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Article

Rapid Oil Pyrolysis in Ediacaran Carbonate Reservoirs in the Central Sichuan Basin Revealed by Analysis of the Unique Optical and Raman Spectral Features of Pyrobitumen

by
Yawei Mo
1,2,
Luya Wu
3,
Peng Yang
1,2,* and
Keyu Liu
1,2,*
1
State Key Laboratory of Deep Oil and Gas, China University of Petroleum (East China), Qingdao 266580, China
2
School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China
3
Exploration and Development Research Institute, PetroChina Southwest Oil & Gas Field Company, Chengdu 610041, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(22), 12272; https://doi.org/10.3390/app152212272
Submission received: 26 October 2025 / Revised: 11 November 2025 / Accepted: 17 November 2025 / Published: 19 November 2025
(This article belongs to the Section Energy Science and Technology)
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Versions Notes

Abstract

Analysis of pyrobitumen in reservoirs can yield key information about hydrocarbon evolution, which may provide vital insights for deep- to ultra-deep hydrocarbon exploration in high- to over-mature petroliferous deep basins. The Ediacaran Dengying Formation in the Penglai area of the Sichuan Basin contains large-scale gas reservoirs, where pyrobitumen is extensively present. To understand the hydrocarbon accumulation and alteration processes in these reservoirs, in this study, we systematically investigated the characteristics of the reservoir pyrobitumen using detailed petrographic analysis and laser Raman spectroscopy. The results indicated that four types of reservoir pyrobitumen are present: pyrobitumen with isotropic (type I), mosaic (type II), fibrous (type III), and honeycomb (type IV) textures. Pyrobitumen in the dolomite reservoirs of the Deng 2 and Deng 4 members of the Dengying Formation often co-occurs with hydrothermal minerals, including saddle dolomite, quartz, and fluorite. The equivalent vitrinite reflectance (Rmc Ro%) calculated indicated that the pyrobitumen is over-mature, with Rmc Ro% values ranging from 3.46% to 3.89%. In addition, significant differences were observed in the Raman parameters between the four types of pyrobitumen: type IV shows the greatest degree of structural ordering, while type II exhibits the highest level of disordering, with types I and III exhibiting intermediate structural ordering. Finally, the spatial distribution of the four types of pyrobitumen indicated that hydrothermal pulses driven by the Emeishan Large Igneous Province toward the end of the Permian Period may be primarily responsible for the extensive cracking of paleo-oil pools, causing the development of types II–IV pyrobitumen and gas generation.

1. Introduction

With advancements in deep and ultra-deep petroleum exploration, reservoir preservation has become vital for successful discoveries. Reservoir pyrobitumen is indicative of thermal pyrolysis and has recently received widespread attention from academia and industry in deep hydrocarbon exploration [1]. Reservoir pyrobitumen, which is an altered organic matter derived from thermal alteration of crude oil [2], is a by-product of hydrocarbon thermal evolution, and its analysis can provide critical insights into hydrocarbon alteration [3,4,5,6,7]. Reservoir pyrobitumen has also been widely used in studying oil–gas source correlations [8,9,10], reconstructing hydrocarbon migration and accumulation [11,12,13,14,15], and identifying paleo-oil accumulation and destruction [16,17]. It is noteworthy that the petrophysical properties and chemical composition of reservoir pyrobitumen can record systematic and irreversible changes with temperature variations and, thus, can be used to reconstruct the thermal evolutionary history of oil reservoirs. Reservoir pyrobitumen is often used as a thermal maturity indicator for strata lacking vitrinite or exhibiting high- to over-mature thermal stages [18,19,20,21,22,23,24,25]. However, solid pyrobitumen reflectance is not only controlled by thermal evolution but also largely influenced by its genesis and optical heterogeneities (such as bireflectance), which may lead to substantial variations in the measured reflectance values [26,27,28,29]. Laser Raman spectroscopy is an effective method for characterizing the thermal maturity of organic matters and obtains structural information of organic molecules by detecting their vibration patterns [3,30,31]. The increase in the molecular ordering of pyrobitumen with increasing thermal maturity can be detected with laser Raman spectroscopic signatures. Compared to traditional optical reflectivity methods, laser Raman spectroscopy has the advantage of non-destructive analysis over micro-areas, a low sample demand, and high signal sensitivity.
The Chuanzhong Ancient Uplift in the Sichuan Basin is an important area with increasing gas reserves in deep marine carbonate strata in China. Following the discovery of the large-scale Anyue gas field in the Central Sichuan Uplift in 2011, PetroChina (Beijing, China) achieved a major breakthrough in Well PT1 in the Penglai area on the northern slope of the Central Sichuan Uplift in 2020, with the daily gas production from the Ediacaran Dengying Formation dolomite reservoir reaching 12.2 × 103 m3. In 2021, Well PT101 showed a high-yield gas flow of 2.31 × 106 m3/d, further indicating the potential for forming large-scale integrated gas reservoirs on the slopes of the Central Sichuan Uplift [32]. Despite the tremendous success in deep gas exploration, there are controversies in the geological community, surrounding the formation of gas in the Central Sichuan Uplift. Some scholars believe that the gas and pyrobitumen in the Dengying Formation reservoirs are mainly formed by thermal cracking of paleo-oil reservoirs in the Late Jurassic [33,34,35]. In contrast, others argue that the Central Sichuan Uplift is adjacent to the Emeishan Large Igneous Province (ELIP) and that hydrothermal activity related to ELIP may have been involved in the thermal evolution of paleo-oil reservoirs, thereby influencing the formation and distribution of current gas reservoirs [36,37,38]. This lack of consensus regarding the mechanism underlying gas accumulation in the Central Sichuan Uplift is a crucial barrier to the formulation of exploration strategies and gas resource assessment.
Pyrobitumen is extensively present in the Dengying Formation dolomite reservoirs of the Penglai area on the Central Sichuan Uplift, recording the thermal history of paleo- oil pools and providing key information for unraveling the gas origin mystery. Considering the Dengying Formation gas reservoir in the Penglai gas field as the research target, this study advances prior Raman assessments [25,28,39], which primarily addressed burial-driven maturity in reservoirs; we conducted a systematic analysis of the optical and laser Raman spectral characteristics of the reservoir pyrobitumen by integrating spatial pyrobitumen micro-textures with ELIP hydrothermal events in ultra-deep carbonates to understand the mechanism underlying oil and gas accumulation and cracking in this area. We aimed to provide theoretical support for exploration risk assessment and target selection in the study area, as well as a reference for reconstructing the formation and destruction process of oil and gas reservoirs in similar high- to over-mature deep basins.

2. Geological Setting

The Penglai gas field is located in the Central Sichuan Basin, bordering the Jiulongshan Tectonic Belt to the north, the Anyue gas field to the south, the Deyang–Anyue Rift Trough to the west, and the Nanchong–Longnusi area to the east, covering an area of approximately 20 × 103 km2 [40], as shown in Figure 1b. Structurally, it belongs to the northern slope of the Central Sichuan Uplift within the stable tectonic unit of the Central Sichuan Basin of the Yangtze Plate, with main structural trends oriented NE–SW. The region was influenced by multi-phase tectonic movements, including the Caledonian Orogeny, the Indosinian Orogeny, the Yanshanian Orogeny, and the Himalayan Orogeny, forming complex fault systems and slope structural backgrounds [41,42].
During the depositional period of the Ediacaran Dengying Formation, the Penglai area generally developed platform margin facies. Based on lithology, the area is divided from bottom to top into four members, Deng 1, Deng 2, Deng 3, and Deng 4, which are dominated by microbial dolomite, algal dolomite, and micritic dolomite (Figure 1c). From the second phase of the Tongwan Movement to the Hercynian Orogeny, the Penglai area underwent multiple phases of uplift and erosion, resulting in complex origins of reservoir spaces [43]. As a result of karstification and fault modification, the Deng 2 and Deng 4 members formed high-quality reservoirs and served as primary target layers. Influenced by tectonic movements and hydrothermal fluid activity [44], paleo-oil reservoirs in the Dengying Formation experienced early biodegradation and late thermal cracking destruction [45,46,47,48], forming the current gas reservoir configuration, with dissolution pores filled with substantial pyrobitumen remnants, intercrystalline pores, and fractures [36].

3. Samples and Methods

A total of 57 core samples of Ediacaran Dengying Formation dolomite from 14 wells in the Penglai gas field were selected for microscopic observation and Raman spectroscopic analysis of pyrobitumen (Figure 1). Sampling targeted spatial/depth variability: 32 samples from Deng 2 (depths 5700–7500 m) and 25 samples from Deng 4 (5800–8000 m), across 20 × 103 km2, with multiple samples from each well, ensuring statistical power (Table 1).
Experimental analyses were conducted at the State Key Laboratory of Deep Oil and Gas, China University of Petroleum (East China). Petrographic observations of the optical characteristics of pyrobitumen in thin sections were performed using a Zeiss Axio Imager A2m polarized light microscope (Oberkochen, Germany). The Raman spectroscopic analysis of pyrobitumen was conducted using a HORIBA LabRAM HR Evolution (Kyoto, Japan) high-resolution laser confocal Raman micro-spectrometer (Punjab, India). Key experimental conditions included the following: an argon laser with a 532 nm excitation wavelength, a 500 nm slit width, and a spectral acquisition range of 100–4000 cm−1, an acquisition time of 10 s, and 3 accumulations. Prior to testing, the instrument was calibrated using a silicon wafer (520.7 cm−1 ± 0.02 cm−1).
During the experiments, to assess the effect of different laser intensities on the Raman spectra of pyrobitumen, Raman spectral analyses were conducted on the same pyrobitumen sample using laser intensities of 0.1%, 1%, 3.2%, 5%, and 10% (Figure 2). To quantify uncertainty: by randomly selecting 3–10 different points on the same sample for repeated sampling to avoid accidental errors, the standard deviation of the finally measured Rmc value is SD = ±0.03. The obtained Raman spectra were processed using LabSpec 6 software (6.5.1.24), smoothed with a window size of 8 and an order of 1 and baseline correction. During the calibration, the straight-line method was chosen to linearly optimize the baseline, with 4 Raman shift points at 800, 1000, 1800, and 2000 cm−1 as control points. The baseline was subtracted, and Peak deconvolution fitting was performed using the Gaussian-Lorentzian function in the LabSpec 6 software to obtain the Raman spectral parameters. The results showed that as the laser intensity increased, the pyrobitumen Raman signal strengthened and the signal-to-noise ratio improved. At 3.2% laser intensity, the Raman spectral intensity peaked, and further increases in laser intensity led to declining Raman signal strength (Figure 2a). Simultaneously, ablation pits appeared in the pyrobitumen samples at 3.2% laser intensity (Figure 2b), with pit areas enlarging with increasing laser intensity. The equivalent vitrinite reflectance calculated from the Raman spectra indicated an increasing trend of reflectance with increasing laser intensity (Figure 2c), possibly due to high-intensity laser-induced thermal baking, raising the measured thermal maturity of pyrobitumen. Based on this analysis, 1% laser intensity was selected for subsequent experiments to obtain Raman signals unaffected by thermal ablation and with a high signal-to-noise ratio.
Figure 3 illustrates the derived Raman spectral parameters, which can be primarily categorized into four groups: (1) position and height parameters, including D peak and G peak positions (WD and WG, respectively) and their intensities (ID and IG, respectively); (2) distance parameters, including the D peak and the G peak full width at half maximum (FWHMD and FWHMG, respectively) and the peak separation (RBS); (3) area parameters, including the D peak and G peak areas (SD and SG, respectively); and (4) ratio parameters, including the D/G peak intensity ratio R1 (ID/IG), the D/G peak FWHM ratio R2 (FWHMD/FWHMG), and the D/G peak area ratio R3 (SD/SG).

4. Results

4.1. Optical Characteristics of Pyrobitumen

A substantial amount of pyrobitumen was found to be filling fractures, dissolution pores, and intercrystalline pores in core samples and thin sections obtained from the Ediacaran Dengying Formation gas reservoirs (Figure 4). Pyrobitumen often co-occurs with hydrothermal minerals, such as saddle dolomite, quartz, and fluorite. Based on its optical characteristics, pyrobitumen in the Dengying Formation reservoirs can be categorized into four types (Figure 4d–l): type I (isotropic texture), type II (mosaic texture), type III (fibrous texture), and type IV (honeycomb texture). Type I pyrobitumen shows no anisotropy under reflected light. Type II pyrobitumen exhibits anisotropic particles and can be further subdivided into granular mosaic (particle size 1–20 μm, with cross-extinction visible on larger mesophase spheres) and domain mosaic (particle size >20 μm). Type III pyrobitumen displays alternating dark and bright bands under reflected light. Type IV pyrobitumen has a typical honeycomb micropore structure. In addition, different types of coke pitch have developed in different locations in the study area. Type I pyrobitumen occurs throughout the entire study area, types II and III pyrobitumen mainly develop close to east–west strike slip faults, and type IV pyrobitumen has primarily developed near deep faults next to the terrace edge. Vertically, the Deng 4 member of the Dengying Formation mainly contains types I–III pyrobitumen, dominated by types I and II, while the Deng 2 member of the Dengying Formation contains types I–IV pyrobitumen, dominated by type III, with types I, IV, and III being comparable (Figure 5).

4.2. Raman Spectra and Quantitative Results

A total of 428 pyrobitumen Raman spectra and their quantitative parameters were obtained (Table 2; also see Supplementary Table S1 for the full dataset). Statistically, the D peak positions were concentrated around 1325~1331 cm−1, and the G peak positions were concentrated around 1590~1593, 1593~1595, and 1595~1598 cm−1. The FWHMD ranged widely from 144 to 228 cm−1, averaging 197 cm−1, while the FWHMG was narrower, in a range of 50~76 cm−1, averaging 62 cm−1. The RBS averaged 265 cm−1, in a range of 257~272 cm−1. For areas, the SD (larger) range was 65,872~135,871, averaging 100,089, while the SG (smaller) range was 32,501~64,301, averaging 49343. For ratio parameters, R1 ranged from 0.63 to 0.84, averaging 0.73; R2 ranged from 2.60 to 3.88, averaging 3.15; and R3 ranged from 1.74 to 2.42, averaging 2.03.

5. Discussion

Given the generally high thermal maturity of pyrobitumen samples in this study, consisting primarily of over-mature carbonaceous solid organic matter [25,28], we used the correlation equation for calculating the thermal evolution degree of pyrobitumen based on Raman spectroscopy parameters established by Liu et al. (2013) [28]:
Rmc Ro% = 1.1659 R1 + 2.7588
The equivalent vitrinite reflectance of pyrobitumen samples obtained from the Ediacaran Dengying Formation gas reservoirs in the Penglai area was obtained using the R1 parameter.

5.1. Thermal Maturity Distribution

The thermal maturity calculation results showed that the equivalent vitrinite reflectance of pyrobitumen in the study area is mainly distributed between 3.46% and 3.89%, with an average value of 3.61% (Figure 6), which is consistent with the results for the Dengying Formation reservoir obtained previously (3.03%~3.71%) [36]. The equivalent vitrinite reflectance of type I pyrobitumen ranges from 3.47% to 3.74%, with a peak value of approximately 3.58%, and that of type II pyrobitumen ranges from 3.48% to 3.69%, peaking at around 3.58%. For type III pyrobitumen, the equivalent vitrinite reflectance ranges from 3.52% to 3.89%, with a peak of about 3.62%, whereas for type IV pyrobitumen, it varies between 3.46% and 3.74%, peaking at approximately 3.70%. These findings indicate that the equivalent vitrinite reflectance values calculated for different types of pyrobitumen are relatively close. A cross-plot of the equivalent vitrinite reflectance of pyrobitumen with the reservoir depth showed no clear correlation between the two parameters (R2 = 0.0011), indicating maturity decoupling from burial (Figure 7).

5.2. Relationships Between Types of Pyrobitumen and Raman Parameters

To reveal Raman spectral differences between different types of pyrobitumen, Raman spectral parameters were compared (Table 3). The four types of pyrobitumen show distinct intergroup differentiation, including peak positions (WD, WG), FWHM (FWHMD, FWHMG), peak separation (RBS), and ratios (R1, R2, R3) (Figure 8 and Figure 9). Overall, the Raman spectral parameters of types I~III pyrobitumen are relatively similar, whereas type IV exhibits marked differences from the other three types. To quantify the differences between the various types of pyrobitumen, the t-test was conducted to assess intergroup disparities (Table 4). The results indicated that type IV pyrobitumen is significantly different from the other three types across all key parameters (|t| > 5, p < 0.0001). Moreover, although there are differences in some parameters between type II and III pyrobitumen (such as FWHMD, R1, R2, R3, p < 0.005), the overall similarity between the two is high (Figure 10).
Regarding the peak position and separation parameters, WD represents the vibrational mode of disordered carbon structures (sp3 carbon), indicating defects at the edges of aromatic rings or amorphous carbon in pitch, while WG represents the vibrational mode of ordered graphitic carbon structures (sp2 carbon), reflecting the planar arrangement of aromatic rings in pitch. RBS is defined as the difference in wavenumber between the D and G peaks, which can quantify the relative separation between ordered and disordered structures. An increase in the difference in wavenumber indicates the evolution of the carbon structure from disorder to graphitization [31,49,50,51]. Among the four types of pyrobitumen samples, the RBS averages 265.81 ± 4.36 cm−1 for type I, 263.72 ± 4.97 cm−1 for type II, 264.59 ± 3.74 cm−1 for type III, and 269.68 ± 2.05 cm−1 for type IV. This finding shows that the RBS of type IV pyrobitumen is the highest, with the highest degree of separation between ordered and disordered structures and the highest degree of graphitization; the RBS of type II pyrobitumen and the degree of graphitization are the lowest; and the RBS of types I and III pyrobitumen is between that of types IV and II. The peak width parameters (FWHMD, FWHMG) are sensitive indicators for characterizing the uniformity and crystallinity of an asphalt structure. The larger the peak width parameter, the more disordered or defective the pyrobitumen crystal structure [49,52]. Type IV pyrobitumen records the narrowest lines (FWHMD = 148.47 ± 8.83 cm−1, FWHMG = 54.50 ± 2.42 cm−1), showing the best crystallinity, whereas type II records the widest lines (FWHMD = 212.89 ± 19.55 cm−1, FWHMG = 65.53 ± 8.59 cm−1), consistent with a highly disordered framework; types I and III occupy the middle ground. Regarding ratio parameters, R1 (intensity ratio ID/IG) represents the intensity ratio of the D peak (disordered/defective structures) to the G peak (ordered graphitic structures). Elevated R1 values typically denote higher disorder and lower thermal maturity, yet at geologically high thermal maturity stages (e.g., Ro > 3%), recrystallization may reverse this trend, with higher values signifying greater ordering and thermal maturity [53,54]. R2, the FWHM ratio (FWHMD/FWHMG), assesses the relative disorder of the D peak compared to the G peak. A higher R2 value indicates a broader D peak, suggestive of greater structural defects commonly associated with rapid thermal alteration [53]. R3, the area ratio (SD/SG), integrates intensity and width to reflect overall aromatic polymerization and the carbon domain size. Elevated R3 values denote more complex and disordered area distributions [54]. R1 is the highest in type IV pyrobitumen (0.80 ± 0.04) and the lowest in type II pyrobitumen (0.71 ± 0.05); R1 is 0.72 ± 0.05 for type I pyrobitumen and 0.74 ± 0.06 for type III, positioning them between types II and IV, with significant intergroup differences (most p < 0.001). Given that the thermal maturity of the pyrobitumen samples in this study ranged from 3.46% to 3.89% (all > 3.0%), as mentioned above, at geologically high maturity stages (e.g., Ro > 3%), recrystallization may reverse this trend, with higher values signifying greater ordering and maturity. Thus, type IV pyrobitumen exhibits higher ordering and thermal maturity. R2 for types I~IV pyrobitumen is 3.20 ± 0.32, 3.28 ± 0.36, 3.12 ± 0.26, and 2.72 ± 0.14, respectively; the lowest R2 value for type IV pyrobitumen indicates more balanced and ordered crystal structures, whereas the highest value for type II pyrobitumen suggests that the disordered D peak is substantially broader than the ordered G peak, further evidencing high structural disorder. R3 is the highest in type IV pyrobitumen and the lowest in type II pyrobitumen, with types I and III showing similarity. Although an elevated R3 for type IV pyrobitumen implies a higher proportion of defective areas, integration with the aforementioned peak width (FWHM) and intensity ratio (R1) parameters suggests that recrystallization in highly mature samples causes the area parameters to be predominantly influenced by peak intensities (ID and IG). In summary, the Raman spectral parameters of type IV pyrobitumen differ most markedly from the other three types (all p < 0.0001), signifying its superior ordering and thermal maturity; type II is the most disordered, with types I and III being intermediate between types IV and II.

5.3. Hydrothermal Effects on Reservoir Pyrobitumen

The thermal maturity of reservoir pyrobitumen represents the cumulative manifestation of time–temperature effects on organic matter during deep burial. Conventionally, the thermal maturity of organic matter progressively increases with increasing burial depth and formation temperature, and so does the corresponding equivalent vitrinite reflectance value of pyrobitumen. The equivalent vitrinite reflectance of pyrobitumen in the Dengying Formation reservoirs in the Penglai gas field is 3.46–3.89%, denoting an over-mature thermal stage. Concurrently, reservoir pyrobitumen displays a spectrum of structural morphologies, encompassing isotropic, mosaic, fibrous, and honeycomb textures, under reflected light. Based on previous research results, there are significant differences in the pyrolysis and condensation temperatures for different types of pyrobitumen [55]: type I exhibits the lowest and type III the highest pyrolysis temperature, while the pyrolysis temperature of type II is in between. Type IV pyrobitumen is believed to have been formed under high-pressure coking conditions [56], where high pressure fills the pyrobitumen with gas, creating internal honeycomb micropores. It is estimated that the reservoir pressure may go up to 40 MPa during honeycomb formation [37]. Prior investigations also indicate that the formation of mosaic and fibrous pyrobitumen should meet the following two conditions: extremely high temperatures and rapid temperature fluctuations [57]. The threshold temperature for the formation of mosaic and fibrous pyrobitumen is generally considered to be greater than 300 °C, whereupon petroleum cracking releases gases in different structural types on the surface of liquid asphalt [58,59]. Furthermore, the reservoir temperature should fluctuate rapidly to preserve the structure of the asphalt surface, as prolonged high-temperature conditions eliminate the bubble marks on the surface of liquid asphalt. However, 1D basin modeling results in the Central Sichuan region reveal that the Dengying Formation in the Penglai area has attained depths exceeding 8000 m, yet the maximum paleo-temperatures endured by the reservoirs remain substantially below 250 °C [39,60,61,62,63]. This basically matches the burial history results of the PT01 well in this study, with the maximum burial depth of the reservoir at 8000 m corresponding to a maximum paleo-temperature around 240 °C (Figure 11), demonstrating that normative sedimentary subsidence and attendant thermal escalation cannot yield the mosaic and fibrous pyrobitumen textures in the Dengying Formation reservoirs. We can deduce that the thermal cracking of crude oil in the Dengying Formation reservoirs in the Penglai gas field, concomitant with the origination of mosaic, fibrous, and honeycomb pyrobitumen textures, was plausibly modulated by hydrothermal incursion.
The Sichuan Basin has experienced multiple magmatic-hydrothermal events, with the Emeishan Large Igneous Province (ELIP) exhibiting the most intense activity and significant mineralization effects [64,65,66]. ELIP is positioned along the western margin of the Yangtze Craton, with its principal activity occurring between 260 Ma and 250 Ma [67]. ELIP basalts are widely distributed across Yunnan, Guizhou, and Sichuan Provinces, encompassing an exposed area of 500 × 103 km2, delimited to the west by the Ailaoshan–Red River Fault and to the northwest by the Longmenshan Fault (Figure 1a). Depending on the varying degrees of influence on regional strata, ELIP can be subdivided from west to east into three zones: inner, intermediate, and outer; the Penglai gas field resides within the outer zone of ELIP. The Emeishan mantle plume activity triggered extensive hydrothermal events across the upper Yangtze region, engendering numerous hydrothermal metallic deposits along the southwestern margin of the Yangtze Plate and causing prolific deposition of saddle-shaped dolomite, quartz, and fluorite hydrothermal minerals. Moreover, fluid inclusion analysis studies on the Dengying Formation reservoirs have documented hydrothermal fluid temperatures reaching 319 °C [63], underscoring the fact that ELIP-driven hydrothermal intrusions likely represent the critical drivers of rapid paleo-oil cracking into gas in the Penglai area, as well as the formation of mosaic and fibrous pyrobitumen. Concurrently, clumped isotope investigations of methane have revealed that the natural gas formation temperature within the Dengying Formation reservoirs exceeds 250 °C [68], which markedly surpasses the maximum burial temperature of these reservoirs, thereby further corroborating the role of hydrothermal fluids in facilitating reservoir crude oil cracking into gas and its subsequent accumulation. In the study area, the platform margin and E-W strike-slip faults promote the migration of ELIP (260–250 Ma) fluids to the Dengying reservoir. Sampling points (Figure 1, Table 1) cluster along these faults: most are within 2 km of the E-W strike-slip fault (favorable for Types II-III), and Type IV asphalt development well locations are close to deep faults at the platform margin. Combined with (Figure 7) the decoupling of asphalt maturity and burial depth, this verifies that the key factor in the spatial distribution pattern of asphalt is hydrothermal intrusion rather than burial thermal gradient. Furthermore, similar textures (mosaic/fibrous/honeycomb) occur in the Gaoshiti-Moxi outer zone [36,55,63], with consistent Raman (Rmc = 3.0–3.7%, R1 = 0.70–0.82), and the pyrobitumen maturity basically matches data obtained in this study, indicating that the hydrothermal activity of the Emeishan Large Igneous Province in the Late Permian has a substantial impact on the hydrocarbon accumulation in the Central Sichuan Basin.
Findings show that high-temperature hydrothermal intrusion may have induced thermal cracking of oil in paleo-oil reservoirs and the formation of types II–IV pyrobitumen (Figure 12). Type IV pyrobitumen predominantly develops adjacent to platform-margin faults, where deep-seated hydrothermal eruptions along major fault zones could engender high-pressure, high-temperature conditions that facilitate ordered structural rearrangement within type IV pyrobitumen, analogous to laboratory coking processes. In contrast, types II and III pyrobitumen chiefly occur near E–W-trending strike-slip faults, wherein hydrothermal pulses infiltrate strata via subsidiary strike-slip faults; under intense high-temperature roasting, these undergo rapid, intense thermal pulse events, yielding type II pyrobitumen characterized by augmented disordered carbon structural defects and heterogeneous aromatization. Type III pyrobitumen exhibits similar characteristics to type II pyrobitumen but has marginally enhanced ordering, potentially signifying protracted hydrothermal activity that permits further internal structural evolution and partial restoration of order. Type I pyrobitumen appears to be subjected to normal burial thermal maturation and is ubiquitously distributed across the study area.
Overall, hydrothermal activity related to the Late Permian ELIP likely exerted a substantial influence on the formation and destruction of oil and gas reservoirs in the Central Sichuan Basin region. The heterogeneity of hydrothermal activity leads to the spatial combination of hydrothermal minerals and different types of pyrobitumen in these reservoirs. In some areas with relatively developed hydrothermal activity, types II–IV pyrobitumen are usually found, while in areas with little or no hydrothermal activity, type I pyrobitumen often dominates.

6. Conclusions

  • Abundant pyrobitumen fills fractures, dissolution pores, and intercrystalline pores in the Ediacaran Dengying Formation reservoirs in the Penglai area, co-occurring with hydrothermal minerals, including saddle dolomite, quartz, and fluorite. Based on optical features, four types of pyrobitumen can be identified: type I (isotropic texture), type II (mosaic texture), type III (fibrous texture), and type IV (honeycomb texture).
  • The equivalent vitrinite reflectance of reservoir pyrobitumen ranges from 3.46% to 3.89%, signifying an over-mature thermal maturity stage; however, this parameter exhibits minimal correlation with burial depths, implying that thermal maturation is not exclusively attributable to burial processes. There are significant differences in the Raman parameters between the four types of pyrobitumen: type IV manifests the highest degree of structural ordering, type II exhibits the greatest structure disordering, types I and III show intermediate structural ordering, types II-IV pyrobitumen are distributed near the fault zones, and Type I pyrobitumen is developed throughout the area, collectively reflecting a divergent structural evolution under varying hydrothermal regimes.
  • Late Permian ELIP-derived hydrothermal pulses constitute the principal drivers of paleo-oil cracking into gas and the origin of hydrothermally structured pyrobitumen, exerting a significant influence on the contemporary gas reservoir configuration.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/app152212272/s1, Table S1: Summary of Raman spectral parameters of different types of pyrobitumen in the Dengying Formation reservoirs in the Penglai area.

Author Contributions

Methods, P.Y. and Y.M.; software, P.Y. and Y.M.; resources, K.L. and L.W.; writing—original draft, Y.M.; writing—review and editing, Y.M., P.Y., and K.L.; project administration, L.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Exploration and Development Research Institute, Southwest Oil and Gas Field Company, China National Petroleum Corporation (code: XNS Exploration Research Institute JS2024-146), and the PetroChina Science & Technology Major Project (no. 2023ZZ02).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available from the corresponding author upon request. The data are not publicly available due to privacy.

Acknowledgments

We gratefully acknowledge Gang Zhou, Che Xie, and Yan Wei of PetroChina Southwest Oil & Gas Field Company for their essential contributions through fruitful discussions and their support in sampling and data collation.

Conflicts of Interest

Authors Luya Wu are employed by the PetroChina Southwest Oil & Gas Field Company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Location map of the study area showing well locations (a), fault development and well location distribution of the Dengying Formation (b), and generalized stratigraphic column of the Ediacaran System in the Penglai area of the Central Sichuan Basin (c).
Figure 1. Location map of the study area showing well locations (a), fault development and well location distribution of the Dengying Formation (b), and generalized stratigraphic column of the Ediacaran System in the Penglai area of the Central Sichuan Basin (c).
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Figure 2. Pyrobitumen characteristics using Raman spectroscopy under different laser intensity (energy) levels. (a) Raman spectra of the same pyrobitumen sample at different laser intensities, (b) pits left by laser ablation on pyrobitumen, and (c) relationship between equivalent vitrinite reflectance of the same pyrobitumen sample and laser intensity during acquisition.
Figure 2. Pyrobitumen characteristics using Raman spectroscopy under different laser intensity (energy) levels. (a) Raman spectra of the same pyrobitumen sample at different laser intensities, (b) pits left by laser ablation on pyrobitumen, and (c) relationship between equivalent vitrinite reflectance of the same pyrobitumen sample and laser intensity during acquisition.
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Figure 3. Schematic diagram illustrating Raman spectra parameters defined for thermal maturity calculation.
Figure 3. Schematic diagram illustrating Raman spectra parameters defined for thermal maturity calculation.
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Figure 4. Petrographic characteristics of pyrobitumen in the Dengying Formation reservoirs, Penglai area, Central Sichuan Basin. (a) Blocky pyrobitumen filling dissolution pores (Well PS106, Deng 4, 5854.28 m); (b) blocky pyrobitumen filling dissolution pores (Well PT101, Deng 2, 5749.80 m); (c) blocky pyrobitumen filling fractures (Well DT1, Deng 2, 7508.75 m); (d,e) type I isotropic pyrobitumen filling fractures (Well DT1, Deng 4, 6083.59 m; (d): transmitted light, (e): reflected light); (f) fluorite filling between dolomite minerals; (g–i) type II mosaic pyrobitumen filling dissolution pores; (h) granular mosaic; (i) domain mosaic; (g,h) Well PT102, Deng 2, 5877.78 m; (i) Well PT102, Deng 2, 5877.35 m; (g,i) reflected light, f: transmitted light; (j,k) type III fibrous pyrobitumen filling intercrystalline pores (Well PT102, Deng 2, 5857.23 m; (j) transmitted light, k: reflected light); (l) type III fibrous pyrobitumen filling intercrystalline pores among quartz and dolomite hydrothermal minerals (Well PT02, Deng 2, 5853.86 m); (m,n) type IV honeycomb pyrobitumen filling intercrystalline pores (Well ZJ2, Deng 2, 6551.35 m; (m) transmitted light, n: reflected light); and (o) pyrite filling fractures between dolomite minerals (Well PT102, Deng 2, 5849.43 m). Bit: pyrobitumen; Bit I: type I isotropic; Bit II: type II mosaic; Bit III: type III fibrous; Bit IV: type IV honeycomb; D: dolomite; SD: saddle dolomite; Q: quartz; Fl: fluorite; Py: pyrite.
Figure 4. Petrographic characteristics of pyrobitumen in the Dengying Formation reservoirs, Penglai area, Central Sichuan Basin. (a) Blocky pyrobitumen filling dissolution pores (Well PS106, Deng 4, 5854.28 m); (b) blocky pyrobitumen filling dissolution pores (Well PT101, Deng 2, 5749.80 m); (c) blocky pyrobitumen filling fractures (Well DT1, Deng 2, 7508.75 m); (d,e) type I isotropic pyrobitumen filling fractures (Well DT1, Deng 4, 6083.59 m; (d): transmitted light, (e): reflected light); (f) fluorite filling between dolomite minerals; (g–i) type II mosaic pyrobitumen filling dissolution pores; (h) granular mosaic; (i) domain mosaic; (g,h) Well PT102, Deng 2, 5877.78 m; (i) Well PT102, Deng 2, 5877.35 m; (g,i) reflected light, f: transmitted light; (j,k) type III fibrous pyrobitumen filling intercrystalline pores (Well PT102, Deng 2, 5857.23 m; (j) transmitted light, k: reflected light); (l) type III fibrous pyrobitumen filling intercrystalline pores among quartz and dolomite hydrothermal minerals (Well PT02, Deng 2, 5853.86 m); (m,n) type IV honeycomb pyrobitumen filling intercrystalline pores (Well ZJ2, Deng 2, 6551.35 m; (m) transmitted light, n: reflected light); and (o) pyrite filling fractures between dolomite minerals (Well PT102, Deng 2, 5849.43 m). Bit: pyrobitumen; Bit I: type I isotropic; Bit II: type II mosaic; Bit III: type III fibrous; Bit IV: type IV honeycomb; D: dolomite; SD: saddle dolomite; Q: quartz; Fl: fluorite; Py: pyrite.
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Figure 5. Histogram of pyrobitumen types occurring in the Dengying Formation reservoirs, Penglai area, Central Sichuan Basin.
Figure 5. Histogram of pyrobitumen types occurring in the Dengying Formation reservoirs, Penglai area, Central Sichuan Basin.
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Figure 6. Histogram of equivalent vitrinite reflectance for different types of pyrobitumen samples obtained from the Dengying Formation reservoirs, Penglai area, Central Sichuan Basin.
Figure 6. Histogram of equivalent vitrinite reflectance for different types of pyrobitumen samples obtained from the Dengying Formation reservoirs, Penglai area, Central Sichuan Basin.
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Figure 7. Cross-plot of equivalent vitrinite reflectance vs. reservoir depth for different types of pyrobitumen from the Dengying Formation reservoirs in the Penglai area.
Figure 7. Cross-plot of equivalent vitrinite reflectance vs. reservoir depth for different types of pyrobitumen from the Dengying Formation reservoirs in the Penglai area.
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Figure 8. Radar chart of average Raman parameters of four types of pyrobitumen.
Figure 8. Radar chart of average Raman parameters of four types of pyrobitumen.
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Figure 9. Violin chart of Raman parameters for four types of pyrobitumen.
Figure 9. Violin chart of Raman parameters for four types of pyrobitumen.
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Figure 10. Radar chart of t-values from t-test results for Raman spectral parameters of different types of pyrobitumen.
Figure 10. Radar chart of t-values from t-test results for Raman spectral parameters of different types of pyrobitumen.
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Figure 11. Burial history of Well PT101 in the Central Sichuan Basin.
Figure 11. Burial history of Well PT101 in the Central Sichuan Basin.
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Figure 12. Schematic diagram of paleo-oil cracking to gas mode during the ELIP event in the Central Sichuan Basin.
Figure 12. Schematic diagram of paleo-oil cracking to gas mode during the ELIP event in the Central Sichuan Basin.
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Table 1. Summary of samples in the Dengying Formation reservoirs from the Penglai area.
Table 1. Summary of samples in the Dengying Formation reservoirs from the Penglai area.
WellCoordinateStageNumberDeepTypesDistance from Fraction
XY
PT101336972218531178Z2dn275743.26II&III2~5 km
5744.25
5749.80
5772.43
5773.33
5775.75
5879.50
PT1337178618526227Z2dn255753.62I&II&III2~5 km
5761.13
5773.12
5781.83
5787.30
PT102337454518531202Z2dn275853.86II&III<2 km
5857.23
5866.60
5874.42
5874.48
5877.83
5877.78
PS106337674018514474Z2dn435854.28II&III2~5 km
5862.34
5870.42
ZJ103337942818510724Z2dn225873.09I2~5 km
5897.48
DS1322186418539106Z2dn216073.3III2~5 km
DB1340504218531999Z2dn426409.29I<2 km
6410.10
ZJ2339261618492860Z2dn256546.65IV2~5 km
6547.13
6550.86
6551.35
6552.81
PS13341558418529656Z2dn416709.80I5~10 km
PS15341290018520554Z2dn476774.03I&II&III2~5 km
6780.01
6787.76
6787.68
6787.76
6928.60
6929.40
PS9343451418580939Z2dn446897.35I2~5 km
6970.66
6979.46
6986.34
PS8343368518567381Z2dn437047.64I2~5 km
7051.8
7061.41
PS1343071518529336Z2dn437258.17II2~5 km
7260.88
7264.85
DT-1340561518482676Z2dn257874.97I&IV2~5 km
7488.94
7508.75
7509.96
PS6345866118542736Z2dn427903.52I<2 km
7909.00
Table 2. Summary of Raman spectral parameters of different types of pyrobitumen in the Dengying Formation reservoirs in the Penglai area.
Table 2. Summary of Raman spectral parameters of different types of pyrobitumen in the Dengying Formation reservoirs in the Penglai area.
WellStageDepthTypesWD (cm−1)WG (cm−1)IDIGFWHM (D/cm−1)FWHMG (/cm−1)SDSGRBS/cm−1R1R2R3Rmc Ro (%)
PT101Z2dn25772.43III 1332.11599.89659.576856.033159.00255.523115,979566,608.3267.790.772.862.403.66
PT1Z2dn25753.62III1331.111593.82313.9419.974205.08865.943181,291.539,897.1262.710.753.112.043.63
5753.62II1331.391592.66287.772381.109217.9470.733675,266.136,925.8261.270.763.082.043.64
5761.13I1327.511594.1391.805527.542197.17461.898399,807.646,957.4266.590.743.192.133.62
PS102Z2dn25857.23III1331.871592.83338.538462.916215.07770.349884,35145,140.5260.960.733.061.873.61
5866.6II1329.451593.04378.895511.192208.42367.199795,202.246,673.3263.590.743.102.043.62
PS106Z2dn45854.28II1332.981589.96315.066435.414228.94474.037680,513.844,354.4256.980.723.091.823.60
5854.28III1330.041591.17414.509590.712228.94469.0704104,70256,226.2261.130.703.311.863.58
ZJ103Z2dn25873.09I1327.891597.81552.33706.586156.53353.128131,79054,324.1269.920.782.952.433.67
DS1Z2dn26073.3III1338.21601.3146.483152.336120.15653.909625,871.911,301.2263.10.962.232.293.88
DB1Z2dn46409.29I1329.431594.67408.351629.028219.08761.285296,902.554,685.8265.240.653.571.773.52
ZJ2Z2dn26546.65IV1330.291596.21302.459363.696153.57858.767870,849.431,255.3265.920.832.612.273.73
PS15Z2dn46774.03I1330.651591.2309.556420.887222.14170.398679,005.840,461.4260.550.743.161.953.62
6786.37II1328.821592.51409.194540.256200.85266.4065102,14650,766.4263.690.763.022.013.64
6787.76III1328.621591.34480.767651.373228.94472.3626121,09463,410.4262.720.743.161.913.62
PS9Z2dn46897.35I1329.631593.51339.336503.497222.70963.326586,591.646,703.6263.880.673.521.853.54
PS8Z2dn47051.8I1329.21593.11369.35552.988226.60463.511993,726.651,130.4263.910.673.571.833.54
PS1Z2dn47258.17II1328.471593.69365.983537.682216.66262.026990,594.648,877.7265.220.683.491.853.55
DT-1Z2dn27488.94IV1328.151597.54401.382490.311152.66755.339493,477.839,682.4269.390.822.762.363.71
7488.94I1326.731597.45415.609501.387140.27451.997389,146.438,053.9270.720.832.702.343.73
PS6Z2dn47903.52I1327.311599.53556.685761.452151.78848.6566128,92054,980.8272.220.733.122.343.61
Note: Only one representative dataset of each type per well is shown here; the complete data are listed in Supplementary Table S1.
Table 3. Summary of Raman spectral parameters of different pyrobitumen types in the Dengying Formation reservoirs in the Penglai area.
Table 3. Summary of Raman spectral parameters of different pyrobitumen types in the Dengying Formation reservoirs in the Penglai area.
TypesWD (cm−1)WG (cm−1)FWHMD (cm−1)FWHMG (cm−1)RBS (cm−1)R1R2R3Rmc Ro (Range) (%)
I
(n = 151)		1328.66 ± 2.241594.47 ± 2.69196.24 ± 34.2961.03 ± 7.59265.81 ± 4.360.72 ± 0.053.20 ± 0.322.03 ± 0.223.60 ± 0.06
(3.47–3.74)
II
(n = 107)		1329.69 ± 3.001593.41 ± 2.71212.89 ± 19.5565.53 ± 8.59263.72 ± 4.970.71 ± 0.053.28 ± 0.361.94 ± 0.153.59 ± 0.06
(3.48–3.69)
III
(n = 128)		1329.92 ± 2.821594.51 ± 2.71200.98 ± 28.0864.28 ± 6.64264.59 ± 3.740.74 ± 0.063.12 ± 0.262.03 ± 0.18 3.62 ± 0.07
(3.52–3.89)
IV
(n = 42)		1327.37 ± 1.741597.05 ± 1.24148.47 ± 8.8354.50 ± 2.42269.68 ± 2.050.80 ± 0.042.72 ± 0.142.27 ± 0.15 3.70 ± 0.04
(3.46–3.74)
Total
(n = 428)		1329.17 ± 2.71 1594.47 ± 2.77 197.13 ± 32.4562.49 ± 7.92 265.30 ± 4.50 0.73 ± 0.06 3.15 ± 0.34 2.03 ± 0.21 3.61 ± 0.07
(3.46–3.89)
Table 4. t-test results for Raman spectroscopy parameters of different types of pyrobitumen in the Dengying Formation reservoirs in the Penglai area.
Table 4. t-test results for Raman spectroscopy parameters of different types of pyrobitumen in the Dengying Formation reservoirs in the Penglai area.
ParameterI vs. III vs. IIII vs. IVII vs. IIIII vs. IVIII vs. IV
WDt = −3.01, p = 0.003
**		t = −4.08, p < 0.001
***		t = 3.98, p < 0.001
***		t = −0.60, p = 0.55
ns		t = 5.87, p < 0.001
***		t = 6.96, p < 0.001
***
WGt = 3.11, p = 0.002
**		t = −0.12, p = 0.90
ns		t = −8.87, p < 0.001
***		t = −3.10, p = 0.002
**		t = −11.22, p < 0.001
***		t = −8.29, p < 0.001
***
FWHMDt = −4.94, p < 0.001
***		t = −1.27, p = 0.21t = 15.38, p < 0.001
***		t = 3.82, p < 0.001
***		t = 27.65, p < 0.001
***		t = 18.55, p < 0.001
***
FWHMGt = −4.35, p < 0.001
***		t = −3.81, p < 0.001
***		t = 9.05, p < 0.001
***		t = 1.23, p = 0.22
ns		t = 12.11, p < 0.001
***		t = 14.06, p < 0.001
***
RBSt = 3.50, p < 0.001
***		t = 2.52, p = 0.012
*		t = −8.14, p < 0.001
***		t = −1.49, p = 0.14
ns		t = −10.36, p < 0.001
***		t = −11.13, p < 0.001
***
R1t = 1.58, p = 0.11
ns		t = −2.99, p = 0.003
**		t = −10.82, p < 0.001
***		t = −4.18, p < 0.001
***		t = −11.48, p < 0.001
***		t = −7.37, p < 0.001
***
R2t = −1.84, p = 0.07
ns		t = 2.30, p = 0.022
*		t = 14.19, p < 0.001
***		t = 3.84, p < 0.001
***		t = 13.67, p < 0.001
***		t = 12.68, p < 0.001
***
R3t = 3.91, p < 0.001
***		t = 0.00, p = 1.00
ns		t = −8.20, p < 0.001
***		t = −4.18, p < 0.001
***		t = −12.08, p < 0.001
***		t = −8.55, p < 0.001
***
Rmc Ro (%)t = 0.32, p = 0.189
ns		t = −2054, p = 0.012
ns		t = −12.71, p < 0.001
***		t = −3.53, p < 0.001
***		t = −12.99, p < 0.001
***		t = −9.15, p < 0.001
***
Note: The t-value denotes the magnitude of the difference (positive values indicate group 1 > group 2), the p-value, and the significance levels (*** p < 0.001; ** p < 0.01; * p < 0.05; ns: not significant).
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Mo, Y.; Wu, L.; Yang, P.; Liu, K. Rapid Oil Pyrolysis in Ediacaran Carbonate Reservoirs in the Central Sichuan Basin Revealed by Analysis of the Unique Optical and Raman Spectral Features of Pyrobitumen. Appl. Sci. 2025, 15, 12272. https://doi.org/10.3390/app152212272

AMA Style

Mo Y, Wu L, Yang P, Liu K. Rapid Oil Pyrolysis in Ediacaran Carbonate Reservoirs in the Central Sichuan Basin Revealed by Analysis of the Unique Optical and Raman Spectral Features of Pyrobitumen. Applied Sciences. 2025; 15(22):12272. https://doi.org/10.3390/app152212272

Chicago/Turabian Style

Mo, Yawei, Luya Wu, Peng Yang, and Keyu Liu. 2025. "Rapid Oil Pyrolysis in Ediacaran Carbonate Reservoirs in the Central Sichuan Basin Revealed by Analysis of the Unique Optical and Raman Spectral Features of Pyrobitumen" Applied Sciences 15, no. 22: 12272. https://doi.org/10.3390/app152212272

APA Style

Mo, Y., Wu, L., Yang, P., & Liu, K. (2025). Rapid Oil Pyrolysis in Ediacaran Carbonate Reservoirs in the Central Sichuan Basin Revealed by Analysis of the Unique Optical and Raman Spectral Features of Pyrobitumen. Applied Sciences, 15(22), 12272. https://doi.org/10.3390/app152212272

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