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Article

Petrographic and Geochemical Insights from Fibrous Calcite Veins: Unraveling Overpressure and Fracture Evolution in the Upper Permian Dalong Formation, South China

by
An Liu
1,2,3,
Lin Chen
1,2,3,*,
Shu Jiang
4,5,*,
Hai Li
1,2,3,
Baomin Zhang
1,2,3,
Yingxiong Cai
1,2,3,
Jingyu Zhang
6,
Wei Wei
7 and
Feiyong Xia
8
1
Wuhan Center, China Geological Survey (Central South China Innovation Center for Geosciences), Wuhan 430205, China
2
Technology Innovation Center for Shale Oil and Gas Accumulation Theory and Engineering in Southern Complex Structural Area, China Geological Survey, Wuhan 430205, China
3
Shale Gas Research Center for Southern Complex Structural Area, China Geological Survey, Wuhan 430205, China
4
Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences (Wuhan), Wuhan 430074, China
5
School of Sustainable Energy, China University of Geosciences (Wuhan), Wuhan 430074, China
6
Centre for Marine Magnetism (CM2), Department of Ocean Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China
7
State Key Laboratory of Geomicrobiology and Environmental Changes, School of Earth Science, China University of Geosciences (Wuhan), Wuhan 430074, China
8
Huanggang Municipal Bureau of Natural Resources and Planning, Huanggang 438000, China
*
Authors to whom correspondence should be addressed.
Minerals 2025, 15(9), 896; https://doi.org/10.3390/min15090896 (registering DOI)
Submission received: 24 June 2025 / Revised: 9 August 2025 / Accepted: 18 August 2025 / Published: 24 August 2025
(This article belongs to the Special Issue Organic Petrology and Geochemistry: Exploring the Organic-Rich Facies)
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Abstract

The characteristics and evolution of fibrous calcite veins in organic-rich shales have gained significant attention due to the recent advancements in shale oil and gas exploration. However, the fibrous calcite veins in the Upper Permian Dalong Formation remain lacking in awareness. To investigate the formation and significance of bedding-parallel fibrous calcite veins in the Dalong Formation, we conducted an extensive study utilizing petrography, geochemistry, isotopic analysis, and fluid inclusion studies on outcrops of the Dalong Formation in South China. Our findings reveal that fibrous calcite veins predominantly develop in the middle section of the Dalong Formation, specifically within the transitional interval between siliceous and calcareous shales, characterized by symmetric, antitaxial fibrous calcite veins. The δ13C values of these veins exhibit a broad range (−4.53‰ to +3.39‰) and display a decreasing trend in the directions of fiber growth from the central part, indicating an increased contribution of organic carbon to the calcite veins. Additionally, a consistent increase in trace element concentrations from the central part toward the fiber growth directions suggests a singular fluid source in a relatively closed environment, while other samples exhibit no distinct pattern, possibly due to the mixing of fluids from multiple layers resulting from repeated opening and closing of bedding-parallel fractures in the shales. The notable difference in δEu between the fibers on either side of the median zone indicates that previously formed veins acted as barriers, impeding the mixing of fluids, with the variation in δEu reflecting the differing sedimentary properties of the surrounding rocks. The in situ U-Pb dating of fibrous calcite veins yields an absolute age of 211 ± 23 Ma, signifying formation during the Late Triassic, which correlates with a shale maturity of 1.0‰ to 1.25‰. This integrated study suggests that the geochemical records of fibrous calcite veins document the processes related to overpressure generation and the opening and healing of bedding-parallel fractures within the Dalong Formation.

1. Introduction

Fibrous veins are typically defined as vein structures characterized by elongated crystal morphology [1,2]. These fibrous veins, often found within bedding-parallel fractures, are prevalent in low-permeability strata of sedimentary basins and have garnered significant interest due to their association with overpressure, oil generation (especially oil crack to gas), and mineralization [3]. Among these, fibrous calcite is the most extensively distributed type of fibrous vein [4]. Based on crystal growth orientation, fibrous calcite veins can be classified as stretched veins, syntaxial veins, and antitaxial veins [5]. Stretched veins form within host rock fractures that contain vein-forming fluids, arising from multiple episodes of fracture opening and closing. Syntaxial veins exhibit vein-forming minerals growing from the host rock toward the central fracture; symmetrical syntaxial veins form when the growth interface is centrally located, allowing vein-forming materials to precipitate on both sides, whereas asymmetrical syntaxial veins develop when the growth interface is displaced toward one side. Antitaxial veins, characterized by mineral crystals growing from the vein’s center toward the host rock, feature two simultaneously growing interfaces [6]. Antitaxial veins are the true fibrous veins in a strict sense and develop in environments devoid of competitive mechanisms; all crystals exhibit nearly identical morphologies and a pronounced fibrous habit, indicating that their formation was not governed by conventional crystal growth theory and that growth competition was effectively suppressed [7].
With the rise of shale oil and gas exploration, fibrous calcite veins in organic-rich shales in China have attracted significant attention from researchers. Notable occurrences of well-developed fibrous calcite veins have been reported in Lower Silurian Longmaxi Formation shales from the Southern Sichuan Basin, Cretaceous shales in the Songliao Basin, shales in the Dongying depression of the Bohai Bay Basin, and Cambrian shales in the Daba Mountains along the northern margin of the Upper Yangtze Block [8,9,10,11,12]. The formation of these veins is closely linked to overpressure, which may arise from hydrocarbon generation, tectonic compression, mineral diagenesis, or a combination of these factors [12].
The Middle and Upper Yangtze region is characterized by the development of organic-rich shales during the Permian period, especially the Late Permian Dalong Formation in the marine troughs, which has become an important target for shale gas exploration [13,14,15]. For instance, well Leiye 1 in the Kaijiang–Liangping marine trough of the Upper Yangtze Block reported a high-yield shale gas flow of 42.66 × 104 m3/day [16]. Moreover, multiple wells in the Western Hubei marine trough of the Middle Yangtze region have also identified shale gas [17]. The author’s investigation found that there are abundant fibrous calcite veins in the interlayers of the Dalong Formation in some outcrops in the Western Hubei area of the Middle Yangtze region. However, the formation mechanisms of these veins remain unclear, and their impact on the generation and accumulation of shale gas is not well understood. This study focuses on the fibrous calcite veins in the Dalong Formation of the Jianshi area. Through petrographic thin-section analysis, cathodoluminescence, laser Raman spectroscopy, fluid inclusion analysis, carbon and oxygen isotopic analysis, LA-ICP-MS in situ trace element analysis, and LA-ICP-MS in situ U-Pb dating, we aim to elucidate the formation mechanisms of these calcite veins and explore their relationship with hydrocarbon generation and accumulation. This research is anticipated to yield valuable insights for shale gas exploration within the Dalong Formation of the Western Hubei region.

2. Geological Setting

During the Late Permian period, the interior of the Yangtze Platform experienced intracratonic rifting and basement fault reactivation. This geological activity led to the formation of various intra-platform secondary deep-water basins, which displayed alternating patterns of elevation and depression (Figure 1). Notable among these basins are the deep-water sedimentary areas, such as the Kaijiang–Liangping Trough and the Western Hubei Trough, which developed organic-rich shales due to their semi-enclosed environments that were connected to the open sea on one side [18,19,20]. The Dalong Formation in the Western Hubei Trough is distinguished by its lower members, which are predominantly composed of siliceous rocks, siliceous mudstone, and carbonaceous mudstone, while the upper members mainly consist of marl and carbonaceous mudstone [21]. The thickness of the Dalong Formation typically ranges from 10 to 60 m, with total organic carbon (TOC) concentrations in the shales varying between 2% and 8%, and a maximum recorded value of 19.51% (data are from this study). Notably, the TOC values decrease progressively upward through the section. The organic matter is primarily classified as Type I and Type II1, with shale vitrinite reflectance (Ro) ranging from 1.80% to 2.79%, and an average value of 2.20% [17]. Since the Indosinian, the Yangtze region entered a phase of tectonic deformation and displacement that terminated marine deposition. During the Early Yanshanian, the fundamental structural framework of the Meso-Paleozoic strata was established, representing the period when the present-day structure was finalized. The Late Yanshanian introduced localized reworking, whereas the Himalayan stage has been relatively stable [22,23].
The study area is located in the village of Shiziping, 15 km north of Jianshi County. The fibrous veins of the Dalong Formation occur at the boundary between the top of the lower member and the upper marl. They display widths ranging from 0.5 to 4 cm and can extend over 10 m in length. These veins are uniformly distributed along the bedding planes within the outcrop, exhibiting a spacing of 10 to 30 cm. The host rocks primarily consist of carbonaceous mudstone, siliceous mudstone, and carbonaceous calcareous mudstone (Figure 2a). Although the stratigraphic dip of the section is generally gentle, instances of local bedding-parallel sliding and vein folding have been observed. Multiple layers of fibrous calcite veins were sampled for analysis (Figure 2a–c).

3. Samples and Methods

The calcite vein samples underwent a series of analytical procedures, including Polarized light and cathodoluminescence microscopy, laser Raman spectroscopy, fluid inclusion analysis, LA-ICP-MS for in situ U-Pb dating, trace element analysis, and carbon and oxygen isotope analysis. Seven calcite vein samples were prepared as doubly polished thin sections and examined under a ZEISS polarizing microscope to investigate their microstructural characteristics. The mineral composition and diagenetic stages were assessed using a Scope Al/RELION III CL cathodoluminescence microscope. The thin sections were also used for fluid inclusion analysis; suitable inclusions were selected and investigated under the ZEISS polarizing microscope. Homogenization temperatures of the inclusions were measured using a LINKAM THMS600 heating/freezing stage attached to the microscope, with a temperature range of −196 °C to 600 °C and a heating rate of 0.1–5.0 °C/min. The precision for homogenization temperature measurements was ±1 °C.
Powder samples for carbon and oxygen isotope analysis were acquired from the residual vein material after the preparation of thin sections. The powder was obtained by scratching the flat surface of the calcite vein with a knife. The carbon and oxygen isotopic compositions of the calcite were analyzed using a GasBench II-MAT 253 system. Dried samples were reacted at 72 °C for 3 h within the GasBench II. Following this, the samples were carried by a gas stream through a 70 °C chromatographic column before entering the MAT 253 analyzer for isotopic analysis. Results were reported relative to the V-PDB standard. The analytical precision was monitored using standards GBW04416 and NBS19, with an error range of ±0.1‰.
In situ U-Pb dating and trace element analysis of the calcite veins were conducted at Sichuan Chuangyuan Microanalysis Technology Co., Ltd. The laser ablation system (LA) employed was the Resolution LR-S155, coupled with a Thermo Fisher iCAP (ICP-MS). The calcite vein samples were first embedded in epoxy resin to create sample targets, which were then polished and cleaned in an ultrasonic bath with deionized water for approximately 4 h. An excimer laser was utilized to ablate the calcite, operating at a frequency of 10 Hz, with an energy density of 3 J/cm2 and a beam spot diameter of 200 μm. Helium gas served as the primary carrier gas for the ablated material, flowing at a rate of 0.35 L/min. Each sample underwent a background collection for 20 s, followed by a 30 s data acquisition and a 7 s sample wash. A standard was measured after every six sample points. The standard NIST614 was applied to correct the instrument’s sensitivity and the 207Pb/206Pb ratio [24]. The primary standard AHX-1 (age of 209.8 ± 1.3 Ma), monitoring standard LD-5 (age of 72.83 ± 0.57 Ma), and PTKD-2a (age of 152.4 ± 2.7 Ma) were utilized to correct the 238U/206Pb ratio [25]. Data processing was carried out using Iolite v3.71 software, and U-Pb age concordia diagrams were generated using Isoplot/EX v3.75 software [26]. The concentrations of trace elements in the carbonate rocks were analyzed using IOLITE software [27]. Analytical uncertainties were within 10%. This study specifically focused on analyzing trace elements in two samples aligned with the direction of fiber growth and completed U-Pb dating in one sample parallel to the median zone.
The burial and thermal history of the study area was assessed utilizing BasinMod-1D software. Stratigraphic thickness, lithology, age, erosion thickness, erosion timing, and the evolution of heat flow values were derived from adjacent wells and sections and analyzed based on existing literature [17,28,29,30]. Vitrinite reflectance simulation was performed using the EASY% Ro numerical modeling method.

4. Results

4.1. Petrographic Characteristics

The fibrous calcite veins within the Dalong Formation exhibit an orientation parallel to the bedding planes. Notably, most veins feature a median zone (central line) that aligns with these bedding planes. Calcite fibers emanate continuously from the median zone to the vein margins, where they are oriented nearly perpendicular to the contact surfaces with the host rocks. The fibrous calcite crystals on either side of the median zone are nearly identical in length and width, exhibiting symmetry and a length-to-width ratio far exceeding 10 (Figure 3a). The central line, which is narrow and dark in color, is primarily composed of continuous bitumen veins or numerous hydrocarbon inclusions (Figure 3b). This median zone, measuring between 3 and 5 mm in width, predominantly comprises granular calcite with minor quartz crystal content (Figure 3c), while bitumen fills the interstitial spaces. Host rock bands are abundant in some samples, primarily extending parallel to the bedding planes, with a few intersecting at oblique angles. These shale bands exhibit widths ranging from 0.05 to 3 mm and lengths from 0.3 to 50 mm (Figure 3a), with the longest extending up to 8 cm. Calcite within the median zone and the fibrous calcite display a range of cathodoluminescence from bright to dark red, with no significant color variation observed along the crystal growth axis (Figure 3d,e).

4.2. Fluid Inclusions

Fluid inclusions are generally scarce in fibrous calcite veins. However, samples JS2 and JS4 contain a relatively high abundance of consistent primary inclusion types, predominantly comprising hydrocarbon inclusions and pure aqueous inclusions, alongside a few gas–liquid two-phase and pure methane inclusions. Hydrocarbon inclusions appear brownish, mostly taking on rectangular or irregular shapes, and range from 5 to 20 μm in size (Figure 4a,b). Pure aqueous inclusions are primarily irregular or rectangular, measuring between 3 and 10 μm, colorless and transparent (Figure 4c). Gas–liquid two-phase inclusions exhibit distinct gas bubbles alongside liquid phases at room temperature, with gas–liquid ratios between 5% and 25% and sizes from 4 to 8 μm (Figure 4d). Pure methane inclusions are characterized by a grayish-black color with a bright central line, ranging from 4 to 10 μm in size, rectangular or negative-crystal in shape (Figure 4e,f), and display the typical characteristics of high-density methane inclusions [31]. The coexistence of hydrocarbon inclusions, methane inclusions, pure aqueous inclusions, and vapor–liquid two-phase inclusions indicates that entrapment occurred within a multiphase immiscible system [8].
The homogenization temperatures of gas–liquid two-phase inclusions within fibrous calcite and quartz crystals from the median zone vary from 128.8 °C to 159.7 °C, peaking between 130 °C and 150 °C. Notably, there are no significant differences between the homogenization temperature peaks of quartz and calcite (Figure 5).

4.3. Trace Elements

In sample JS-2, the iron (Fe) content ranges from 138.71 ppm to 555.37 ppm, while the manganese (Mn) content ranges from 336.04 ppm to 1177.65 ppm, yielding an Mn/Fe ratio from 1.21 to 2.93, with an average of 1.92. Conversely, in sample JS-4, the Fe content ranges from 120.47 ppm to 213.91 ppm, while Mn content ranges from 399.23 ppm to 873.53 ppm, resulting in an Mn/Fe ratio between 3.11 and 4.24, with a mean of 3.58. Sample JS-4 displays a higher Mn/Fe ratio and lower Fe content in comparison to JS-2 (Figure 6 and Figure 7). The magnesium (Mg) content in JS-2 fluctuates from 1304.74 ppm to 2825.53 ppm, gradually decreasing from the median zone to the fibrous calcite on either side. Similarly, overall trends for strontium (Sr) and scandium (Sc) coincide with those observed for Mg (Figure 6 and Figure 7). In JS-4, Mg content ranges from 1080.03 ppm to 2877.58 ppm, revealing no distinct trend from the median zone to context with fibrous calcite. Sr and Sc do not demonstrate a consistent trend and are uncorrelated with Mg (Table 1; Figure 6 and Figure 7).
In JS-2, the total rare earth element (REE) content (ΣREE) ranges from 1.41 ppm to 12.39 ppm. The europium anomaly (δEu) generally presents a positive anomaly (0.88 to 2.60), with the upper fibrous calcite (average 1.17) significantly lower than the lower side (average 2.26). Conversely, the cerium anomaly (δCe) generally reflects a negative anomaly (0.49 to 0.80), where the upper fibrous calcite (average 0.59) is lower than the lower side (average 0.78). The light rare earth element/heavy rare earth element (LREE/HREE)N ratio varies from 0.33 to 2.73, with the upper fibrous calcite (average 0.41) lower than the lower side (average 0.65) (Table 1, Figure 6).
For sample JS-4, the total REE content ranges from 1.25 to 19.10 ppm. The europium anomaly generally presents a positive anomaly (0.71 to 1.38), with the upper fibrous calcite (average 0.91) being similar to the lower side (average 1.00). The cerium anomaly generally displays a negative anomaly (0.53 to 0.76), where the upper fibrous calcite (average 0.66) is similar to the lower side (average 0.68). The (LREE/HREE)N ratio ranges from 0.26 to 0.69, with the upper fibrous calcite (average 0.51) being similar to the lower side (average 0.51) (Table 1, Figure 7).

4.4. Carbon and Oxygen Isotopes

The carbon and oxygen isotopic compositions of calcite veins, along with their variations, are presented in Figure 6 and Figure 7 and Table 2. In sample JS-2, δ13C ranges from −4.53‰ to 3.39‰, with higher values concentrated in the median zone (1.71‰ to 3.39‰). A decreasing trend is evident from the median zone to the fibrous calcite on both sides. δ18O ranges from −7.19‰ to −6.41‰; the upper fibrous vein averages −6.57‰, while the lower side averages −6.95‰, indicating that the lower side is slightly lower in value. In sample JS-4, δ13C varies from 2.41‰ to 3.95‰, exhibiting a more pronounced decreasing trend from the median zone to the fibrous calcite on both sides, demonstrating symmetry. δ18O ranges from −7.85‰ to −6.99‰, exhibiting greater variation on the lower side compared to the upper side. Compared to JS-2, sample JS-4 displays higher δ13C values and slightly lower δ18O values.

4.5. LA-ICP-MS In Situ U-Pb Dating

In situ trace element analysis of calcite yielded 238U/206Pb ratios, with the selection of spots exhibiting higher 238U/206Pb ratios and relatively lower 206Pb content for U-Pb dating purposes. The absolute age of the fibrous calcite in sample JS-4 was determined to be 211 ± 23 Ma, with an average standard deviation of 1.9 (Figure 8).

5. Discussion

5.1. Fluid Properties and Sources

5.1.1. Trace Elements as Indicators of Fluid Properties and Origins

During calcite growth, significant fluctuations in the contents of trace elements such as Mg, Mn, and Fe invariably coincide with changes in the fluid source, making these trace elements reliable indicators of the origin of vein-forming materials [36,37]. Variations in trace element concentrations during calcite vein growth can be categorized into four modes [36,38]: (1) In an open system with a low rock-to-water ratio, where either only a small amount of calcite precipitates or the fluid is continuously replenished from an external source, keeping the solution composition constant; under these conditions, the trace-element content in the growing calcite remains unchanged. (2) In a closed system, concentrations are primarily determined by the partition coefficient (DTE = (trace element/Ca)calcite/(trace element/Ca)calcite) of trace elements between calcite and the aqueous solution, when DTE < 1, trace element concentrations in calcite increase with reaction progress, (3) and while concentrations decrease when DTE > 1. (4) Fluid mixing during calcite vein growth results in fluctuations in trace element concentrations. In sample JS-2, trace elements, including Mg, Sr, and Sc, generally increase from the median zone to the fibrous calcite on both sides, indicating DTE < 1. This observation suggests that within a relatively closed fracture fluid system, trace element concentrations in newly formed calcite crystals and the solution increase along the growth direction of the crystals. However, in the lower fibrous calcite near the vein wall (points 13–15), the concentrations of Mg, Mn, Fe, Sr, and Sc slightly decrease, indicating a possible influence from external fluids. Conversely, sample JS-4 does not exhibit a clear trend in trace element concentrations along the growth direction of the calcite crystals, showing multiple abrupt fluctuations in Mg content, indicative of mixing between low-Mg and high-Mg fluids. The poor correlation between Mg content and other trace elements suggests that fluids from different sources possess distinct trace element compositions.
The rare earth element (REE) characteristics of calcite veins reflect the REE composition of the fluids during vein precipitation [39]. Consequently, the REE abundance patterns in calcite can indicate fluid sources and flow paths [37]. Extensive data indicate that Dalong Formation shales are generally enriched in light REEs, with δEu values ranging from 0.72 to 3.01, demonstrating predominantly positive anomalies [40,41]. In contrast, the calcite veins analyzed in this study are generally depleted in light REEs and enriched in heavy REEs. The strong adsorption of light REEs by clay minerals in shales accounts for their enrichment, whereas heavy REEs are more likely to migrate through complexation with carbonates [42]. The calcite vein samples exhibit predominantly positive δEu anomalies with substantial variation, analogous to those observed in the Dalong Formation shales. δEu anomalies in sediments are primarily influenced by temperature and the nature of the source rock. At temperatures exceeding 200 °C, Eu in aqueous fluids exists in the +2 oxidation state, leading to significant positive Eu anomalies in chemically precipitated rocks [43,44]. The fibrous calcite veins, with fluid inclusion data indicating formation temperatures below 200 °C, are primarily influenced by the source rock regarding δEu values. The notable differences in δEu and δCe values between the two sides of the vein in sample JS-2 indicate discrepancies in the paleofluid environments on either side. The median zone of fibrous calcite veins extends laterally, forming a planar barrier within the shale [11]. Thus, the fibrous calcite on either side of the median zone develops in different fluid systems. Given that the burial depth of the shale sections during vein formation is similar, the variations in internal fluid compositions primarily result from the material composition of the shales. The pronounced vertical heterogeneity of Permian shales, shaped by climate change, volcanic activity, and sea-level fluctuations, is the principal cause of the compositional differences in the fluids surrounding the vein. In contrast, siliceous layers, volcanic tuff, and limestone interlayers exhibit significantly lower δEu values [45,46].

5.1.2. Carbon and Oxygen Isotopes as Tracers of Fluid Properties and Sources

The dissolution and reprecipitation of carbonates do not result in significant carbon isotope fractionation; therefore, carbon isotopes serve as effective tracers for identifying carbon sources [47]. The carbon isotopic composition of vein samples from the Dalong Formation closely resembles that of the limestones present during the formation’s deposition [33]. Both the carbon and oxygen isotopic compositions of these samples fall within the range characteristic of marine carbonates (Figure 9), suggesting that the carbon predominantly originates from the dissolution of carbonates in adjacent rocks. Additionally, these results indicate the possible mixing of fluids from other sources, including meteoric water and organic-rich fluids [48].
Considering the prior observations, particularly that calcite fluid inclusions formed during significant oil generation and that the Mn and Fe contents, Mn/Fe ratios, and cathodoluminescence colors of the veins indicate a reducing environment, the potential for meteoric water mixing can be ruled out [49]. The study area is also situated far from major faults, further eliminating the likelihood of mixing with mantle-derived inorganic carbon. Consequently, CO2 produced through the thermal degradation of organic matter emerges as a crucial carbon source in the calcite veins. The contribution of CO2 from organic matter evolution can be quantified using the formula developed by Worden et al., 2015 [32].
During the evolution of organic matter in shales, the generated CO2 exhibits very low δ13C values, typically around −25‰ [34,35]. Various isotopic geochemical profiles indicate that the δ13C values of limestones during the deposition of the Dalong Formation remain stable, averaging approximately 4‰ [33]. Accordingly, the contribution of organic carbon in sample JS-2 varies from a minimum of 2.1‰ in the median zone to a maximum of 29.4‰ in the fibrous calcite (Figure 6, Table 2). In sample JS-4, the contribution of organic carbon increases from 0.2‰ to 5.5%, with a median zone value of 2.0‰ (Figure 7, Table 2), consistent with the presence of bitumen-rich bands in the median zone (Figure 3a).
The marked changes in carbon isotopes from the median zone to the fibrous calcite in sample JS-2 suggest that vein growth was temporarily halted after median zone formation. As oil generation persisted, the composition of fluids within the shale underwent considerable change, with the organic carbon contribution in shale fluid CO2 escalating from approximately 5% to over 25%. During the fibrous calcite’s growth, the contribution of organic carbon increased only slightly. In contrast, the carbon isotopic composition of sample JS-4 demonstrates continuous changes in shale fluid from the median zone, with organic carbon contributions in CO2 gradually rising as oil generation progressed. Fibrous calcite extended from the median zone into the shale, displaying progressively decreasing δ13C values. The substantial differences in carbon isotopes between the fibrous calcite of JS-2 and JS-4 result primarily from variations in the composition of surrounding rocks. Sample JS-2 is encased in siliceous shale with limited calcite content, which was depleted after the formation of the median zone, leading to a marked increase in the proportion of organically sourced carbon during the growth of fibrous calcite. Conversely, sample JS-4 is surrounded by calcareous shale, where calcite content remained relatively stable. Although both samples experienced significant mixing with organic CO2 during oil generation, the proportions differed. The oxygen isotopic composition of calcite veins is influenced by various factors (including source rock (organic vs. inorganic), the water-to-rock ratio, and temperature), rendering it more complex and less reliable for fluid tracing compared to carbon isotopes.

5.2. Formation Time of Calcite Veins

Observations of fluid inclusions indicate that samples JS-2 and JS-4 display consistent inclusion types, predominantly hydrocarbon inclusions. Notably, there is no significant difference in inclusion types between the median zone and fibrous calcite within sample JS-2. The similar homogenization temperatures of fluid inclusions across different samples imply that fibrous calcite veins formed nearly simultaneously. U-Pb isotopic dating of sample JS-4 reveals that the veins originated during the Middle Triassic (211 Ma), corresponding to a burial depth of approximately 3500 m for the Dalong Formation, with a maturity Ro range of 1.00% to 1.25%, denoting the primary oil generation stage (Figure 10). Consequently, the formation of fibrous calcite veins is synchronous with the main oil generation phase of the shales.

5.3. Formation Mechanism of Fibrous Calcite Veins

5.3.1. Material Basis for Vein Formation

Fibrous calcite veins in the Upper Permian Dalong Formation predominantly occur within the transitional zone between siliceous mudstone and calcareous mudstone. In contrast, both the lower siliceous shale section and the upper marly section exhibit little to no development of fibrous calcite veins. In lacustrine basins such as the Songliao Basin and Dongying Depression in China, the host rocks of fibrous calcite veins are organic-rich mudstones that are either calcareous or contain interlayers of limestone and ostracod shell laminae [9,10]. Peter et al. documented 110 global occurrences of fibrous calcite veins, indicating their formation in organic-rich shales derived from a marine carbonate origin and highlighting the significance of lithology in the development of fibrous calcite veins [4]. The lithological transition zone within the Dalong Formation supplies the requisite calcium for calcite vein formation [3]. Additionally, the significant hydrocarbon generation potential of organic-rich shales generates overpressure, which is considered a critical factor in the formation of fibrous calcite veins.

5.3.2. Formation of the Median Zone

The formation of the median zone involves the opening of fractures and the deposition of materials. The presence of bitumen in the median zone of calcite veins from the Dalong Formation (Figure 3a) and the abundance of hydrocarbon inclusions within the median zone minerals (Figure 3b) indicate that hydrocarbon generation and migration occurred during this phase. The extremely low permeability of mudrocks partially restricts hydrocarbon expulsion, fostering conditions favorable for overpressure and fracture opening [50]. The transformation of clay minerals, particularly the illitization of montmorillonite, also contributes significantly to the development of overpressure [51]. The Permian shales in the study area are predominantly rich in illite, and the overpressure resulting from the large-scale conversion of montmorillonite to illite plays a crucial role in the opening of horizontal fractures [52]. During the Late Triassic, the Mianlue Paleo-ocean Basin underwent a closure process from east to west. As a result of tectonic activity associated with ocean basin closure and continental collision, the northern margin of the Yangtze Block formed a foreland fold-and-thrust belt and a foreland basin system [53]. This tectonic activity coincides temporally with the fracture formation in the Dalong Formation. However, since the study area is situated within the foreland basin and is remote from the orogenic belt, tectonic activity is not the primary driver of overpressure [54].
In sample JS-2, the median zone is predominantly composed of calcite, with minor amounts of quartz and bitumen. The calcite originates from the dissolution of surrounding rock calcite, while the quartz results from the transformation of montmorillonite to illite during diagenesis [55]. In contrast, the median zone of sample JS-4 is dominated by hydrocarbon products of the organic matter maturation process (liquid hydrocarbons during the injection stage, which later evolve into bitumen through deep burial and thermal maturation). This indicates differences in fluid composition during fracture opening. Organic acids produced during the maturation of organic matter interact with the surrounding rock, facilitating the migration of metal ions such as Ca and reducing the carbonate mineral content of the surrounding rock. The CO2 generated during organic matter maturation elevates CO2 concentrations in shale fluids, reducing the pH of the fluid system and promoting carbonate dissolution [56]. Upon encountering fractures, the reduced pressure encourages further precipitation of CaCO3. Geochemical evidence suggests that the carbon in calcite formed during this stage primarily originates from the dissolution of carbonate minerals in shales, with minimal mixing of CO2 from organic matter maturation (Figure 11a,c).

5.3.3. Growth of Fibrous Calcite

The formation of characteristic fibrous calcite veins is closely linked to the supersaturation of solutions in the pores of the surrounding rock, with vein growth occurring at the interface between the surrounding rock and the vein [6,57]. When fractures are not open or the rate of opening is slower than the growth rate of the slowest-growing crystal face, the crystal interface remains in contact with the surrounding rock, inhibiting crystal growth [58]. Vein-forming materials are transported through diffusion, resulting in the formation of fibrous crystals. In samples such as JS-4, multiple host rock bands parallel to the median zone develop within the fibrous crystals, indicating that the shale underwent multiple cycles of opening and healing during vein formation, thereby creating these host rock bands [59]. As previously discussed, the opening of horizontal fractures in shales is closely related to overpressure. Therefore, the entire sequence, from the opening of horizontal fractures in shales through the formation of the median zone to the growth of fibrous crystals, is expected to occur in a continuous overpressure environment. The presence of high-density CH4 inclusions supports the conclusion that fibrous calcite growth occurred in an overpressure fluid environment, even with pressure coefficients exceeding 2.6 during the overmature stage of shale [8].
The growth of fibrous calcite coincides with the continuous generation of hydrocarbons in shales. As hydrocarbon generation progresses, the concentration of organic-derived CO2 in shale fluids increases, leading to a decrease in δ13C values of calcite from the median zone toward the fibrous growth direction. Variations in trace element concentrations in fibrous calcite are influenced by both the source of the fluids and the partition coefficient of trace elements between calcite and the aqueous solution (Figure 11b,d). Under constant fluid-sourcing conditions, trace element concentrations systematically increase with fiber growth (Figure 11b). However, as bedding-parallel fractures in shales undergo multiple openings and closings, with varying locations of fracture initiation, some shale fragments enter the veins and become encapsulated by growing calcite fibers. Additionally, when fractures open and shales rupture perpendicular to bedding, fluids from adjacent layers are introduced into the fiber growth space, resulting in a relatively open fluid environment. Consequently, variations in trace element concentrations become irregular (Figure 11d). This phenomenon has been confirmed in shales that have experienced multiple fracture openings and closings due to hydrocarbon generation [11]. The median zone and the already formed fibrous calcite crystals act as interlayer barriers within the shale, creating independent environments for the formation of fibrous calcite veins on either side. Therefore, the δEu in the fibers primarily reflects differences in fluid environments on either side of the vein. It has been shown that δEu is predominantly controlled by the source rock when formation temperatures are below 200 °C [43,44].

5.4. Geological Significance of Fibrous Calcite Veins for Oil and Gas

The formation of fibrous calcite veins is closely associated with overpressure in mudstones and the migration of oil and gas. The presence of bitumenic bands within the median zone of fibrous calcite veins in the Dalong Formation, as well as hydrocarbon inclusions within calcite crystals, confirms their strong association with hydrocarbon generation. U-Pb dating of calcite, in conjunction with burial history analysis, indicates that these veins formed within the oil generation window (Figure 10). Variations in the carbon isotopes of fibrous calcite veins document the continuous hydrocarbon generation in shales and the increasing contribution of organic-derived CO2 to the fluid system. Preservation conditions represent a crucial factor for shale gas enrichment in the Middle Yangtze region [60,61]. Abundant fibrous calcite veins in Dalong Formation shales suggest that the shales underwent multiple episodes of horizontal fracture opening and subsequent healing throughout geological history. However, since these veins formed early and originated within the shale section, pre-Yanshanian horizontal fractures do not impact the preservation of shale gas. Fibrous calcite veins are closely related to shale hydrocarbon generation; consequently, some researchers consider them to be indicators of “sweet spots” [62]. Nevertheless, the fibrous calcite veins in the Dalong Formation are located in the transitional zone between siliceous and calcareous mudstones, rather than in the lower siliceous shale section characterized by a high total organic carbon (TOC) content (the vertical variation in TOC of the Dalong Formation is shown in Figure 6 of [45]). Therefore, fibrous calcite veins cannot serve as indicators of “sweet and brittle” zones for shale gas within the Permian of the Dalong Formation.

6. Conclusions

(1)
The fibrous calcite veins in the Dalong Formation, situated in the Jianshi area of Western Hubei, are found in the transitional zone between siliceous and calcareous shales in the central section of the formation and are predominantly composed of antitaxial fibrous calcite veins.
(2)
The δ13C values of fibrous calcite veins exhibit a decreasing trend from the median zone toward the directions of fibrous growth. This trend is associated with continuous hydrocarbon generation, wherein CO2 produced during the maturation of organic matter increasingly contributes to the carbon source of the calcite veins. Previous studies have shown that the carbon isotopes of limestones deposited during the Dalong Formation exhibit vertical stability. Consequently, differences in δ13C values among fibrous calcite veins are attributed to variations in the calcareous content of the surrounding rocks and the hydrocarbon generation potential of the shales.
(3)
Two distinct types of trace element variations are observed in fibrous calcite veins. One type displays a consistent increase in trace element concentrations from the median zone toward the fibrous growth directions, indicating a single fluid source in a closed environment with a trace element partition coefficient (DTE) of less than 1 between calcite and the aqueous solution. The other type reveals substantial and irregular variations in trace element concentrations, suggesting that the fluids within the fractures are mixtures from multiple layers within the shales. Repeated fracture opening and healing, resulting in vertical shale rupture, are the primary causes of fluid mixing. The median zone and pre-formed veins act as barriers, preventing fluid mixing across the two sides. Variations in δEu values of the fibrous calcite on either side reflect the differences in the compositional makeup and the depositional environment of the surrounding rocks.
(4)
In situ U-Pb dating of fibrous calcite veins yields an absolute age of 211 ± 23 Ma, indicating their formation during the Late Triassic. This period aligns with the oil generation window of the Dalong Formation and is consistent with the prevalence of hydrocarbon inclusions within the veins. The formation of fibrous calcite veins is intrinsically linked to the overpressure generated during hydrocarbon maturation.

Author Contributions

Conceptualization, A.L., L.C. and S.J.; methodology, A.L., L.C. and S.J.; software, A.L. and H.L.; validation, A.L. and L.C.; formal analysis, A.L. and H.L.; investigation, A.L., L.C., Y.C. and S.J.; data curation, A.L., L.C. and S.J.; writing—original draft preparation, A.L. and L.C.; writing—review and editing, A.L., L.C., S.J., H.L., B.Z., J.Z., W.W. and F.X.; visualization, A.L., L.C. and S.J.; supervision, A.L. and L.C.; project administration, A.L. and L.C.; funding acquisition, A.L., L.C. and S.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Open Fund of Technology Innovation Center for Shale oil and Gas Accumulation Theory and Engineering in Southern Complex Structural Area of China Geological Survey, grant number SOG202404; the National Natural Science Foundation of China, grant number 42302134; and the Geological Survey Projects of China Geological Survey, grant numbers DD20240047 and 2024ZRYJDC017.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

We thank the anonymous reviewers for their helpful comments.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Late Permian–Early Triassic (260–245 Ma) global paleogeography. (b) Late Permian Changhsingian paleogeography of South China showing the positions of studied section. (c) Stratigraphy and sedimentary facies of Permian in the Western Hubei Trough.
Figure 1. (a) Late Permian–Early Triassic (260–245 Ma) global paleogeography. (b) Late Permian Changhsingian paleogeography of South China showing the positions of studied section. (c) Stratigraphy and sedimentary facies of Permian in the Western Hubei Trough.
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Figure 2. (a) Field photograph showing the distribution of bedding-parallel calcite vein samples in the Dalong Formation of Jianshi section. The bedding-parallel fibrous calcite vein samples of JS4 (b) and JS2 (c) are shown.
Figure 2. (a) Field photograph showing the distribution of bedding-parallel calcite vein samples in the Dalong Formation of Jianshi section. The bedding-parallel fibrous calcite vein samples of JS4 (b) and JS2 (c) are shown.
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Figure 3. Petrography of calcite veins in the Dalong Formation in Jianshi section. (a) Median zone, bitumen veins, and shale bands developed along a fibrous vein (sample JS-4), with plane-polarized light; (b) hydrocarbon inclusions in median zone (sample JS-6), with plane-polarized light; (c) calcite and quartz in median zone (sample JS-2), with cross-polarized light; (d) cathodoluminescence images of the veins, with calcite showing dark red (sample JS-4); (e) bright red, dark red, and quartz does not luminesce (sample JS-2).
Figure 3. Petrography of calcite veins in the Dalong Formation in Jianshi section. (a) Median zone, bitumen veins, and shale bands developed along a fibrous vein (sample JS-4), with plane-polarized light; (b) hydrocarbon inclusions in median zone (sample JS-6), with plane-polarized light; (c) calcite and quartz in median zone (sample JS-2), with cross-polarized light; (d) cathodoluminescence images of the veins, with calcite showing dark red (sample JS-4); (e) bright red, dark red, and quartz does not luminesce (sample JS-2).
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Figure 4. Petrography of fluid inclusions in calcite veins. (a) Hydrocarbon inclusions in median zone calcite (sample JS2), (b) hydrocarbon inclusions in fibrous veins (JS4 calcite), (c) pure aqueous inclusions in fibrous veins (sample JS4); (d) aqueous two-phase fluid inclusions in fibrous veins (sample JS2); (e) methane inclusions in fibrous veins (sample JS4); (f) methane inclusions in in fibrous veins (sample JS2). The yellow arrow indicates the inclusion.
Figure 4. Petrography of fluid inclusions in calcite veins. (a) Hydrocarbon inclusions in median zone calcite (sample JS2), (b) hydrocarbon inclusions in fibrous veins (JS4 calcite), (c) pure aqueous inclusions in fibrous veins (sample JS4); (d) aqueous two-phase fluid inclusions in fibrous veins (sample JS2); (e) methane inclusions in fibrous veins (sample JS4); (f) methane inclusions in in fibrous veins (sample JS2). The yellow arrow indicates the inclusion.
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Figure 5. Histogram of homogenization temperatures for aqueous two-phase fluid inclusions in samples JS2 and JS4.
Figure 5. Histogram of homogenization temperatures for aqueous two-phase fluid inclusions in samples JS2 and JS4.
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Figure 6. Variation in isotopic composition and trace element concentrations across the vein of JS2. δEu = 2 × EuN/(SmN + GdN), δCe = 2 × CeN/(LaN + PrN). N means using North American shale standardization.
Figure 6. Variation in isotopic composition and trace element concentrations across the vein of JS2. δEu = 2 × EuN/(SmN + GdN), δCe = 2 × CeN/(LaN + PrN). N means using North American shale standardization.
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Figure 7. Variation in isotopic composition and trace element concentrations across the vein of JS4. δEu = 2 × EuN/(SmN + GdN), δCe = 2 × CeN/(LaN + PrN). N means using North American shale standardization.
Figure 7. Variation in isotopic composition and trace element concentrations across the vein of JS4. δEu = 2 × EuN/(SmN + GdN), δCe = 2 × CeN/(LaN + PrN). N means using North American shale standardization.
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Figure 8. Tera–Wasserburg Concordia plot of U-Pb dating (sample JS4).
Figure 8. Tera–Wasserburg Concordia plot of U-Pb dating (sample JS4).
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Figure 9. Carbon and oxygen isotopic compositions and diagenetic fluid sources of the calcite veins and surrounding host rocks. (a) Plot of C versus O isotopes. Mantle fluid data are from Hugh and Taylor (1974) [6]; Permian carbonate data are from Shen et al., 2013 [33]. (b,c) Comparison of the C and O isotopes for the veins.
Figure 9. Carbon and oxygen isotopic compositions and diagenetic fluid sources of the calcite veins and surrounding host rocks. (a) Plot of C versus O isotopes. Mantle fluid data are from Hugh and Taylor (1974) [6]; Permian carbonate data are from Shen et al., 2013 [33]. (b,c) Comparison of the C and O isotopes for the veins.
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Figure 10. Simulation of Permian burial history and thermal history of well X1 in Western Hubei.
Figure 10. Simulation of Permian burial history and thermal history of well X1 in Western Hubei.
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Figure 11. Growth of bedding-parallel calcite veins and changes of geochemical feature. (a) Formation of the median plane with calcite; (b) growth of fibrous calcite in closed system; (c) formation of the median plane with bitumen; (d) growth of fibrous calcite in multi-source mixed fluid.
Figure 11. Growth of bedding-parallel calcite veins and changes of geochemical feature. (a) Formation of the median plane with calcite; (b) growth of fibrous calcite in closed system; (c) formation of the median plane with bitumen; (d) growth of fibrous calcite in multi-source mixed fluid.
Minerals 15 00896 g011
Table 1. The trace element contents (×10−6) and related parameters in fibrous calcite veins detected using LA-ICP-MS.
Table 1. The trace element contents (×10−6) and related parameters in fibrous calcite veins detected using LA-ICP-MS.
SamplesMgMnFeMn/FeSrScLaCePrNdSmEuGdTbDyHoErTmYbLuΣREEδEuδCeL/H
JS2-12502.98875.11379.452.3111,583.284.903.7513.923.1616.224.101.234.300.734.550.932.450.311.790.2257.651.280.740.51
JS2-22280.771177.66402.042.939580.754.341.065.081.518.762.790.642.690.553.590.742.090.281.840.2431.861.020.620.33
JS2-32359.75805.11475.531.6910,766.037.801.187.642.5515.894.421.134.660.815.751.233.480.493.090.4152.731.090.580.33
JS2-41898.93551.29426.251.299147.383.690.695.512.2514.893.781.073.990.734.770.982.580.332.000.2643.821.210.490.39
JS2-51529.51452.38341.381.336922.973.290.584.931.9911.482.730.772.610.462.820.561.460.201.160.1731.921.260.500.49
JS2-61752.42476.69220.702.1610,456.504.822.4627.448.3443.177.731.416.391.035.981.102.680.321.950.24110.250.880.660.75
JS2-71305.08336.04138.712.422433.590.9012.3439.977.8034.344.421.352.270.261.390.260.600.070.380.05105.481.790.802.73
JS2-81892.27472.74323.301.467866.400.750.131.240.392.270.700.180.750.130.770.150.420.060.320.047.551.110.640.41
JS2-91304.74370.07166.992.229135.671.951.578.912.019.682.530.752.460.372.190.411.040.130.740.1032.891.320.800.65
JS2-101643.00414.09343.331.218473.984.491.8610.392.4411.912.921.292.850.442.850.571.480.201.250.1740.621.970.780.66
JS2-112301.20583.63444.071.3112,802.776.433.2816.453.8418.474.732.324.880.794.951.042.820.372.200.2966.432.110.770.62
JS2-122538.091133.62555.372.0413,384.795.463.0715.043.3816.474.692.354.430.754.670.992.690.332.130.2761.252.260.790.64
JS2-132825.531025.52459.232.2313,631.294.664.6518.073.8719.455.363.125.150.855.281.052.790.342.020.2672.282.600.780.74
JS2-142676.19938.42389.082.4114,152.385.194.4915.793.2516.915.252.735.460.946.051.253.410.452.660.3569.012.230.780.56
JS2-152567.40636.05359.541.7712,157.073.205.5316.043.0115.244.442.384.320.734.600.982.570.331.890.2562.302.390.790.66
JS4-11590.88873.53213.914.089049.356.570.9411.334.4027.157.951.237.351.429.522.156.420.977.111.0789.010.710.530.26
JS4-21905.74559.78132.154.243527.202.071.7611.083.1721.728.421.307.431.147.191.403.800.503.480.4772.860.720.670.40
JS4-32119.48521.06133.423.913361.222.132.0611.042.9420.259.581.616.971.015.951.142.900.382.470.3368.620.860.690.54
JS4-41318.63450.65129.683.482553.613.983.2216.584.3629.4213.131.988.611.277.451.423.840.533.620.5395.950.800.700.55
JS4-51609.87422.89128.773.283676.012.731.919.382.6217.619.771.826.510.915.651.052.790.372.550.3563.290.990.660.57
JS4-61367.65433.95128.493.382081.093.632.5612.033.2221.099.371.455.910.855.141.022.800.422.980.4469.280.840.680.54
JS4-72058.32482.05141.593.402950.540.200.422.310.624.141.600.481.430.191.170.200.470.060.330.0413.461.380.690.68
JS4-81292.03425.90120.473.541794.434.874.1418.234.6727.179.031.717.031.056.131.203.300.463.300.4987.900.940.690.54
JS4-92877.58539.08131.834.092102.572.951.917.531.7510.444.721.154.890.774.800.922.510.332.090.3044.121.050.730.39
JS4-101440.04425.80121.403.512191.944.546.3421.974.7528.0410.572.118.851.368.011.523.920.563.630.55102.160.960.750.51
JS4-111373.49447.66132.403.381978.886.536.3124.535.4530.7711.542.099.041.398.491.634.550.644.410.65111.490.900.760.50
JS4-121214.38427.67137.373.111757.294.913.8117.224.2625.307.831.326.521.006.051.223.410.503.500.5582.490.810.720.46
JS4-131330.34399.23129.373.092858.08/0.281.440.382.670.870.240.790.120.670.120.270.030.160.018.041.300.700.69
JS4-142712.09517.85125.014.142314.07/0.281.630.503.461.500.260.970.150.900.180.440.050.300.0310.650.930.620.61
JS4-152749.68571.37166.273.443734.290.680.161.160.393.031.430.381.640.251.760.330.910.110.760.1012.431.090.590.33
JS4-161080.03431.34130.843.302329.532.612.8414.703.6921.165.430.824.320.754.510.902.610.392.750.4265.300.750.720.45
Note: δEu = 2 × EuN/(SmN + GdN), δCe = 2 × CeN/(LaN + PrN); ΣREE means the total REE content; L/H = (LREE/HREE)N. N means using North American shale standardization. The position of the measuring point is shown in Figure 6 and Figure 7.
Table 2. Carbon, oxygen, and the percentage of carbon atoms in recrystallized calcite from carbonic acid data for veins in the Dalong Formation.
Table 2. Carbon, oxygen, and the percentage of carbon atoms in recrystallized calcite from carbonic acid data for veins in the Dalong Formation.
SampleNo.δ13CV-PDB (‰)δ18OV-PDB (‰)δ18OV-SMOW (‰)∆
JS21−4.30−6.5524.1728.62
2−4.29−6.6424.0728.57
3−4.30−6.6324.0828.61
4−4.08−6.7523.9727.88
5−2.38−6.4624.2622.01
6−3.42−6.5224.2025.59
7−3.51−6.4124.3125.89
82.96−6.4324.293.60
92.84−6.7723.944.01
102.72−7.1023.604.41
113.16−6.7623.952.90
123.39−6.5924.132.11
133.10−7.0523.653.09
141.71−6.9723.737.90
15−4.06−6.7823.9327.80
16−4.50−6.9023.8129.32
17−4.52−6.9723.7429.38
18−4.53−6.7323.9829.43
19−4.44−7.1323.5729.09
20−4.38−7.1923.5128.89
JS412.48−7.4323.265.23
22.41−7.2423.465.49
32.99−7.3923.303.49
43.37−7.2723.432.18
53.64−7.1623.541.25
63.87−7.4323.260.46
73.42−6.9923.711.98
83.82−7.3023.390.63
93.95−7.8522.830.16
103.86−7.2823.420.48
113.53−7.6922.991.63
123.04−7.3423.353.32
132.54−7.0223.685.03
Note: The position of the measuring point is shown in Figure 6 and Figure 7. ∆ = the percentage of carbon atoms inrecrystallized calcite from carbonic acid data = (δ13C (calcite vein) − δ13C (original calcite))/(δ13C (CO2 generated from hydrocarbon source rocks) − δ13C (original calcite)). (The formula is from [32]; δ13C (original calcite) is from [33]; δ13C (CO2 generated from hydrocarbon source rocks) is from [34,35].
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Liu, A.; Chen, L.; Jiang, S.; Li, H.; Zhang, B.; Cai, Y.; Zhang, J.; Wei, W.; Xia, F. Petrographic and Geochemical Insights from Fibrous Calcite Veins: Unraveling Overpressure and Fracture Evolution in the Upper Permian Dalong Formation, South China. Minerals 2025, 15, 896. https://doi.org/10.3390/min15090896

AMA Style

Liu A, Chen L, Jiang S, Li H, Zhang B, Cai Y, Zhang J, Wei W, Xia F. Petrographic and Geochemical Insights from Fibrous Calcite Veins: Unraveling Overpressure and Fracture Evolution in the Upper Permian Dalong Formation, South China. Minerals. 2025; 15(9):896. https://doi.org/10.3390/min15090896

Chicago/Turabian Style

Liu, An, Lin Chen, Shu Jiang, Hai Li, Baomin Zhang, Yingxiong Cai, Jingyu Zhang, Wei Wei, and Feiyong Xia. 2025. "Petrographic and Geochemical Insights from Fibrous Calcite Veins: Unraveling Overpressure and Fracture Evolution in the Upper Permian Dalong Formation, South China" Minerals 15, no. 9: 896. https://doi.org/10.3390/min15090896

APA Style

Liu, A., Chen, L., Jiang, S., Li, H., Zhang, B., Cai, Y., Zhang, J., Wei, W., & Xia, F. (2025). Petrographic and Geochemical Insights from Fibrous Calcite Veins: Unraveling Overpressure and Fracture Evolution in the Upper Permian Dalong Formation, South China. Minerals, 15(9), 896. https://doi.org/10.3390/min15090896

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