Next Article in Journal
YOLO-EDH: An Enhanced Ore Detection Algorithm
Previous Article in Journal
Petrogenesis of Tholeiitic Basalts from CZK06 Drill Core on the Tianchi Volcano, China–North Korea Border
Previous Article in Special Issue
Mineralogical and Geochemical Characteristics of the Deeply Buried Wufeng–Longmaxi Shale in the Southern Sichuan Basin, China: Implications for Provenance and Tectonic Setting
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Minerals
Volume 15
Issue 9
10.3390/min15090951
minerals-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Clay Mineral Characteristics and Smectite-to-Illite Transformation in the Chang-7 Shale, Ordos Basin: Processes and Controlling Factors

by
Kun Ling
1,2,
Ziyi Wang
1,2,
Yaqi Cao
1,2,
Yifei Liu
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.
Minerals 2025, 15(9), 951; https://doi.org/10.3390/min15090951
Submission received: 5 August 2025 / Revised: 28 August 2025 / Accepted: 2 September 2025 / Published: 5 September 2025
(This article belongs to the Special Issue Reservoir and Geochemistry Characteristics of Black Shale, 2nd Edition)
Browse Figures
Versions Notes

Abstract

As critical components in continental shale systems, the composition and evolution of clay minerals are fundamental to their diagenetic processes and petrophysical properties. The Chang-7 shales in the Ordos Basin exhibit abundant clay mineral content, offering a valuable case study for clay mineral research under moderate diagenetic conditions. This study employed XRD analysis to determine the whole-rock mineralogy, clay mineral composition, and the evolution characteristics of illite-smectite mixed-layer minerals (I/S). Comprehensive clay mineral datasets compiled from 13 newly analyzed wells and existing literature revealed distinct lateral distribution patterns. Total Organic Carbon (TOC) analysis and vitrinite reflectance (Ro) measurements provided systematic quantification of organic matter abundance and thermal maturation parameters in the studied samples. The results reveal that the Chang-7 shale exhibits a characteristic clay mineral assemblage, with I/S (average 44.2%) predominating over illite (34.7%), followed by chlorite (15.6%) and limited kaolinite (5.4%). Frequent volcanic activities provided substantial precursor materials for smectite formation, which actively participated in subsequent illitization processes, while chlorite and kaolinite distributions were predominantly controlled by provenance inputs and sedimentary facies, respectively. Inconsistencies exist between diagenetic stages inferred from I/S mixed-layer ratios and Ro values, particularly in low-maturity samples exhibiting accelerated illitization. The observed negative correlation between TOC content and mixed-layer ratios in Well YY1 and YSC Section samples demonstrates the catalytic role of organic matter in facilitating smectite-to-illite transformation. These results systematically clarify the coupled effects of sedimentary-diagenetic processes, offering new insights into the mutual interactions between inorganic and organic phases during illitization under natural geological conditions. The findings advance the understanding of Chang-7 shale oil and gas systems and offer practical guidance for future exploration.

1. Introduction

Clay minerals are of substantial research importance in petroleum geology due to their widespread distribution, complex physicochemical properties, diverse crystal structures, and multifaceted formation mechanisms [1,2]. On a regional scale, they serve as critical indicators for reconstructing basin tectonic evolution, sedimentary processes, and burial histories, thereby enabling paleoenvironmental interpretation and stratigraphic correlations [3,4,5]. As key constituents of source rocks, clay minerals play essential roles in controlling hydrocarbon generation, expulsion, and migration processes [6,7,8]. The regulatory effects of clays on reservoir properties (e.g., porosity and permeability) [9,10], combined with characteristic attributes including adsorption capacity, surface area, ion exchange capability, and swelling behavior, profoundly influence hydrocarbon occurrence and storage potential in shale reservoirs [11,12,13]. Given the characteristically high clay mineral content in continental shales compared to marine counterparts [14], particularly in the Chang-7 Member of the Ordos Basin, where clay minerals constitute approximately two-thirds of the total mineral composition [15], conducting systematic research on clay minerals holds important scientific significance for deepening the understanding of the Chang-7 shale oil system.
The formation, transformation, and dissolution of clay minerals represent a complex geological process influenced by multiple factors, with their multiple genetic origins further contributing to compositional uncertainties [16]. Within similar tectonic settings, the initial clay mineral assemblages are primarily controlled by sedimentary provenance and depositional environments, including facies distribution and aqueous chemical conditions [3,11,17]. In addition, discrete geological events may also modify these assemblages through the episodic input of detrital or volcanic materials. During diagenesis, pervasive water-rock interactions drive mineral transformations encompassing both inter-clay evolution and reactions involving other minerals, which may act either as reactants or authigenic products [18,19,20]. These superimposed effects progressively alter the original mineral composition and meanwhile, induce vertical heterogeneity in the mineralogical profile. Given that the diagenetic evolution of clay minerals, primarily characterized by smectite-to-illite transformation, exhibits strong thermal dependence, the compositional variations and evolutionary trajectories of illite, smectite, and illite-smectite mixed-layer minerals (I/S) can serve as effective relative geothermometer and provide critical criteria for diagenetic stage classification [21,22].
More importantly, the temperature-dominated nature establishes a genetic linkage between clay mineral diagenesis and organic matter thermal maturation [7,12]. Significantly, the smectite-to-illite transition temperature range generally coincides with the activation threshold of the oil window. During the early maturation stage with vitrinite reflectance (Ro) values approximating 0.5% and illite content stabilizing around 25%, clay minerals predominantly undergo dehydration processes accompanied by limited hydrocarbon generation. The principal oil generation zone (Ro = 0.5%–1.0%) corresponds to illite layer proportions of 25%–50% in I/S [2]. In fact, clay minerals and organic matter exhibit more intrinsic associations in sedimentary systems, forming co-occurring assemblages within shale matrices [23,24]. The interplay between clay–organic complexes constitutes a fundamental control on hydrocarbon generation mechanisms [25,26]. Specifically, clay minerals facilitate the accumulation of organic precursors through selective adsorption processes, thereby generating hydrocarbon-prone source materials [27,28]. These minerals further function as catalytic agents during thermal decomposition of organic matter [29]. Theoretically, this relationship should be reciprocal: organic matter, in turn, may influence the diagenetic evolution of clay minerals. However, the impact of organic matter on clay mineral transformation—particularly on illitization—has received relatively limited attention.
In specific stratigraphic units of petroliferous basins, distinct clay mineral assemblages often develop, typically dominated by one or two mineral species. A crucial foundation for conducting clay mineral research lies in comprehensive analysis of the spatial distribution patterns of these assemblages. To this end, we conducted whole-rock and clay mineral composition analyses on the Chang-7 shales from 13 wells distributed across the Ordos Basin, supplemented by a comprehensive literature review and data compilation to delineate lateral variability and its primary controls. In parallel, continuous vertical sampling from Well YY1 allowed for detailed vertical correlation. By coupling clay mineralogical profiles with organic geochemical parameters, we aim to explore the long-term co-evolution of clay minerals and organic matter during progressive burial and thermal maturation. Ultimately, these datasets offer new insights into the reciprocal interactions between inorganic and organic phases during illitization under natural geological conditions.

2. Geological Settings

The Ordos Basin is a multi-cycle superimposed sedimentary basin covering approximately 25 × 104 km2 in northern China. It is also recognized as one of the most significant continental petroliferous basins in China, hosting abundant conventional and unconventional hydrocarbon resources [30,31]. Developed on Archaean granulites and Paleoproterozoic greenschists, this typical cratonic basin experienced multi-stage tectonic evolution from the Paleozoic through the Mesozoic eras [32,33]. Based on its current structural configuration and evolutionary history, the basin is divided into six tectonic units: Western Thrust Belt, Tianhuan Depression, Yishan Slope, Jinxi Fault-Fold Belt, Yimeng Uplift, and Weibei Uplift (Figure 1a).
The Yanchang Formation constitutes the key stratigraphic target of this study, having been deposited during the Middle-Late Triassic and dominated by fluvial, deltaic, and lacustrine deposits [35,36]. This formation has been subdivided into 10 members (Chang-1 to Chang-10, from top to bottom) based on lithological association, sedimentary cycles and typical beds (Figure 1b) [37,38]. Among them, the Chang-7 Member corresponds to the most extensive lacustrine transgressive phase during the Mesozoic, characterized by peak values in both areal extent and water depth [39]. During this period, the Chang-7 Member developed a suite of organic-rich black shales and dark mudstones-dominated strata exceeding 100 m in cumulative thickness, forming the most critical hydrocarbon source rock succession and the principal oil-producing interval within the Mesozoic succession [40,41].
Since the deposition of the Chang-7 Member, the Ordos Basin has undergone four distinct paleo-geothermal evolutionary phases [42]. The Triassic thermal event (203–300 Ma) was regionally extensive, with a paleo-geothermal gradient of 2.2–3.0 °C/100m. The Middle Jurassic event primarily affected the western and southwestern basin, exhibiting a gradient of 2.8–4.0 °C/100m. The Late Jurassic to Early Cretaceous event (124–153 Ma) was the most intense, with a gradient reaching 3.5–5.5 °C/100m. In contrast, the Late Cretaceous to Quaternary event (~72 Ma) showed a moderated gradient of 2.7–3.1 °C/100m. These episodic thermal events profoundly influenced the diagenetic evolution and hydrocarbon generation processes of the organic-rich Chang-7 shale. The Ordos Basin received continuous terrigenous supplies from multiple peripheral ancient lands during the deposition of the Chang-7 Member, with five distinct provenance areas identified—dominated by the northeastern and southwestern source regions (Figure 1c) [43,44,45]. Additional secondary provenances encompass the northwestern and southern source regions and restricted western contributions [34]. Notably, intensive volcanic activity accompanied Yanchang Formation deposition, as evidenced by the widespread tuffaceous intervals [46]. These volcanic processes were not only intimately linked to the development of the Chang-7 Member hydrocarbon source rocks through enhancing primary productivity and establishing persistent anoxic conditions [47,48,49], but also exerted fundamental controls on mineral assemblages via volcanic glass alteration and authigenic clay formation processes [50].

3. Materials and Methods

3.1. Materials

In this study, a total of 95 samples collected from 13 wells and 1 outcrop section (Figure 1a) were selected for mineral composition analysis and organic geochemical testing. With the exception of Well M53 (located in the Tianhuan Depression and classified as delta front-shallow lacustrine facies), all other wells and outcrop sections belong to semi-deep to deep lacustrine facies, situated within either the Yishaan Slope or the Weibei Uplift. In particular, continuous sampling was carried out in Well YY1 and the Yishicun outcrop section to investigate the vertical composition and evolution of clay minerals.
Based on the samples collected for this study, and to further investigate the lateral distribution characteristics of clay minerals in the Chang-7 shale, we also surveyed 16 published relevant literature sources and oilfield databases (Table S1) [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66]. We systematically collected X-ray diffraction (XRD) test data on clay minerals from representative shale samples across 42 wells. Utilizing this robust experimental dataset, we conducted a comprehensive statistical analysis of their specific composition.

3.2. X-Ray Diffraction (XRD) Analysis

We conducted XRD analysis to determine mineral composition, specifically targeting the assemblages and structural characteristics of clay minerals. As the primary character-ization step, whole-rock XRD analysis was employed to obtain comprehensive miner-alogical data encompassing both clay and non-clay constituents. Samples were ground to powders with particle sizes smaller than 100 mesh (150 μm) and prepared as non-oriented mounts using back-pressing technique in sample holders. Based on China Petroleum Standard SY/T 5163-2018, the <2 μm clay fraction was isolated from homogenized bulk rock samples via suspension-centrifugation methodology [65]. All samples underwent preparation of oriented mounts following separation. Where high organic/carbonate content hindered clay suspension, pretreatment with H2O2 and dilute HCl was applied to remove interfering components prior to separation. Subsequently, clay XRD analyses were conducted under three standardized treatment conditions: (1) air-dried state (AD), (2) ethylene-glycol saturated state (EG, 60 °C, 8 h), and (3) heat-treated state (HT, 550 °C, 3 h) [67]. XRD patterns were acquired using an Ultima IV diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu-Kα radiation (λ = 1.5406 Å) at 40 kV and 30 mA, scanning at 2°/min with 0.02° step increments. Mineral mass fractions were quantified using integrated diffraction peak areas, with subsequent Lorentz correction procedures as outlined by Chalmers and Bustin [68].
The XRD pattern modeling and fitting analysis were conducted using XrayRun (2018.0) software following the Standard SY/T 5163-2018. Chlorite was calibrated based on its characteristic (001), (002) and (003) reflections at 14.2 Å, 7.1 Å and 4.7 Å, respectively, while kaolinite was identified using its characteristic (001) and (002) peaks at 7.15 Å and 3.58 Å. The composite diffraction peak near 10.0 Å was attributed to the overlapping contributions from illite and I/S. Through parameter adjustment, optimal fitting between the experimental test pattern and the modeled pattern is achieved in terms of peak height, peak width, peak position, and other characteristics. Subsequently, the program will extract quantitative information by analyzing the individual curves of each mineral phase in the modeled pattern, while simultaneously calculating the mixed-layer ratios (i.e., the proportion of smectite layers within I/S).

3.3. Organic Geochemical Analyses

To characterize the organic matter richness and thermal maturity of the samples, we conducted total organic carbon (TOC) analysis and measured vitrinite reflectance (Ro) respectively.
Total organic carbon content was determined using a LECO CS230 carbon/sulfur analyzer. Sample preparation involved pulverization to <100 mesh particle size (150 μm), followed by decarbonation through immersion in dilute hydrochloric acid (1 M HCl) and subsequent rinsing with deionized water to remove inorganic carbonates [31]. The treated samples were combusted at 550 °C under high-purity oxygen conditions for TOC quantification.
Vitrinite reflectance analysis was carried out using a Zeiss Scope A1 incident-light microscope equipped with a photomultiplier tube and 546 nm monochromatic illumination. Sample preparation followed a standardized protocol: crushing to <10 mesh particle size (2 mm), homogenization with agar (1:1 mass ratio), and consolidation for thin-section preparation. For each polished section analyzed under oil immersion, 20–55 valid measurement points were systematically acquired by excluding weathered or altered domains.

4. Results

4.1. Clay Mineral Composition and Basin-Wide Distribution

The comprehensive XRD-derived statistics from both experimental analyses in this study and compiled literature data systematically demonstrate the clay mineral content and specific compositional characteristics of the Chang-7 shale (Table 1, Tables S1 and S2). Clay minerals constitute 15.6% to 75.0% of the bulk mineral content, predominantly concentrated between 30% and 55%, with an average of 43.4% (Figure 2). Specifically, illite and I/S are the most widespread components. Illite accounts for 3.0% to 83.0% of the total clay content, displaying relatively uniform distribution across most values except for a minor concentration peak between 15% and 40%, averaging 34.7%. I/S exhibits a broad range of 0% to 86.0%, with two distinct concentration intervals: 0% and 50%–65%, reflecting varying controlling factors on clay composition, and an average of 44.2%. Kaolinite shows limited abundance, representing 0% to 41.0% of clay minerals, primarily concentrated at 0–5%, averaging 5.4%. Chlorite content ranges widely from 0% to 73.9%, with a notable concentration between 5% and 15%, while samples exceeding 40% are rare, yielding an average of 15.6%.
Figure 3 utilizes radar charts to delineate the relative abundance relationships of specific clay mineral components in well samples. Integrated with well location data superimposed on the Ordos Basin palaeogeographic map, this visualization facilitates comparative analysis of planar distribution characteristics of the clay minerals. The illitization of smectite serves as a primary pathway for illite formation, with authigenic I/S developing as transitional phases during this process. Comparative analysis reveals that I/S dominates as the most abundant clay mineral type in the majority of studied wells (blue radar chart), particularly in the southern region (darker blue), with their abundance gradually converging with other clay components toward the northern basin margin (lighter blue). Notably, certain minerals are absent in specific wells within localized zones (dark red and dark green). Kaolinite is preferentially distributed along the margins of the lacustrine basin but is largely absent in the central basin. In contrast, chlorite not only has detrital input but also forms authigenically through Fe-Mg enrichment derived from parent materials in provenance areas, exhibiting significantly higher abundances in proximal wells, with a regional trend of increased concentrations toward the eastern part of the basin (green radar chart).

4.2. Mineralogical Characteristics and Total Organic Carbon Content of Well YY1

Supplementary Table S2 compiles bulk mineral composition, clay mineral assemblages, and Total Organic Carbon (TOC) content data from continuous sampling in Well YY1. Figure 4 also illustrates the variations in TOC and clay minerals with depth. Clay minerals constitute one of the predominant mineral components in shale samples, exhibiting concentrations spanning 17.7% to 54.4% with a mean value of 34.4%. As noted in the previous section, I/S dominates the assemblage, accounting for 61%–94% (average 79.9%), followed by illite (average 13.3%), chlorite (average 4.6%), and kaolinite (average 2.2%). A discernible interval is present within the 225–245 m depth range. Within this specific interval, both the TOC content and the clay mineral assemblages exhibit characteristics significantly distinct from those of the overlying and underlying intervals. Notably, the I/S content decreases progressively with depth, while the illite content exhibits a corresponding increase. This inverse correlation indicates progressive I/S-to-illite conversion during burial diagenesis.
Quartz is the most abundant non-clay mineral component across most samples, with content ranging from 11.0% to 38.4%. Notably, pyrite abundance ranges from 0% to 48.0%, with marked enrichment observed in a specific interval, indicating distinct genetic controls. Plagioclase exceeds potassium feldspar in abundance. This compositional disparity stems from both a dominant supply of plagioclase in the parent materials and diagenetic alteration, specifically the preferential consumption of potassium feldspar during clay mineral transformation. Additionally, the samples contain quantities of carbonate minerals (mainly calcite and dolomite).
Organic matter exhibits pronounced heterogeneity among samples, with TOC contents ranging from 7.21 to 26.90 wt% (mean = 17.14 wt%) in certain samples, whereas other comparable specimens display markedly lower TOC levels (0.44–1.92 wt%). Organic-rich strata demonstrate a persistent stratigraphic association with the pyrite-enriched interval, suggesting a possible genetic linkage between organic matter accumulation and pyrite formation.

4.3. Characteristics of Illite-Smectite Mixed-Layer Minerals and Organic Matter Evolution

The mixed-layer ratios of I/S in studied samples were determined through XRD pattern analysis (Table 1, Figure 5), with these values serving as reliable proxies for assessing diagenetic progression stages. Smectite layer proportions in I/S minerals register 15%–30%, predominantly clustering at 15%–20%, with only specimens from Wells G347 and Z80 exhibiting peak values of 30%. The dataset conclusively places the diagenetic progression within the middle diagenesis stage A2 [69], progressively approaching the stage B transition boundary defined by the 15% smectite-layer threshold.
Complementary vitrinite reflectance measurements were conducted as an independent thermal maturity indicator, providing cross-validation with the I/S mixed-layer ratios for refined diagenetic stage assessment. The Well G347 exhibits the lowest organic maturity at 0.48%Ro, transitional to the middle diagenetic stage A1, while Well B522 demonstrates peak thermal evolution with Ro reaching 1.04%. Despite the significant variability in organic matter thermal evolution indicated by Ro data, XRD analyses reveal striking structural similarities in I/S across the sample suite. Critical evidence emerges from Well Z8 (Ro = 0.58%) and Well B522 (Ro = 1.04%) specimens, both maintaining identical 15% smectite-layer proportions. Within the integrated framework of diagenetic stage classification and organic matter thermal evolution analysis, the illitization extent of smectite in specific samples surpasses theoretical predictions, particularly evident in low-maturity specimens as strikingly demonstrated by Well G347.
Subset XRD pattern modeling and fitting analysis enables enhanced resolution of I/S minerals, revealing subtle heterogeneity among specimens sharing equivalent mixed-layer ratios (Figure 5). Unlike conventional petroleum geological studies that typically assume uniform I/S structures, our results reveal discrete coexistence of two distinct I/S populations within individual samples. For instance, sample M53-24 (Ro = 0.55%) exemplifies a bimodal I/S phase: a subordinate R0 ordering I/S population (7.3%) with 56.6% smectite layers, and a dominant R1 ordering population (39.6%) with 31.0% smectite, reflecting heterogeneous illitization pathways within the clay mineral assemblage (Figure 5a). The I/S mixed-layer assemblages from the remaining specimens—Z233-3 (Ro = 0.58%), L211-15 (Ro = 0.72%), H269-27 (Ro = 0.79%), B522-38 (Ro = 1.02%), and low-maturity G347-24 (Ro = 0.48%)—are systematically documented in Figure 5b–f. Quantitative phase analysis reveals a predominant R1 ordering I/S phases in all the analyzed samples. Notably, the less illitized I/S populations exhibit a stronger correlation with thermal maturity indicators (Ro = 0.58%–0.67%) than their highly illitized counterparts, suggesting that early-stage I/S phases may serve as more sensitive tracers of diagenetic evolution.

5. Discussion

5.1. Controls of Surficial Geological Processes on Clay Mineral Spatial Distribution

The differences in clay mineral assemblages and contents are jointly controlled by depositional and diagenetic processes, making them valuable indicators for reconstructing basin evolution [70,71]. Among most shale samples of the Chang-7 Member across the entire basin, illite/smectite mixed-layer (I/S) minerals and illite are the two dominant clay mineral components. This highlights the critical role of smectite illitization during diagenesis in shaping the final clay mineral assemblage. When integrated with thermal maturity proxies—such as vitrinite reflectance (Ro) and smectite-layer proportion (S%)—the data reveal widespread epigenetic modifications to clay minerals. Although the predominance of R1-ordered I/S suggests that diagenetically formed illite should be the dominant clay mineral component, Figure 3 reveals that I/S minerals remain significantly enriched in most samples. This observation implies that smectite originally held an absolute predominance in the depositional clay mineral assemblage, preceding major illitization during burial diagenesis. This feature is particularly pronounced in well samples or section samples from the southern basin, as exemplified by Well YY1 and Well Jinghe13 (Figure 6a,b), where I/S content significantly exceeds illite abundance, with chlorite and kaolinite remaining scarce [62]. Although the lower Ro values in these samples indicate limited thermal maturation, similar maturity levels in Well G347 do not yield comparable mineralogical characteristics, suggesting that the primary clay mineral assemblage exerts a major control on diagenetic evolution. Specifically, the high abundance of smectite in the initial composition is likely attributed to the intense volcanic activity during deposition, where in situ alteration of volcanic ash through hydrolysis and crystallization processes led to its widespread neoformation [50]. The frequent occurrence of tuffaceous intervals in Well YY1 provides direct evidence of extensive volcanic ash deposition in this area. Integrated with previous studies identifying the Qinling orogenic belt as a volcanic source along the southern basin margin [47], these findings provide a coherent explanation for the pronounced spatial heterogeneity in the distribution of illite and I/S in the Chang-7 Shale across the basin.
Exceptions to this general pattern are observed in the Zhenjing block and the DZ section (Figure 6c,d) [53,63], where the clay mineral assemblages are dominated by illite and chlorite, while I/S and kaolinite are largely absent or occur only sporadically in isolated samples. The underlying controls in these two areas differ significantly. In the Zhenjing Block, drill cores from gas-bearing shale reservoirs exhibit high thermal maturity, with Ro values ranging from 1.02% to 1.30%, indicating advanced diagenetic alteration. Under such conditions, smectite has undergone complete illitization, reaching the illite end-member, and kaolinite has been entirely consumed through transformation into either illite (via reactions with K+) or chlorite (via Mg2+ incorporation). In contrast, the DZ Section, characterized by a lower thermal maturity (Ro ≈ 0.7%), also lacks detectable I/S minerals. This absence can be attributed to the scarcity of smectite or its volcanic-ash-derived precursors within the depositional system. Instead, detrital clay minerals are dominated by illite and chlorite, sourced from parent rocks containing substantial quantities of feldspars and mafic minerals. These primary minerals can directly serve as precursor phases for the authigenic formation of illite and chlorite—a process independent of smectite intermediation.
Kaolinite exhibits a basin-wide depletion in the study area, with only localized enrichments observed in marginal lacustrine settings such as Wells Honghe21 [62] and H269 (this study). Significantly, kaolinite scarcity is not limited to the Chang-7 shale but persists throughout the Middle-Lower Yanchang Formation, forming a stratigraphically extensive kaolinite-depleted interval. The presence of kaolinite in deeper stratigraphic units indicates that its absence in the Chang-7 and overlying intervals cannot be solely attributed to diagenetic alteration. Instead, the vertical distribution pattern implies a primary control related to limited detrital input during deposition. Detrital kaolinite abundance is significantly influenced by sedimentary facies and hydrodynamic sorting processes [17]. Among the major clay minerals, kaolinite typically displays the coarsest particle size, followed by illite and smectite. This grain-size hierarchy leads to a spatial segregation during transport and deposition: kaolinite tends to accumulate preferentially in proximal, nearshore settings, while finer-grained illite and smectite are more readily transported to distal lacustrine environments. The relatively high kaolinite content in Well H269, sourced from multiple proximal sources in the northeast and west, supports the interpretation that kaolinite depletion in most samples is primarily controlled by depositional factors rather than diagenetic processes.
At the regional scale, chlorite content demonstrates a distinct east–west distribution pattern, with higher concentrations in the eastern part of the basin and localized enrichments in certain southwestern areas. Chlorite abundance in the Chang-7 shale is governed by both authigenesis and detrital input. The chlorite originates from authigenic processes in sedimentary basin strongly relies on weathering products of mafic source materials, which provide abundant Fe2+ and Mg2+ ions—key components of chlorite’s crystal structure [72,73]. Provenance analysis of the Chang-7 Member indicates that the northeastern and eastern areas of the basin are mainly influenced by sediment input from the Yinshan-Daqingshan provenance system, located along the northeastern margin [34]. Source rocks from this region comprise intermediate-basic magmatic rocks and garnet-bearing khondalite series. Terrigenous clastic rocks derived from this system also contain notably abundant detrital chlorite components, which specifically contribute to the chlorite enrichment observed in the northeastern part of the basin. In the southwestern sectors, localized chlorite enrichment is linked to a different provenance system, dominated by parent rocks metamorphosed under amphibolite–greenschist facies conditions [15].
Collectively, although the ultimate clay mineral assemblages are shaped by diagenetic overprinting, epigenetic geological processes still exert profound influence on both their composition and spatial distribution. The enrichment patterns of the four clay mineral components, governed by distinct genetic pathways, result from the coupled effects of provenance characteristics, sedimentary dynamics, tectonic activity and prevailing paleoenvironmental-paleohydrological conditions.

5.2. The Correlation Between Clay Mineral Abundance and Other Mineral Constituents

From deposition through deep burial, inorganic minerals evolve as interdependent assemblages, with mutual transformations and ion exchange—often mediated by dissolution-reprecipitation processes—modulating their diagenetic pathways. In Well YY1, the correlation between clay minerals and quartz exhibits a bimodal pattern: no significant correlation is observed when clay content is below 35%, which shifts to a negative correlation beyond this threshold (Figure 7a). A similar pattern has been documented in the Da’anzhai shale of the eastern Sichuan Basin [74]. In continental shale systems, clay minerals and quartz usually exhibit a negative correlation [75], reflecting their dominance as the two primary mineral constituents and their inherent volumetric competition within the rock matrix. However, this characteristic negative clay-quartz correlation can be disrupted by the presence of a third major mineral phase. In the analyzed samples, pyrite serves as this tertiary component, and its abundance significantly alters the clay-quartz relationship. The enrichment of pyrite is closely linked to high organic matter content, indicating strong redox-driven authigenesis. Conversely, authigenic quartz may precipitate at the micrometer scale during the transformation of smectite to illite [76,77], further modifying the original mineral correlations established during diagenesis.
The correlation between K-feldspar and clay minerals is similarly influenced by diagenetic process. During burial, K-feldspar is progressively depleted either through direct transformation of K-feldspar into clay minerals under acidic conditions, or via dissolution, which releases K+ that facilitates smectite illitization [78]. Given the high content of illite-smectite mixed-layer clay minerals in the Chang-7 shale, the elevated degree of illitization, and the relatively low K+ concentration in ancient formation water, diagenesis has likely exerted significant influence on K-feldspar stability. This is further manifested in the negative correlation observed between K-feldspar and clay mineral content (Figure 7b). In contrast, plagioclase alteration shows limited interaction with clay evolution. Although plagioclase alteration can generate kaolinite under certain conditions, such transformation is constrained by its competitive interactions with K-feldspar alteration and by the limited K+ yield from plagioclase dissolution [79]. As a result, plagioclase contributes minimally to clay mineral transformation, and its content shows weak or negligible correlation with clay minerals (Figure 7c). A similar pattern is observed for carbonate minerals (Figure 7d). While authigenic carbonate formation may occur through pathways involving organic matter decarboxylation and Ca2+ release during smectite-to-illite transformation [20], the carbonate content generated through this specific mechanism appears to be minor. Consequently, carbonate content does not significantly influence clay mineral distributions. Furthermore, intervals characterized by carbonate scarcity and organic matter enrichment, the generation of organic acids can trigger carbonate dissolution, further weakening the association between carbonates and clay minerals.

5.3. Impact of Organic Matter on Clay Mineral Composition and Diagenetic Evolution

Organic matter, a key constituent of the Chang-7 shale, has been intimately associated with clay minerals from incipient stages of particle transport and deposition, forming structurally integrated clay–organic nanocomposites [24,25]. The co-evolution of these two components further suggests that organic matter significantly influences both the composition and diagenetic processes of clay minerals [7]. TOC data from Well YY1 reveals significant organic matter enrichment within the 215–235 m interval, where the total clay mineral content shows a marked decrease (Figure 4). As discussed in Section 4.2, pyrite enrichment coincides with elevated organic matter content within this interval, which dilutes the clay mineral proportion in the inorganic fraction. This observation contrasts with the commonly reported positive clay–organic correlation, which has been attributed to early-stage adsorption and organo-mineral binding [80,81,82,83]. In the high-TOC interval of Well YY1, however, increased pyrite and organic matter reduce the volumetric proportion of clays, effectively reversing this typical correlation (Figure 8, green data points). Conversely, in low-TOC intervals, this dilution effect operates in the reverse direction, with clay minerals suppressing organic matter abundance (Figure 8, orange data points, [84]).
Specifically, the clay mineral compositions in organic-rich samples exhibit a marked depletion in kaolinite and chlorite proportions, with kaolinite being undetectable in some cases (Figure 4). The observed mineralogical changes can be attributed to two primary factors. During the tectonically active phase, substantial volcanic ash deposition was introduced into the lacustrine basin. These volcanogenic inputs not only created favorable conditions for organic matter enrichment but also served as critical precursor materials for smectite formation [47,48,49,50], thereby fundamentally modifying clay mineral assemblages. Furthermore, the consumption of kaolinite and authigenic formation of chlorite are also influenced by organic matter. The elevated organic acid flux generated during organic matter thermal maturation promotes the transformation of kaolinite into illite [85]. Chlorite formation, whether through direct precipitation from aqueous solutions or precursor mineral transformations, requires Fe-Mg-enriched diagenetic fluids or detrital components supplying these cations [86]. However, widespread pyrite formation consumes substantial Fe2+, suppressing chloritization of feldspars, kaolinite and smectite. Additionally, low-pH conditions generated by organic acids destabilize chlorite’s crystalline structure [87].
Modifications in I/S, such as changes in ordering types and a gradual reduction in smectite content, have served as effective mineral indicators to constrain thermal evolution histories [5]. This approach is grounded in the well-documented consensus that temperature exerts dominant control over smectite-to-illite transformation [88]. However, in studies of the Chang-7 shale, this methodology has demonstrated limited reliability. The diagenetic stages indicated by I/S and organic matter thermal maturity exhibit notable inconsistencies. Moreover, equivalent mixed-layer ratios (e.g., 15% smectite layers within I/S) correspond to samples exhibiting substantial variations in vitrinite reflectance (Ro) values, ranging from 0.58% to 1.04%. These findings suggest that temperature is not the predominant controlling factor influencing illitization during specific diagenetic phases.
Instead, analysis of Well YY1 reveals a notable pattern: samples from the high-TOC interval exhibit a lower proportion of smectite within I/S compared to adjacent low-TOC intervals (Figure 4). This trend is more pronounced in the Yishicun Section shale specimens, where Figure 9 clearly demonstrates an inverse correlation between mixed-layer ratios and TOC content. This observation implies that the elevated organic matter content may actively promote the transformation of smectite to illite during illitization. The underlying mechanism likely involves the co-evolution of smectite (the precursor phase) and organic matter during diagenesis. Under hydrothermal simulation conditions, this coherent evolutionary process can be broadly divided into two distinct phases, corresponding to organic matter desorption/decarboxylation and cracking/cross-linking reactions [89]. Experimental investigations into kerogen pyrolysis reveal that organic acid production initiates at immature stages with Ro < 0.5%, preceding the peak acid generation phase, but already yielding 20%–40% of the maximum acid output [90]. Carbon dioxide is simultaneously produced through organic acid breakdown and direct kerogen decarboxylation processes. Analyses of natural samples further demonstrate that the immature stage represents a critical phase for organic acid evolution in the Chang-7 shale [91]. Integrating these findings, the organic-rich nature of the Chang-7 shale facilitates pronounced decarboxylation processes prior to the onset of the oil window, which in turn accelerates illitization of smectite under relatively low-maturity conditions. As thermal maturity increases, organic cracking and polymerization dominate, marking a second stage of accelerated transformation [89]. This leads to the development of uniform I/S mixed-layer structures (R1 ordering) and consistent diagenetic signatures in shale samples with Ro > 0.6%.
Revisiting the XRD profile fitting analyses (Figure 5), the coexistence of two distinct I/S populations with divergent mixed-layer ratios within individual samples reflects differential dominant controls. The R0 Ordering I/S phases provide greater accuracy in constraining diagenetic stages and exhibit higher sensitivity to thermal gradients. In contrast, the R1-ordered I/S populations predominantly reflect the accelerating influence of organic matter enrichment on mineral transformation in the Chang-7 shale, resulting in their dominance within the assemblage.

6. Conclusions

This study investigates the clay mineral characteristics in the Chang-7 shale through integrated systematic mineralogical analyses and comprehensive data investigation, revealing the joint control of sedimentary depositional and diagenetic processes on clay mineral assemblages. The coupled analysis of clay mineralogical profiles and organic geochemical parameters further elucidates the influence of organic matter on smectite-to-illite transformation. By advancing a novel framework for inorganic-organic co-regulation of clay mineral compositional evolution, these findings fundamentally deepen the understanding of Chang-7 shale oil and gas systems while delivering actionable strategies for future exploration.
(1)
Frequent volcanic activity during the depositional period of the Chang-7 Member supplied abundant source materials for smectite formation, resulting in the predominance of I/S in the majority of samples. Provenance characteristics and sedimentary systems jointly govern the differentiated planar distribution patterns of clay minerals, serving as principal controls for kaolinite and chlorite distribution, respectively.
(2)
Inter-mineral water-rock interactions exert significant controls on clay mineral evolution. The transformation processes necessitate the dissolution of K-feldspar and plagioclase (with K-feldspar being preferentially consumed), resulting in a pronounced negative correlation between clay mineral content and K-feldspar abundance. Although quartz and carbonate minerals can precipitate as authigenic phases, such diagenetic modifications remain insignificant in the Chang-7 shale.
(3)
Organic matter acts as a critical component in the organic-rich Chang-7 shale, driving co-evolution with clay minerals during progressive burial and thermal maturation. The accelerated illitization of smectite and the negative correlation between mixed-layer ratios and TOC content demonstrate that organic matter plays a dominant role in mineral transformation, primarily through decarboxylation, hydrocarbon cracking during thermal maturation, and enhanced dissolution of potassium feldspar.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15090951/s1, Table S1: Clay mineral composition of the Chang-7 shale in published literature and databases.; Table S2: Clay mineralogical composition and TOC content of samples from Well YY1; Table S3: Analytical data of I/S and TOC content in samples from Yishicun Section.

Author Contributions

Conceptualization, K.L. and L.D.; methodology, K.L. and Z.W.; formal analysis, K.L. and L.D.; investigation, K.L. and Z.W.; resources, K.L. and Z.W.; data curation, K.L., Y.C. and Y.L.; writing—original draft preparation, K.L.; writing—review and editing, K.L.; 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 NSFC, grant numbers: 42090021 and 42373050, and Deep Earth Probe and Mineral Resources Exploration-National Science and Technology Major Project, grant number: 2024ZD1001002.

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.

References

  1. Meunier, A. Clays; Springer Science & Business Media: Berlin, Germany, 2005; ISBN 3-540-21667-7. [Google Scholar]
  2. Jiang, S. Clay Minerals from the Perspective of Oil and Gas Exploration. In Clay Minerals in Nature—Their Characterization, Modification and Application; InTech: London, UK, 2012; pp. 21–38. ISBN 978-953-51-0738-5. [Google Scholar]
  3. Dilli, K.; Pant, R.K. Clay Minerals as Indicators of the Provenance and Palaeoclimatic Record of the Kashmir Loess. J. Geol. Soc. India 1994, 44, 563–574. [Google Scholar] [CrossRef]
  4. Franke, D.; Ehrmann, W. Neogene Clay Mineral Assemblages in the AND-2A Drill Core (McMurdo Sound, Antarctica) and Their Implications for Environmental Change. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2010, 286, 55–65. [Google Scholar] [CrossRef]
  5. Ola, P.S.; Aidi, A.K.; Bankole, O.M. Clay Mineral Diagenesis and Source Rock Assessment in the Bornu Basin, Nigeria: Implications for Thermal Maturity and Source Rock Potential. Mar. Pet. Geol. 2018, 89, 653–664. [Google Scholar] [CrossRef]
  6. Wu, L.; Zhou, C.; Keeling, J.; Tong, D.; Yu, W. Towards an Understanding of the Role of Clay Minerals in Crude Oil Formation, Migration and Accumulation. Earth-Sci. Rev. 2012, 115, 373–386. [Google Scholar] [CrossRef]
  7. Pevear, D.R. Illite and Hydrocarbon Exploration. Proc. Natl. Acad. Sci. USA 1999, 96, 3440–3446. [Google Scholar] [CrossRef]
  8. Bu, H.; Yuan, P.; Liu, H.; Liu, D.; Liu, J.; He, H.; Zhou, J.; Song, H.; Li, Z. Effects of Complexation between Organic Matter (OM) and Clay Mineral on OM Pyrolysis. Geochim. Cosmochim. Acta 2017, 212, 1–15. [Google Scholar] [CrossRef]
  9. Loucks, R.G.; Reed, R.M.; Ruppel, S.C.; Hammes, U. Spectrum of Pore Types and Networks in Mudrocks and a Descriptive Classification for Matrix-Related Mudrock Pores. AAPG Bull. 2012, 96, 1071–1098. [Google Scholar] [CrossRef]
  10. Zhang, W.; Zhou, S.; Yu, Z.; Liu, X.; Wang, S.; Miao, H.; Liu, D.; Tian, J.; Wang, H. Controls of Clay Mineral Transformation and Organic Matter on Pore Networks of the Paleogene Lacustrine Shale Oil System in the Yitong Basin, NE China. J. Asian Earth Sci. 2025, 280, 106469. [Google Scholar] [CrossRef]
  11. Worden, R.H.; Morad, S. (Eds.) Clay Mineral Cements in Sandstones, 1st ed.; Wiley: Hoboken, NJ, USA, 1999; ISBN 978-1-4051-0587-3. [Google Scholar]
  12. Wilson, M.J.; Wilson, L.; Shaldybin, M.V. Clay Mineralogy and Unconventional Hydrocarbon Shale Reservoirs in the USA. II. Implications of Predominantly Illitic Clays on the Physico-Chemical Properties of Shales. Earth-Sci. Rev. 2016, 158, 1–8. [Google Scholar] [CrossRef]
  13. Kang, Y.; Liu, K.-Q.; Zhu, R.-K.; Yin, G.-G.; Zhang, J.-Y.; Zhang, S.-R. 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. Pet. Sci. 2024, 21, 3867–3883. [Google Scholar] [CrossRef]
  14. De Silva, P.N.K.; Simons, S.J.R.; Stevens, P.; Philip, L.M. A Comparison of North American Shale Plays with Emerging Non-Marine Shale Plays in Australia. Mar. Pet. Geol. 2015, 67, 16–29. [Google Scholar] [CrossRef]
  15. Deng, H.; Xie, X.; Chen, K.; Vij, C.; Pang, Y.; Li, H.A. Mineralogical Characteristics of Continental Shale: A Case Study in Yan-Chang Formation, Ordos Basin. Aust. J. Earth Sci. 2018, 65, 851–862. [Google Scholar] [CrossRef]
  16. Pozo, M.; Calvo, J.P. An Overview of Authigenic Magnesian Clays. Minerals 2018, 8, 520. [Google Scholar] [CrossRef]
  17. Smoot, T.W. Clay Mineralogy of Pre-Pennsylvanian Sandstones and Shales of the Illinois Basin. Part III: Clay Minerals of Various Facies of Some Chester Formations; Circular 293; Illinois State Geological Survey: Champaign, IL, USA, 1960; pp. 1–19. [Google Scholar]
  18. Keller, W.D. Diagenesis in Clay Minerals—A Review. Clays Clay Miner. Natl. Conf. Clays Clay Miner. 1962, 11, 136–157. [Google Scholar] [CrossRef]
  19. Liu, W.; Zhu, Q.; Qiao, Y.; Wu, W.; Pan, J.; Mou, C.; Yao, J.; Chen, Y. Origin of Authigenic Quartz in Organic-Rich Shales of the Dalong Formation (NW Middle Yangtze Basin), South China: Implications for Pore Network Development. Energy Fuels 2024, 38, 6912–6926. [Google Scholar] [CrossRef]
  20. Liang, C.; Wang, J.; Cao, Y.; Xiong, Z.; Liu, K.; Hao, F.; Han, W. Authigenic Calcite in Shales: Implications for Tracing Burial Processes and Diagenetic Fluid Evolution in Sedimentary Basins. Earth-Sci. Rev. 2024, 258, 104935. [Google Scholar] [CrossRef]
  21. Środoń, J. Nature of Mixed-Layer Clays and Mechanisms of Their Formation and Alteration. Annu. Rev. Earth Planet. Sci. 1999, 27, 19–53. [Google Scholar] [CrossRef]
  22. Zanoni, G.; Šegvić, B.; Moscariello, A. Clay Mineral Diagenesis in Cretaceous Clastic Reservoirs from West African Passive Margins (the South Gabon Basin) and Its Impact on Regional Geology and Basin Evolution History. Appl. Clay Sci. 2016, 134, 186–209. [Google Scholar] [CrossRef]
  23. Ahmat, A.M.; Boussafir, M.; Le Milbeau, C.; Guégan, R.; De Oliveira, T.; Le Forestier, L. Organic Matter and Clay Interaction in a Meromictic Lake: Implications for Source Rock OM Preservation (Lac Pavin, Puy-de-Dôme, France). Org. Geochem. 2017, 109, 47–57. [Google Scholar] [CrossRef]
  24. Gu, Y.; Wan, Q.; Li, X.; Han, T.; Yang, S.; Hu, Q. Structure and Evolution of Clay-Organic Nanocomposites in Three Leading Shales in China. J. Earth Sci. 2023, 34, 824–837. [Google Scholar] [CrossRef]
  25. Berthonneau, J.; Grauby, O.; Abuhaikal, M.; Pellenq, R.J.-M.; Ulm, F.J.; Van Damme, H. Evolution of Organo-Clay Composites with Respect to Thermal Maturity in Type II Organic-Rich Source Rocks. Geochim. Cosmochim. Acta 2016, 195, 68–83. [Google Scholar] [CrossRef]
  26. Rahman, H.M.; Kennedy, M.; Löhr, S.; Dewhurst, D.N. Clay-Organic Association as a Control on Hydrocarbon Generation in Shale. Org. Geochem. 2017, 105, 42–55. [Google Scholar] [CrossRef]
  27. Collins, M.J.; Bishop, A.N.; Farrimond, P. Sorption by Mineral Surfaces: Rebirth of the Classical Condensation Pathway for Kerogen Formation? Geochim. Cosmochim. Acta 1995, 59, 2387–2391. [Google Scholar] [CrossRef]
  28. Adams, R.S.; Bustin, R.M. The Effects of Surface Area, Grain Size and Mineralogy on Organic Matter Sedimentation and Preservation across the Modern Squamish Delta, British Columbia: The Potential Role of Sediment Surface Area in the Formation of Petroleum Source Rocks. Int. J. Coal Geol. 2001, 46, 93–112. [Google Scholar] [CrossRef]
  29. Lao, Y.; Korth, J.; Ellis, J.; Crisp, P.T. Heterogeneous Reactions of 1-Pristene Catalysed by Clays under Simulated Geological Conditions. Org. Geochem. 1989, 14, 375–379. [Google Scholar] [CrossRef]
  30. Jin, Z.; Wang, G.; Liu, G.; Gao, B.; Liu, Q.; Wang, H.; Liang, X.; Wang, R. Research progress and key scientific issues of continental shale oil in China. Acta Pet. Sin. 2021, 42, 821–835. [Google Scholar] [CrossRef]
  31. Wang, Z.; Dong, L.; Jin, Z.; Zou, S.; Fu, J.; Zhu, R. Efforts to Untie the Multicollinearity Knot and Identify Factors Controlling Macropore Structures in Shale Oil Reservoirs. Adv. Geo-Energy Res. 2024, 11, 194–207. [Google Scholar] [CrossRef]
  32. Chu, R.; Wu, H.; Zhu, R.; Fang, Q.; Deng, S.; Cui, J.; Yang, T.; Li, H.; Cao, L.; Zhang, S. Orbital Forcing of Triassic Megamonsoon Activity Documented in Lacustrine Sediments from Ordos Basin, China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2020, 541, 109542. [Google Scholar] [CrossRef]
  33. Tang, X.; Zhang, J.; Wang, X.; Yu, B.; Ding, W.; Xiong, J.; Yang, Y.; Wang, L.; Yang, C. Shale Characteristics in the Southeastern Ordos Basin, China: Implications for Hydrocarbon Accumulation Conditions and the Potential of Continental Shales. Int. J. Coal Geol. 2014, 128, 32–46. [Google Scholar] [CrossRef]
  34. Zhang, C.; Zhang, L.; Chen, T.; Zhag, Y.; Yang, Y.; Huang, J.; He, J.; Li, H. Provenance and Parent-Rock Types of Member 7 of Yanchang Formation(Triassic), Ordos Basin. Acta Sedimentol. Sin. 2013, 31, 430–439. [Google Scholar] [CrossRef]
  35. Zou, C.; Wang, L.; Li, Y.; Tao, S.; Hou, L. Deep-Lacustrine Transformation of Sandy Debrites into Turbidites, Upper Triassic, Central China. Sediment. Geol. 2012, 265, 143–155. [Google Scholar] [CrossRef]
  36. Chen, P.; Xian, B.; Li, M.; Fang, L.; Rahman, N.U.; Liu, J.; Chen, S.; Tian, R.; Wu, Q. Intensified Lacustrine Turbidite Deposition as a Response to the Carnian Pluvial Episode: Insights from the Triassic Ordos Basin in North China Plate. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2023, 623, 111599. [Google Scholar] [CrossRef]
  37. Qiu, X.; Liu, C.; Mao, G.; Deng, Y.; Wang, F. Enrichment Feature of Thorium Element in Tuff Interlayers of Upper Triassic Yanchang Formation in Ordos Basin, China. Geol. Bull. China 2010, 29, 1185–1191. [Google Scholar] [CrossRef]
  38. Deng, S.; Lu, Y.; Luo, Z.; Fan, R.; Li, X.; Zhao, Y.; Ma, X.; Zhu, R.; Cui, J. Subdivision and Age of the Yanchang Formation and the Middle/Upper Triassic Boundary in Ordos Basin, North China. Sci. China Earth Sci. 2018, 61, 1419–1439. [Google Scholar] [CrossRef]
  39. Fu, J.; Li, S.; Guo, Q.; Guo, W.; Zhou, X.; Liu, J. Enrichment conditions and favorable area optimization of continental shale oil in Ordos Basin. Acta Pet. Sin. 2022, 43, 1702–1716. [Google Scholar] [CrossRef]
  40. Yang, H.; Zhang, W.; Wu, K.; Li, S.; Peng, P.; Qin, Y. Uranium Enrichment in Lacustrine Oil Source Rocks of the Chang 7 Member of the Yanchang Formation, Erdos Basin, China. J. Asian Earth Sci. 2010, 39, 285–293. [Google Scholar] [CrossRef]
  41. Wang, C.; Wang, Q.; Chen, G.; He, L.; Xu, Y.; Chen, L.; Chen, D. Petrographic and Geochemical Characteristics of the Lacustrine Black Shales from the Upper Triassic Yanchang Formation of the Ordos Basin, China: Implications for the Organic Matter Accumulation. Mar. Pet. Geol. 2017, 86, 52–65. [Google Scholar] [CrossRef]
  42. Wang, T.; Yang, B. Source-Rock Burial History of Triassic Yanchang Group Chang 7 Formation in Ordos Basin. Liaoning Chem. Ind. 2011, 40, 375–377. [Google Scholar] [CrossRef]
  43. Wei, B.; Wei, H.; Chen, Q.; Zhao, H. Sediment Provenance Analysis of Yanchang Formation in Ordos Basin. J. Northwest Univ. Sci. Ed. 2003, 4, 447–450. [Google Scholar] [CrossRef]
  44. Wang, R.; Li, W.; Liao, Y.; Guo, Y.; Liu, H. Provenance Analysis of Chang 7 Member of Triassic Yanchang Formation in Ordos Basin. Geol. Bull. China 2013, 32, 671–684. [Google Scholar] [CrossRef]
  45. Cui, J.; Zhu, R.; Li, S.; Qi, Y.; Shi, X.; Mao, Z. Development Patterns of Source Rocks in the Depression Lake Basin and Its Influence on Oil Accumulation: Case Study of the Chang 7 Member of the Triassic Yanchang Formation, Ordos Basin, China. J. Nat. Gas Geosci. 2019, 4, 191–204. [Google Scholar] [CrossRef]
  46. Zhu, R.; Cui, J.; Deng, S.; Luo, Z.; Lu, Y.; Qiu, Z. High-precision Dating and Geological Significance of Chang 7 Tuff Zircon of the Triassic Yanchang Formation, Ordos Basin in Central China. Acta Geol. Sin.-Engl. Ed. 2019, 93, 1823–1834. [Google Scholar] [CrossRef]
  47. Zhang, W.; Yang, H.; Peng, P.; Yang, Y.; Zhang, H.; Shi, X. The Influence of Late Triassic Volcanism on the Development of Chang 7 High Grade Hydrocarbon Source Rock in Ordos Basin. Geochemica 2009, 38, 573–682. [Google Scholar] [CrossRef]
  48. Qiu, X.; Liu, C.; Mao, G.; Deng, Y.; Wang, F.; Wang, J. Major, Trace and Platinum-Group Element Geochemistry of the Upper Triassic Nonmarine Hot Shales in the Ordos Basin, Central China. Appl. Geochem. 2015, 53, 42–52. [Google Scholar] [CrossRef]
  49. Zou, C.; Zhu, R.; Chen, Z.-Q.; Ogg, J.G.; Wu, S.; Dong, D.; Qiu, Z.; Wang, Y.; Wang, L.; Lin, S.; et al. Organic-Matter-Rich Shales of China. Earth-Sci. Rev. 2019, 189, 51–78. [Google Scholar] [CrossRef]
  50. Mortland, M.M. Clay Minerals. Earth-Sci. Rev. 1982, 18, 182. [Google Scholar] [CrossRef]
  51. Xiao, D. Characteristics of Shale Diagenesis and Pore Evolution of Chang 73 Sub-member of Yanchang Formation in the West Center of Yishan Slope in Ordos Basin. Master’s Thesis, East China University of Technology, Shanghai, China, 2023. [Google Scholar]
  52. Feng, X.; Ao, W.; Tang, X. Characteristics of Pore Development and Its Main Controlling Factors of Continental Shale Gas Reservoirs: A Case Study of Chang 7 Member in Ordos Basin. J. Jilin Univ. Sci. Ed. 2018, 48, 678–692. [Google Scholar] [CrossRef]
  53. Dong, G. The Evaluation of Chang7 Shale Gas Reservoir Characteristics of YanChang Formation in Zhenyuan and Jingchuan Area, Ordos Basin. Master’s Thesis, Chengdu University of Technology, Chengdu, China, 2012. [Google Scholar]
  54. Zeng, Q. Characteristics and Evaluation of the Organic-Rich Shale Reservoir of Yanchang Formation, Southeast in Ordos Basin. Master’s Thesis, China University of Geosciences (Beijing), Beijing, China, 2013. [Google Scholar]
  55. Wang, H. Diagenesis and Pore Evolution of Chang 7 Shale Strata in Zhidan Area of Ordos Basin. Master’s Thesis, Chengdu University of Technology, Chengdu, China, 2022. [Google Scholar]
  56. Guo, R.; Yu, G.; Wen, K.; Liu, F.; Zhao, J.; Bai, Y.; Wu, W.; Cao, L. Pore Characteristics and Its Impact on Shale Oil Occurrence of Lacustrine Shale from Chang 7, Ordos Basin. Pet. Sci. Technol. 2024, 43, 2501–2520. [Google Scholar] [CrossRef]
  57. Huang, W.; Zhou, X.; Liu, J.; He, T.; Ma, X.; Tao, H.; Li, S.; Hao, L. Characteristics and controlling factors of pore structure of shale in the seventh member of Yanchang Formation in Huachi area, Ordos Basin. Nat. Gas Geosci. 2022, 33, 1951–1968. [Google Scholar] [CrossRef]
  58. Li, D.; Shi, Q.; Mi, N.; Xu, Y.; Wang, X.; Tao, W. The Type, Origin and Preservation of Organic Matter of the Fine-Grain Sediments in Triassic Yanhe Profile, Ordos Basin, and Their Relation to Paleoenvironment Condition. J. Pet. Sci. Eng. 2020, 188, 106875. [Google Scholar] [CrossRef]
  59. Teng, J.; Deng, H.; Liu, B.; Chen, W.; Fu, M.; Xia, Y.; Yu, H. Insights of the Pore System of Lacustrine Shales from Immature to Late Mature with the Aid of Petrology, Mineralogy and Porosimetry: A Case Study of the Triassic Yanchang Formation of the Ordos Basin, North China. J. Pet. Sci. Eng. 2021, 196, 107631. [Google Scholar] [CrossRef]
  60. Sun, N.; Chen, T.; Zhong, J.; Gao, J.; Shi, X.; Xue, C.; Swennen, R. Petrographic and Geochemical Characteristics of Deep-Lacustrine Organic-Rich Mudstone and Shale of the Upper Triassic Chang 7 Member in the Southern Ordos Basin, Northern China: Implications for Shale Oil Exploration. J. Asian Earth Sci. 2022, 227, 105118. [Google Scholar] [CrossRef]
  61. Zhao, Y.; Zhang, C.; Lu, J.; Zhu, X.; Li, L.; Si, S. Sedimentary Environment and Model for Organic Matter Enrichment: Chang 7 Shale of Late Triassic Yanchang Formation, Southern Margin of Ordos Basin, China. Energies 2022, 15, 2948. [Google Scholar] [CrossRef]
  62. Han, H.; Pang, P.; Li, Z.; Shi, P.; Guo, C.; Liu, Y.; Chen, S.; Lu, J.; Gao, Y. Controls of Organic and Inorganic Compositions on Pore Structure of Lacustrine Shales of Chang 7 Member from Triassic Yanchang Formation in the Ordos Basin, China. Mar. Pet. Geol. 2019, 100, 270–284. [Google Scholar] [CrossRef]
  63. Liu, B.; Teng, J.; Li, C.; Li, B.; Bie, S.; Wang, Y. The Control of Shale Composition on the Pore Structure Characteristics of Lacustrine Shales: A Case Study of the Chang 7 Member of the Triassic Yanchang Formation, Ordos Basin, North China. Energies 2022, 15, 8353. [Google Scholar] [CrossRef]
  64. Ju, W.; You, Y.; Chen, Y.; Feng, S.; Xu, H.; Zhao, Y.; Liu, B. Nanoscale Pore Structure and Fractal Characteristics of the Continental Yanchang Formation Chang 7 Shale in the Southwestern Ordos Basin, Central China. Energy Sci. Eng. 2019, 7, 1188–1200. [Google Scholar] [CrossRef]
  65. Wang, G.; Zhang, Q.; Zhu, R.; Tang, X.; Liu, K.; Jin, Z. Geological Controls on the Pore System of Lacustrine Unconventional Shale Reservoirs: The Triassic Chang 7 Member in the Ordos Basin, China. Geoenergy Sci. Eng. 2023, 221, 111139. [Google Scholar] [CrossRef]
  66. Xiao, L.; Tian, W.; Yu, L.; Zhao, M.; Wei, Q. Chemical Characteristics and Distribution Prediction of Hydrocarbon Source Rocks in the Continental Lacustrine Basin of the Chang 7 Member in the Heshui Area of the Ordos Basin, China. Minerals 2024, 14, 303. [Google Scholar] [CrossRef]
  67. Brown, G.; Brindley, G.W. X-Ray Diffraction Procedures for Clay Mineral Identification. In Crystal Structures of Clay Minerals and Their X-Ray Identification; Mineralogical Society of Great Britain and Ireland: Colchester, UK; London, UK, 1980; Volume 5, pp. 305–360. ISBN 978-0-903056-08-3. [Google Scholar]
  68. Chalmers, G.R.L.; Bustin, R.M. Lower Cretaceous Gas Shales in Northeastern British Columbia, Part I: Geological Controls on Methane Sorption Capacity. Bull. Can. Pet. Geol. 2008, 56, 1–21. [Google Scholar] [CrossRef]
  69. Ying, F.; Luo, P.; He, D. Diagenesis and Diagenetic Numerical Modeling of Clastic Reservoir Rocks in Petroliferous Basins of China; Petroleum Industry Press: Beijing, China, 2004; ISBN 7-5021-4816-7. [Google Scholar]
  70. Lynch, F.L.; Mack, L.E.; Land, L.S. Burial Diagenesis of Illite/Smectite in Shales and the Origins of Authigenic Quartz and Secondary Porosity in Sandstones. Geochim. Cosmochim. Acta 1997, 61, 1995–2006. [Google Scholar] [CrossRef]
  71. Chen, B.; Wang, F.; Shi, J.; Chen, F.; Shi, H. Origin and Sources of Minerals and Their Impact on the Hydrocarbon Reservoir Quality of the PaleogeneLulehe Formation in the Eboliang Area, Northern Qaidam Basin, China. Minerals 2019, 9, 436. [Google Scholar] [CrossRef]
  72. Zhu, S.; Wang, X.; Qin, Y.; Jia, Y.; Zhu, X.; Zhang, J.; Hu, Y. Occurrence and Origin of Pore-Lining Chlorite and Its Effectiveness on Preserving Porosity in Sandstone of the Middle Yanchang Formation in the Southwest Ordos Basin. Appl. Clay Sci. 2017, 148, 25–38. [Google Scholar] [CrossRef]
  73. Li, K.; Xi, K.; Cao, Y.; Niu, X.; Wu, S.; Feng, S.; You, Y. Chlorite Authigenesis and Its Impact on Reservoir Quality in Tight Sandstone Reservoirs of the Triassic Yanchang Formation, Southwestern Ordos Basin, China. J. Pet. Sci. Eng. 2021, 205, 108843. [Google Scholar] [CrossRef]
  74. Wang, R.; Hu, Z.; Lai, F.; Liu, Y.; Wu, Z.; He, J.; Zou, G.; Wang, P.; Li, Z. Brittleness features and controlling factors of continental shale from Da’anzhai Member of the Lower Jurassic Ziliujing Formation, northeastern Sichuan Basin. Oil Gas Geol. 2023, 44, 366–378. [Google Scholar] [CrossRef]
  75. Nie, H.; Liu, Q.; Li, P.; Li, P.; Ding, J.; Sun, C.; Zhai, C.; Zhao, J.; Jin, Z.; Dang, W. Quartz Types, Formation Mechanism, and Its Effect on Shale Oil and Gas Enrichment: A Review. Earth-Sci. Rev. 2025, 261, 105011. [Google Scholar] [CrossRef]
  76. Inoue, A.; Kohyama, N.; Kitagawa, R.; Watanabe, T. Chemical and Morphological Evidence for the Conversion of Smectite to Illite. Clays Clay Miner. 1987, 35, 111–120. [Google Scholar] [CrossRef]
  77. Peltonen, C.; Marcussen, Ø.; Bjørlykke, K.; Jahren, J. Clay Mineral Diagenesis and Quartz Cementation in Mudstones: The Effects of Smectite to Illite Reaction on Rock Properties. Mar. Pet. Geol. 2009, 26, 887–898. [Google Scholar] [CrossRef]
  78. Hower, J.; Eslinger, E.V.; Hower, M.E.; Perry, E.A. Mechanism of Burial Metamorphism of Argillaceous Sediment: 1. Mineralogical and Chemical Evidence. Geol. Soc. Am. Bull. 1976, 87, 725–737. [Google Scholar] [CrossRef]
  79. Huang, S.; Huang, K.; Feng, W.; Tong, H.; Liu, L.; Zhang, X. Mass Exchanges among Feldspar, Kaolinite and Illite and Their Influences on Secondary Porosity Formation in Clastic Diagenesis—A Case Study on the Upper Paleozoic, Ordos Basin and Xujiahe Formation, Western Sichuan Depression. Geochimica 2009, 38, 498–506. [Google Scholar] [CrossRef]
  80. Cai, C.; Cai, J.; Liu, H.; Wang, X.; Zeng, X.; Wang, Y. Occurrence of Organic Matter in Argillaceous Sediments and Rocks and Its Geological Significance: A Review. Chem. Geol. 2023, 639, 121737. [Google Scholar] [CrossRef]
  81. Wu, Z.; He, S.; He, X.; Zhai, G.; Xia, X.; Yang, R.; Dong, T.; Peng, N. Pore Structure Characteristics and Comparisons of Upper Permian Longtan and Dalong Formation Transitional Facies Shale in Xiangzhong-Lianyuan Depression. Earth Sci. 2019, 44, 3757. [Google Scholar] [CrossRef]
  82. Yu, B.; Dong, H.; Jiang, H.; Lv, G.; Eberl, D.; Li, S.; Kim, J. The Role of Clay Minerals in the Preservation of Organic Matter in Sediments of Qinghai Lake, NW China. Clays Clay Miner. 2009, 57, 213–226. [Google Scholar] [CrossRef]
  83. Jia, J.; Bechtel, A.; Liu, Z.; Strobl, S.A.I.; Sun, P.; Sachsenhofer, R.F. Oil Shale Formation in the Upper Cretaceous Nenjiang Formation of the Songliao Basin (NE China): Implications from Organic and Inorganic Geochemical Analyses. Int. J. Coal Geol. 2013, 113, 11–26. [Google Scholar] [CrossRef]
  84. Zhao, T.; Xu, S.; Hao, F. Differential Adsorption of Clay Minerals: Implications for Organic Matter Enrichment. Earth-Sci. Rev. 2023, 246, 104598. [Google Scholar] [CrossRef]
  85. Worden, R.H.; Morad, S. Clay Minerals in Sandstones; John Wiley & Sons, Ltd: Hoboken, NJ, USA, 1999; ISBN 978-1-4443-0433-6. [Google Scholar]
  86. Chuhan, F.A.; Bjørlykke, K.; Lowrey, C. The Role of Provenance in Illitization of Deeply Buried Reservoir Sandstones from Haltenbanken and North Viking Graben, Offshore Norway. Mar. Pet. Geol. 2000, 17, 673–689. [Google Scholar] [CrossRef]
  87. Gustafsson, Å.B.; Puigdomenech, I. The Effect of pH on Chlorite Dissolution Rates at 25 °C. MRS Proc. 2002, 757, 316. [Google Scholar] [CrossRef]
  88. Elliott, W.C.; Edenfield, A.M.; Wampler, J.M.; Matisoff, G.; Long, P.E. The Kinetics of the Smectite to Illite Transformation in Cretaceous Bentonites, Cerro Negro, New Mexico. Clays Clay Miner. 1999, 47, 286–296. [Google Scholar] [CrossRef]
  89. Du, J.; Cai, J.; Lei, T.; Li, Y. Diversified Roles of Mineral Transformation in Controlling Hydrocarbon Generation Process, Mechanism, and Pattern. Geosci. Front. 2021, 12, 725–736. [Google Scholar] [CrossRef]
  90. Andresen, B.; Throndsen, T.; Barth, T.; Bolstad, J. Thermal Generation of Carbon Dioxide and Organic Acids from Different Source Rocks. Org. Geochem. 1994, 21, 1229–1242. [Google Scholar] [CrossRef]
  91. Pang, W.; Li, J.; Zhou, S.; Li, Y.; Liu, L.; Wang, H.; Chen, G. Distribution, Origin, and Impact on Diagenesis of Organic Acids in Representative Continental Shale Oil. Processes 2024, 12, 2092. [Google Scholar] [CrossRef]
Minerals 15 00951 g001
Figure 1. (a) Geological map of the Ordos Basin (Chang-7 Member) showing lacustrine facies distribution, source rock thickness variations, and spatial distribution of sampling wells and outcrop sections (modified from Wang et al., 2024 [31]). (b) Representative stratigraphic column of the Upper Triassic Yanchang Formation (modified from Chu et al., 2020 [32]). (c) Provenance distribution of the Chang-7 shale (modified from Zhang et al., 2013 [34]).
Figure 1. (a) Geological map of the Ordos Basin (Chang-7 Member) showing lacustrine facies distribution, source rock thickness variations, and spatial distribution of sampling wells and outcrop sections (modified from Wang et al., 2024 [31]). (b) Representative stratigraphic column of the Upper Triassic Yanchang Formation (modified from Chu et al., 2020 [32]). (c) Provenance distribution of the Chang-7 shale (modified from Zhang et al., 2013 [34]).
Minerals 15 00951 g001
Minerals 15 00951 g002
Figure 2. Raincloud plot characterizing clay mineral compositions in the Chang-7 Shale: integrated visualization of kernel density estimation illustrating global distribution frequency (top), boxplot displaying medians and interquartile ranges (25th–75th percentiles) (middle), and scatter points demonstrating actual spatial distribution of all data points with opacity scaling reflecting sample size magnitude (bottom).
Figure 2. Raincloud plot characterizing clay mineral compositions in the Chang-7 Shale: integrated visualization of kernel density estimation illustrating global distribution frequency (top), boxplot displaying medians and interquartile ranges (25th–75th percentiles) (middle), and scatter points demonstrating actual spatial distribution of all data points with opacity scaling reflecting sample size magnitude (bottom).
Minerals 15 00951 g002
Minerals 15 00951 g003
Figure 3. Planar mapping of clay mineral assemblages using radar diagrams in the Chang-7 Shale. Radar chart color coding reflects clay mineral assemblages: blue indicates I/S-dominated assemblages, with darker hues representing elevated illite/smectite content; red designates illite-dominant assemblages, where dark red corresponds to minimal I/S content (<5%); green signifies chlorite-enriched assemblages, with dark green indicating chlorite-dominated compositions; purple characterizes balanced clay mineral assemblages without dominant phases.
Figure 3. Planar mapping of clay mineral assemblages using radar diagrams in the Chang-7 Shale. Radar chart color coding reflects clay mineral assemblages: blue indicates I/S-dominated assemblages, with darker hues representing elevated illite/smectite content; red designates illite-dominant assemblages, where dark red corresponds to minimal I/S content (<5%); green signifies chlorite-enriched assemblages, with dark green indicating chlorite-dominated compositions; purple characterizes balanced clay mineral assemblages without dominant phases.
Minerals 15 00951 g003
Minerals 15 00951 g004
Figure 4. Profiles of TOC and clay mineral assemblages in Well YY1. S% represents the proportion of smectite layers within I/S.
Figure 4. Profiles of TOC and clay mineral assemblages in Well YY1. S% represents the proportion of smectite layers within I/S.
Minerals 15 00951 g004
Minerals 15 00951 g005
Figure 5. Modeling and fitting analysis of XRD patterns of the clay fraction for six representative samples ((a) M53-24, Ro = 0.55%; (b) Z233-3, Ro = 0.58%; (c) L211-15, Ro = 0.72%; (d) H269-27, Ro = 0.79%; (e) B522-38, Ro = 1.02%; (f) G347-24, Ro = 0.48%). Upper section: Integrated XRD patterns with experimental curve (black), fitted curve of total phases (red), and baseline (yellow); Lower section: Individual phase curves: illite (blue), chlorite (dark red), kaolinite (green), I/S with higher smectite layer content (brown), and I/S with lower smectite layer content (pink).
Figure 5. Modeling and fitting analysis of XRD patterns of the clay fraction for six representative samples ((a) M53-24, Ro = 0.55%; (b) Z233-3, Ro = 0.58%; (c) L211-15, Ro = 0.72%; (d) H269-27, Ro = 0.79%; (e) B522-38, Ro = 1.02%; (f) G347-24, Ro = 0.48%). Upper section: Integrated XRD patterns with experimental curve (black), fitted curve of total phases (red), and baseline (yellow); Lower section: Individual phase curves: illite (blue), chlorite (dark red), kaolinite (green), I/S with higher smectite layer content (brown), and I/S with lower smectite layer content (pink).
Minerals 15 00951 g005
Minerals 15 00951 g006
Figure 6. Histograms of the clay fraction in representative wells/sections/blocks illustrating assemblage characteristics and dominant mineral types. I/S enrichment type: (a) Well YY1 and (b) Well Jinghe13; I/S enrichment type: (c) Zhenjing block and (d) DZ section; four-component distribution type: (e) Well Honghe21 and (f) Well H269.
Figure 6. Histograms of the clay fraction in representative wells/sections/blocks illustrating assemblage characteristics and dominant mineral types. I/S enrichment type: (a) Well YY1 and (b) Well Jinghe13; I/S enrichment type: (c) Zhenjing block and (d) DZ section; four-component distribution type: (e) Well Honghe21 and (f) Well H269.
Minerals 15 00951 g006
Minerals 15 00951 g007
Figure 7. Cross-plots of clay minerals versus other mineral constituents in Well YY1: (a) quartz (Qtz), orange: high clay-content; green: low clay-content; (b) K-feldspar (Kfs); (c) plagioclase (Pl); (d) carbonate (Carb = calcite + dolomite).
Figure 7. Cross-plots of clay minerals versus other mineral constituents in Well YY1: (a) quartz (Qtz), orange: high clay-content; green: low clay-content; (b) K-feldspar (Kfs); (c) plagioclase (Pl); (d) carbonate (Carb = calcite + dolomite).
Minerals 15 00951 g007
Minerals 15 00951 g008
Figure 8. The clay mineral abundance versus TOC content cross-plot in high-TOC (green data points) and low-TOC (orange data points) series from Well YY1.
Figure 8. The clay mineral abundance versus TOC content cross-plot in high-TOC (green data points) and low-TOC (orange data points) series from Well YY1.
Minerals 15 00951 g008
Minerals 15 00951 g009
Figure 9. Plot of TOC versus mixed-layer ratio (S%) for Chang-7 Shale Samples from Yishicun Section.
Figure 9. Plot of TOC versus mixed-layer ratio (S%) for Chang-7 Shale Samples from Yishicun Section.
Minerals 15 00951 g009
Table 1. Clay mineralogical composition and vitrinite reflectance of the tested shale samples.
Table 1. Clay mineralogical composition and vitrinite reflectance of the tested shale samples.
Well NameNumber of SamplesClays 1 (%)I (%)I/S (%)K (%)C (%)Mixed-Layer Ratio 2 (S%)Ro%
B522344.139.755.91.43.015–200.88–1.04
G347250.333.248.111.37.4525–300.48
H269358.427.221.430.421.015–200.77–0.78
H36258.749.843.506.7150.84–0.86
L211353.053.136.02.18.815–200.72–0.75
L82239.535.047.1017.915–200.81–0.82
M53148.833.541.315.69.6200.55
T211256.235.058.506.515NA 3
W336460.129.445.812.712.215–200.73–0.77
Z233261.645.334.43.716.7150.57–0.58
Z40258.520.276.20.23.5200.55–0.58
Z8347.533.362.30.34.115–200.57–0.58
1 Clays (%) represents the proportion of clay minerals in the total rock, while I (%), I/S (%), K (%) and C (%) denote the relative contents of illite, I/S, kaolinite and chlorite within the clay mineral fraction, respectively, all being average values for the well samples. 2 The proportion of smectite layers within I/S. 3 Not Available.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Ling, K.; Wang, Z.; Cao, Y.; Liu, Y.; Dong, L. Clay Mineral Characteristics and Smectite-to-Illite Transformation in the Chang-7 Shale, Ordos Basin: Processes and Controlling Factors. Minerals 2025, 15, 951. https://doi.org/10.3390/min15090951

AMA Style

Ling K, Wang Z, Cao Y, Liu Y, Dong L. Clay Mineral Characteristics and Smectite-to-Illite Transformation in the Chang-7 Shale, Ordos Basin: Processes and Controlling Factors. Minerals. 2025; 15(9):951. https://doi.org/10.3390/min15090951

Chicago/Turabian Style

Ling, Kun, Ziyi Wang, Yaqi Cao, Yifei Liu, and Lin Dong. 2025. "Clay Mineral Characteristics and Smectite-to-Illite Transformation in the Chang-7 Shale, Ordos Basin: Processes and Controlling Factors" Minerals 15, no. 9: 951. https://doi.org/10.3390/min15090951

APA Style

Ling, K., Wang, Z., Cao, Y., Liu, Y., & Dong, L. (2025). Clay Mineral Characteristics and Smectite-to-Illite Transformation in the Chang-7 Shale, Ordos Basin: Processes and Controlling Factors. Minerals, 15(9), 951. https://doi.org/10.3390/min15090951

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop