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Original Research

Frontiers in Earth Science ·

Volume 14 - 2026 | https://doi.org/10.3389/feart.2026.1958365

Gas preservation conditions of Cambrian Niutitang formation shale in western margin of Xuefeng uplift, SE China-based on the analysis of paleofluid

  • Zhiwei Fan1
  • Xiang Ge2*corresponding author
  • Jing Han1
  • Qingqing Lei2
  • Hao Wu1
  • Yi Zhang2
  1. 1Sinopec Exploration Company, Chengdu, China
  2. 2Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan, China
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Abstract

The lower Cambrian Niutitang Formation shale in the western margin of the Xuefeng uplift (WMXU), South China, possesses favorable material conditions including significant thickness, high organic carbon content, and high thermal maturity. However, industrial shale gas has not been discovered in this region, suggesting that preservation conditions may be the key limiting factor. In this study, we comprehensively analyzed the preservation conditions of shale gas in the Niutitang Formation using paleofluid analysis, including calcite carbon–oxygen isotope analysis, fluid inclusion analysis, and in situ calcite LA-ICP-MS U–Pb dating. The results show that calcite veins in WMXU exhibit negative carbon and oxygen isotopes, low Z values, and low-salinity brine inclusions, indicating exogenous fluid infiltration and poor sealing conditions. Calcite U–Pb dating reveals one reliable paleofluid episode at 84.4 ± 2.9 Ma, corresponding to the middle–late Yanshanian strike-slip movement. A Neogene episode at ca. 17 Ma is inferred from apatite fission-track thermochronology and fluid inclusion data. We propose that large-scale fractures and detachment surfaces at the base of the Niutitang Formation provided pathways for gas escape, with the Yanshanian and Himalayan tectonic movements being the critical periods for shale gas loss. These findings provide important insights for shale gas exploration in structurally complex areas.

Keywords

1 Introduction

Shale gas currently plays an increasingly important role as an alternative oil and gas resource, and extensive focus is being devoted to its exploration and exploitation (Bellani et al., 2021; Sun et al., 2021; Baiyegunhi et al., 2022; Li et al., 2022). South China Block, where several favorable shale formations were developed between the Precambrian era and the Neogene era, possesses a strong material foundation and is a promising exploration area (Zou et al., 2015; Zhao et al., 2016; Wang et al., 2020; Zhang et al., 2022). The exploration of marine shale gas in the Upper Ordovician–Lower Silurian Wufeng–Longmaxi formation has been remarkably successful (Wu et al., 2016; Zou et al., 2019; Qiu and Zou, 2020). Weiyuan–Changning, Fushun–Yongchuan, and Fuling–Jiaoshiba in the Sichuan basin and its surroundings have realized exploitation of industrial gas flow (Zhou et al., 2014; Zou et al., 2015; Wang, 2015; Jin et al., 2018; Guo et al., 2019). Along with the Wufeng–Longmaxi shale, early Cambrian Niutitang formation shale formed mainly in platform and basin facies with high organic carbon content. The type I and II1 organic matter indicates that the material base of the source rocks in this region is extensive (Wang et al., 2015; Zhao et al., 2016; Xiang et al., 2018; Fang et al., 2019). Presently, the industrial shale gas flow of the Niutitang formation has been discovered in Weiyuan, Jianwei in the Sichuan basin, and the surroundings of the Huangling uplift, with the daily gas production reaching 25.86 × 103 m3/d in the Jinshi 103HF well. However, no industrial shale gas flow has been found along the Xuefeng uplift (Zhao et al., 2016; Zhai et al., 2018). Several wells have been drilled into the Niutitang Formation in the WMXU, but the gas content is generally low, typically less than 1 m3/t, except for the XZD1 and XJD1 wells, which exceed 2 m3/t.

Although the influencing and controlling factors of the shale gas content in the Cambrian Niutitang formation are inconsistent in different areas, many scholars believe that the preservation conditions may be the key factors affecting shale gas enrichment in WMXU (Cao et al., 2018; Xiao et al., 2018; Li et al., 2022). Based on the exploration and development of shale gas in South China, North America, and Australia, establishing the characteristics of faults and fractures as well as the structural styles are critical for the evaluation of shale gas preservation conditions (Clarkson et al., 2016; Davudov and Moghanloo, 2018; Gao et al., 2018; Rysak et al., 2022). Compared with the Niutitang Formation shale in the Sichuan Basin and its surrounding areas, the generation and expulsion history of shale gas in the WMXU is obviously different (Wang et al., 2018; Peng, 2019; Li et al., 2021; Qiu et al., 2021). Moreover, the structural characteristics in the WMXU are more complicated than those in the Sichuan Basin (Deng, 2014; Li et al., 2020; Liu et al., 2021b). As a result, the preservation conditions of the lower Cambrian shale gas in the WMXU differ considerably, making the enrichment or escape of shale gas relatively complex.

However, when and how the low Cambrian shale gas in WMXU escaped is still unclear. Many scholars believe that the tectonic movement since the Indosinian movement had a strong destructive effect on the accumulation of oil and gas in WMXU (Deng, 2014; Zhang et al., 2017; Peng, 2019), but some scholars reckon that tectonic movement of Neogene is the main period of shale gas escaping (Xiao et al., 2023; He et al., 2023). As for the escape of shale gas, Zheng et al. (2019) reckon that the destruction of shale gas along WMXU may result from the vertical large-scale fault dimensions and detachment planes. According to fracture analyses in wells and outcrops, fractures can improve the reservoir characteristics of shale. Nevertheless, the development of several shear fractures, extension fractures, and shear-tension fractures in shale may have a destructive effect on the migration and preservation of Niutitang formation shale gas (Wang et al., 2018; Miao et al., 2020). The combined properties of veins within fractures as well as the content and composition of the gas in the Niutitang formation—an open environment that could lead to shale gas escape as well as atmospheric infiltration, were identified as key reasons for the damage to the shale gas reservoir (Liu et al., 2021a).

2 Geologic settings

Based on the previous studies, paleofluid properties of veins filled fractures in the G well, D well, and field outcrops in WMXU were selected as the research target in this study. Various types of experiments, including gas content within the shale, carbon and oxygen isotopes, fluid inclusions as well as in situ U-Pb dating of calcite veins, were conducted to provide absolute age constraints on the timing of fault activation and paleofluid migration. By integrating information on the source, temperature, and timing of paleofluid activity, this study systematically analyzes the gas preservation conditions of the lower Cambrian shale and proposes a model for shale gas generation and escape. These findings attempt to provide favorable guidance for shale gas exploration in the complex geological areas of the Xuefeng uplift.

2.1 Tectonic characteristics

The research area was located within the fold belt between the Xuefeng uplift and the Cili–Baojing fault zone (Figure 1a). This area located in the junction of the Yangtze plate and the Cathaysian plate featured with an NNE–NE strike, protruding NW in an arc shape (Figure 1A) (Deng, 2014; Yang et al., 2016).

Geological characteristic in WMXU. **(a)** Geological map; **(b)** Seismic section of BB’
Figure 1. Geological characteristic in WMXU. (a) Geological map; (b) Seismic section of BB’

2.2 Stratum of the G well

The study area has experienced multiple stages of tectonic activity since the Mesoproterozoic. During the early Caledonian (ca. 500–420 Ma), extensive bimodal magmatism and rifting occurred throughout South China, accompanied by the deposition of the Cambrian black shale (Sun et al., 2017; Xiao et al., 2018; Peng, 2019). The Caledonian movement (Late Silurian) subsequently led to tectonic uplift and amalgamation of the Yangtze and Cathaysia blocks into the South China plate, forming an angular unconformity between the lower and upper Paleozoic strata (Charvet et al., 2010; Wang et al., 2010; Wang et al., 2013). The Hercynian stage (Devonian to Permian) was dominated by extensional tectonics, during which a multi-stage rift basin developed and carbonate-clastic platform-shelf deposits accumulated (Wang et al., 2013; Deng, 2014). The Indosinian movement (Triassic) was marked by the collision of the North China block with the South China block, which resulted in contrasting deformation styles across the Xuefeng uplift. The eastern margin developed fault-fold uplift structures with compressive fault blocks and thrust folds, whereas the western part was characterized by uplift-depression structures with variable uplift magnitudes, creating an unconformity between the Late Triassic and overlying strata (Wang et al., 2013; Qiu et al., 2014; Qiu et al., 2016). The Yanshanian movement (Jurassic to Cretaceous), mainly controlled by subduction of the Paleo-Pacific Plate, can be divided into three stages. Early-stage magmatic intrusion and eruption caused strong folding and thrusting, which significantly modified or destroyed preexisting structures. In the middle-late stages, the WMXU experienced folding, thrusting, and intense strike-slip deformation, followed by late-stage uplift and short-term extension that formed the Yuanma Basin (He et al., 2018; Wu et al., 2020; Yang et al., 2016). The Himalayan movement (since Paleocene) in South China was characterized by strong uplift in the west and weak deformation in the east, with uplift remaining the dominant process in the WMXU during this period (Kuang et al., 2020).

The overlying strata of the Niutitang formation in WMXU is mudstone of the Palang/Qingxudong formation, and the underlying strata are dolomite and siliceous mudstone of the Sinian Dengying formation, all of which are integrated with the Niutitang formation strata (Figures 1b, 2). The upper part of the Niutitang formation is mostly shallow shelf facies, and only the top is shallow-mixed shelf facies. The limestone and argillaceous limestone in the middle are deep-water-mixed shelf facies and those of the lower part are deep-water shelf facies. Based on the lithological characteristics and the changing total organic carbon (TOC) trend, the Niutitang formation is divided into the Niu 1, Niu 2, and Niu 3 sections from bottom to top. The well logging cures of the Niu 1 and Niu 2 sections have the characteristics of “high gamma, low acoustic wave, and low density,” TOC >2% (Figure 2), and shale maturity of 2.17%–2.48%, which is in the over-mature stage.

Comprehensive column of G well.
Figure 2. Comprehensive column of G well.

3 Samples and experimental methods

3.1 Samples

Samples were collected from the D and G wells as well as from outcrops in WMXU. The samples included nine samples from Ordovician strata, seven samples from the lower Cambrian (except Niutitang formation), 22 samples from the lower Cambrian Niutitang formation, and 6 samples from the Dengying formation of Sinian (Figure 1a). The samples collected from Ordovician strata included calcite (n = 7; No. 4, No. 6, No. 7, No. 9, No. 10, No. 13, No. 17) and limestone (n = 2; No. 11, No. 14), which were mainly distributed in the fields. The samples collected from Є3-Є1p included calcite (n = 7), four of which were collected from the field (No. 9, No. 12, No. 15, No. 16), and two were collected from G well, the rest collected from D well. The samples collected from the Niutitang formation included calcite (n = 20), and quartz (n = 2), all of which were collected from the G and D well. The samples collected from the Dengying formation of Sinian included calcite (n = 5), four of which were collected from the D and G well, another one collected from fields (No.3), and quartz (n = 1), collected from D well (Figure 1a). In this study, calcite samples suitable for carbon and oxygen isotope analysis and U-Pb geochronological analysis were primarily obtained from wells G and D. However, as suitable vein materials for isotope and geochronological analysis were not available from other wells (A, B, C, F, H, I, J), these wells contributed mainly gas content data.

3.2 Experimental methods

3.2.1 Field testing for shale gas content

The field desorption method was used to test the shale gas content, referring to the shale gas content determination method of “SY/T6940-2013” (National Energy Administration of China, 2014). An intelligent in situ shale gas desorption instrument (created by Sinopec Jianghan Oilfield Company) was used for testing. The core was loaded into a closed desorption tank, and the amount of desorption gas was measured directly using a metering device. The United States Bureau of Mines method was adopted to calculate the lost gas volume based on the desorption gas volume found in the initial stage. The core was crushed to measure the residual gas volume after final desorption. The gas content of the shale was the sum of the desorption gas, lost gas, and residual gas volumes (Curtis, 2002; Jarvie et al., 2007; Yao et al., 2016).

3.2.2 Carbon and oxygen isotope analysis

The carbon (δ13C) and oxygen isotope (δ18O) analyses were conducted in the State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences. The samples were ground to 200 mesh and heated to remove absorbed water. The samples were processed in a vacuum reactor with phosphoric acid for 24 h at 25 °C, and the generated CO2 was collected. The carbon and oxygen isotopic compositions were determined using MAT253 according to the relative carbon stable isotope reference material (Vienna–PeeDee belemnite (V–PDB)) and the PDB standard, respectively. The test accuracies of δ13C and δ18O were <0.01‰ and <0.02‰, respectively.

3.2.3 Fluid inclusion analysis

To characterize the primary microstructure of the veins and the habit and size of the filling crystals, vein samples were cut into thin sections (∼30 μm thickness) for a detailed microscale petrographic investigation. Fluid inclusion petrography was conducted using a Nikon 80I dual-channel microscope equipped with a 100W mercury lamp manufactured by ThermoFisher, USA. Fluid inclusion microthermometric measurements were performed based on the concept of fluid inclusion assemblages as described by Goldstein (Goldstein and Reynolds, 1994). Temperature measurements were performed using a THMS-600G hot-cold stage manufactured by Linkam of the UK, with Fluid Inclusion Microthermometry analysis conducted in two stages. Heating to determine the homogenization temperatures of oil and brine inclusions in the first step and cooling to determine the ice melting temperatures in the second. The measurement accuracies ranged from ±1 °C to ±0.X °C (where X is a variable representing the less precise range) and ±0.1 °C to ±0.2 °C, respectively. The temperature measurement range was from −196 °C to 600 °C, with freezing/heating rates varying between 0.01 °C/min and 150 °C/min, while the precision and stability were maintained within 0.01 °C. The salinity was converted from the ice melting temperature (final melting temperature, Tm) to the salinity of a binary H2O-CaCl2 system and expressed as weight percentage (%wt) (Bodnar, R., 1993).

Laser Raman analysis and the temperature measurements of the inclusions were conducted at the China University of Geosciences (Wuhan) Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education. Laser Raman spectroscopic analysis was performed using a Horiba Laser Raman spectrometer with a 532-nm argon-ion laser source and a slit width of 12.5 µm, which was equipped with 10×, ×20, and ×50 long-focus objectives. Gaseous fluid inclusions were investigated using a 200-μm confocal aperture and 300-g⋅mm−1 grating with a 2,650cm−1 grating center. To acquire the optimal signal-to-noise ratio, the duration of the data accumulation ranged from 10 to 500 s (Huang et al., 2022).

3.2.4 In situ calcite LA–ICP–MS U–Pb dating

The calcite samples filled in the fractures were cut, polished, and prepared into standard 2.5 cm diameter mounts. The cleaned mounts were assembled into a Laurin Technic sample holder of an ASI resolution SE 193 nm ArF-excimer laser. A dual-volume Laurin Technic ablation cell was employed with a fluence at ∼3 J/cm2. The protocols described by Nuriel et al. (2019) and Ge et al. (2022) were followed for in situ LA–ICP–MS U–Pb dating of carbonates. The testing conditions included a laser spot diameter ranging from 150 to 200 (μm), a frequency of 15 Hz, and an energy density of 1.0–3.0 J/cm2. Helium served as the primary carrier gas, with argon and nitrogen used as auxiliary gases to enhance the signal. The analysis of each sample spot comprised a 20-s background acquisition, a 30-s data acquisition, and a 10-s cleaning period. NIST-612 glass standard was used for instrument calibration and as a reference for elemental concentration calculation, with 43Ca as the internal standard. NIST614 was employed to correct the instrument sensitivity and 207Pb/206 Pb fractionation. Raw data were processed using Iolite software v3.64 (Paton et al., 2011) to calculate elemental concentrations (ppm) and U/Pb ratios. Age calculation was performed by regression on Tera–Wasserburg concordia plots (238U/206Pb vs. 207Pb/206Pb) using Isoplot (Ludwig, 2012). Common Pb correction was applied through regression of initial 207Pb/206Pb intercepts. All reported ages include propagated 2σ uncertainties.

4 Results

4.1 Gas-bearing characteristics

In general, the gas content of the Niutitang formation in WMXU was low. Except for the D and J wells, whose maximum gas contents exceeded 2 m3/t, the gas content of the other wells was <1 m3/t, and some even contained no gas or less gas (Table 1). In terms of the gas composition, the nitrogen content of the Niutitang formation shale gas in the Xuefeng uplift was generally high (>20%), with most wells having a content of >70% (Table 1). The gas content values reported in Table 1 are based on core samples from the Niutitang Formation, with the ranges reflecting depth-dependent variations within the formation. Due to data confidentiality agreements with Sinopec Exploration Company, detailed depth intervals and exact measurement numbers per well cannot be disclosed. However, only general range data are available for each well, and these ranges sufficiently capture the principal variations in gas content across the study area. The average values used in our discussion are calculated from all available measurements.

TABLE 1. Shale gas content and composition of Niutitang formation in WMXU.

WellGas content (m3/t)N2 content (%)CH4 content (%)CO2 content (%)
G0.063–0.53475.69.661.61
F0.11–0.7191.458.55—
H<0.02983.8712.5—
B0.5–2.172.098.65≈18
A0.32–0.942080—
D0.02–2.29———
J0.89–2.53———
I0.05–0.51———

4.2 Characteristics of carbon and oxygen isotopes

The carbon isotope compositions of the surrounding rocks in the WMXU ranged from −5.18 to −0.71‰ and were generally >−4‰. The oxygen isotope compositions ranged from −15.68 to −7.98‰, with a small portion being <−10‰ (Table 2). Compared with the background value of the surrounding rocks, the carbon and oxygen isotope distributions in the calcite veins were more varied, showing an obvious negative skew. The carbon isotope compositions of the veins ranged from −8.19 to −3.82‰, most of which were <−4‰, and almost all the oxygen isotope compositions were <−10‰, ranging from −16.55 to −9.92‰.

TABLE 2. Carbon-oxygen isotopes values in G well and fields in WMXU.

LocationDepth (m)StrataCalcite veinsSurrounding rocks
δ13C‰δ18O‰Z valuδ13C‰δ18O‰Z
G well2,824.96Dengying(Z)−5.18−10.15111.64
2,819.81Niutitang(Є)−1.31−9.92119.67−4.32−10.65113.14
2,816.38Niutitang(Є)−6.09−15.46107.14
2,778.02Niutitang(Є)−3.72−10.16114.63
2,773.58Niutitang(Є)−7.27−10.17107.34
2,767.05Niutitang(Є)−6.72−10.09108.51
2,766.63Niutitang(Є)−9.20−9.95103.50
2,764.55Niutitang(Є)−6.01−13.20108.41
2,740.03Niutitang(Є)−2.21−10.98117.30
2,734.51Niutitang(Є)−2.15−12.23116.81
2,722.33Niutitang(Є)−4.8−13.7110.74
2,631.86Niutitang(Є)−1.98−12.51117.02
2,629.38Niutitang(Є)−1.6−13.71117.19
2,611.56Palang(Є)−4.62−14.00110.86
NO.4—Lower ordovician0.504−15.09120.89
NO.6—Lower ordovician−8.19−10.95105.07
NO.7—Lower ordovician−7.88−12.41104.98
NO.9—Є−1.09−15.95117.12
NO.11—Lower ordovician−0.71−7.98121.87
NO.13—O−9.61−10.86102.21
NO.13—O−7.33−10.96106.83
NO.14—Dawan(O)−4.36−16.55110.13−2.82−15.68113.72
NO.16—Huaqiao(Є)0.96−14.64121.98
NO.16—Huaqiao(Є)3.82−14.12128.10

4.3 Characteristics of fluid inclusion

Fluid inclusions in calcite- or quartz-filled opening fractures in WMXU are mainly brine inclusions, with few hydrocarbon-bearing brine and gas inclusions, and in some samples, there were almost no gas inclusions. The gas inclusions were mainly developed in the calcite-filled fractures in the Niu 3 and Niu 2 sections, Palang formation of lower Cambrian (G and D wells) and the lower Ordovician in the fields, with clustered patterns and varied diameters of 4–8 µm (Fig. 3a, c, d, f, and g), and undeveloped in the Niu 1 section and Dengying formation in G well and fields (Figures 3j–l). The Raman spectra showed that some gas inclusions contained CH4 (Figures 3b,e), and parts of the gas inclusions included bitumen solid particles (Figure 3h). Brine inclusions were widely developed in the calcite- and quartz-filled opening fractures in WMXU, which were distributed directionally or in small groups. The brine inclusion sizes were between 5 and 20 µm and the shapes were quadrihorizontal, irregular, or rice-shaped (Figures 3a,c–g,l). Hydrocarbon-bearing brine inclusions were observed in the calcite-filled fractures of the Niutitang formation (Figure 3g), and the sizes were between 3 and 5 μm, the shapes were mainly oval or irregular polygons, and the distributions were isolated (Figure 3g). The inclusions were undeveloped in the top of the Dengying formation at the G well (Figure 3j), and in the fields and D well, mainly developed brine inclusion in the top of Dengying formation. The proportion of gas inclusions shows systematic variation with depth. They are most common in the Niu 3 section, decrease in the Niu 2 section, and are almost entirely absent in the Niu 1 section and the Dengying Formation.

Characteristic of fluids inclusion in veins from wells and fields in WMXU. **(a)** large number of gas inclusions and brine inclusions nearby were developed in calcite filled dissolved cavities at lower Ordovician at filed, **(b)** The laser Raman optical spectra of gas inclusion at lower Ordovician in filed; **(c)** gas and brine inclusions developed in calcite veins filled fractures in Palang formation of G well; **(d)** numerous of gas inclusions and brine inclusion developed in calcite veins filled fractures in section Niu 3 of well G, **(e)** The laser Raman optical spectra of gas inclusion in section Niu 3 of G well; **(f,g)** gas inclusions and brine inclusions in calcite veins filled fractures in section Niu 2 of G well; **(h)** The laser Raman optical spectra of gas inclusion in section Niu 2 of G well; **(i)** brine inclusion in the Dengying formation of D well; **(j)** inclusions were not developed in veins of fractures in Dengying formation of G well; **(k)** The laser Raman optical spectra of gas inclusion in Dengying formation at field; **(l)** lots of brine inclusions were developed in calcite filled fractures in Dengying formation at field.
Figure 3. Characteristic of fluids inclusion in veins from wells and fields in WMXU. (a) large number of gas inclusions and brine inclusions nearby were developed in calcite filled dissolved cavities at lower Ordovician at filed, (b) The laser Raman optical spectra of gas inclusion at lower Ordovician in filed; (c) gas and brine inclusions developed in calcite veins filled fractures in Palang formation of G well; (d) numerous of gas inclusions and brine inclusion developed in calcite veins filled fractures in section Niu 3 of well G, (e) The laser Raman optical spectra of gas inclusion in section Niu 3 of G well; (f,g) gas inclusions and brine inclusions in calcite veins filled fractures in section Niu 2 of G well; (h) The laser Raman optical spectra of gas inclusion in section Niu 2 of G well; (i) brine inclusion in the Dengying formation of D well; (j) inclusions were not developed in veins of fractures in Dengying formation of G well; (k) The laser Raman optical spectra of gas inclusion in Dengying formation at field; (l) lots of brine inclusions were developed in calcite filled fractures in Dengying formation at field.

The inclusion thermometry results of the gas–liquid brine inclusions in the G and D well and the fields are shown in Table 3. Gas–liquid brine inclusions were developed densely in the veins of the G and D well and the fields. A total of 22 fluid inclusion assemblages (FIAs) were measured in the G well, 17 in the D well, and 14 in the field outcrops, as listed in Table 3. The homogenization temperatures of the G well ranged from 93.4 °C to 165.5 °C, with peaks in the ranges of 100 °C–120 °C, 130 °C–145 °C, and 155 °C–165 °C. The salinity of the inclusions in G well were between 0.2 and 14.4. The homogenization temperatures of the D well ranged from 71.3 °C to 224 °C, with peaks in the ranges of 70 °C–86 °C, 90 °C–128 °C, 135 °C–155 °C and 160 °C–186 °C. The salinity of the inclusions in D well between 3.23 and 21.24. The homogenization temperature of the brine inclusions in the fields ranged from 63.7 °C to 173.5 °C. The homogenization peak temperatures of the brine inclusions in the fields were universally lower than those of G well, which were in the ranges of 80 °C–95 °C, 105 °C–135 °C, and 140 °C–165 °C. The salinity of the inclusions in the fields were scattered and mainly ranged from 1.7 to 9.6 and 15.7 to 24.4.

TABLE 3. Homogenization temperature and salinity of fluid inclusion in WMXU.

LocationDepth (m)Host mineralsStrataHomogenization temperature (°C)Salinity (°C)
G well2,816.38CalciteZbdn100.1–111.1
126.6–133.9
162.2–165.52.4–5.6
2,787.44CalciteЄ1n109.1–128.78.4
137.8–157.41.9–8.9
165.9–187.57.9–9.2
2,774.57CalciteЄ1n114.6–123.10.4
130.2–138.33.4–9.2
2,773.58CalciteЄ1n108.5–120.80.7–3.5
140.1–155.83.1–9.8
2,764.55CalciteЄ1n93.4–105.40.7–4.6
2,734.67CalciteЄ1n110.0–121.30.5–3.4
141.9–147.07.9–14.4
2,734.51CalciteЄ1n103.6–113.9
116.2–123.50.2–0.4
2,640.53QuartzЄ1n114.4–121.62.9–6.4
141.4–163.52.6–5.7
2,629.38CalciteЄ1n102.4–113.1
115.1–126.8
131.2–144.60.7–6.3
2,611.56CalciteЄ1p100.4–115.6
Quartz109.7–124.1
D well1,661CalciteЄ1p78.9–96.519.11
102.5–115.6
144.4–148.718.65
1789.3CalciteЄ1n90.3–92.2
125.4
1848CalciteЄ1n143–1864.34–6.45
1877CalciteЄ1n154–2015.2–15.1
1891CalciteЄ1n156–1753.23–12.6
1947.7CalciteЄ1n71.3–86.120.2
108.6–127.818.66
144.7–160.221.24
1966.2CalciteЄ1n128–1594.65–11.9
1981.5QuartzЄ1n128–2243.39–11.8
2004CalciteZbdn89.6–104.211.94
135.2–165.8
2013CalciteZbdn134–1583.25–17.6
2057.5QuartzZbdn156–1965.86–11.9
NO.3CalciteZbdn121.9–130.53.4–7.7
146.9–155.15.7–9.6
NO.9CalciteO1n123.5–140.64.5–16.5
157.2–163.57.0–9.6
NO.10CalciteO1d63.7
97.1–113.52.7
143.9–147.18.3
NO.12CalciteЄ2+3Ls76.5–81.6
103.3–110.615.9–21.2
127.6–149.220.1–24.1
NO.15CalciteЄ1p80.2–90.9
96.8–109.81.7–5.4
NO.17CalciteO180.7–91.310.6–15.7
156.7–173.521.3–24.4

4.4 LA–ICP–MS U–Pb data

Determining the absolute ages of the calcite is crucial for reconstructing the diagenetic evolution of paleofluid and tectonic movement (Shen et al., 2019). Three calcite samples were analyzed using U–Pb dating; however, only one sample yielded a reliable age determination (Figure 4a), while the other two samples yielded ages with very large uncertainties that preclude meaningful geological interpretation (Figures 4b,c). According to previous research, closed isotopic systematics, homogeneous initial “common” Pb compositions, and sufficient spread in the U–Pb ratio are the key factors for successful calcite dating (Roberts and Walker, 2016; Drost et al., 2018). The Cili-Baojing fault sample yielded an age of 84.4 ± 2.9 Ma, with a mean square weighted deviation (MSWD) of 3.7 (Figure 4a). The 2 G well samples (2,765.94 m and 2,766.63 m) yielded ages of 17 ± 18 Ma (MSWD = 1.8) and 16 ± 29 Ma (MSWD = 12), respectively (Figures 4b,c). The relatively large uncertainties for the two samples are mainly attributable to their low U abundance (<0.1 ppm) and narrow 238U/206Pb ratios (0–40). Given the large uncertainties and the high MSWD of the 2 G well samples, these ages should be interpreted with caution and are considered tentative.

The U-Pb data of calcite vein in fields and G well (**a and a’**. calcite U-Pb data of Cili-Baojing faults; **b and b’**. calcite U-Pb data of G well (2,765.94 m); **c and c’**. calcite U-Pb data of G well (2,766.63 m).
Figure 4. The U-Pb data of calcite vein in fields and G well (a and a’. calcite U-Pb data of Cili-Baojing faults; b and b’. calcite U-Pb data of G well (2,765.94 m); c and c’. calcite U-Pb data of G well (2,766.63 m).

5 Discussion

5.1 Analysis of gas preservation conditions

According to previous studies on ancient carbonate rocks, the carbon isotope variation in syndepositional carbonate rocks ranged from −5‰ to 5‰ or −4‰ to 4‰ (Veizer et al., 1986; Wang et al., 2011). Magmatic activity, oxidation of organic matter, and infiltration of atmospheric freshwater are the main reasons for the negative carbon isotope of calcite, and δ13C value of carbonate rock originating from meteoric water can as low as −15‰ (Jacobsen and Kaufman, 1999; Boles et al., 2004; Liu et al., 2022). The oxygen isotope in carbonate rock is unstable, and the composition is easily affected by various factors, such as infiltration of meteoric water, water/rock ratio and high-temperature hydrothermal fluid (Derry et al., 1992; Kaufman and Knoll, 1995; Boles et al., 2004; Liu et al., 2022).

As shown in Figure 5a, the carbon isotopes of calcite veins in J well are large than −4‰ mainly, but the oxygen isotopes of calcite veins in J well are almost less than 10‰, which may be related to mixed thermal fluids (upwelling and seawater) (Xiao et al., 2023), leading to oxidation of organic matter and increase of temperature. The carbon and oxygen isotopes of calcite veins in G well and fields were more dispersed in the calcite veins than in the surrounding rocks, being negatively skewed mainly. This indicates that the veins were not formed entirely from the surrounding rocks and there was a source of exogenous fluid, such as meteoric water or high-temperature hydrothermal fluid. The paleosalinity index (Z value) was calculated following Keith and Weber (1964): Z = 2.048 × (δ13C + 50) + 0.498 × (δ18O+ 50), where δ13C and δ18O are expressed relative to the V-PDB standard. This empirical index has been widely applied to distinguish diagenetic environments (Kerrich et al., 1984). The δ13C–Z value graphical method was employed to distinguish the diagenetic environment. Areas with Z < 120 represent carbonate rocks mixed with exogenous fluid while those with Z > 120 represent marine carbonate. The Z values of the veins had a wide distribution range: a small number of the vein Z values were >120 but most were <110 (Figure 5b), indicating the mixing of meteoric water greatly affected the veins filled fractures in WMXU, and high-temperature hydrothermal fluid may also affect calcite veins properties.

Distribution diagram of Carbon-oxygen isotopes and Z in WMXU. **(a)** Carbon and oxygen isotope scatter plots of veins and surrounding rocks, **(b)** The δ<sup>13</sup>C-Z scatter plots of veins and surrounding rocks. (Data of J well collected from Xiao et al. (2023)).
Figure 5. Distribution diagram of Carbon-oxygen isotopes and Z in WMXU. (a) Carbon and oxygen isotope scatter plots of veins and surrounding rocks, (b) The δ13C-Z scatter plots of veins and surrounding rocks. (Data of J well collected from Xiao et al. (2023)).

The salinity of brine inclusions can reflect the source of fluid. Low salinity (<3.5%) is caused by the connectivity of channels between the strata and atmospheric water, leading to the infiltration of meteoric water, which indirectly indicates poor sealing of the strata. Intermediate salinity (3.5%–10%) and high salinity (>10%) of the brine inclusions indicate that meteoric water had little or no effect on the diagenetic fluids, demonstrating the strong sealing of the strata (Sorkhabi, 2005; Li et al., 2016; Teixeira et al., 2018). These thresholds, originally established in other sedimentary basins, are considered valid for the WMXU as they are here supported by the consistent association of low-salinity inclusions with negative δ13C and δ18O signatures (Figure 5). As shown in Table 3, the salinity of some brine inclusions in G well were even as low as 0.2%, with many brine inclusions having lower salinity. The brine inclusions in the G and D wells and fields all exist some low salinities inclusions (Figure 6), indicating the possibility of a meteoric water source.

Scatter diagram of homogenous temperature with salinity of fluid inclusion of veins in WMXU: **(a)** G well; **(b)** D well; **(c)** fields. The dashed lines indicate salinity thresholds: <3.5% (meteoric water influence), 3.5%–10% (mixed fluids), and >10% (formation waters).
Figure 6. Scatter diagram of homogenous temperature with salinity of fluid inclusion of veins in WMXU: (a) G well; (b) D well; (c) fields. The dashed lines indicate salinity thresholds: <3.5% (meteoric water influence), 3.5%–10% (mixed fluids), and >10% (formation waters).

The carbon and oxygen isotopes and salinity of the calcite veins in the fields and the G, D wells in the western margin of the Xuefeng uplift indicate that they were greatly influenced by exogenous fluids during their formation. Furthermore, the formation of calcite veins was accompanied by mixing meteoric water or high-temperature hydrothermal fluid, which may destroy the strata sealing. It should be acknowledged that negative δ13C and δ18O values and low-salinity fluid inclusions are not uniquely diagnostic of meteoric water infiltration. Organic matter oxidation, thermal decarboxylation, and hydrothermal fluid mixing can also produce negative isotopic signatures (Boles et al., 2004; Liu et al., 2022). In this study, however, the low salinity (<3.5%) and its systematic association with fractures connected to the basal detachment zone (Figure 7) strongly favor meteoric water infiltration as the primary mechanism, as the alternative processes cannot fully explain the observed salinity and structural relationships. Only a small number of gas inclusions were found in the fluid inclusions in the calcite-filled opening fractures in the fields and the G, D wells. Additionally, few methane inclusions developed in the upper part of the Niutitang formation and its overburdened strata, whereas the Niu 1 section and its underlying strata contained brine inclusions mainly. Even an absence of inclusions was observed (Figure 3), indicating that the fluid inclusions were undeveloped in the high gas–bearing stage. Further, the brine inclusions were densely developed (Figure 3), showing that the degree of shale gas escaping was high throughout the geological history.

Characteristic of shale fractures at Niutitang formation in WMXU (part of data collected from Miao et al. (2020); Liu et al. (2021a); Xiao et al. (2023)).
Figure 7. Characteristic of shale fractures at Niutitang formation in WMXU (part of data collected from Miao et al. (2020); Liu et al. (2021a); Xiao et al. (2023)).

As shown in Figure 2, the gas content of the G well was characterized by “high in the bottom of the Niutitang formation and low in the top of the Niutitang formation.” The gas contents of the Niu 1 and Niu 2 sections were relatively high, with an average of 0.254 m3/t, whereas the Niu 3 section had an overall gas content of <0.2 m3/t. In terms of the fracture distribution, the fractures were mainly developed in the Niu 1 and Niu 2 sections (2,726–2,824 m) of the G well, with a few of them being discovered at the top of Niutitang formation (Figure 7a). In the water immersion experiments, the air bubbles were observed in the section of fracture development at the lower part of the Niutitang formation, indicating that the gas content had a positive correlation with the fractures. Except for the G wells, the J, D wells in WMXU showed that large-scale development of the fracture or broken core sections had higher gas contents than the nonfractured sections (Figures 7b,c). The fractures may play an important role as a channel for shale gas to escape, resulting in a certain amount of gas being trapped in the fractures later.

Only a small amount of hydrocarbon inclusions was found in G, D and J Wells, indicating that fluid inclusions did not develop in the high gas content stage, but developed in the uplift and denudation stage. And calcite veins in wells and fields dominated by brine inclusions (Figure 7), reflecting that the degree of shale gas loss in geological history has been very high. The proportion of brine inclusions in Niutitang formation shale veins tends to increase downwards, while the pure gas inclusions are fewer in lower part of Niutitang formation (Figure 7). And the carbon and oxygen isotopes of calcite veins filled in fractures in the lower part of Niutitang formation in G, D and J wells are generally negative and lower than in the upper part, and the Z value of calcite veins also shows the same trend (Figure 7), indicating that the formation of calcite veins in the lower part of Niutitang formation may have the input of exogenous fluids, like meteoric water, high-temperature hydrothermal fluid. At the same time, shale gas may escape from the bottom of Niutitang formation.

Although bedding detachment, shear, and extensional fractures may promote the horizontal migration of shale gas, which can improve the porosity and permeability of shale, large-scale fractures can lead to the escape of shale gas and destroy shale gas reservoirs (Luo et al., 2021; Wang et al., 2022). The study area was close to the Xuefeng uplift and the existing Cili–Baojing faults, which are large and deep faults (Figures 1A,B). Because the area is close to the fault zones or areas with strong tectonic activity, the shale may have been pierced (Zhang et al., 2019; Zhang et al., 2020), accelerating the escape of shale gas in WMXU.

Overall, the capping ability of the bottom and top strata of the Niutitang formation is strong, which is conducive to the preservation of shale gas (Li et al., 2016). Nevertheless, based on the analysis of the gas content, fractures, combined with carbon and oxygen isotopes and fluid inclusion of veins filled fractures, we speculate that the escape of Niutitang formation shale gas is severe. Regionally, the lower part of the Niutitang formation is a detachment zone and the mineral content is high in brittle minerals, which favors the development of fractures between the lower part of the Niutitang formation and the underlying strata (Peng, 2019; Liu et al., 2021a). Except for the G well, high-angle shear fractures and interlayer extensional fractures were also densely developed at the lower part of the Niutitang formation in other areas at WMXU (Peng, 2019; Miao et al., 2020), and the fractures filled with veins in the lower part of Niutitang formation have the input of exogenous fluids, leading to the negative bias of carbon and oxygen isotopes and development of massive brine inclusions (Figure 7). The combined effect of the opening faults, large-scale high-angle shear fractures, and extensional fractures may lead to the escape of shale gas directly, and the detachment zone of the Niutitang formation promotes horizontal dissipation of shale gas (Li et al., 2020), forming a channel for the infiltration of external meteoric water or high-temperature hydrothermal fluid, resulting in changes in the paleofluid properties in WMXU.

5.2 Model of shale gas formation and escaping in WMXU

As shown in Figure 3, and the research of Liu et al. (2021a), Xiao et al. (2023), He et al. (2023), Various types of inclusions were developed in the lower Cambrian shale fractures filled with vein in WMXU, including primary brine inclusions and secondary brine inclusions contemporaneous with bitumen, primary brine inclusions contemporaneous with bitumen and methane, primary and secondary brine inclusions contemporaneous with methane in calcite veins, primary and secondary brine inclusions contemporaneous with bitumen, primary brine inclusions contemporaneous with bitumen/methane, secondary brine inclusions contemporaneous with methane in quartz veins. Among them, the homogeneous temperature distribution of primary brine inclusions in calcite veins contemporaneous with bitumen is distributed between 80 °C and 220 °C; the homogeneous temperature of secondary brine inclusions contemporaneous with bitumen is in 90 °C–130 °C; The homogeneous temperature of primary brine inclusions contemporaneous with bitumen and methane is distributed between 140 °C and 205 °C; the homogeneous temperature of primary brine inclusions contemporaneous with methane is in the range of 100 °C–160 °C; the homogeneous temperature of secondary brine inclusions contemporaneous with methane is in the range of 100 °C–170 °C; The homogeneous temperature distribution of primary brine inclusions in quartz veins contemporaneous with bitumen is between 120 °C and 170 °C; the homogeneous temperature distribution of secondary brine inclusions contemporaneous with bitumen is 75 °C–110 °C; The homogeneous temperature of primary brine inclusions contemporaneous with bitumen and methane is between 130 °C and 220 °C, and the homogeneous temperature of secondary brine inclusions contemporaneous with methane is between 100 °C and 160 °C (Figure 8). As listed in Table 3 and research date of Xiao et al. (2023), the inclusions captured in quartz and calcite veins have the same homogeneous temperature range roughly and a small difference in salinity range. It can be considered that the hydrochemical conditions and geothermal conditions were similar when the quartz and calcite veins were formed, and the formation time was similar, but the precipitation order is different.

Statistical histogram of fluid inclusion homogenization temperatures of the Niutitang Formation of G **(a)** and J **(b)** wells (data of fluid inclusion homogenization temperatures of J well collected form Xiao et al., 2023).
Figure 8. Statistical histogram of fluid inclusion homogenization temperatures of the Niutitang Formation of G (a) and J (b) wells (data of fluid inclusion homogenization temperatures of J well collected form Xiao et al., 2023).

The sedimentary and thermal histories of the Niutitang formation shale were analyzed by collecting geological field reports in the study area and adjacent areas as well as data related to the Niutitang formation thermal evolution history (Peng, 2019), using the maturity of the Niutitang formation shale in the G well as the boundary condition. Based on the tectonic evolution research, the distribution characteristics and homogenization temperature of the lower Cambrian shale brine inclusion (Figure 8), calcite U–Pb data of the veins filled with fractures in Niutitang formation (Figure 4), and thermochronological data analysis in WMXU (Mei et al., 2010; Li and Shan, 2011; Deng, 2014; Wu, 2019; Zheng et al., 2019; Shen et al., 2020), the tectonic–burial–thermal evolution history of the Niutitang formation in WMXU was constructed (Figure 9).

Tectonic-Burial-Thermal evolution of Niutitang formation in WMXU (collected from Mei et al., 2010; Li and Shan, 2011; Deng, 2014; Wu, 2019; Zheng et al., 2019; Shen et al., 2020; He et al., 2023; Xiao et al., 2023).
Figure 9. Tectonic-Burial-Thermal evolution of Niutitang formation in WMXU (collected from Mei et al., 2010; Li and Shan, 2011; Deng, 2014; Wu, 2019; Zheng et al., 2019; Shen et al., 2020; He et al., 2023; Xiao et al., 2023).

In general, the Niutitang formation shale in WMXU has the characteristic of “early hydrocarbon generation and early uplift.” In the middle to late Ordovician, the Guangxi movement caused the intrusion of acidic magma, and the Guzhang fault formed at same time provided a vertical migration channel for the upwelling of hydrothermal fluids (Liu et al., 2021a), leading to the migration of paleofluid. The shale of the Niutitang formation entered the peak oil generation period in the late Silurian period, and the Caledonian movement led to the massive migration of liquid hydrocarbons accompanied with paleofluid migration. With the continuous increase in burial depth, the Niutitang formation shale entered a gas generation stage in the middle and late Devonian eras, and then, the oil and gas reservoirs were reformed or destroyed by the NE–SW compression of the Indosinian movement. And large number of bitumen and gas inclusion captured by calcite or quartz in same stage. In the early Jurassic, the Niutitang formation reached the maximum burial depth. However, the Yanshanian movement subsequently had a considerable destructive effect on the preservation conditions of shale gas, accompanied with continuously paleofluid migration, and gas inclusion and brine inclusion were captured by calcite and quartz meantime. Both the ∼84Ma U-Pb age of the fault related calcite veins and Late Cretaceous fluid activity constrained by the homogenization temperatures (130 °C–145 °C) of the calcite from the G Well recorded this event. The Himalayan movement was dominated by the uplift of strata, which further led to the escape of shale gas. No oil and gas inclusions were detected at this time, speculated that captured oil inclusions cracked to methane and bitumen at later stage, and the inclusions were destroyed with the strata uplifting (Xiao et al., 2023). Although dates obtained from the 2 G well samples possess large uncertainties, the good consistency between the two samples (17 Ma vs. 16 Ma) may suggest that there is an fluid activity during the Neogene (Figure 9). In addition to the U-Pb ages, previous apatite fission-track thermochronology modeling data from the WMXU (Mei et al., 2010; Li and Shan, 2011; Zheng et al., 2019; Shen et al., 2020) and the projection of low-salinity brine inclusions (with homogenization temperatures <100 °C) onto the burial history curve (Figure 9) collectively suggest a Neogene episode of paleofluid migration at ca. 17 Ma.

The low salinity temperature ranges of the brine inclusions in the fractures of the G and D wells and the fields were 95 °C–125 °C and 135 °C–155 °C (Figure 6), indicating that the two periods of the tectonic movement were strong, which may lead to the connection between the target strata and the surface. Therefore, the brine inclusions had a source of meteoric water, and the salinity was low. The homogeneous temperatures of the brine inclusions with low salinity (Figure 6) were projected onto the burial history map, corresponding to the tectonic movement during the periods of 108–132 Ma and 62–83 Ma, the NW–SE-trending extrusion and the North-Northeast trending strike–slip structure in the middle and late Yanshanian movement, respectively. Tectonic movement in Neogene inferred from apatite fission-track thermochronology and calcite U-Pb dating also have destructive effect on shale gas preservation, for the carbon and oxygen isotopes of the calcite are negative bias and Z value less than 120 (Table 2) (Xiao et al., 2023), revealing the formation of calcite veins was accompanied by imputing of exogenous fluid, and forming the channel for shale gas escaping.

Additionally, the U–Pb data of the calcite-filled fractures of Cili-Baojing faults is 84 ± 2.9 Ma. As shown in Figure 10, the Cili–Baojing faults showed thrust strike–slip or strike–slip structures (Figures 10a–c), and the faults had an immensely destructive effect on the strata from the Ordovician to Sinian periods (Figures 10a–d). Some outcrops even developed near-vertical fractures (30–50 cm wide) (Figure 10d). Combined with the electron spin resonance dating of the Cili–Baojing fault zones (Qin et al., 2019), calcite U-Pb data of our research, we reckon that the Cili-Baojing faults experienced NW–SE compression and strike–slip tectonic movement at least during the Yanshanian movement stage. The sealing ability of the Cili–Baojing faults may have been damaged in this stage, providing a channel for gas to escape for the wells near the faults, as the I, G, F, and C wells. While in Yuanma depression at the edge of the Xuefeng uplift, the late Yanshanian movement was primarily a NW–SE-trending extensional movement, forming a faulted basin and receiving Cretaceous sandstone deposition. This may have been beneficial to the preservation of shale gas, as evidenced by the gas contents of the D and J wells being slightly higher than average in WMXU (Peng, 2019).

Characteristics of the Cili-Baojing faults in WMXU. **(a–c)** Thrust strike–slip or strike–slip structures of the Cili-Baojing faults, which had a destructive effect on the strata from the Ordovician to Sinian periods. **(d)** Near-vertical fractures (30–50 cm wide) developed in some outcrops.
Figure 10. Characteristics of the Cili-Baojing faults in WMXU. (a–c) Thrust strike–slip or strike–slip structures of the Cili-Baojing faults, which had a destructive effect on the strata from the Ordovician to Sinian periods. (d) Near-vertical fractures (30–50 cm wide) developed in some outcrops.

As shown in Figure 9, the Caledonian and the Indosinian movements were mainly displayed with the SE–NW extrusion in the research area. The Caledonian movement had a low influence on the tectonic characteristics of WMXU and only showed stratigraphic unconformable contact in some areas (Deng, 2014). The Indosinian movement, dominated by faulting and folding movements, led to the formation of the large uplift and depression tectonic patterns in WMXU, and the faults were mainly NNW trending thrust faults (Wang et al., 2013; Qiu et al., 2014). Thus, the Yanshanian and Himalayan movement have great impact on the Niutitang shale gas enrichment in WMXU.

In the early Jurassic period, the structural deformation intensity of the WMXU was not strong enough. Fractures of the Niutitang formation may have been relatively undeveloped, and the shale gas mainly migrated horizontally within the reservoir (Figure 11a). The Cili–Baojing fault zone may have provided a channel for shale gas escaping on the upper side of the fault, whereas the underside of the fault had a strong sealing ability. In addition, the preservation conditions of the shale gas were good in areas far from the Cili–Baojing fault zone (Figure 11a).

Shale gas dispersion mode of the Niutitang Formation in WMXU (the location of the wells is shown in Figure 1a). **(a)** Early Jurassic; **(b)** Late Cretaceous (Yanshanian movement); **(c)** Neogene–Now (Himalayan movement).
Figure 11. Shale gas dispersion mode of the Niutitang Formation in WMXU (the location of the wells is shown in Figure 1a). (a) Early Jurassic; (b) Late Cretaceous (Yanshanian movement); (c) Neogene–Now (Himalayan movement).

During the Yanshanian movement stage, WMXU showed progressive deformation from the SE to NW, the tectonic intensity decreased, and the folds and faults that formed during the early stage were reformed, which had an important impact on the current stratigraphic framework. The Yanshanian movement was dominated by strong folding, thrusting, and strike–slip tectonic movement. The strata were severely damaged, and the fractures in the Niutitang formation abundantly developed, including high-angle shear fractures, extensional fractures, and network fractures (Figure 7), which were conducive to the escape of shale gas in the horizontal and vertical directions (Zhang et al., 2019; Luo et al., 2021; Wei et al., 2021). In addition, several detachment fractures at the bottom of the Niutitang formation, combined with the extensional and shear fractures, formed a channel for shale gas to escape downward along the fractures (Figure 11b). Combined with the thrust strike–slip or strike–slip effect of Cili-Baojing faults in this stage (Figures 4, 10), accelerated the escape of shale gas around the fault, as C, F, G and I wells (Figure 1; Table 1). Far from the Cili–Baojing fault, the gas contents of the J and D wells were relatively high but did not meet the standard of industrial shale gas flow. The escape of shale gas may be mainly related to the dense development of extensional fractures and shear fractures in the lower part of the Niutitang formation as well as the connection with detachment fractures at the bottom of the Niutitang formation, and shale gas escaping may be less affected by peripheral thrust faults.

The Himalayan movement in WMXU was mainly characterized by the uplift of strata, accompanied with migration of paleofluid and infiltration of exogenous fluid. Previous apatite fission-track thermochronology (Mei et al., 2010; Li and Shan, 2011; Zheng et al., 2019; Shen et al., 2020) and the projection of fluid inclusion data onto the burial history curve suggest a Neogene paleofluid migration event at ca. 17 Ma, although this age is not directly constrained by U-Pb dating and should be considered tentative. The uplift and erosion of strata accelerated the speed of shale gas escaped, the gas content of the Niutitang formation in WMXU was universally lower (Figure 11c).

6 Conclusion

Integrating U-Pb dating, fluid inclusion analysis, and burial-thermal modeling, five episodes of large-scale fluid movement are recognized in the study area. Among these, the Late Jurassic to Early Cretaceous (ca. 84.4 ± 2.9 Ma) episode is directly constrained by U-Pb dating of the fault-related calcite vein, corresponding to thrusting and strike-slip movement that activated the Cili-Baojing fault and created fracture networks facilitating significant shale gas escape. The remaining four episodes are mainly inferred from burial-thermal modeling and fluid inclusion data rather than from direct absolute dating. These include early-middle Ordovician source rock maturation, early Silurian hydrocarbon expulsion, early-middle Triassic gas generation, and Neogene (ca. 17 Ma) rapid uplift. The Neogene age is inferred from calcite U-Pb dating, apatite fission-track thermochronology modeling and the projection of fluid inclusion data onto the burial history curve, and should be considered tentative given the large uncertainties of the individual U-Pb analyses. Despite limited direct age constraints, the combination of gas content, fluid inclusion characteristics, isotope geochemistry, U-Pb geochronology, and apatite fission-track thermochronology provides a coherent picture of the preservation conditions and gas escape history of the Niutitang Formation shale in the WMXU. The results indicate that preservation conditions, rather than source rock quality alone, control shale gas accumulation in this structurally complex region. Future exploration should focus on areas distal to major faults where fracture networks are less developed and sealing conditions are more favorable.

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Footnotes

Edited by: George Kontakiotis, National and Kapodistrian University of Athens, Greece

Reviewed by: Shahab Aman E Room, China University of Petroleum, China

Longyi Wang, University of Chinese Academy of Sciences, China

Statements

Conflict of interest

Authors ZF, JH, and HW were employed by Sinopec Exploration Company.

The remaining author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This work was supported by the Outstanding Youth Funding of Natural Science Foundation of Hubei Province (No. 2016CFA055), the Natural Science Foundation of China (No. 41672140). The authors gratefully acknowledge the support of their institutions.

Data availability

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

ZF: Conceptualization, Methodology, Writing – original draft. XG: Methodology, Validation, Writing – review and editing. JH: Data curation, Investigation, Visualization, Writing – review and editing. QL: Data curation, Investigation, Visualization, Writing – review and editing. HW: Data curation, Investigation, Visualization, Writing – review and editing. YZ: Data curation, Investigation, Visualization, Writing – review and editing.

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Citation

Fan, Z., et al. (2026). Gas preservation conditions of Cambrian Niutitang formation shale in western margin of Xuefeng uplift, SE China-based on the analysis of paleofluid. Frontiers in Earth Science, Volume 14. https://doi.org/
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Copyright © 2026 Fan, Ge, Han, Lei, Wu and Zhang.

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*Correspondence: Xiang Ge

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