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Article

Organic Matter Source, Fluid Migration, and Geological Significance of Stylolites in Organic-Lean Carbonate Rocks: A Case from the Sichuan Basin

by
Shengnan Liu
1,2,3,
Shiju Liu
1,2,3,
Gang Gao
2,3,* and
Rukai Zhu
1
1
PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China
2
School of GeoSciences, China University of Petroleum, 18 Fuxue Road, Beijing 102249, China
3
State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, 18 Fuxue Road, Beijing 102249, China
*
Author to whom correspondence should be addressed.
Processes 2023, 11(10), 2967; https://doi.org/10.3390/pr11102967
Submission received: 25 June 2023 / Revised: 8 September 2023 / Accepted: 25 September 2023 / Published: 13 October 2023
(This article belongs to the Special Issue Physical, Chemical and Biological Processes in Energy Geoscience)
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Abstract

:
Carbonate rocks exhibit significant heterogeneity as both a source rock and reservoir. Stylolite formation plays a crucial role in the enrichment of organic matter and the migration of geofluids within carbonate rocks. In order to study the enrichment mechanism of organic matter and the geofluid migration mode in the stylolites developed in carbonate rocks, stylolite-bearing core samples from the Dachigan structural belt in the eastern Sichuan Basin were collected. The stylolites and matrix were subjected to the total organic carbon (TOC) test and Rock-Eval pyrolysis, thin-section observation under fluorescent light, whole-rock X-ray diffraction, carbon and oxygen isotope analysis, and scanning electron microscopy. The organic matter occurring in the stylolites is mainly in the form of three types: soluble organic matter, pyrobitumen, and bitumen. This suggests that the organic matter within the stylolites mainly consists of secondary migrated organic matter. The stylolites also exhibit well-developed secondary dolomite and pyrite resulting from late-diagenetic recrystallization. These minerals contribute to the preservation of intercrystalline pore spaces and fractures, providing favorable conditions for oil and gas accumulation and migration. The strong cementation observed at the contact between the stylolites and matrix, along with the presence of secondary minerals nearby, may be attributed to the fractionation of light and heavy oil components during the migration of hydrocarbon fluids from the matrix to the stylolites. The thicknesses of the stylolites vary within the bulk, indicating severe diagenesis in thinner areas. Consequently, this leads to significant fractionation effects. The fractionation of crude oil components by stylolites poses challenges for the study of definitive oil–source correlations. To overcome these challenges, future research could investigate biomarker compounds to attempt oil–source correlations. Additionally, future efforts should take into consideration the spatial variation in the crude oil properties. Understanding the role of stylolites in organic matter enrichment and geofluid migration is crucial for optimizing exploration strategies in the Sichuan Basin, a region of growing importance in the energy industry. Moreover, our findings shed light on the complex interactions within stylolite-bearing rocks, which are not limited to this specific basin. These insights offer valuable contributions to the broader field of geology and reservoir characterization, enhancing our ability to predict and interpret similar geological formations globally.

1. Introduction

In the realm of geological investigation, the distinctiveness of carbonate rock as both a source rock and reservoir type has garnered considerable attention from scholars [1,2,3,4,5,6,7,8]. However, the intrinsic attributes of carbonate rocks, marked by low organic matter content and intricate heterogeneity [9,10,11,12,13], pose challenges, resulting in reservoirs characterized by their inherent tightness. This intricacy has led to a prolonged discourse encompassing hydrocarbon generation, expulsion, and migration within carbonate formations [14,15,16,17,18,19].
Of particular intrigue are stylolites, a distinctive feature inherent to carbonate rocks [18]. The inception of their observation by Mylius in 1751 instigated an array of research endeavors aimed at unraveling their origins, giving rise to diverse genetic theories, including those rooted in organics, crystallization, pitch, pressure, gas, denudation, dissolution, and contraction pressure [20,21,22]. Among these, the pressure-solution theory has gained broad acceptance. In the evolution of stylolites, the enrichment of the varying minerals within these structures has undergone meticulous scrutiny [23,24,25,26], as has the presence of organic matter [27,28,29,30,31,32].
Concurrently, a pivotal discourse questions whether the characteristics of hydrocarbon generation and migration mirror those prevalent in mudstone [4,9,15,33]. Emerging studies suggest that stylolites, distinguished by a heightened presence compared to fractures in carbonate rocks, contain reservoir-related materials, like petroleum and bitumen, indicating a discernible role in hydrocarbon migration [33,34,35,36,37]. Perspectives offered by Lind et al. (1994), Heap et al. (2014), and Rustichelli et al. (2015) [38,39,40] propose that stylolites indeed serve as migration conduits rather than impediments. Complementing this, micro-CT data by Heap et al. (2018) [41] spotlight the stylolite porosity, further endorsing their potential as pathways for oil and gas migration. Biomarker compound analysis also lends insight into the impact of stylolites on migration, with Liu et al. (2020) [42] juxtaposing the organic fluid migration from the matrix to the stylolites and along these structures, revealing pronounced migration fractionation, particularly along the latter.
In summation, the ubiquity of stylolites within carbonate formations, distinguished by their assorted morphologies and pronounced variations in thickness, length, and composition, emerges from the interplay of stress and dissolution processes. While investigations into stylolites span a spectrum of themes—distribution, typology, composition, and provenance—their implications for reservoir porosity and fluid migration have encountered considerable exploration. Yet, an appreciable void persists in the realm of studies concerning the petrological nuances of stylolites, the origins of their organic matter, and the mechanisms governing hydrocarbon generation and expulsion in contexts characterized by low organic matter content.
In this study, we examined the stylolites and matrix in Carboniferous and Ordovician marine carbonate rocks from the Sichuan Basin. Several analyses were performed, including total organic carbon content (TOC) testing, Rock-Eval pyrolysis, X-ray diffraction (XRD), carbon and oxygen isotope composition analyses, as well as petrological analysis. The objective was to investigate the mechanism behind the accumulation and migration of the organic matter within the stylolites.

2. Geologic Setting and Stratigraphy

The Sichuan Basin is situated in the middle and eastern parts of Sichuan Province, encompassing a significant portion of Chongqing as well. Geologically, it is a morphologically rhomboic basin. The basin is surrounded by fault belts, adjacent to the Longmenshan platform margin depression zone in the east and bounded by the Dabashan and Daloushan platform margin depression zones in the west, closely adjacent to the Daliangshan fault zone, and connected to the Micangshan platform fold belts in the north, totally covering about 260,000 km2 (Figure 1A) [43]. Structurally, the Sichuan Basin belongs to an important first-level unit in the western part of the Yangtze platform. It is a large petroliferous basin developed on the foundation of the Upper Yangtze Craton, which has undergone multiple tectonic cycles [44]. Throughout its geological history, the Sichuan Basin has experienced various periods of tectonic movement, primarily characterized by subsidence and sedimentation. These stages can be classified into two main phases of sedimentary and tectonic evolution: (1) The Sinian–Middle Triassic, which was the sedimentary–tectonic evolution stage of the marine craton basin with the deposition of carbonate rocks; the basin is dominated by vertical movement; (2) the sedimentary–tectonic evolution stage, which was dominated by continental clastic rocks that had entered the foreland basin since the Late Triassic, forming a “foreland-cratonic” type of superimposed basin [45].
The strata in the Sichuan Basin are mainly composed of marine carbonate rocks deposited from the Sinian to the Middle Triassic (Figure 1B). These strata have a total thickness of several thousand meters, and while some layers contain continental sandstone and carbonate rocks, the main reservoirs consist of marine carbonate rock intervals [46]. Evaporite deposits, such as open-platform facies, restricted-platform facies, and intra-platform beaches, are prevalent in the basin. These beach facies anomalies are conducive to the development of early karsts, which is conducive to the occurrence of the pressure solution in carbonate rocks to form stylolites. For this study, the researchers focused on the Dachigan structural belt located in the eastern part of the Sichuan Basin. This area was chosen due to the discovery of thicker stylolites in the carbonate rocks. The carbonate samples from this region exhibit a rich occurrence of stylolites, making them suitable for the purposes of this study.

3. Samples and Methods

3.1. Samples

A total of 7 core samples of carbonate rocks were collected from three drilling wells in the Dachigan structural belt in the eastern Sichuan Basin: Zuo3 (3), Chi53 (2), and Wuke1 (2) (Figure 1, Table 1). In the study, the stylolites and corresponding matrix on both sides were compared and analyzed. Therefore, it was essential to separate the stylolites and matrix in the samples. The collection of pure matrix samples is relatively simple. [47] showed that the porosity and specific area are different close to stylolites but remain unaffected beyond a distance of 1 cm. Hence, breaking the core and grinding it with pure matrix debris near the stylolites (within 1 cm) was sufficient to obtain the matrix sample. However, collecting stylolites proved more challenging due to their irregularities and varying thicknesses in different parts. In the process of collecting, we tried to select the thicker parts of the stylolites, and we first disintegrated them along the stylolites with a dentist drill. Then, special pliers were used to separate the filling in the stylolites and remove the evident matrix with a file for a finer separation. The pure stylolite filling was then ground with a mortar. Subsequently, all rock samples were crushed and sieved using an 80-mesh sieve before various tests were conducted. They were then placed in sample bags, appropriately numbered, and prepared for further experiments (Table 1). In addition, sections were sliced along the vertical stratification plane of the rock with a thickness of about 0.5 cm, and fluorescent thin slices and scanning electron microscope samples were prepared with this part of the sample.

3.2. Methods

3.2.1. TOC and Rock-Eval Experiments

Using a balance sensitive to one ten-thousandth of a gram, weigh the samples (0.1 g for suture samples, 0.25 g for matrix samples) and place them into crucibles. Then, add 10% diluted hydrochloric acid to decompose the carbonate in the samples until no gas bubbles are produced. Subsequently, rinse the samples with distilled water, taking care to prevent any sample overflow from the crucibles. After drying the samples in the crucibles, use the Leco CS-230 instrument to determine the organic carbon content [32,48,49]. Rock-Eval analyses were performed on an OGE-Ⅱ instrument, which was developed by the Experimental Center of Petroleum Geology of the Research Institute of China Petroleum Exploration and Development, and it can mainly obtain the data of the S1 (mg HC/g rock) (<300 °C), S2 (mg HC/g rock) (300 °C < t < 600 °C), and Tmax (°C) (the maximum temperature at which S2 pyrolyzate can generate) [50,51]. The standard sample used for the TOC was LOT NO.0602 (CARBON% = 0.691 ± 0.006), and the standard samples used for the Rock–Eval pyrolysis were IFP 160000 and Chinese standard reference materials.

3.2.2. Petrographic Analyses

In the study, a total of 7 carbonate rock samples containing stylolites were used to prepare thin sections, and these samples were observed with the transmitted light, reflection, and fluorescence of the Leica microscope. Blue fluorescence was used to observe the organic matter of different samples, with an excitation wavelength ranging from 420 nm to 485 nm and an emission wavelength of 515 nm [52]. Based on the observation of thin sections, the fresh surfaces and argon ion polishing of three samples were selected and observed under the scanning electron microscope. The scanning electron microscope model was a Hitachi SU8010 field-emission SEM-EDS. The experiment was carried out in the State Key Laboratory of the China University of Petroleum, Beijing.

3.2.3. XRD Analysis

The X-ray diffraction (XRD) was analyzed via a Bruker D2 PHASER diffractometer system with operating conditions of 30 kV, 10 mA, a scanning rate of 2° 2θ/min, and with Cu Kα radiation and whole-rock random-powder patterns recorded from 4.5° to 50° 2θ, which were conducted at the State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing and State Key Laboratory, and the dosage of the experimental sample was about 100 mg [53].

3.2.4. Isotope Composition Analyses

The carbon and oxygen isotope analyses were conducted at the State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum (Beijing), and performed using a MAT 253 gas mass spectrometer equipped with a Kiel IV carbonate device [54]. The results, based on replicate analyses of GBW 04405, are given using conventional δ13C and δ18O notations with respect to the Vienna Pee Dee Belemnite (VPDB) standard, with the precision and reproducibility better than ±0.030‰ and ±0.080‰, respectively [55].

4. Results

4.1. TOC and Rock-Eval Pyrolysis

The data on the TOC content and pyrolysis parameters for the stylolites and matrix samples of carbonate rocks are illustrated in Table 2. The TOC values of the stylolites varied between 0.63 wt.% and 0.88 wt.%, with an average of 0.77 wt.%. The TOC values of the matrix varied between 0.03 wt.% and 0.24 wt.%, with an average of 0.12 wt.% (Table 3). The Rock-Eval pyrolysis S1 (free hydrocarbon), S2 (pyrolyzed hydrocarbon), and S1 + S2 of the stylolites, respectively, were in the ranges of 0.09~1.05 mg HC/g rock, 1.92~6.50 mg HC/g rock, and 2.29~6.64 mg HC/g rock, with corresponding averages of 0.39 mg HC/g rock, 4.27 mg HC/g rock, and 4.66 mg HC/g rock, respectively. The Rock-Eval pyrolysis S1, S2, and S1 + S2 of the matrix were, respectively, in 0.05~0.53 mg HC/g rock, 0.16~0.82 mg HC/g rock, and 0.21~1.17 mg HC/g rock, with corresponding averages of 0.23 mg HC/g rock, 0.35 mg HC/g rock, and 0.57 mg HC/g rock, respectively (Table 3). The ratios of the HCI (S1/TOC) values of the stylolites were 10.2~148.5 mg HC/g TOC, with an average of 566.5 mg HC/g TOC (Table 3). The ratios of the HCI of the matrix were 120.5~307.2 mg HC/g TOC, with an average of 185 mg HC/g TOC (Table 3). The PI (S1/(S1 + S2)) values of the stylolites were 0.02~0.35, with an average of 0.1, and those of the matrix were 0.21~0.75, with an average of 0.39 (Table 3). The Tmax values of the stylolites were 373 °C~500 °C (averaging 420 °C), and the Tmax values of the matrix varied from 420 °C to 600 °C (averaging 481 °C) (Table 3).

4.2. Petrographic Analyses

The petrological characteristics record the heterogeneity of the sedimentation and diagenesis, which can be used to initially evaluate the preservation of the original sedimentary environment [56,57]. From the handpicked core samples, the thicknesses of the stylolites are not uniform, showing toruloid distribution (Figure 2B). Peeling off the cores along the stylolite lines, it could be observed that the surfaces of the stylolites are uneven (Figure 2A). Under microscopic observation, it could be seen that the particles in the matrix are densely cemented; however, the stylolites were dark-colored and showed toruloid distribution (Figure 2C), indicating that they were formed via pressure dissolution. Under reflective light, the distribution of secondary pyrite was observed (Figure 2D,E) in massive occurrence, which is quite different from the framboidal occurrence indicating the formation of primary pyrite. Under the condition of blue fluorescence excitation, the stylolites are characterized by extremely strong fluorescence luminescence with a green color, indicating that they are rich in light oil (Figure 2F,G). Through the SEM observation of the fresh surfaces of the samples, a large amount of secondary organic matter (oil, bitumen, and pyrobitumen) was observed in the stylolites, but no primary organic matter was observed (Figure 3), which is consistent with the phenomenon observed in the thin sections. In addition, a large number of intercrystalline pores and fractures could also be observed with the scanning electron microscope (Figure 3).

4.3. XRD and Isotopic Analyses

The data on the XRD of the stylolites and matrix for the same carbonate rock samples are listed in Table 4. The matrix contained more calcite than stylolites; however, the dolomite, quartz, pyrite, and clay minerals were more abundant in the stylolites (Table 4). The carbon and oxygen isotopic compositions of the stylolites and matrix for the same carbonate rock samples are shown in Table 5. The obtained δ13CV-PDB of the stylolite and matrix had characteristic values of −4~3‰ and −2.7~3.2‰ (Table 5). In different carbonate rock samples, the δ18OV-PDB of the stylolites was evidently lower than that of the matrix (Table 5). The δ18OV-PDB of the stylolites and matrix were, respectively, −12.6~−1.3‰ and −13.2~−2‰ (Table 5).

5. Discussion

5.1. Source of Organic Matter in Stylolites

5.1.1. Comparison of Organic Geochemical Characteristics between Stylolites and Matrix

Organic matter is fundamental to hydrocarbon generation from source rocks, whether in conventional or unconventional petroleum systems, and including in emerging hydrocarbon resources, such as gas hydrates [1,59,60,61]. Generally, the parameters used to assess the organic matter abundance of source rocks include the total organic carbon (TOC), Rock-Eval pyrolysis S1 + S2, and organic solvent extract content [1,62,63,64]. According to data obtained from the samples analyzed (Table 2 and Table 3), it is evident that the TOC, S2, and S1 + S2 values of the stylolites were higher than those of the corresponding matrix in the same carbonate rock (Figure 4a,b,d). The values of the Rock-Eval pyrolysis S1 between the stylolites and matrix for the same sample were not consistent; this may be attributed to the influence of migrated hydrocarbons in the matrix or the poor hydrocarbon expulsion efficiency in the matrix [42,65].
Parameters such as the Rock-Eval S1 + S2 (hydrocarbon generation potential) and HI (hydrogen index) were used as auxiliary indices to assess the hydrocarbon generation potential of the source rocks [1,59,66]. The residual hydrocarbon content of source rocks, a crucial parameter characterizing the generation and expulsion of hydrocarbons, can be reflected by the Rock-Eval pyrolysis S1. When no hydrocarbon expulsion occurs, it can represent the total hydrocarbon amount generated in the source rock and reflect the hydrocarbon-generating characteristics of organic matter. Therefore, for source rocks with the same organic matter type and maturity, if no hydrocarbon expulsion occurs, then the S1 values increase with the increasing TOC content, whereas there is no variation and slight variations in the ratios of the HCI (S1/TOC). When hydrocarbon expulsion occurs, the S1 values, solvent extract contents, and ratios of the HCI decrease [32]. In Figure 5A,B, some samples show extremely low TOC values for the matrix, indicating higher HI and hydrocarbon generation potential (S1 + S2), suggesting that the matrix near the stylolites was influenced by migrated hydrocarbons. Figure 5C reveals that the HCI (S1/TOC) of the stylolites was lower than that of the matrix in the same sample, even though the corresponding TOC content was higher than that of the matrix. This indicates that the hydrocarbon expulsion efficiency of the stylolites is much higher than that of the matrix. In carbonate source rocks, stylolites serve as the primary channel for hydrocarbon discharge; therefore, the hydrocarbons formed in the matrix aggregated near the stylolites and caused a higher hydrocarbon index of the matrix adjacent to the stylolites [42].

5.1.2. Occurrence and Source of Organic Matter in Stylolites

Organic matter can be indirectly observed through fluorescence microscopes and scanning electron microscopes. Under the excitation of blue fluorescence, different types of organic matter will show different fluorescence characteristics. The fluorescence color of the hydrocarbon-generating parent material often varies with the evolution of the source rock. The fluorescence luminescence of organic matter tends to vary from yellow to brown to black as the thermal maturity of the organic matter evolves from low to high maturity [67,68,69]. Light oils tend to be green when excited by blue fluorescence, while heavier oils tend to be reddish brown. In scanning electron microscopy observation, organic matter can be directly identified through the analysis of energy spectrum data. From Figure 2F,G, it can be seen that the fluorescence intensity in the stylolite was extremely strong with green luminescence, indicating an enrichment in the light oil in the stylolite. The observation of the same sample under a scanning electron microscope revealed that the organic matter in the stylolite mainly migrated from the carbonate rock matrix, also known as migrated organic matter. For example, the organic matter observed in Figure 3A shows strong homogeneity and a high carbon content (Figure 3B), characteristic of typical solid bitumen. Figure 3C shows a spherical morphology, consistent with the intermediate state of bitumen, as previously published (Figure 3F) [58]. Figure 3D,E represents a relatively common form observed under scanning electron microscopy, characterized by small black dots. When excited by the energy spectrum, the dots disappear and bulges appear. Combined with the observation of thin sections under fluorescent luminescence, these dots are likely to be soluble organic matter adsorbed onto mineral surfaces. Ref. [42] compared the biomarkers of a stylolite and the matrix, and their findings suggested that the organic matter in the stylolite was inherited from the matrix, where it formed liquid hydrocarbons that migrated to the stylolite. The Rock-Eval pyrolysis Tmax is used as an auxiliary index for figuring out the thermal-maturity levels [1,59,66]. However, a high content of soluble organic matter can lead to a decrease in the Tmax [70]. Figure 6A shows that the Tmax values of the stylolites were lower than those of the matrix, and the ∆Tmax displays a positive correlation with the ∆TOC, which is consistent with the understanding that the organic matter of stylolites is the result of the migration and accumulation of liquid hydrocarbon from the matrix to the stylolite.

5.2. Influence Mechanism of Stylolites on Migration and Accumulation of Geofluid

5.2.1. Mineral Composition Characteristics of Stylolites and Matrix

Through the study of the characteristics and genesis of carbonate rock stylolites, it is universally realized that the formation of stylolites is a process of the continuous dissolution of carbonate rocks and the gradual enrichment of insoluble residues [71]. Most of the stylolites in carbonate rocks are distributed in a beaded shape, with highly uneven thicknesses [39] (Figure 2). Thicker stylolites tend to have zones of stress relief, dissolution, and mineral precipitation, while thinner stylolites often exhibit stress concentration zones with strong diagenesis [39]. The dissolution and precipitation zones in stylolites provide favorable conditions for the formation of secondary minerals during the later stage of diagenesis. As stylolites form and carbonate rocks continuously dissolve, the salinity within the stylolites increases, creating a conducive environment for the formation of dolomite [72]. Numerous studies have explored the material compositions of stylolites, revealing that they are mainly composed of insoluble clay minerals, solid organic matter, asphalt, secondary pyrite, and dolomite [73]. In carbonate rocks, the precipitation of dolomite requires specific spatial conditions for the growth of soluble minerals and a high-salinity environment [74,75,76]. By comparing the mineral compositions of the stylolite and matrix in the same sample, it becomes evident that the matrix has a higher content of calcite, while the stylolite contains higher levels of insoluble quartz, clay minerals, and secondary dolomite and pyrite (Figure 7). The presence of dolomite and pyrite, with well-defined crystalline morphologies observed under scanning electron microscopy, shows the secondary genesis of minerals (Figure 3). Also, the clay minerals that are more abundant in stylolites can catalyze oil cracking [77].

5.2.2. δ13C and δ18O Characteristics of Stylolites and Matrix

The analysis of stable isotopes, such as δ13C and δ18O, is valuable for reconstructing the fluid properties during dolomite precipitation, especially the δ13C value, which is influenced by the diagenesis after deposition [78,79,80,81]. The δ18O value alongside the associated crossplot of δ13C and δ18O, as the oxygen isotope exchanges between seawater/marine carbonate and meteoric/burial water, has tended to occur easily relative to other stable isotopes during diagenetic alteration [79,80,82,83,84,85,86]. By comparing the carbon isotope characteristics of the stylolites and matrix, it is found that the carbon isotope characteristics of the stylolites are lighter than that of the matrix (Figure 8A). During the formation of stylolites, the dissolution of carbonate minerals in the matrix leads to the release of lighter δ12C, resulting in the enrichment of lighter stable carbon isotopes in the stylolites. The formation of secondary dolomite in the stylolites indicates the high salinity of the geofluid, which can be reflected by the salinity of the diagenetic fluid [87]. The high content of secondary dolomite in the stylolites suggests higher δ18O values than those of the matrix, but there were some outsiders for the variation law of the δ18O values in the stylolites and matrix (Figure 8B). This is possibly due to the precipitation of extremely light oxygen isotopes in the matrix, resulting in greater δ18O values. Additionally, the diagenesis is not as strong as the isotopic fractionation in certain cases. The Δδ13C and Δδ18O values of the stylolites and matrix show a certain positive correlation (Figure 8C), further supporting the differences in the compositions of the carbon and oxygen isotopes of the studied samples.

5.2.3. Microscopic Characteristics and Migration Effect of Stylolites

A stylolite is a three-dimensional structure with a certain thickness, and its ability to serve as an enrichment zone and migration channel of organic fluid can be deciphered by the observation of the microscopic characteristics of its pore structure. Scanning electron microscopy is an effective method for studying the microscopic pore structure [88,89]. From Figure 3, it is evident that the stylolites show well-developed secondary dolomites with intact crystalline morphologies (Figure 3G,H), which can retain good intercrystalline pores. The brittleness of dolomite allows for the formation of many intergranular cracks due to tectonic stress (Figure 2G and Figure 3H,I,L). These secondary pores and cracks serve as important storage spaces and migration channels for geofluid.
Liu et al. (2020) [32] compared the molecular markers of a matrix and stylolites and found that the light-to-weight ratio parameters in saturated hydrocarbons (C21−/C22+, tricyclic/pentacyclic terpane, and (C21 + C22) pregnane/regular sterane index) revealed that small molecular compounds are more likely to enter stylolites than macromolecular compounds, and it has been found that the migration differentiation is more pronounced when hydrocarbons migrate along stylolites. During the formation of stylolites, fluids carrying dissolved carbonate move along the stylolites or to the matrix adjacent to the stylolites. When conditions change, the dissolved mineral ions will recrystallize and act as cementation, leading to a decrease in the porosity of the matrix around the stylolites [90,91]. By observing the thin sections under reflected light, the precipitation of tight minerals can be observed at the contact between the stylolites and matrix (Figure 9A,B), accompanied by the formation of secondary pyrite (Figure 9B). The strong cementation at the contact interface and the formation of secondary minerals nearby may be the cause of the fractionation effect when the hydrocarbon fluid migrates from the matrix to the stylolites. Considering an individual stylolite as a whole, its thickness is not uniform. Through the observation of the stylolite under fluorescence light, it can be found that the wider part of the stylolite exhibits a stronger fluorescence intensity, and the central part of the stylolite is evidently stronger than the contact between the stylolite and matrix (Figure 9C,D). Therefore, the regions where the stylolite is thinner will cause an evident fractionation effect (Figure 10).

5.3. Geological Significance of Stylolites in Carbonate Rocks for Oil and Gas–Source Correlation

Stylolites are relatively developed in carbonate rocks. The abovementioned research shows that stylolites are beneficial to the enrichment and migration of soluble organic matter. In carbonate rock oil and gas reservoirs, when stylolites are well developed, the thermal cracking of the enriched soluble organic matter can lead to the generation of significant amounts of natural gas. The source of natural gas in the gas reservoir is affected by the natural gas formed by the thermal cracking of the organic matter in the stylolites, resulting in the enrichment of the δ13C in the gas reservoir, which exerts a certain influence on the identification of natural gas sources [92,93].
Stylolites are essentially important for the migration of crude oil, and they can form a network channel for fluid migration by combining with fracture. However, crude oil undergoes a strong fractionation effect during the migration process in stylolites. As the migration distance increases, the properties and biomarkers of crude oil change to a certain extent, and the contents of the light components and light molecules change, making it challenging to determine the sources of crude oils accurately. In such cases, it is safer to comprehensively explain the spatial variation in the crude oil properties.

6. Conclusions

Stylolites are a common geological phenomenon found in carbonate rocks. The analyses of the total organic carbon, Rock-Eval pyrolysis, and petrography show that the organic matter in the stylolites is primarily composed of EOM, pyrobitumen, and bitumen, indicating that the organic matter in the stylolites is mainly secondary organic matter.
Comparisons of the mineral composition, δ13C and δ18O characteristics, and microscopic characteristics between the stylolites and the matrix reveal that the secondary dolomite and pyrite minerals in the stylolites are more developed, and the secondary intercrystalline pores and intercrystalline fractures in the stylolites provide storage space and migration channels for oil and gas. The contact interface between the stylolites and matrix has a strong cementation, which impedes the migration of oil and gas from the matrix to the stylolites. This causes the migration fractionation effect.
For the oil and gas in carbonate rock reservoirs, the influence of stylolites on the oil and gas properties should be considered. The enrichment of soluble organic matter in stylolites can lead to the continuous production of thermally cracked gas at high maturity levels, impacting the group composition and isotope characteristics of the natural gas in the gas reservoir. When stylolites serve as effective migration channels, the fractionation effect on the crude oil during the migration process can cause the loss of the biomarkers of crude oil, making it challenging to determine the oil–source relationship.

Author Contributions

Writing—original draft, S.L. (Shengnan Liu); Writing—review & editing, S.L. (Shiju Liu) and G.G.; Supervision, R.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Natural Science Foundation of China (No. 411372142 and No. U22B6004) and research project of PetroChina Research Institute of Exploration and Development (2022yjcq03). G.G. (Gang Gao) is the founder of Natural Science Foundation of China (No. 411372142); W.Z. (Wenzhi Zhao) is the founder of Natural Science Foundation of China (No. U22B6004); R.Z. (Rukai Zhu) is the founder of research project of PetroChina Research Institute of Exploration and Development (2022yjcq03).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ethical restrictions.

Acknowledgments

Thanks to the funds provided by the Natural Science Foundation of China (No. 411372142 and No. U22B6004) and the research project of the PetroChina Research Institute of Exploration and Development (2022yjcq03), and to the guidance provided by the China University of Petroleum (Beijing), as well as the experiment and testing support sponsored by the State Key Laboratory of Petroleum Resources and Prospecting, Beijing.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Sampling location of the Carboniferous and Ordovician carbonate rocks and (B) stratigraphic column in the Sichuan Basin.
Figure 1. (A) Sampling location of the Carboniferous and Ordovician carbonate rocks and (B) stratigraphic column in the Sichuan Basin.
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Figure 2. (A) Photograph of core showing the uneven surfaces of stylolites in Well Z3, 4317.60 m, O22; (B) photograph of core showing stylolites are beaded in Well Wk1, 4251.68 m, C2; (C) photomicrograph showing carbonate rock samples containing stylolites under plane-polarized light (the red color is to highlight the stylolites), Wk1, 4251.68 m, C2; (D) photomicrograph of a closed zone of stylolite-rich laminae in (C) under plane-polarized light; (E) same field as (D) under reflective light; (F) photomicrograph of a closed zone of stylolite-rich laminae in (C) under plane-polarized light; (G) same field as (F) under fluorescent light.
Figure 2. (A) Photograph of core showing the uneven surfaces of stylolites in Well Z3, 4317.60 m, O22; (B) photograph of core showing stylolites are beaded in Well Wk1, 4251.68 m, C2; (C) photomicrograph showing carbonate rock samples containing stylolites under plane-polarized light (the red color is to highlight the stylolites), Wk1, 4251.68 m, C2; (D) photomicrograph of a closed zone of stylolite-rich laminae in (C) under plane-polarized light; (E) same field as (D) under reflective light; (F) photomicrograph of a closed zone of stylolite-rich laminae in (C) under plane-polarized light; (G) same field as (F) under fluorescent light.
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Figure 3. Microphotographs showing (A–F) the occurrence of organic matter and (G–L) the pores and cracks in stylolites: (A) bitumen with smooth surface, WK1, 4251.68 m, C2; (B) the energy spectrum of the OM of A; (C) solid globular pyrobitumen pieces with a rough surface, pasted on the crystal face of dolomite, WK1, 4251.68 m, C2; (D) dispersed EOM; when the energy spectrum is excited, the black spot disappears and bulges appear, which are combined with the fluorescent flakes and are considered to be soluble organic matter and secondary pyrite, WK1, 4251.68 m, C2; (E) photomicrograph of a closed zone of dispersed soluble organic matter in (D); (F) photograph of pyrobitumen pieces with a rough surface, Adapted with permission from Ref. [58]. 2018, Chengyu Yang; (G) intergranular pores of secondary dolomite and with euhedral morphology, Z3, 4317.60 m, O22; (H) the crack formed by secondary minerals, WK1, 4251.68 m, C2; (I–L) intergranular pores and cracks of secondary dolomite and secondary pyrite in stylolites (the sample with argon ion polishing).
Figure 3. Microphotographs showing (A–F) the occurrence of organic matter and (G–L) the pores and cracks in stylolites: (A) bitumen with smooth surface, WK1, 4251.68 m, C2; (B) the energy spectrum of the OM of A; (C) solid globular pyrobitumen pieces with a rough surface, pasted on the crystal face of dolomite, WK1, 4251.68 m, C2; (D) dispersed EOM; when the energy spectrum is excited, the black spot disappears and bulges appear, which are combined with the fluorescent flakes and are considered to be soluble organic matter and secondary pyrite, WK1, 4251.68 m, C2; (E) photomicrograph of a closed zone of dispersed soluble organic matter in (D); (F) photograph of pyrobitumen pieces with a rough surface, Adapted with permission from Ref. [58]. 2018, Chengyu Yang; (G) intergranular pores of secondary dolomite and with euhedral morphology, Z3, 4317.60 m, O22; (H) the crack formed by secondary minerals, WK1, 4251.68 m, C2; (I–L) intergranular pores and cracks of secondary dolomite and secondary pyrite in stylolites (the sample with argon ion polishing).
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Figure 4. Comparison of geochemical parameters between different samples: (a) TOC (wt.%); (b) S2 (mg HC/g rock); (c) S1 (mg HC/g rock); (d) S1 + S2 (mg HC/g rock).
Figure 4. Comparison of geochemical parameters between different samples: (a) TOC (wt.%); (b) S2 (mg HC/g rock); (c) S1 (mg HC/g rock); (d) S1 + S2 (mg HC/g rock).
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Figure 5. Diagrams of (A) TOC vs. S1 + S2, (B) TOC vs. HI, and (C) TOC vs. HCI of stylolites and matrix in the same carbonate rock samples.
Figure 5. Diagrams of (A) TOC vs. S1 + S2, (B) TOC vs. HI, and (C) TOC vs. HCI of stylolites and matrix in the same carbonate rock samples.
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Figure 6. Diagrams of (A) Tmax between stylolites and matrix and (B) ∆Tmax vs. ∆TOC for the same carbonate rock sample (∆Tmax = (Tmax-M) − (Tmax-S), ∆TOC = (TOC-S) − (TOC-M)).
Figure 6. Diagrams of (A) Tmax between stylolites and matrix and (B) ∆Tmax vs. ∆TOC for the same carbonate rock sample (∆Tmax = (Tmax-M) − (Tmax-S), ∆TOC = (TOC-S) − (TOC-M)).
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Figure 7. Diagrams of the contents of different minerals between stylolites and matrix for the same carbonate rock sample.
Figure 7. Diagrams of the contents of different minerals between stylolites and matrix for the same carbonate rock sample.
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Figure 8. Diagrams of (A) δ13C and (B) δ18O between stylolites and matrix and (C) ∆δ13C versus ∆δ18O for the same carbonate rock samples (∆δ13C = (δ13C − S) − (δ13C − M), ∆δ18O = (δ18O-S) − (δ18O-M)).
Figure 8. Diagrams of (A) δ13C and (B) δ18O between stylolites and matrix and (C) ∆δ13C versus ∆δ18O for the same carbonate rock samples (∆δ13C = (δ13C − S) − (δ13C − M), ∆δ18O = (δ18O-S) − (δ18O-M)).
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Processes 11 02967 g009
Figure 9. (A,B) Thin-section photomicrograph showing the structure of stylolites under plane-polarized light (A) and reflected light, Well Z3, 4317.60 m, O22; (C) photomicrograph showing the structure of stylolites under plane-polarized light, Well Wk1, 4251.68 m, C2; (D) same field as (C) under fluorescent light.
Figure 9. (A,B) Thin-section photomicrograph showing the structure of stylolites under plane-polarized light (A) and reflected light, Well Z3, 4317.60 m, O22; (C) photomicrograph showing the structure of stylolites under plane-polarized light, Well Wk1, 4251.68 m, C2; (D) same field as (C) under fluorescent light.
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Figure 10. Model of organic matter source and fluid migration of stylolites in carbonate source rock.
Figure 10. Model of organic matter source and fluid migration of stylolites in carbonate source rock.
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Table 1. Basic information and numbers of samples.
Table 1. Basic information and numbers of samples.
Well IDLayerDepth (m)No.
Zuo3 O224316.30 m1-Ssr
1-Msr
4318.84 m2-Ssr
2-Msr
4317.60 m3-Ssr
3-Msr
Chi53C23030.13 m4-Ssr
4-Msr
3053.88 m5-Ssr
5-Msr
WuKe1C24254.28 m6-Ssr
6-Msr
4251.67 m7-Ssr
7-Msr
Table 2. TOC and Rock-Eval pyrolysis of stylolites and matrix in carbonate rock samples.
Table 2. TOC and Rock-Eval pyrolysis of stylolites and matrix in carbonate rock samples.
Sample No.TOC
(wt.%)		Tmax
(°C)		S1
(mg HC/g Rock)		S2
(mg HC/g Rock)		S1 + S2
(mg HC/g Rock)		PIHI (mg HC/g TOC)HCI (mg HC/g TOC)∆Tmax∆TOC
1-Ssr0.713731.051.922.970.35271.5148.5470.53
1-Msr0.174200.530.180.710.75104.3307.2
2-Ssr0.884240.092.22.290.041221.550.01160.64
2-Msr0.245400.350.821.170.30343.5146.6
3-Ssr0.813910.454.344.790.09535.855.6620.70
3-Msr0.104530.20.290.490.41280.5193.4
4-Ssr0.734310.146.506.640.02893.619.2110.67
4-Msr0.054420.090.230.320.28443.2173.4
5-Ssr0.815000.195.916.10.03733.423.61000.77
5-Msr0.036000.050.160.210.24462.4144.5
6-Ssr0.634400.144.945.080.03786.922.3420.50
6-Msr0.124820.150.560.710.21449.8120.5
7-Ssr0.813800.644.054.690.14494.478.1530.71
7-Msr0.104330.210.190.40.53189.2209.2
Note: PI = S1/(S1 + S2); HI = S2/TOC; HCI = S1/TOC.
Table 3. Statistical data on TOC and Rock-Eval parameters for the stylolites and matrix in carbonate rock samples.
Table 3. Statistical data on TOC and Rock-Eval parameters for the stylolites and matrix in carbonate rock samples.
IndexStylolitesMatrix
MinMaxMeanNo.MinMaxMeanNo.
TOC (wt.%)0.630.880.7770.030.240.127
S1 (mg HC/g rock)0.091.050.3970.050.530.237
S2 (mg HC/g rock)1.926.54.2770.160.820.357
S1 + S2 (mg HC/g rock)2.296.644.6670.211.170.577
PI0.020.350.170.210.750.397
HI (mg HC/g TOC)271.5893.6566.57104.3462.4324.77
HCI (mg HC/g TOC)10.2148.551.27120.5307.21857
Tmax (°C)37350042074206004817
Table 4. The data from the XRD for the stylolites and matrix in carbonate rock samples.
Table 4. The data from the XRD for the stylolites and matrix in carbonate rock samples.
Sample No.Type and Content of Minerals (%)Clay Minerals (%)
QuartzPlagioclaseCalciteDolomitePyriteAnhydrite
1-Ssr5.12.611.847.614.1017.5
1-Msr2.42.656.127.23.405.3
2-Ssr5.71.88.348.112.51.719.2
2-Msr2.42.566.117.32.607.1
3-Ssr5.32.112.445.510.6022.5
3-Msr2.61.558.726.45.703.1
4-Ssr6.91.911.843.411.5022.7
4-Msr3.42.468.416.83.21.14.1
5-Ssr6.62.711.541.39.3026.3
5-Msr2.41.459.327.92.202.8
6-Ssr6.22.912.248.68.71.518.5
6-Msr1.62.166.715.21.808.2
7-Ssr6.82.89.744.813.1018.1
7-Msr2.22.763.418.12.706.9
Table 5. The δ13C and δ18O for the stylolites and matrix in carbonate rock samples.
Table 5. The δ13C and δ18O for the stylolites and matrix in carbonate rock samples.
Sample No.δ13CV-PDB (‰)δ18OV-PDB (‰)∆δ13C (‰)∆δ18O (‰)
1-Ssr1.4−10.80.4−0.3
1-Msr1.8−11.1
2-Ssr1.6−10.60.6−0.4
2-Msr2.2−11
3-Ssr3−11.10.20.6
3-Msr3.2−10.5
4-Ssr1−12.60−0.6
4-Msr1−13.2
5-Ssr−2.3−8.1−0.4−0.4
5-Msr−2.7−8.5
6-Ssr0.3−1.30.3−0.7
6-Msr0.6−2
7-Ssr−4−6.41.72.9
7-Msr−2.3−3.5
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Liu, S.; Liu, S.; Gao, G.; Zhu, R. Organic Matter Source, Fluid Migration, and Geological Significance of Stylolites in Organic-Lean Carbonate Rocks: A Case from the Sichuan Basin. Processes 2023, 11, 2967. https://doi.org/10.3390/pr11102967

AMA Style

Liu S, Liu S, Gao G, Zhu R. Organic Matter Source, Fluid Migration, and Geological Significance of Stylolites in Organic-Lean Carbonate Rocks: A Case from the Sichuan Basin. Processes. 2023; 11(10):2967. https://doi.org/10.3390/pr11102967

Chicago/Turabian Style

Liu, Shengnan, Shiju Liu, Gang Gao, and Rukai Zhu. 2023. "Organic Matter Source, Fluid Migration, and Geological Significance of Stylolites in Organic-Lean Carbonate Rocks: A Case from the Sichuan Basin" Processes 11, no. 10: 2967. https://doi.org/10.3390/pr11102967

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