The Role of the Emeishan Large Igneous Province in Hydrocarbon Formation in the Anyue Gas Field, Sichuan Basin, China

Abstract

This study investigates the impact of the Emeishan Large Igneous Province (ELIP) on hydrocarbon formation within the Anyue gas field in the Sichuan Basin. As a major Middle to Late Permian large igneous province, the ELIP hosted intense mantle plume activity that reshaped regional tectonics and thermal structures, indirectly influencing hydrocarbon accumulation. This paper examines three primary factors in hydrocarbon evolution linked to the ELIP: its thermal influence, induced fluid activity, and role in hydrocarbon cracking. Data reveal that the thermal effects of the ELIP extend to the central Sichuan Basin, where an elevated paleogeothermal gradient has driven hydrocarbon evolution in the Anyue gas field. Petrographic characteristics, chronological data, fluid inclusion features, and C–O, S, and Pb isotopic signatures collectively indicate that around 260 Ma, a hydrothermal event occurred in the Sichuan Basin, closely aligned with a natural gas charging event. The combined effects of a heightened geothermal gradient and hydrothermal fluids (with temperatures up to 320 °C) suggest that paleo-oil reservoirs had already cracked into natural gas during the peak ELIP activity.

1. Introduction

The Emeishan Large Igneous Province (ELIP) is recognized as one of the world’s major large igneous provinces, with its formation closely associated with mantle plume activity during the Middle to Late Permian [1]. This plume activity led to widespread volcanic eruptions and extensive basalt formation [2,3,4]. Large-scale magmatic processes associated with the ELIP significantly influenced the geodynamic evolution of the region and controlled the formation and spatial distribution of ore deposit resources [5,6,7]. While magmatic processes cannot directly contribute to hydrocarbon formation due to the relatively low temperatures required for hydrocarbons to form, recent studies on the ELIP have highlighted their direct and significant role in regional tectonics and ore deposits, as well as their indirect influence on hydrocarbon accumulation processes in surrounding areas [8,9,10]. The activity of the mantle plume generated significant crustal uplift, particularly in the southwestern Sichuan Basin, as it was closer to the core of the plume. This tectonic process resulted in a distinctive paleogeographic pattern, with a transgression developing sequentially from northeast to southwest [11,12]. In addition, the tectonic events associated with the ELIP generated numerous fractures and fissures within the Sichuan Basin and its surrounding regions, creating crucial pathways for hydrocarbon migration and offering favorable structural conditions for hydrocarbon accumulation [13,14]. While the role of the ELIP in influencing ore deposit formation and regional tectonics has been extensively studied, its impact on petroleum systems, particularly hydrocarbon accumulation and evolution, remains underexplored. In the Sichuan Basin, the Anyue gas field, located within the “outer belt” of the ELIP, provides a unique opportunity to investigate the interplay between mantle plume activity and hydrocarbon systems. Previous studies have focused primarily on gradual geological processes, such as burial and thermal evolution, to explain hydrocarbon formation. However, emerging evidence highlights the potential significance of episodic geological events, like those associated with the ELIP, in shaping petroleum systems. These events, characterized by thermal anomalies and induced fluid activity, may have profoundly altered the maturation, migration, and cracking processes of hydrocarbons. This study aims to bridge the knowledge gap by presenting new mineralogical and geochemical evidence that elucidates the role of the ELIP in hydrocarbon evolution within the Anyue gas field. Specifically, the thermal effects, fluid dynamics, and hydrocarbon cracking mechanisms associated with the ELIP are analyzed to establish their influence on natural gas formation from three key aspects: ELIP’s thermal impact, ELIP-induced fluid activity, and its effects on hydrocarbon cracking. Our findings reveal that vitrinite reflectance (Ro) data from the Sichuan Basin show a distinct inflection point between the Middle and Upper Permian, which aligns with the main phase of ELIP activity around 260 Ma. These observations suggest a causal link between the ELIP and the inflection point in Ro [15,16]. Additionally, C–O, S, and Pb isotopic characteristics confirm the presence of a hydrothermal event influencing the Sinian–Cambrian reservoirs in the Anyue gas field. Isotopic dating indicates that this hydrothermal event occurred around 260 Ma, and the presence of abundant methane (CH4) inclusions in hydrothermal dolomite from this period suggests a close association with natural gas formation [17,18]. The relevance of this research lies in its potential to shift traditional paradigms in hydrocarbon exploration. By demonstrating that episodic geological events can significantly modify pre-existing petroleum systems, this study advocates for a broader perspective in exploring ancient strata. This approach underscores the importance of evaluating reservoir modifications caused by geological events rather than solely focusing on hydrocarbon generation and migration processes. Despite its significant findings, the existing body of literature on the influence of large-scale episodic events on hydrocarbon systems remains limited. This study seeks to address this gap by providing robust data and interpretations, thereby contributing to the understanding of how volcanic and tectonic activities can shape petroleum systems. Ultimately, this work aims to advance scientific knowledge and offer practical implications for hydrocarbon exploration strategies in regions influenced by large igneous provinces.

2. Geological Background

The Emeishan Large Igneous Province (ELIP), located along the western margin of the Yangtze Craton in China, extends across Yunnan, Sichuan, and Guizhou provinces. This province consists primarily of sub-alkaline and slightly alkaline mafic volcanic rocks along with pyroclastic deposits. The area of the ELIP is significant, covering approximately 250,000 km². This large igneous province is notable for its extensive volcanic activity during the Permian period, which played a key role in shaping the regional geology and may have influenced global environmental changes during that time [19,20,21]. In addition to extensive basaltic formations, the region contains ultramafic and mafic intrusions, syenite, granite, and large amounts of magmatic and hydrothermal deposits [22,23,24]. The ELIP was categorized into inner, intermediate, and outer zones based on the degree of erosion in the Maokou Formation limestone and the crustal uplift [25,26] (Figure 1).

The Sichuan Basin, located in the intermediate and outer belts of the ELIP along the western edge of the Yangtze Plate, is a superimposed basin, which has undergone a Paleozoic–Early Mesozoic cratonic depression stage and a Late Triassic–Cenozoic foreland basin stage [27,28]. The Sichuan Basin has undergone multiple tectonic events, leading to complex deformation and erosion of its sedimentary layers. Different stages in the basin’s evolution have produced various paleo-uplifts, including the Caledonian, Hercynian, Indosinian, and Yanshanian uplifts. These uplifts, along with regional unconformities, have collectively influenced the formation of carbonate reservoirs and the accumulation of hydrocarbons [29,30]. The basin is known for its abundant hydrocarbon resources, featuring numerous highly prospective hydrocarbon systems, and has become the basin with the largest number of discovered gas fields in China (>300) and the highest number of gas-producing layers (>20) [31,32]. In recent years, substantial discoveries have been made in the Anyue gas field (Moxi–Gaoshiti area) in the central region of the Sichuan Basin (Figure 1). Large gas reserves have been identified within the Sinian–Cambrian formations, one of China’s oldest hydrocarbon-bearing sequences and a current focal point for exploration and development efforts. The Sinian gas-bearing layers span an area of over 7000 km² at depths of 5000–5500 m, with gas reservoir thicknesses ranging from 20 to 60 m. The estimated geological reserves in these layers exceed 700 × 10⁹ m³ [33]. The Cambrian Longwangmiao Formation reservoir has confirmed gas reserves of 440 × 10⁹ m³ across an area of 800 km², with geological reserves anticipated to exceed 600 × 10⁹ m³ [33,34]. Reservoir spaces in the Sinian and Cambrian formations mainly consist of intergranular pores and fractures, with most of these spaces filled by residual pyrobitumen. This pyrobitumen predominantly occupies vein-like fractures, pores, and voids, and is typically granular or spherical in shape [35]. The pyrobitumen content decreases markedly from the apex of the paleo-uplift toward its flanks. Pyrobitumen has been identified in drilling cores throughout the gas field, indicating that the current gas reservoir was originally part of a much larger ancient oil reservoir. Additionally, the optical characteristics of the pyrobitumen indicate exposure to high temperatures, suggesting that it is the product of high-temperature cracking of liquid hydrocarbons [36]. In the region, the Sinian Doushantuo Formation, the third member of the Dengying Formation, and the Cambrian Qiongzhusi Formation serve as primary potential hydrocarbon source rocks. The Doushantuo Formation, which lies above the pre-Sinian crystalline basement, consists of deep-water fine-grained sediments, primarily black shale, siltstone, and dolomite, with a high organic carbon content and thickness ranging from 9 to 20 m. The third member of the Dengying Formation mainly comprises terrigenous clastic deposits, including dark gray to gray-green carbonaceous shale and silty shale, with notable lateral lithologic variation and high-organic-carbon zones observed locally in wells such as GK1 and GS1. The Cambrian Qiongzhusi Formation is predominantly composed of black mudstone, with high organic carbon content and a widespread distribution, typically reaching thicknesses of 100–400 m, making it the most favorable hydrocarbon source rock in the area [37,38]. The natural gas accumulation in the Anyue gas field is thought to have evolved from an ancient oil reservoir, undergoing crude oil cracking followed by uplift adjustments [29,33]. The basin’s thermal evolution history has been a key factor in the generation and evolution of hydrocarbons, with the activity of the Emeishan Large Igneous Province (ELIP) during the Middle–Late Permian playing a significant role in shaping this thermal history.

3. Methods

A sample was collected from a depth of 5517.5 m (Zdn₄) in well GS101 in the Auyue gas field of the Sichuan Basin and made into ~0.03 mm thick doubly polished thin sections for petrographic analysis and imaging. Thin sections were examined using a Leica polarizing microscope (Leica Microsystems, Wetzlar, Germany) under transmitted and reflected light in China University of Petroleum (CUP). Representative examples of pyrobitumen-bearing dolomite were gold-coated and analyzed on Quanta200F (FEI Company, Hillsboro, OR, USA) and Hitachi SU8010 field emission scanning electron microscopes (SEMs) (Hitachi High-Tech, Tokyo, Japan) in CUP. Eight vein-like dolomite samples from the Zdn₄ in well GS101 were selected for C–O isotope analysis at ALS Chemex Co. Ltd. (ALS, Guangzhou, China). A 50 mg sample was placed into borosilicate glass vials sealed with butyl rubber septa and reacted with concentrated phosphoric acid at 72 °C for 4 h in a heated aluminum block. During the reaction of carbonate minerals with phosphoric acid, oxygen isotopes exchange between the phosphoric acid and the liberated CO₂ and water. The evolved CO₂ gas was then analyzed using a Thermo-Finnigan Gas Bench coupled with a Thermo-Finnigan MAT Delta Plus XP Continuous-Flow Isotope Ratio Mass Spectrometer (CF-IRMS) (Thermo Electron Corporation, Waltham, MA, USA). The δ¹³C and δ¹⁸O values are reported using the delta (δ) notation in permil (‰), relative to Vienna Pee Dee Belemnite (VPDB) for carbon and Vienna Standard Mean Ocean Water (VSMOW) [39] for oxygen, with precision of 0.1‰ for carbon and 0.5‰ for oxygen. Calibration was conducted using CaCO₃ minerals and international isotopic standards NBS18 and NBS19 [39]. Sulfur isotope analysis was conducted on eight metallic sulfides associated with the dolomites at ALS. Samples were weighed into tin capsules, and the sulfur isotopic composition was measured using a MAT 253 Stable Isotope Ratio Mass Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) coupled with a Costech ECS 4010 Elemental Analyzer (Costech Analytical, Valencia, CA, USA). δ³⁴S values were calculated by normalizing the ³⁴S/³²S ratios in the sample to that of the Vienna Canyon Diablo Troilite (VCDT) [40] international standard. The results, expressed in delta (δ) notation in permil (‰), have a reproducibility of 0.2‰. Lead isotopes in the metallic sulfides were analyzed at ALS. The samples were treated with nitric acid, hydrochloric acid, and hydrofluoric acid, followed by microwave digestion. Lead (Pb) content was measured using high-resolution inductively coupled plasma sector field mass spectrometry (HR-ICP-SFMS). If Pb content was low, separation was required: the digested solution was evaporated to dryness and re-dissolved in 3 mol/L nitric acid, and Pb was separated using Eichrom ion exchange resin (Eichrom Technologies Inc., Lisle, IL, USA). The Pb concentration in the sample solution was adjusted, and an internal standard (Tl) was added to correct for Pb mass fractionation. Pb isotopes were then measured by HR-ICP-SFMS, with data standardized using both the internal standard (Tl isotope ratio) and external calibration (natural Pb standard reference material). Each digested sample was tested twice to obtain its standard deviation (SD), and results are reported together. If the Pb content in the sample was sufficient, the relative standard deviation (RSD) was generally less than 0.1–0.2%. The laboratory-controlled acceptable RSD is <0.2%.

4. Results and Discussion

4.1. Thermal Effects of the ELIP in the Sichuan Basin

The ELIP had a significant impact on the thermal history of the Sichuan Basin, primarily through elevated paleothermal flow and increased paleogeothermal gradients. Notably, during the Late Permian, paleothermal flow in the basin peaked, with heat flow values reaching as high as 124 mW/m² in certain areas. This intense thermal activity played a crucial role in the hydrocarbon generation and maturation processes within the basin [41]. This thermal anomaly closely coincides with the period of activity of the Emeishan mantle plume, suggesting a likely correlation between the anomaly and the ELIP. The elevated heat flow during this period also raised paleothermal indicators, such as vitrinite reflectance (Ro), a widely recognized and extensively used maturity index in hydrocarbon basins, which is crucial for reconstructing thermal histories [40]. Data from boreholes in the Sichuan Basin reveal a notable inflection in vitrinite reflectance (Ro) between the Middle and Upper Permian strata, indicating significant variations in paleothermal regimes. For example, in borehole NJ in central Sichuan, the geothermal gradient shifts from 19 °C/km above the unconformity to 22 °C/km below it. Similarly, in the Y1 well, the gradient increases from 20 °C/km above the unconformity to 29 °C/km below. In the fault-active region of eastern Sichuan, the JS28 well shows a shift from 20 °C/km above the unconformity to 42 °C/km below (Figure 2).

The Ro values provide insights into the maximum temperatures that these strata have experienced, and these variations suggest that the upper and lower sections of the strata underwent distinct thermal histories or achieved maximum paleotemperatures at different geological times [15,42,43]. The significant erosion at the unconformity is likely a factor contributing to the observed variations in Ro. Based on Ro differences, the calculated erosion thickness in these wells ranges from 1000 to 2000 m. However, previous studies suggest that the typical erosion thickness in this area is generally below 500 m [44]. This discrepancy suggests that other factors may also contribute to the Ro variations. In addition to erosion, thermal events associated with dynamic geological processes could lead to Ro discontinuities. The Ro variation at the Middle–Upper Permian boundary (~259 Ma) closely coincides with the peak activity of the Emeishan Large Igneous Province (ELIP) (~260 Ma), indicating that the thermal effects of ELIP likely elevated paleogeothermal gradients in the central and northeastern Sichuan Basin to abnormally high levels during the Middle–Late Permian. These findings highlight the significant role of ELIP in intensifying the basin’s paleothermal conditions, with important implications for hydrocarbon maturity and evolution [45,46].

4.2. Influence of the ELIP on Fluid Activity in the Sichuan Basin

4.2.1. Petrographic Evidence

The Sichuan Basin is rich in oil and gas resources, featuring multiple petroleum reservoirs. These include the second and fourth members of the Sinian Dengying Formation, the Cambrian Longwangmiao Formation, the Permian Changxing Formation, the Triassic Feixianguan and Xujiahe Formations, and the Jurassic Ziliujing and Da’anzhai Formations. The primary activity period of the Emeishan Large Igneous Province (ELIP), which occurred around 260 Ma [1], may have influenced older reservoirs but did not significantly alter younger strata. Consequently, reservoirs older than 260 Ma are currently receiving more focus in exploration efforts. Currently, exploration and development in the Sichuan Basin is primarily focused on the Sinian–Cambrian reservoirs. This study uses the Sinian fourth member of the Dengying Formation (Zdn₄) in the GS101 well of the Anyue gas field as an example to explore the modification effects of fluid activity on reservoir rocks. The Zdn₄ in the GS101 well is mainly composed of mudstone dolomite and algal dolomite, formed in a carbonate reef at the edge of a platform. It exhibits reservoir characteristics such as dissolution pores, caves, and fractures. The dolomite can be categorized into at least three types: Type I Dolomite: This includes mudstone and algal dolomite, which represent the earliest sedimentary processes. It contains mud particles and bioclastic debris, likely influenced by initial surface dolomitization, generally preserving the original rock structure (Figure 3a,b).

Type II Dolomite: This dolomite grows within pores and fractures, often exhibiting “cloudy center-bright rim” structures, reflecting compositional changes during burial diagenesis (Figure 3b). Type III Dolomite: This often occurs in vein forms, characterized by larger crystals and curved crystal faces (Figure 3b), and frequently coexists with quartz, pyrobitumen, and hydrothermal minerals such as galena and sphalerite. Sphalerite appears brownish-red under transmitted light (Figure 3c), galena displays distinct black triangular voids under reflected light (Figure 3d), and pyrobitumen shows strong anisotropy under reflected light (Figure 3e). Scanning electron microscopy reveals flaky structures in the pyrobitumen (Figure 3f), suggesting that the pyrobitumen formed under elevated temperatures. These structures are believed to have resulted from the high-temperature cracking of liquid hydrocarbons, possibly facilitated by hydrothermal activity. Whether this hydrothermal activity is related to the ELIP requires further chronological evidence.

4.2.2. Chronological Evidence of Hydrothermal Minerals

Advancements in isotopic dating techniques have enabled the U–Pb dating of low-U-content carbonate minerals. Researchers have applied laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technology to date hydrothermal dolomite in the Sichuan Basin, and their results are consistent with the active period of the ELIP. For example, Shen et al. (2019) conducted laser in situ U–Pb isotopic dating of coarse crystalline dolomite cement filling cavities in the Dengying Formation at the Gucheng profile, obtaining an age of 259.4 ± 3.0 Ma [47]. Similarly, Su et al. (2020) used LA-ICP-MS technology to date hydrothermal dolomite in the Dengying Formation of the MX119 well in the Anyue gas field, also yielding an age of 259.4 ± 3.0 Ma [48]. Zhang et al. (2024) performed laser in situ U–Pb isotopic dating on dolomite filling low-angle faults in cores from the GS1 (4950.6 m) and MX23 (5214 m) wells, obtaining isotopic ages of 260.1 ± 1.5 Ma, 260 ± 11 Ma, 257.5 ± 1.1 Ma, and 256 ± 17 Ma [13]. In addition to hydrothermal dolomite, Zhu et al. (2023) conducted in situ U–Pb dating on hydrothermal authigenic monazite in the Neoproterozoic Datangpo Formation shale in the Sichuan Basin, obtaining an age of 262.1 ± 6.8 Ma [49]. In particular, Zhu et al. (2023) reported that the Re–Os isotopic analysis of bitumen from the Anyue gas field yielded an isochron age of 492 ± 49 Ma [49]. Although this result carries considerable uncertainties, it offers valuable direct evidence constraining the timing of the earliest stage of oil charging in the paleo-oil pools of the Sichuan Basin. These ages of hydrothermal minerals coincide with the main activity period of the Emeishan Large Igneous Province, indicating that fluid migration events occurred around 260 Ma.

4.2.3. Fluid Inclusion Evidence

Fluid inclusions trapped in minerals provide insights into fluid activity. This study uses the Zdn₄ reservoir from the GS-6 well in the Anyue gas field as an example, showcasing evidence of high-temperature hydrothermal events recorded by fluid inclusions. As shown in Section 4.1, the Zdn₄ in the GS-6 well consists of at least three types of dolomites. Algal dolomite is poorly translucent, making it impossible to observe fluid inclusions. Numerous inclusions are visible in burial dolomite, hydrothermal dolomite, and quartz. In burial dolomite, fluid inclusions are distributed along mineral fractures (Figure 4a,f), indicating that these inclusions formed after the burial dolomite was established. The fluid inclusions are predominantly water inclusions (w-type), with a small number of CH₄ inclusions (C-type) (Figure 4a). In hydrothermal dolomite, fluid inclusions are found along mineral growth zones (Figure 4b,f), indicating concurrent formation with the hydrothermal dolomite. In addition to w-type inclusions, numerous C-type fluid inclusions suggest that natural gas fluids were largely contemporaneous with this phase of hydrothermal dolomite. CH₄ inclusions are also developed in the growth rims of quartz coexisting with dolomite (Figure 4c,f). Laser Raman analysis confirmed the presence of pyrobitumen in some of the CH₄ inclusions (Figure 4d,e), indicating that the cracking of crude oil occurred roughly simultaneously with this hydrothermal activity. Microthermometric measurements on w-type and C-type fluid inclusions yielded trapping temperatures of 249–319 °C and 185–227 °C, respectively. The salinity of w-type inclusions exhibits relatively minor variation, ranging from 18.1% to 23.3% [18]. The trapping temperatures were determined by intersecting of isochores calculated from coexisting saline and methane inclusions within the same quartz grain, likely representing the temperature range during significant methane generation in the reservoir. The higher temperatures correspond to the initial formation of high-temperature hydrothermal fluids at greater depths, before reaching thermal equilibrium with the surrounding strata, or alternatively, w-type fluid inclusions that may have been altered by leakage or deformation, rendering their trapping temperatures unreliable. The lower temperatures reflect conditions in which fluids migrated to the Zdn₄ formation and reached thermal equilibrium with the surrounding strata. Notably, simulations of the burial history of the GS6 well indicate that the maximum temperature at the greatest depth corresponds to 220 °C [18], suggesting that the cracking process related to natural gas formation is not solely dependent on the burial heating process of the reservoir. Furthermore, the highest strata temperatures (220 °C) occurred around 252 Ma, coinciding with a period of rapid sedimentary deep burial and peak geothermal flow, consistent with the primary activity period of the ELIP.

4.2.4. Isotope Evidence of Hydrothermal Mineral

Carbon and oxygen isotopes are crucial geochemical markers for interpreting the genesis of dolomite, as they help us to assess the nature and source of dolomitizing fluids and infer their formation environments and origins. In this study, we obtained the carbon and oxygen isotope characteristics of vein-like dolomite in the Zdn₄ Formation of the GS101 well (5517.5 m), and the results are shown in

Table 1. The C–O, S, and Pb isotope characteristics of vein-like mineral in the Zdn₄ Formation (5517.5 m) of the GS101 well (Min.: minerals; Dol.: dolomite; Sph.: sphalerite; Gal.: galena).

No.	Min.	δ13CVPDB‰	δ18OVPDB‰	No.	Min.	δ13SVCDT‰	208Pb/204Pb	207Pb/204Pb	206Pb/204Pb
GS101-1	Dol.	0.6	−10.0	GS101-S-1	Sph.	37.4	38.019	15.708	17.909
GS101-2	Dol.	0.7	−9.9	GS101-S-2	Sph.	37.1	37.919	15.670	17.888
GS101-3	Dol.	0.8	−10.1	GS101-S-3	Sph.	36.7	37.940	15.664	17.879
GS101-4	Dol.	0.7	−10.1	GS101-G-1	Gal.	33.8	37.991	15.694	17.915
GS101-5	Dol.	0.7	−10.2	GS101-G-2	Gal.	33.4	38.020	15.715	17.936
GS101-6	Dol.	0.6	−10.1	GS101-G-3	Gal.	33.8	38.005	15.702	17.927
GS101-7	Dol.	0.6	−10.0	GS101-G-4	Gal.	33.2	37.993	15.698	17.912
GS101-8	Dol.	0.7	−11.1	GS101-G-5	Gal.	33.5	37.980	15.689	17.904

.

Combining with previous research, dolomites from the Anyue gas field exhibit significant differences in C–O isotope compositions (Figure 5a). Early dolomitization and burial dolomitization processes yield oxygen isotope values that generally fall within or slightly above the seawater oxygen isotope range. Sinian seawater oxygen isotopes (δ¹⁸O_VPDB) range from −36.8‰ to −34.8‰ (δ¹⁸O_VSMOW, −7.0‰ to −5.0‰) [50]. Using the formula summarized by Land (1983), 10³ lnα_{dolomite-seawater} = 2.78 × 10⁶ T⁻² + 0.11 [51], and assuming seawater temperatures of approximately 15–20 °C, the oxygen isotope values of dolomites in equilibrium with Sinian seawater were calculated, yielding δ¹⁸O_VPDB values between −4.3‰ and −1.2‰ (Figure 5a).

However, vein-like dolomites exhibit significantly lower δ¹⁸O values compared to early and burial dolomites, indicating a hydrothermal origin. Meanwhile, carbon isotope values of contemporaneous seawater–sedimentary dolomites range from 4.8‰ to 5.8‰ [55], with hydrothermal dolomites showing markedly lower δ¹³C values.

To confirm the formation conditions of these vein-like minerals, sulfur isotope analysis was conducted on eight metallic sulfides associated with the dolomites at ALS. As shown in Table 1, metallic sulfides coexisting with hydrothermal dolomite in the GS101 well exhibit the following characteristics: Sphalerite’s sulfur isotope values (δ³⁴S) range from 36.1‰ to 37.4‰ (mean 37.1‰), while galena’s sulfur isotope values (δ³⁴S) range from 33.2‰ to 33.8‰ (mean 33.5‰). The δ³⁴S values of sphalerite are notably higher than those of galena, consistent with the δ³⁴S enrichment sequence of the sulfides [56], indicating that sulfur isotope fractionation during the precipitation of these metal sulfides has nearly reached equilibrium. Using the sulfur isotope balance fractionation equation for sphalerite, galena, and H₂S [57]:

10³lnα_sph-H2S = 0.1 × 10⁶/T² (50–700 °C)

(1)

10³lnα_gal-H2S = −0.63 × 10⁶/T² (50–700 °C)

(2)

Substituting δ³⁴S_sph = 37.1‰ and δ³⁴S_gal = 33.5‰ into Equations (1) and (2) yields an equilibrium temperature for the sulfur isotopes of the galena–sphalerite system of approximately 177 °C. This temperature estimate aligns closely with fluid temperatures obtained from fluid inclusions (185–223 °C). These results both verify the reliability of fluid inclusion and sulfur isotope data and further confirm the hydrothermal origin of these vein minerals.

To further investigate the source of the hydrothermal fluid, lead isotopes in the metallic sulfides were analyzed at ALS. As shown in Table 1, the ²⁰⁶Pb/²⁰⁴Pb ratio of sulfides varies between 17.879 and 17.936 (mean 17.909), the ²⁰⁷Pb/²⁰⁴Pb ratio ranges from 15.664 to 15.715 (mean 15.693), and the ²⁰⁸Pb/²⁰⁴Pb ratio is between 37.919 and 38.020 (mean 37.983). The relatively concentrated lead isotope values suggest a uniform source of lead. The lead isotope data were projected onto the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram [54], and the results indicate that the sulfide samples lie close to or above the upper crustal evolution line (Figure 5b), suggesting that the lead source has characteristics consistent with upper crustal materials, most likely from the pre-Sinian metamorphic basement via hydrothermal fluid transport [58].

4.3. Constraints of the ELIP on the Timing of Crude Oil Cracking in the Sinian–Cambrian Reservoirs of the Sichuan Basin

The Anyue gas field, located in the Sichuan Basin, is currently a key focus of exploration and development. The natural gas in the Sinian–Cambrian reservoirs is generally regarded as oil-cracked gas, indicating that it has undergone two stages of reservoir evolution: first as a liquid paleo-oil reservoir, followed by a cracked gas reservoir stage (Figure 6). However, the timing of paleo-oil reservoir formation and the subsequent cracking of oil remains a topic of debate, which presents challenges for future exploration efforts [33,34,38,59,60]. This article seeks to narrow down the timing of oil cracking based on thermal anomalies induced by the Emeishan Large Igneous Province (ELIP) and associated fluid activity. The thermal evolution history of the region plays a critical role in controlling hydrocarbon generation in the source rocks. Qiu et al. (2019) used apatite fission track data to derive heat flow curves for the W28 and NJ wells, reconstructing the thermal evolution history of the Anyue gas field. Their study divides this history into three stages: a low-stable period from 700 Ma to 290 Ma (Pre-Sinian to Early Permian), a peak period from 290 Ma to 200 Ma (Early Permian to Triassic), and a decline period from 200 Ma to the present (Early Jurassic to now) (Figure 6). These stages are characterized by fluctuations in regional heat flow and geothermal gradients [61]. Due to the differing kerogen types in the source rocks, the temperatures at which each enters the oil-generation window vary. However, burial history simulations across multiple wells in the region indicate that the Cambrian Qiongzhusi Formation source rocks had entered the oil-generation phase before 260 Ma. Two additional potential source rocks in the area, the Sinian Doushantuo Formation and the third member of the Dengying Formation, predate the Qiongzhusi Formation and would also have entered the oil generation stage. This suggests that paleo-oil reservoirs existed prior to the main activity of the Emeishan Large Igneous Province. Around 260 Ma, the regional thermal history shifted from a low-stable to a peak period, corresponding with the main activity phase of the Emeishan Large Igneous Province. This shift is marked by a clear inflection point in vitrinite reflectance (Ro) values, which serves as an indicator of thermal maturity in the region. Fluid inclusion data show formation temperatures exceeding 180 °C [18], which would have been sufficient to trigger the cracking of existing paleo-oil reservoirs into natural gas. Additionally, petrological features of quartz–dolomite–sulfide veins, along with C–O isotopic compositions of dolomite, S, and Pb isotope signatures of sulfides and chronology of hydrothermal minerals, provide further evidence of a widespread hydrothermal event in the region around 260 Ma [13,47,48,49]. The occurrence of CH₄ fluid inclusions in hydrothermal dolomite, along with fluid inclusion thermometry showing maximum temperatures of hydrothermal fluid up to 320 °C [18], also indicates that paleo-oil reservoirs were already cracked into natural gas triggering by hydrothermal fluid at the main activity period of the ELIP. In summary, the formation of the Anyue gas field is a classic example of igneous activities reshaping a petroleum system. Before the End-Triassic ELIP event, the Anyue gas field reservoir, located at the apex of a large anticline uplift, accumulated significant crude oil from the surrounding thick source rocks, forming an extensive ancient oil reservoir. It was the ELIP event, rather than high temperatures from deep burial, that triggered rapid and comprehensive cracking of this ancient oil reservoir around 260 million years ago. During this process, the pyrobitumen remained within the reservoir, while natural gas replaced crude oil. The evolution of the Anyue gas field highlights the critical role of episodic geological events as major drivers of reservoir transformation.

5. Conclusions

This study highlights the significant role of the Emeishan Large Igneous Province (ELIP) in shaping the thermal regime, fluid activity, and hydrocarbon evolution in the Anyue gas field of the Sichuan Basin. The key findings are as follows:

• The thermal effects of the ELIP significantly elevated paleogeothermal gradients across the Sichuan Basin, driving the maturation and evolution of hydrocarbons. This intensified thermal regime played a crucial role in cracking pre-existing paleo-oil reservoirs into natural gas.
• Hydrothermal events associated with ELIP activity, as evidenced by mineralogical, isotopic, and fluid inclusion data, facilitated the migration and transformation of fluids. These events were critical in modifying reservoir properties and in the cracking of liquid hydrocarbons under high-temperature conditions.
• The integration of isotopic signatures, fluid inclusion thermometry, and chronological evidence confirms the close temporal relationship between ELIP activity and the rapid evolution of the Anyue gas sield. Around 260 Ma, hydrothermal fluids and elevated temperatures triggered the transition of ancient oil reservoirs into predominantly natural gas reservoirs.

This research underscores the importance of considering large-scale episodic geological events, such as ELIP activity, in petroleum system analysis. Shifting the focus of exploration from traditional hydrocarbon generation processes to the impact of geological modifications on pre-existing reservoirs may provide new insights for hydrocarbon exploration in ancient strata globally.