/S mixed-layer minerals demonstrates that increased organic matter content leads to expanded d-spacing, consistent with suppressed illitization. These findings imply that elevated organic loading inhibits smectite illitization under hydrothermal conditions. We propose that this inhibitory effect arises from the dual control of organic matter on K+ mobility and its accessibility to interlayer sites, through two key aspects.
On one hand, organic matter interacts with clay minerals at the nanometer scale through multiple physicochemical pathways, including ligand interactions, ion exchange, cation bridging, hydrophobic interactions, hydrogen bonding, and van der Waals forces [59,60]. A portion of organic compounds can even intercalate into the interlayers of swelling clay minerals (e.g., smectite), forming tightly bound organic-clay complexes. These adsorbed organics will enhance interlayer stability, reduce cation exchange capacity (CEC), and delay the release of interlayer water via ‘water bridges’, collectively hindering K+ exchange [26]. In this study, the majority of organic matter exists as macromolecules that adsorb preferentially at edge sites of clay minerals. This edge-bound configuration acts as a physical barrier, reducing water–rock contact efficiency and further delaying K+ migration into interlayers.
On the other hand, organic matter competes directly with smectite for K+ through binding mechanisms [61,62]. In organic-rich systems, functional groups such as carboxyl and phenolic hydroxyl impart negative surface charges that facilitate the electrostatic adsorption of K+ ions [63]. The oxygen-bearing functional groups also form coordination bonds with K+, leading to effective K+ sequestration. Although the binding affinity for K+ is weaker than that for polyvalent cations, this interaction remains effective in reducing aqueous K+ concentration given the predominance of K+ in the system. Consequently, the ion-exchange incorporation of K+ into smectite interlayers experiences proportional reduction.
Thus, organic matter fundamentally regulates K+ partitioning in hydrothermal experiments. Elevated organic content imposes a strong constraint on K+ availability, especially under low-K+ aqueous conditions, and can effectively stall smectite illitization. The heightened organic matter/K+ ratio drives tripartite competition for adsorption sites between organic ligands, K+ ions, and clay minerals. Consequently, the smectite-to-illite transformation is suppressed because of an insufficient supply of K+. It is important to emphasize that these effects are not static over geological timescales but instead evolve as diagenesis progresses. With increasing burial, thermal degradation of organic matter becomes increasingly important. As thermal maturity rises, decarboxylation, dehydrogenation, and aromatization reactions begin to break down macromolecular organics, liberating previously occupied mineral surfaces and creating secondary porosity. This transformation mitigates the initial inhibitory effect of organic matter and enables renewed K+ access to interlayers, facilitating illitization. This shift from inhibition to facilitation reflects a dynamic feedback mechanism, in which organic matter first constrains and later promotes clay mineral diagenesis as a function of burial depth and temperature.
In summary, organic matter exerts a temporally evolving influence on smectite illitization, initially impeding and subsequently facilitating the transformation. Recognizing this time-dependent organic–inorganic feedback is essential for accurately reconstructing diagenetic pathways and predicting the evolution of shale reservoir quality under different thermal regimes.

5. Conclusions

This study investigates smectite-to-illite transformation under coupled organic-inorganic conditions through a combined approach of semi-open pyrolysis experiments on natural specimens and hydrothermal simulations using synthesized analogs. The experimental results capture the key geological controls across distinct diagenetic stages, demonstrating strong comparability with naturally evolved systems. The study provides a new perspective on multifactorial, synergistic controls over illitization in lacustrine shale settings and has important implications for evaluating shale reservoir quality and maturity.
  • When aqueous media are deficient in potassium ions, the smectite-to-illite transformation is nearly stagnant. Given that potassium ion (K+) supply is essential for this process, the extensive illitization observed in low-salinity depositional environments (e.g., the Chang-7 shale) strongly suggests that K-feldspar dissolution plays a critical role in the transformation.
  • Illitization progresses with increasing temperature and is accompanied by the thermal evolution of organic matter. As the primary kinetic regulator, temperature also exerts indirect influences on critical diagenetic processes—including organic matter thermal evolution and K-feldspar dissolution.
  • Organic matter exerts a temporally evolving influence on smectite-to-illite transformation. During early diagenesis, enrichment in organic matter correlates with delayed illitization, likely due to reduced K+ accessibility. At higher thermal maturity stages, however, organic matter degradation coincides with significantly enhanced transformation rates, facilitated by secondary porosity generation and increased mineral surface exposure. This transition demonstrates a feedback mechanism whereby organic matter functions as both a mediator and a modulator within the diagenetic system.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13092966/s1, Table S1: Clay mineral content of the initial sample and pyrolysis products; Table S2: Organic geochemical parameters of the initial sample and pyrolysis products.

Author Contributions

Conceptualization, K.L. and L.D.; methodology, K.L. and Z.W.; formal analysis, K.L., Z.W. and L.D.; investigation, K.L. and C.Z.; resources, K.L. and C.Z.; data curation, K.L. and Z.W.; writing—original draft preparation, K.L.; writing—review and editing, K.L. and L.D.; supervision, L.D.; project administration, L.D.; funding acquisition, L.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Deep Earth probe and Mineral Resources Exploration-National Science and Technology Major Project, grant number: 2024ZD1001002 and NSFC, grant number: 42090021 and 42373050.

Data Availability Statement

The original contributions presented in the study are included in the article and Supplementary Materials; further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Geological map of the Ordos Basin showing lacustrine facies distribution of the Ch7 Member and locations of the sampling well and section (modified from Wang et al., 2023 [43]). “Reproduced with permission from Guanping Wang, Journal of Asian Earth Sciences; published by Elsevier, 2023”.
Figure 1. Geological map of the Ordos Basin showing lacustrine facies distribution of the Ch7 Member and locations of the sampling well and section (modified from Wang et al., 2023 [43]). “Reproduced with permission from Guanping Wang, Journal of Asian Earth Sciences; published by Elsevier, 2023”.
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Figure 2. The heating procedure of the semi-open pyrolysis experiments.
Figure 2. The heating procedure of the semi-open pyrolysis experiments.
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Figure 3. Evolution curves of clay mineral content and organic geochemical parameters in the initial sample (Z233-0) and pyrolysis products. Clay (%) represents the proportion of clay minerals in the total rock, while I (%), I/S (%), and C (%) denote the relative contents of illite, I/S, and chlorite within the clay mineral fraction, respectively.
Figure 3. Evolution curves of clay mineral content and organic geochemical parameters in the initial sample (Z233-0) and pyrolysis products. Clay (%) represents the proportion of clay minerals in the total rock, while I (%), I/S (%), and C (%) denote the relative contents of illite, I/S, and chlorite within the clay mineral fraction, respectively.
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Figure 4. SEM images of natural fracture surfaces in pyrolysis products. (a) I/S and matrix-like clays at 280 °C; (b) interwoven I/S and illite with carbonate minerals (Carb) indicating clay-organic coevolution at 320 °C; (c) acicular chlorite and potential intermediate kaolinite at 320 °C; (d) platy illite with higher proportion and authigenic quartz at 380 °C; (e) two generations of chlorite: 1 μm-size acicular microcrystals (Chlorite 1) and 10 μm-scale rosette-like aggregates (Chlorite 2) at 380 °C; (f) pyrobitumen filling partial pores at 380 °C.
Figure 4. SEM images of natural fracture surfaces in pyrolysis products. (a) I/S and matrix-like clays at 280 °C; (b) interwoven I/S and illite with carbonate minerals (Carb) indicating clay-organic coevolution at 320 °C; (c) acicular chlorite and potential intermediate kaolinite at 320 °C; (d) platy illite with higher proportion and authigenic quartz at 380 °C; (e) two generations of chlorite: 1 μm-size acicular microcrystals (Chlorite 1) and 10 μm-scale rosette-like aggregates (Chlorite 2) at 380 °C; (f) pyrobitumen filling partial pores at 380 °C.
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Figure 5. SEM images of argon-ion-polished pyrolysis products. (a,a′) The initial sample with interlaminated organic-rich and clay-rich laminae; (b,b′) the essentially unchanged pyrolysis product at 280 °C; (c,c′) gradually consumed organic-rich laminae at 350 °C; (d,d′) banded pores replacing organic-rich laminae at 420 °C. (a–d) are secondary electron images and (a′–d′) are backscattered electron images.
Figure 5. SEM images of argon-ion-polished pyrolysis products. (a,a′) The initial sample with interlaminated organic-rich and clay-rich laminae; (b,b′) the essentially unchanged pyrolysis product at 280 °C; (c,c′) gradually consumed organic-rich laminae at 350 °C; (d,d′) banded pores replacing organic-rich laminae at 420 °C. (a–d) are secondary electron images and (a′–d′) are backscattered electron images.
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Figure 6. XRD patterns of the untreated Na-smectite and solid residues generated through hydrothermal reactions of Na- and K-smectite with 1 M KCl at 250 °C and 300 °C. N: oriented air-dried mounts; EG: ethylene glycol (EG)-solvated mounts.
Figure 6. XRD patterns of the untreated Na-smectite and solid residues generated through hydrothermal reactions of Na- and K-smectite with 1 M KCl at 250 °C and 300 °C. N: oriented air-dried mounts; EG: ethylene glycol (EG)-solvated mounts.
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Figure 7. XRD patterns of solid residues generated through hydrothermal reactions of Na-smectite with 0.1 M KCl at 250 °C and 0.01 M KCl at 300 °C. N: oriented air-dried mounts; EG: ethylene glycol (EG)-solvated mounts.
Figure 7. XRD patterns of solid residues generated through hydrothermal reactions of Na-smectite with 0.1 M KCl at 250 °C and 0.01 M KCl at 300 °C. N: oriented air-dried mounts; EG: ethylene glycol (EG)-solvated mounts.
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Figure 8. Oriented air-dried XRD patterns of solid residues generated through hydrothermal reactions of Na-smectite with 1 M KCl for various K-feldspar contents at 250 °C. Sample 250-12%Kfs is equivalent to sample 250-Na-1 M.
Figure 8. Oriented air-dried XRD patterns of solid residues generated through hydrothermal reactions of Na-smectite with 1 M KCl for various K-feldspar contents at 250 °C. Sample 250-12%Kfs is equivalent to sample 250-Na-1 M.
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Figure 9. Oriented air-dried XRD patterns of solid residues generated through hydrothermal reactions of Na-smectite with 1 M KCl for various organic matter contents. Sample 250-10%OM is equivalent to sample 250-Na-1 M, the same applies to sample 300-10%OM.
Figure 9. Oriented air-dried XRD patterns of solid residues generated through hydrothermal reactions of Na-smectite with 1 M KCl for various organic matter contents. Sample 250-10%OM is equivalent to sample 250-Na-1 M, the same applies to sample 300-10%OM.
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Table 1. Geochemical characterization of the original Z233 sample.
Table 1. Geochemical characterization of the original Z233 sample.
Original Rock
TOC (%)11.69
Tmax (°C)435
S1 (mg/g)2.17
S2 (mg/g)38.81
S3 (mg/g)0.25
RO (%)0.51
Kerogen typeType-II
Table 2. Experimental Conditions of Hydrothermal Experiments and Composition Information of Powder Samples.
Table 2. Experimental Conditions of Hydrothermal Experiments and Composition Information of Powder Samples.
Naming
Convention 		Simulated
Temperature
(°C)		SolutionPowder Sample Composition Content (wt%)
KerogenSmectite *KaoliniteChloriteK-FeldsparQuartz
250-Na-1 M2501 M KCl1036 (Na)331236
250-K-1 M1 M KCl1036 (K)331236
250-0.1 M0.1 M KCl1036 (Na)331236
250-0.01 M0.01 M KCl1036 (Na)331236
250-0Kfs1 M KCl1036 (Na)33048
250-6%Kfs1 M KCl1036 (Na)33642
250-15%Kfs1 M KCl1036 (Na)331533
250-5%OM1 M KCl536 (Na)331241
250-15%OM1 M KCl1536 (Na)331231
300-Na-1 M3001 M KCl1036 (Na)331236
300-K-1 M1 M KCl1036 (K)331236
300-0.1 M0.1 M KCl1036 (Na)331236
300-0.01 M0.01 M KCl1036 (Na)331236
300-0Kfs1 M KCl1036 (Na)33048
300-6%Kfs1 M KCl1036 (Na)33642
300-15%Kfs1 M KCl1036 (Na)331533
300-5%OM1 M KCl536 (Na)331241
300-15%OM1 M KCl1536 (Na)331231
* The Na or K in parentheses represents the type of interlayer cations in smectite.
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Ling, K.; Wang, Z.; Zhang, C.; Dong, L. Coupled Evolution of Clay Minerals and Organic Matter During Diagenesis: Mechanisms of Smectite Illitization in Organic-Rich Shale. Processes 2025, 13, 2966. https://doi.org/10.3390/pr13092966

AMA Style

Ling K, Wang Z, Zhang C, Dong L. Coupled Evolution of Clay Minerals and Organic Matter During Diagenesis: Mechanisms of Smectite Illitization in Organic-Rich Shale. Processes. 2025; 13(9):2966. https://doi.org/10.3390/pr13092966

Chicago/Turabian Style

Ling, Kun, Ziyi Wang, Changhu Zhang, and Lin Dong. 2025. "Coupled Evolution of Clay Minerals and Organic Matter During Diagenesis: Mechanisms of Smectite Illitization in Organic-Rich Shale" Processes 13, no. 9: 2966. https://doi.org/10.3390/pr13092966

APA Style

Ling, K., Wang, Z., Zhang, C., & Dong, L. (2025). Coupled Evolution of Clay Minerals and Organic Matter During Diagenesis: Mechanisms of Smectite Illitization in Organic-Rich Shale. Processes, 13(9), 2966. https://doi.org/10.3390/pr13092966

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