Pyrite morphological evidence for sedimentary conditions of the Wufeng formation–Longmaxi formation in south-east Chongqing area–insights for high-quality shale gas reservoir formation mechanism

Abstract

As one of the important minerals in marine shale reservoirs, the development characteristics of pyrite can provide guidance for the exploration of deep shale gas resources and the study of high-quality reservoir development mechanisms. In this study, samples of the Wufeng and Longmaxi Formation in the southeast Chongqing area were selected, and the development characteristics of framboidal pyrite were revealed through a combination of qualitative and quantitative method. High-quality reservoirs of the upper Wufeng Formation and the bottom of the Longmaxi Formation are developed with smaller sizes, weaker variation and a narrower size distribution of pyrite framboids compared with other units of the strata, suggesting relatively stable euxinic conditions during deposition. The development characteristics of framboidal pyrite and some of the key reservoir evaluation parameters such as organic matter content and brittle mineral content, etc., are controlled by similar factors. Therefore, pyrite morphological evidence can be used as a potential indicator of high-quality shale reservoirs.

Keywords

framboidal pyrite black shale shale reservoir sedimentary environment Wufeng formation Longmaxi formation

Introduction

In recent years, great progress has been made in the exploration and exploitation of shale gas resources in China (Cui et al., 2023; Tan et al., 2014; Zou et al., 2019; Zou et al., 2015; Sun et al., 2021). The Sichuan basin is the predominant contributor to shale gas productivity among the major shale gas-bearing basins in China (Sun et al., 2021; Jiang et al., 2017; Li et al., 2022). The Longmaxi Formation, whose underlying black shale is from the Wufeng Formation, formed the predominant shale gas-bearing stratum in the Sichuan basin (Wu et al., 2021). The black shale in the Wufeng Formation and Longmaxi Formation is widely distributed in the Sichuan basin with stable thickness and good gas-bearing characteristics (Ye et al., 2017). During the exploitation of shale gas, reservoirs with the highest gas content are commonly distributed at the Wufeng Formation and the bottom section of the Longmaxi Formation reservoir (Guo and Zhang, 2014; Li et al., 2017; Zhang et al., 2018). Although these two layers form a set of fracturing target layers, research has revealed differences in the factors affecting the quality of them (Li et al., 2017). The Wufeng Formation shale reservoir was deposited in an anoxic and occlusive environment restricted by palaeogeographic conditions, while the reservoirs with better quality in the bottom Longmaxi Formation benefited from a reductive environment formed by transgression (Li et al., 2022; Li et al., 2017). The reservoirs have strong small-scale heterogeneity characteristics in compositions, structure characteristics, and physical properties controlled by the sedimentary conditions and diagenesis process (Wang et al., 2022; Guo and Zhao, 2015; Xiong et al., 2015). Although great achievements have been made in the exploration of shale gas, the geological theory of gas accumulation and high-quality reservoir formation in the Wufeng and Longmaxi Formations needs further study.

Pyrite is one of the basic mineral compositions of organic-rich marine shale (Ross and Bustin, 2009; Zhang et al., 2013; Anderson et al., 2011; Liu et al., 2018; He et al., 2021). In the reservoir, there are genetic differences and different types of pyrite occurs in variant forms (Singh et al., 2013). The mineralogical characteristics of framboidal pyrite contain abundant sedimentary and petrogenesis information of organic rich fine-grained sediment including shale, coal and mudstone (Singh et al., 2012; Xi et al., 2021; Song et al., 2021; Pan et al., 2022). The size of pyrite framboid is related to its sedimentary environment and diagenesis during its formation process (Singh et al., 2013; Pan et al., 2022). Framboidal pyrite is commonly used as an indicator of redox conditions for paleo-environment using the size distribution characteristics because the size of framboidal pyrite is related to the dysoxic degree of water (Wilkin et al., 1997; Wignall and Newton, 1998; Wilkin et al., 1997; Zou et al., 2018). Positive correlations were commonly found between pyrite content, organic matter content, and gas-bearing capacity of the reservoir (Guo and Zhao, 2015; He et al., 2021; He et al., 2022). As the research on unconventional energy and paleoenvironment progress, the importance of pyrite in recording paleoenvironment and indicating sedimentary conditions has widely attracted geological scholars’ attention in recent years. Reservoirs of the Wufeng Formation and the Longmaxi Formation contribute to an important proportion of shale gas production capacity in both China and world. In terms of geological age, the sedimentation and formation periods of the Wufeng Formation and the Longmaxi Formation crossed the Ordovician-Silurian boundary. It should be noted that the high-quality reservoirs for exploration and exploitation are mainly distributed above and below this boundary. As a severe global extinction event, the Late Ordovician mass extinction (LOME) also occurred during this period (Zou et al., 2018). It is crucial to further enrich the relevant geological evidence in order to explore the relationship between the ancient environment, ancient events, and the development mechanism of high-quality reservoirs. Considering the particular material source the pyrite in shale origins from, it has the potential to play a key-role in understanding the paleoenvironmental characteristics and high-quality reservoir development mechanisms (Wu et al., 2021; Xi et al., 2021; Wignall and Newton, 1998; Wilkin et al., 1997; Zou et al., 2018).

The sedimentary environment and diagenetic transformation information reflected by the morphological characteristics of framboidal pyrite can provide important information for further understanding the development mechanism of high-quality reservoirs (Xi et al., 2021; Song et al., 2021; Wilkin et al., 1997; Wignall and Newton, 1998; Wilkin et al., 1997; Zou et al., 2018; He et al., 2022). This study investigates the morphological characteristics by conducting quantitative analysis of particle size characteristics of pyrite framboids in shale reservoirs of the Wufeng Formation and Longmaxi Formation in the south-east Chongqing area. A total of 90 samples were selected from representative outcrops and wells and observed using a high-resolution microscope and Scanning Electron Microscopy (SEM). Argon Ion Polishing Field Emission Scanning Electron Microscopy (AIP-FESEM) with an Energy Dispersive Spectroscopy (EDS) was further used to collect images and data of pyrite aggregates and microcrystals in representative samples. The objectives of this study are to (1) derive the size distribution and morphological characteristics of framboidal pyrite of Wufeng and Longmaxi Formation shale in the research area; (2) make comparisons of mineralogical characteristics between the Wufeng Formation and the bottom, the lower, the middle, and the upper part of the Longmaxi Formation; (3) determine the sedimentary paleo-environmental significance using the size distribution evidence of framboidal pyrite and provide new proofs for the reconstruction of paleo-sedimentary environment; (4) discuss the indicative effect of pyrite development characteristics on the distribution of high-quality reservoirs.

Sampling and methods

Study area

The study area is located in the eastern part of the Sichuan basin. As shown in Figure 1, the study area is on the southeast part of the Upper Yangtze plate and north of the central Guizhou Uplift. During the deposition of the Wufeng formation and the Longmaxi formation from Late Ordovician to Early Silurian, the Yangtze plate was surrounded by the Yangtze Sea, which has a certain degree of connectivity with the external oceans (Li et al., 2017; Wang et al., 1997). A semi-enclosed bay formed in the Sichuan basin during the Caledonian movement, surrounded by the Dianqian Uplift and Xuefeng Uplift on the south, Chengdu Uplift, and Cathaysian Land on the north (Chen et al., 1987). The black shale in the Wufeng formation was deposited within a restricted water environment in the study area surrounded by these uplifts (Li et al., 2017). The Wufeng formation consists of two members with different lithological characteristics: the Graptolite Shale Member with lithology of organic-rich and siliceous-rich shale on the lower part and the Guanyinqiao Member with lithology of black shale and carbonaceous limestone on the upper part (Liang et al., 2012).

Figure 1.

Research area and sampling locations.

Figure 2.

Information extraction using high-resolution FE-SEM images.

The lower Longmaxi formation shale is rich in both organic matter and brittle minerals. Two large-scale transgressions occurred during the transition from Ordovician to Silurian and contributed to the organic accumulation of shale (Li et al., 2017; Luo et al., 2016). From the lower to the upper Longmaxi formation, the TOC content of shale decreased with a faster deposition rate and lower water depth within a deepening shelf environment (Zou et al., 2018; Xu et al., 2004; Ran et al., 2015). The development degree of horizontal beddings is also decreasing from the bottom to the top of the Longmaxi formation. Sedimentary structure and graptolite assemblage evidence reflect a relatively shallower environment (Liao et al., 2021). During the exploration of shale gas in this black shale strata, the Wufeng formation and lower Longmaxi formation were proved to be the major target with higher organic matter content and better mechanical properties for water fracturing (Guo and Zhang, 2014).

Sampling

A total of over 200 bulk rock samples were collected from the Guanyinqiao outcrop section, Quanqian-1 well, and J-12 well in the research area. The Guanyinqiao section and J-12 well are stratigraphically continuous from the Wufeng formation to the upper Longmaxi formation. The Guanyinqiao section is located in the Qijiang area, Chongqing. 10 and 15 samples were chosen from the Wufeng formation and Longmaxi formation in the Guanyinqiao section, respectively. The J-12 well is a shale gas-producing well in the Jiashiba area, Fuling, Chongqing. The Quanqian-1 well is located in Nanchuan area of Chongqing, with a depth of 40 m. Note that the selected 10 samples from the Quanqian-1 well all belong to the lower section of the Longmaxi formation. Most of the samples were black graphitic-rich shale and mudstone with high content of organic matter from the Wufeng Formation and the lower-bottom Longmaxi Formation, as they are the high-quality reservoirs and major development targets. As indicated by pyrolysis measurements, samples have a TOC value ranging from 1.2%∼6.6% (Table 1). The kerogen type falls into Type I with maturity values ranging from 2.07% to 2.38% (equivalent vitrinite reflectance values ranging from 3.26% to 3.98%), indicating that the organic matter is mature to over the mature stage for hydrocarbon generation.

Table 1.

Pyrolysis analysis of representative shale samples.

Sample ID	Location	Formation	TOC(%)	Maturity(%)	Kerogen type
G4	Guanyinqiao section	Wufeng	2.8	2.34	TypeＩ
G10		Wufeng	3.4	-	TypeＩ
G11		Lower Longmaxi	4.6	2.22	TypeＩ
G22		Upper Longmaxi	1.6	2.14	TypeＩ
Q1	Quanqian-1 well	Lower Longmaxi	3.4	2.12	TypeＩ
Q6		Lower Longmaxi	4.2	2.32	TypeＩ
Q16		Middle Longmaxi	1.2	2.07	TypeＩ
J1	J12 well	Wufeng	2.9	2.38	TypeＩ
J6		Guanyinqiao Bed	-	-	-
J7		Lower Longmaxi	6.6	2.21	TypeＩ

Analytical procedures and apparatus

In the mineralogy and geology research of shale pyrite, it is necessary to use a variety of analytical techniques and means to have a comprehensive understanding of the properties and characteristics of shale pyrite. At present, scanning electron microscope and other related technologies are mainly used to explore the origin and formation environment in the relevant research on the mineral microstructure and morphology of shale pyrite (Zhao et al., 2018; Berner, 1970; Raiswell and Berner, 1985).

All 200 shale hand specimens were described and identified, among them, 47 samples were selected for XRD tests and micro-observing under High-Resolution Microscope and FE-SEM. In the XRD test, the sample preparation procedure is based on the SY/ T5163-1995 standard: all samples were pulverized to powders of ∼300 mesh, then the powders were placed in a 10 ml tube and mixed with distilled water to produce suspension liquid under ultrasonic dispersion treatment. After that, the suspension liquid was naturally dried for 24 h. The apparatus used for the test is the D8 ADVANCE XRD analyzer, which has a measurement accuracy of angle reproducibility ± 0.0001°, goniometer radius larger than 200 mm, minimum step size of 0.0001° and angle range (2θ) of −100° ∼ 168°. Phase analysis was based on the material standard powder diffraction data and component analysis was carried out according to standard analytical methods and diffraction criteria.

Besides, as the mineral separation method can destroy the original structure of the sample, 20 samples (from Quanqian-1 well and J-12 well) were selected for FE-SEM with imaging processing to provide more original and quantitative data on pyrite (Figure 2). After being polished by an ion beam and coated with a thin layer of gold, the polished cross-section was imaged by Helios Nanolab 600i electronic double-beam microscope and S-4700 FE-SEM. Moreover, typical highest-quality images were selected for image processing in each cross-section. The measured size distribution was slightly smaller than the true size, but the deviation is proved to be unlikely to exceed 10% (Zhao et al., 2018; Raiswell and Berner, 1985).

Results

Mineral compositions and content of pyrite

Typical samples were selected to study the mineral compositions of shale reservoirs. XRD results of representative samples from the Guanyinqiao section, Quanqian-1 well, and J-12 well are shown in Table 2. The results show that the Wufeng formation, the lower, middle, and upper parts of the Longmaxi formation have similar mineral types and different proportions of mineral components. The minerals in shale can be divided into clay minerals, brittle minerals, and other minerals like pyrite. The clay mineral compositions of shale samples are mostly illite and illite-montmorillonite. Chlorite and montmorillonite with small content values were also confirmed by XRD. Brittle minerals are mainly quartz, followed by feldspar, calcite, and dolomite. Pyrite is commonly found in tested samples. Wufeng formation and lower Longmaxi formation are developed with a higher proportion of pyrite. The Wufeng formation shale has a mean pyrite content value of 5.95% (3.99%–6.49%), and 4.12% (2.8%–6.2%) in the Guanyinqiao section and J-12 well, respectively. The lower Longmaxi formation shale has higher contents of brittle minerals and pyrite than the middle and upper parts, with a mean brittle mineral content value of 58.74% (51.60%–67.50%), 46.60% (40.40%–70.20%), 72.17%(68.8%–76.5%) and a mean pyrite content value of 3.08% (2.5%–3.9%), 3.8%(2.4%–5.2%), 3.97%(3.7%–4.4%), respectively. Thus, the pyrite content has a decreasing trend from the Wufeng formation and lower Longmaxi formation to the middle and upper part of the Longmaxi formation shale.

Table 2.

The test results of mineral compositions using XRD.

Sampling location	Formation/Section	Clay mineral content (%)	Brittle mineral content (%)	Pyrite content (%)
Guanyinqian section	Wufeng	$\frac{56.03 - 66.80}{62.41}$	$\frac{24.16 - 29.91}{27.78}$	$\frac{3.99 - 6.49}{5.95}$
	Lower Longmaxi	$\frac{32.50 - 48.40}{41.26}$	$\frac{51.60 - 67.50}{58.74}$	$\frac{2.50 - 3.90}{3.08}$
	Middle-upper Longmaxi	$\frac{45.60 - 48.20}{47.52}$	$\frac{50.8 - 52.4}{52.28}$	$\frac{0.1 - 1.1}{0.5}$
Quanqian-1 well	Lower Longmaxi	$\frac{28.80 - 54.40}{39.80}$	$\frac{40.40 - 70.20}{46.60}$	$\frac{2.4 - 5.2}{3.8}$
Quanqian-1 well	Middle-upper Longmaxi	$\frac{38.6 - 76.4}{46.4}$	$\frac{21.2 - 58.8}{45.4}$	$\frac{1.2 - 4.6}{1.9}$
J-12 well	Wufeng	$\frac{44.8 - 80.1}{64.03}$	$\frac{48.7 - 80.1}{65.25}$	$\frac{2.8 - 6.2}{4.12}$
	Lower Longmaxi	$\frac{19.8 - 26.8}{23.87}$	$\frac{68.8 - 76.5}{72.17}$	$\frac{3.7 - 4.4}{3.97}$
	Middle-upper Longmaxi	$\frac{36.4 - 50.8}{43.53}$	$\frac{48.8 - 61.2}{54.8}$	$\frac{0.4 - 2.4}{1.67}$

Clay, brittle minerals, and pyrite contents are shown in form of (minimum value – maximum value (average value)).

Pyrite types and morphologies

According to the XRD test results, the shale reservoirs of the Wufeng Formation and Longmaxi Formation generally contain pyrite components. However, the results of FE-SEM and EDS show that there are several types of pyrite with different morphologies and origins. As shown in Figure 3, pyrite appears in various shapes. The distribution of pyrites shows strong heterogeneity. Stratiform or lenticular pyrites are developed along bedding surfaces and horizontal fractures, showing a layered distribution and controlled by sedimentary structures or tectonic fractures (Figure 3(a)-(b)). The development of stratiform or lenticular pyrites is often related to sedimentary structures. Wufeng Formation and the bottom of the Longmaxi Formation shale reservoir have more abundant euhedral pyrites concentrated in tectonic fractures and intraformational sliding fractures developed within horizontal beddings. Nodular form pyrites can be occasionally observed at the location where sedimentary structures are developed, such as laminae (Figure 3(c)). Pyrite with larger crystalline particles can be observed in fractures, often associated with calcite crystallization (Figure 3(d)-(e)). It can be observed that the fracture is affected by vertical compaction during the diagenesis of the reservoir. In the shale matrix, the most common type of pyrite is framboidal pyrite (Figure 3(f)).

Figure 3.

Pyrite types and morphologies based on core observation, microscope, and SEM observation. (a-b) Stratiform and lenticular pyrites observed in the upper Longmaxi Formation shale reservoir, Quanqian-1 well; (c) Nodular form pyrite observed in the lower Longmaxi Formation shale reservoir, Quanqian-1 well; (d-e) Microscopic and SEM observation of pyrites developed in tectonic fractures in the Wufeng Formation shale reservoir, Guanyinqian section; (f) SEM observation of framboidal pyrite in the lower Longmaxi shale matrix, J-12 well.

Pyrite in shale can be divided into synsedimentary pyrite and diagenetic pyrite according to different formation periods and mechanisms (Raiswell and Berner, 1985). The former was formed under shallow burial and weak compaction conditions in the early sedimentation diagenesis period. The aggregate of framboidal pyrite is a special type of pyrite in the synsedimentary period, and its development is mainly affected by the water environment and hydrodynamic conditions (Wignall et al., 2005). In the shale matrix, framboids are the dominant form of pyrite in the Wufeng and lower Longmaxi Formation (Liu et al., 2019). The relatively closed water body with high hydrogen sulfide content is generally conducive to the development of framboidal pyrite (Shen et al., 2016; Wang et al., 2022; Rickard, 2019). Different from the formation period of framboidal pyrite, euhedral pyrite developed in spatial positions of sedimentary structures, structural fractures and biological remains in the reservoir generally belongs to diagenetic pyrite. During diagenesis, the reservoir experienced compaction, metasomatism and other diagenesis, which made the reservoir more compact. Therefore, the reservoir space available for euhedral pyrite growth is limited. Therefore, the space in fractures, microfractures and sedimentary structures is the major space for its growth, which controls and affects the shape and size of pyrite crystals.

FE-SEM method was applied to the observation of pyrite morphology in the shale matrix (Figure 4). Various types of pyrite with different morphologies were observed, including framboidal pyrite (Figure 4(a)-(e)), infilled framboidal pyrite (Figure. 4(f)) and euhedral pyrite (Figure 4(f)-(h)). Note the significant difference in pyrite content, occurrence forms and morphologies in shale matrix of different layers. In the Wufeng and bottom Longmaxi Formation, the pyrites in shale matrix are generally small normal framboidal pyrite (Figure 4(a)-(e)). Euhedral pyrite and infilled framboidal pyrite become the predominant type in some shale layers (Figure 4(f)). Compared with normal framboidal graphite, infilled framboidal pyrite has a larger size of pyrite microcrystals and smaller intergranular pores. On the basis of normal framboidal pyrite, this type of pyrite undergoes further authigenic mineralization in the sedimentary reservoir during diagenesis. Thus, infilled framboids keep the shape of sphericity or sub-sphericity (Liu et al., 2019). In the middle-upper part of Longmaxi Formation, the pyrite content becomes lower. Large framboidal pyrite and euhedral pyrite can be observed in Figure 4h-i. Pyrite types observed in the Wufeng and Longmaxi Formation shale reservoir are shown in Table 3.

Figure 4.

Micro-scale observation of pyrite developed in shale matrix using FE-SEM images on typical Longmaxi Formation samples from the Guanyinqiao section. (a-b) Bottom Longmaxi Formation; (c-d) Lower Longmaxi Formation; (e-g) Middle Longmaxi Formation; (h-i) Upper Longmaxi Formation.

Table 3.

Pyrite types observed in the Wufeng and Longmaxi Formation shale reservoir.

Pyrite type	Formation period	Developmental position	Morphology
Euhedral pyrite in fractured reservoir space	During diagenesis of reservoir	Fractures and sedimentary structures	Aggregated pyrite crystals
Euhedral pyrite in shale matrix	During diagenesis of reservoir	Shale matrix	Isolated pyrite crystal
Framboidal pyrite	During synsedimentary period	Shale matrix	Framboidal shape
Infilled framboidal pyrite	During diagenesis of reservoir	Shale matrix	Infilled framboidal shape

Figure 5 shows the pyrite morphologies and distribution of framboidal pyrite using Longmaxi Formation samples from the Quanqian-1 well. Pyrite is commonly present in tested samples. Figure 5(a)/(b) belongs to the lower Longmaxi Formation. Figure 5(c) and (d) belong to the middle and upper Longmaxi Formation, respectively. A recrystallized framboidal pyrite with a size of larger than 15 μm is shown in Figure 5(a)/(e). Its microcrystals in pyrite aggregates have a bigger size and different shape compared with other pyrite framboids in this sample (Figure 5(f)). Note that although this aggregate belongs to framboidal pyrite, its size has changed during diagenesis. Thus, the size of this recrystallized framboidal pyrite cannot be used as an indicator of paleosedimentary environment during the collection and analysis of framboidal pyrite size data. In the middle-upper samples, the size of the framboids becomes relatively larger (Figure 5(g)-(h)).

Figure 5.

Observation of framboidal pyrite by FE-SEM images using typical samples from Quanqian-1 well.

The size distribution of framboidal pyrite

The box-and-whisker plots of framboid populations of the Guanyinqiao section, the Quanqian-1 well, and the J-12 well were shown in Figure 6–8, respectively. Polished sections of the samples were prepared and coated with gold film for micro-scale observations using FE-SEM. Pyrite framboids displayed different morphologies during observation. Normal and infilled framboidal pyrite were observed in different samples (Figure 5). Size distributions and other structural parameters of normal pyrite framboids were obtained through image processing.

Figure 6.

Box-and-whisker plots of framboid populations of the Wufeng-Longmaxi formation from the Guanyinqiao section using box-and-whisker plots

Figure 7.

Size-frequency plots of framboid populations from the lower to middle and upper Longmaxi Formation of the Quanqian-1 well

Figure 8.

Size distribution and formation mechanism of framboidal pyrite in different environmental conditions (modified after Wang et al., 2022, size distribution data from Wilkin et al., 1996 and Rickard, 2019).

The pyrite framboid size distributions of framboidal of the Wufeng Formation -Longmaxi Formation from the Guanyinqiao section were shown in Figure 6. 10 Wufeng Formation fresh samples (G1-G10) and 15 Longmaxi Formation samples were collected from the Guanyinqiao section. Most of the Longmaxi Formation shale samples (G11-G13) belong to the lower and middle parts of the Longmaxi Formation. G14-G15 samples were taken from the upper part of the Longmaxi Formation. Data statistics show that the mean value of framboidal size ranges from 0.2 to 7.6 μm with an average value of 3.4 μm. The maximum values of framboidal size range from 5.9 to 33.5 μm (with an average value of 14.9 μm).

Figure 7 shows framboid populations of 10 samples (Q1-Q10) from the lower to middle and upper Longmaxi Formation of the Quanqian-1 well. Abundant pyrite framboids are found in tested samples, but the size distribution characteristics varies between samples. From the lower to middle and upper Longmaxi Formation, the framboid size range becomes wider, the mean value and SD value of aggregate size grows. It is notable that the bottom shale sample has the smallest mean framboid size value of 3.1 μm and SD value of 0.90.

Framboid populations from the Wufeng Formation and lower-middle Longmaxi Formation of the J-12 well are shown in Figure 8. JY-12 well is a shale gas-producing well located in the Jiashiba area, Fuling, Chongqing. 11 samples were collected from the JY-12 well, 5 of which belong to the lower Longmaxi Formation, 1 sample from the middle Longmaxi Formation and 5 from the Wufeng Formation (including 2 from the Guanyinqiao bed). The results show that the distribution of framboid size in the Wufeng Formation has a certain degree of fluctuation. The samples from the lower Wufeng Formation yielded relatively larger-sized framboids with an increasing trend towards the middle Wufeng Formation. However, from the middle to the upper Wufeng Formation shale, the samples yielded relatively smaller-sized framboids with a decreasing trend towards the Guanyinqiao bed. Some larger-sized pyrite framboids were observed in the shelly limestone sample from the Guanyinqiao bed. From the Guanyinqiao bed to the bottom Longmaxi Formation, the samples showed a narrower distribution in size and yielded smaller-sized framboids again. Note that the samples from the bottom Longmaxi Formation displayed the smallest sizes and narrowest size distribution compared with the middle-upper Longmaxi Formation and Wufeng Formation.

The framboidal pyrite in the samples of the bottom Longmaxi Formation displayed small sizes and narrow size distribution, suggesting euxinic environments. The samples in the lower Longmaxi Formation (Excluding bottom samples) are mostly in the overlapping area range of euxinic environment and dysoxic-oxic environment (Figure 9). Compared with the Wufeng Formation and the middle-upper part of the Longmaxi Formation, samples from the bottom Longmaxi Formation show less variable in size distribution, suggesting relatively stable euxinic environments.

Pyrite framboids with a size of >20 μm were observed in the middle-upper Longmaxi Formation. Authigenic pyrite crystals are commonly observed in fractures and matrices in the upper Longmaxi Formation samples, but framboidal pyrite is rarely observed in some of the samples, reflecting a relatively unstable sedimentary environment. Samples from the upper Longmaxi Formation generally fell into the dysoxic-oxic environment (Figure 9). Thus, fewer pyrite framboids with larger particle sizes provide evidence for the shallower water and a dysoxic-oxic environment in the middle-upper Longmaxi Formation, which is consistent with field evidence and geochemical analysis results (Ye et al., 2017; Li et al., 2017; Zhao et al., 2016). From the bottom to the middle-upper of the Longmaxi Formation, shale reservoirs show a wider range and fewer variables in size distribution, suggesting relatively more oxidized environments during deposition.

Insights for high-quality shale gas reservoir formation mechanism

The exploration target of shale gas is mainly the reservoirs with good physical properties, gas-bearing properties and brittleness (Tian et al., 2013). At present, China's shale gas exploration targets have gradually shifted to deep shale gas resources and shale gas resources in complex tectonic regions. Due to the significant increase in the difficulty and cost of obtaining geological data, understanding the development mechanism of high-quality shale reservoirs has become a key basis for guiding scientific exploration of shale gas. The shale reservoirs of Wufeng formation and Longmaxi formation contribute to the main productivity of shale gas in the Sichuan Basin. The exploration and development progress of this set of strata indicates that the Wufeng formation and the bottom Longmaxi formation (during the transitional period of Late Ordovician to Early Silurian) are the exploration targets with the best quality within the strata (Hu et al., 2019; Zhao et al., 2020; Li et al., 2023). Physical properties and gas-bearing properties of shale reservoirs are comprehensively affected by the sedimentary environment, reservoir diagenesis, and tectonic transformation. The sedimentary environment provides the original material components for the reservoir, including minerals, organic matter, and primitive fluids. These material components are the material basis of reservoir evolution under diagenesis.

Figure 9.

Box-and-whisker plots of framboid populations from the Wufeng formation and lower-middle Longmaxi Formation of the J-12 well.

Thus, high-quality shale gas reservoirs of the Wufeng and Longmaxi Formation shale mainly consist of three sub-layers during the Ordovician-Silurian transitional period, including the Wufeng Formation shale, the Guanyinqiao bed, and the lower Longmaxi Formation shale. Usually, we consider the black shale during this period as a mining target; however, there are significant differences in sedimentary conditions between these three sub-layers (Li et al., 2017). The size distribution evidence shows that during the deposition of the Wufeng Formation shale (Excluding the Guanyinqiao bed), the heterogeneity of framboids size distribution is relatively strong. The lower Wufeng Formation shale has a wider framboid size distribution and larger average size than the upper part (Figures 6/8). The decreasing trend indicates a more reducing water environment in the upper Wufeng Formation shale and a transformation from dysoxic-oxic conditions to euxinic conditions during deposition. Scholars have confirmed that the extinction events that occurred during the Ordovician-Silurian transition were influenced by euxinic conditions, based on evidence from the Pyrology and Pyric Sulfur Isotopic Composition (Zou et al., 2018). The link between euxinic conditions indicated by pyrite evidence and extinction was clearly presented in the Shuanghe section, indicating that extinction during this period is a result of euxinic conditions influenced by paleoclimate change (Zou et al., 2018). Similar size distribution trends can be found in the Guanyinqiao section and the J-12 well suggesting euxinic conditions during the deposition of the upper Wufeng Formation shale are regional phenomenon. The shale reservoirs of the Wufeng Formation are mainly composed of siliceous shale, carbonaceous black shale and gray argillaceous shale (Figure 10(a)). Among them, the carbonaceous shale with well-developed horizontal bedding sedimentary structure is developed in the upper part of the Wufeng Formation shale and is underlying the Guanyinqiao bed.

Figure 10.

Field geological characteristics of the Wufeng Formation-Longmaxi Formation in the Guanyinqiao section. (a) Shale reservoirs and the Guanyinqiao Bed of the Wufeng Formation; (b) The boundary between the Wufeng Formation and the Longmaxi Formation; (c) Graptolite fossils on bedding planes of the bottom Longmaxi Formation; (d) Horizontal laminations in the lower Longmaxi Formation; (e) Shale reservoirs of the middle Longmaxi Formation.

The Guanyinqiao bed is a relatively special layer segment at the top of the Wufeng formation. In general, when evaluating the Wufeng Formation-Longmaxi Formation reservoirs, geologists tend to overlook this interval because of its differences in lithology and small thickness (Figure 10(b)). However, the experimental studies on the argillaceous limestone samples and calcareous shale samples from the Guanyinqiao bed show that this layer also has certain organic and other types of pores, as well as good mechanical brittleness. Therefore, when conducting reservoir evaluation and engineering technology design, the Guanyinqiao bed should also be an important component of high-quality reservoirs.

Unlike the shale formed in anoxic environments under strong water mass restriction in the Wufeng formation, the sedimentary environment of the Longmaxi formation shale reservoir changed during sedimentation (Li et al., 2017). During this period, the sea level rapidly rose, and a static marine environment favorable for the enrichment and preservation of organic matter was formed at the bottom of the water body. During this sedimentary period, organic-rich siliceous shale with developed graptolite-rich horizontal bedding was formed (Figure 10(b-d)). Pyrite evidence confirmed that the bottom shale of Longmaxi formation was deposited in relatively stable euxinic environments. Except for the samples at the bottom of the Longmaxi formation that are deposited in euxinic environments, all other layers of the Longmaxi formation are formed in dysoxic-oxic conditions. The element geochemical evidence shows that the reducibility of the water body at the bottom of the Longmaxi Formation is stronger than that of the Wufeng Formation (a static marine environment with a certain amount of H₂S), while the middle and upper parts of the Longmaxi Formation are dominated by oxic environments (Li et al., 2017). The development of sandy layers and interlayers has significantly increased in the middle-upper Longmaxi Formation (Figure 10(e)). The deposition rate is relatively faster and the enrichment condition of the organic matter becomes worse (Li et al., 2017). This is also the reason for the worse physical and gas-bearing properties of reservoirs in the middle-upper Longmaxi Formation.

The prediction of high-quality reservoirs and geological sweet spots requires theoretical analysis and data analysis of influencing factors with formation mechanism relationships (Zhao et al., 2019; Xu et al., 2020). High-quality shale gas reservoirs are comprehensively controlled by reservoir material composition, diagenesis evolution, and tectonic transformation. Therefore, through the comprehensive analysis of the sedimentary environment and reservoir characteristics, it can be concluded that in the high-quality shale gas reservoirs at the upper Wufeng Formation and the bottom of the Longmaxi Formation, framboidal pyrite characteristics are significantly different from other units of the strata, which can be used as a good indicator of high-quality shale reservoirs on the basis of obtaining quantitative pyrite data (Figure 11). The evidence of pyrite points to two important sedimentary factors of euxinic conditions and stability of the water environment, shared by the upper Wufeng formation and the bottom of the Longmaxi formation. This evidence provides a new scientific basis for the development mechanism of high-quality shale reservoirs in the Wufeng Formation-Longmaxi Formation of the Sichuan Basin.

Figure 11.

Sedimentary environment characteristics of high-quality shale reservoir reflected by framboidal pyrite evidence.

In previous studies, the study on the geological significance of shale pyrite mainly focused on the indicative significance of paleoenvironmental characteristics (Wignall and Newton, 1998; Wilkin et al., 1997; Zou et al., 2018; He et al., 2022; Zhao et al., 2018; Wignall et al., 2005; Liu et al., 2019; Zhao et al., 2016; Wignall et al., 2005). Some studies have pointed out its role in reservoir exploration, transformation, and other aspects (Zhao et al., 2016). Compared with previous studies on pyrite evidence for sedimentary conditions of the Wufeng Formation and the Longmaxi Formation, this research further enriched the relatively few shale pyrite research data in the study area, and established the correlation between pyrite evidence and high-quality shale reservoir development mechanism to provide scientific guidance for further supporting shale exploration and development. This study also further reveals the influence of euxinic conditions and environmental stability on the development of high-quality reservoirs based on pyrite evidence. Previous studies mainly focused on the impact of redox conditions in sedimentary environments. The importance of environmental stability for the development of high-quality shale reservoirs is not emphasized. In-depth study of the mineralogy and geology of shale pyrite to further understand its genesis and distribution rules is of great significance for shale gas exploitation and exploration. Significant correlation exists between important reservoir evaluation indicators (such as organic matter content, brittle mineral content) and framboidal pyrite characteristics (Liu et al., 2019), indicating that they are controlled by similar sedimentary environmental factors. It should be noted that the relationship between framboidal pyrite information and these reservoir evaluation indicators is not directly controlled or affected.

Conclusions

The shale reservoirs of the Wufeng Formation-Longmaxi Formation generally contain pyrite components, but the occurrence form, genesis, and content of pyrite in different layers have significantly different characteristics. The pyrite in the shale reservoir matrix of the Wufeng Formation and the lower Longmaxi Formation is mainly framboidal pyrite.

Size distributions and other structural parameters of normal pyrite framboids were obtained through image processing. The upper Wufeng Formation and the bottom of the Longmaxi Formation are developed with smaller sizes, weaker variation, and a narrower size distribution of pyrite framboids compared with other units of the strata. The Guanyinqiao bed is developed with bigger sizes and wider size distribution. From the bottom layer upwards, fewer pyrite framboids with larger particle sizes are developed with more significant heterogeneity.

High-quality reservoirs of the upper Wufeng Formation and the bottom of Longmaxi Formation are developed with smaller sizes, weaker variation and a narrower size distribution of pyrite framboids compared with other units of the strata, suggesting relatively stable euxinic conditions during deposition. The evidence of pyrite points to two important sedimentary factors of euxinic conditions and stability of the water environment as important components of the development mechanism of high-quality shale reservoirs.

Framboidal pyrite and key reservoir evaluation parameters (organic matter content, brittle mineral content, etc.) are affected by common sedimentary environment and other factors, leading to the correlation of framboidal pyrite morphological data and reservoir evaluation parameters. Pyrite morphological evidence can be used as a potential indicator of high-quality shale reservoirs based on quantitative pyrite data, and is a key factor for understanding the paleosedimentary conditions of sedimentary environments.

Footnotes

Acknowledgements

Our special thanks go to editors and reviewers for their meaningful comments and help, which inspired us and helped us to improve the quality of our paper. The author Difei Zhao also acknowledges the support from the Qihang Project for Young Scholars and the Fundamental Research Funds for the Central Universities (No. 2021QN1061) from China University of Mining and Technology.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Science and Technology Research Project of Chongqing Education Commission, the Natural Science Foundation of Jiangsu Province, the Natural Science Foundation of Chongqing City, (grant number KJZD-K202103201, BK20210521, cstc2021jcyj-msxmX0624).

ORCID iDs

Difei Zhao

Zhibo Zhang

References

Anderson

Schiffbauer

Xiao

(2011) Taphonomic study of Ediacaran organic-walled fossils confirms the importance of clay minerals and pyrite in Burgess Shale-type preservation. Geology 39(7): 643–646.

Berner

(1970) Sedimentary pyrite formation. American Journal of Science 268(1): 1–23.

Chen

Xiao

Chen

(1987) Wufenian (Ashgillian) graptolite faunal differentiation and anoxic environment in South China. Acta Palaeontologica Sinica 26(3): 326–338. (in Chinese with English abstract).

Cui

Niu

Zhao

, et al. (2023) High-temperature hydrothermal resource exploration and development: Comparison with oil & gas resource. Gondwana Research 122(10): 306–314.

Guo

Zhang

(2014) Formation and enrichment mode of Jiaoshiba shale gas field, Sichuan basin. Petroleum Exploration and Development 41(1): 31–40.

Guo

Zhao

(2015) Analysis of micro-scale heterogeneity characteristics in marine shale gas reservoir. Journal of China University of Mining & Technology 44(2): 300–307. (in Chinese with English abstract).

Yang

Shi

, et al. (2022) Genetic mechanism of pyrite in the shale of the Longmaxi Formation and its influence on the pore structure: A case study of the Changning area, South Sichuan basin of SW China. Frontiers of Earth Science 10(6): 919923.

Qin

, et al. (2021) Influence of mineral compositions on shale pore development of Longmaxi Formation in the Dingshan area, Southeastern Sichuan basin, China. Energy & Fuels 35(6): 10551–10561.

Chen

, et al. (2019) Marine shale reservoir evaluation in the Sichuan basin-A case study of the lower Silurian Longmaxi marine shale of the B201 well in the Baoluan area, southeast Sichuan basin, China. Journal of Petroleum Science and Engineering 182(11): 106339.

10.

Jiang

Tang

Long

, et al. (2017) Reservoir quality, gas accumulation and completion quality assessment of Silurian Longmaxi marine shale gas play in the Sichuan basin, China. Journal of Natural Gas Science and Engineering 39(3): 203–215.

11.

Yang

, et al. (2022) Geological characteristics and controlling factors of deep shale gas enrichment of the Wufeng-Longmaxi Formation in the Southern Sichuan basin, China. Lithosphere 2022(12): 4737801.

12.

Zhou

Nie

, et al. (2023) Organic matter accumulation mechanisms in the Wufeng-Longmaxi shales in western Hubei Province, China and paleogeographic implications for the uplift of the Hunan-Hubei submarine high. International Journal of Coal Geology 270(4): 104223.

13.

Zhang

Ellis

, et al. (2017) Depositional environment and organic matter accumulation of upper Ordovician–lower Silurian marine shale in the upper Yangtze platform, South China. Palaeogeography, Palaeoclimatology, Palaeoecology 466(1): 252–264.

14.

Liang

Jiang

Yang

, et al. (2012) Shale lithofacies and reservoir space of the Wufenge-Longmaxi Formation, Sichuan basin, China. Petroleum Exploration and Development 39(6): 736–743.

15.

Liao

Zhao

Chen

, et al. (2021) Constraints of sedimentary structures on key parameters of shale gas reservoirs: A case study of the Wufeng and Longmaxi Formations of the Ordovician-Silurian transition in Luzhou area, Sichuan basin, southwestern China. Geological Journal 56(9): 5691–5711.

16.

Liu

Ostadhassan

Gentzis

, et al. (2018) Characterization of geochemical properties and microstructures of the Bakken shale in North Dakota. International Journal of Coal Geology 190(4): 84–98.

17.

Liu

Chen

Zhang

, et al. (2019) Pyrite morphology as an indicator of Paleoredox conditions and shale gas content of the Longmaxi and Wufeng shales in the Middle Yangtze area, South China. Minerals 9(7): 428.

18.

Luo

Zhong

Dai

, et al. (2016) Graptolite-derived organic matter in the Wufeng–Longmaxi Formations (upper Ordovician–lower Silurian) of southeastern Chongqing, China: Implications for gas shale evaluation. International Journal of Coal Geology 153(1): 87–98.

19.

Pan

Zhao

Wei

, et al. (2022) Characteristics of deep shale minerals in Longmaxi Formation in western Chongqing and their reservoir geological significance. Unconventional Oil & Gas 9(2): 8–14. 33.

20.

Raiswell

Berner

(1985) Pyrite formation in euxinic and semi-euxinic sediments. American Journal of Science 285(8): 710–724.

21.

Ran

Liu

Jansa

, et al. (2015) Origin of the upper Ordovician-lower Silurian cherts of the Yangtze block, South China, and their palaeogeographic significance. Australian Journal of Earth Sciences 108(8): 1–17.

22.

Rickard

(2019) Sedimentary pyrite framboid size-frequency distributions: A meta-analysis. Palaeogeography, Palaeoclimatology, Palaeoecology 522(5): 62–75.

23.

Ross

DJK

Bustin

(2009) The importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs. Marine and Petroleum Geology 26(6): 916–927.

24.

Shen

Feng

Algeo

, et al. (2016) Two pulses of oceanic environmental disturbance during the Permian-Triassic boundary crisis. Earth and Planetary Science Letters 443(6): 139–152.

25.

Singh

Kumar

, et al. (2013) Control of different pyrite forms on desulfurization of coal with bacteria. Fuel 106: 876–879.

26.

Singh

, et al. (2012) Petrographic and geochemical characterization of coals from Tiru valley, Nagaland, NE India. Energy Exploration & Exploitation 30(2): 171–191.

27.

Song

Bhattacharya

Webb

, et al. (2021) Preservation of organic carbon in the cretaceous hue shale on the North slope of Alaska: Insights from pyrite morphology. International Journal of Coal Geology 235(2): 103678.

28.

Sun

Nie

Dang

, et al. (2021) Shale gas exploration and development in China: Current Status, geological challenges, and future directions. Energy & Fuels 35(8): 6359–6379.

29.

Tan

Horsfield

Fink

, et al. (2014) Shale gas potential of the major marine shale Formations in the upper Yangtze platform, South China, part III: Mineralogical, lithofacial, petrophysical, and rock mechanical properties. Energy & Fuels 28(4): 2322–2342.

30.

Tian

Pan

Xiao

, et al. (2013) A preliminary study on the pore characterization of lower Silurian black shales in the Chuandong thrust fold belt, Southwestern China using low pressure N₂ adsorption and FE-SEM methods. Marine and Petroleum Geology 48(12): 8–19.

31.

Wang

Zhang

, et al. (2022) Formation mechanism of framboidal pyrite and its theory inversion of paleo-redox conditions. Geology in China 49(1): 36–50. (in Chinese with English abstract).

32.

Wang

Chatterton

BDE

Wang

(1997) An organic carbon isotope record of late Ordovician to early Silurian marine sedimentary rocks, Yangtze sea, South China: Implications for CO₂ changes during the Hirnantian glaciation. Palaeogeography, Palaeoclimatology, Palaeoecology 132(8): 147–158.

33.

Wang

Cheng

, et al. (2022) Pore structure heterogeneity of Wufeng-Longmaxi shale, Sichuan basin, China: Evidence from gas physisorption and multifractal geometries. Journal of Petroleum Science and Engineering 208(1): 109313.

34.

Wignall

Newton

(1998) Pyrite framboid diameter as a measure of oxygen deficiency in ancient mudrocks. American Journal of Science 298(9): 537–552.

35.

Wignall

Newton

Brookfield

(2005) Pyrite framboid evidence for oxygen-poor deposition during the Permian–Triassic crisis in Kashmir. Palaeogeography, Palaeoclimatology, Palaeoecology 216(2): 183–188.

36.

Wilkin

Arthur

Dean

(1997) History of water-column anoxia in the Black Sea indicated by pyrite framboid size distributions. Earth and Planetary Science Letters 148(5): 517–525.

37.

Wilkin

Barnes

Brantley

(1996) The size distribution of framboidal pyrite in modern sediments: An indicator of redox conditions. Geochimica et Cosmochimica Acta 60(10): 3897–3912.

38.

Liang

Yang

, et al. (2021) The significance of organic matter-mineral associations in different lithofacies in the Wufeng and longmaxi shale-gas reservoirs in the Sichuan basin. Marine and Petroleum Geology 126(4): 104866.

39.

Tang

Lash

, et al. (2021) Geochemical characteristics of organic carbon and pyrite sulfur in Ordovician-Silurian transition shales in the Yangtze latform, south China: Implications for the depositional environment. Palaeogeography, Palaeoclimatology, Palaeoecology 563: 110173.

40.

Xiong

Jiang

Tang

, et al. (2015) Characteristics and origin of the heterogeneity of the lower Silurian Longmaxi marine shale in southeastern Chongqing, SW China. Journal of Natural Gas Science and Engineering 27(11): 1389–1399.

41.

Rong

, et al. (2004) Facies patterns and geography of the Yangtze region, south China, through the Ordovician and Silurian transition. Palaeogeography, Palaeoclimatology, Palaeoecology 204(2): 353–372.

42.

Hao

Zhang

, et al. (2020) High-quality marine shale reservoir prediction in the lower Silurian Longmaxi Formation, Sichuan basin, China. Interpretation 8(4): T453.

43.

Liu

Ran

, et al. (2017) Characteristics of black shale in the upper Ordovician Wufeng and lower Silurian Longmaxi Formations in the Sichuan basin and its periphery, China. Australian Journal of Earth Sciences 64(5): 667–687.

44.

Zhang

Jiang

, et al. (2018) Heterogeneity characterization of the lower Silurian Longmaxi marine shale in the Pengshui area, South China. International Journal of Coal Geology 195(7): 250–266.

45.

Zhang

Shen

, et al. (2013) Mineral compositions and organic matter occurrence modes of lower Silurian Longmaxi Formation of Sichuan basin. Journal of China Coal Society 38(5): 766–771. (in Chinese with English abstract).

46.

Zhao

Guo

Shuai

, et al. (2019) Prediction of geomechanical sweet spots in a tight gas sandstone reservoir: A case study of lower Permian strata in the southern Qinshui basin, China. Interpretation 7(1): 207–219.

47.

Zhao

Guo

Wang

, et al. (2020) Quantitative characterization of nano-scale pores in shale reservoirs of Wufeng-Longmaxi Formation based on image processing. Fresenius Environmental Bulletin 29(5): 3992–3999.

48.

Zhao

Guo

Zhu

, et al. (2016) The theoretical basis of shale gas exploration with complex resistivity in Longmaxi Formation. Unconventional Oil & Gas 3(6): 15–20.

49.

Zhao

Guo

Zhu

, et al. (2018) Micropore characteristics and geological significance of pyrite in shale rocks of Longmaxi Formation. Acta Sedimentologica Sinica 36(5): 864–876. (in Chinese with an English abstract).

50.

Zhao

Jin

, et al. (2016) Applying sedimentary geochemical proxies for paleoenvironment interpretation of organic-rich shale deposition in the Sichuan basin, China. International Journal of Coal Geology 163(6): 52–71.

51.

Zou

Dong

Wang

, et al. (2015) Shale gas in China: Characteristics, challenges and prospects (I). Petroleum Exploration and Development 42(6): 753–767.

52.

Zou

Qiu

Wei

, et al. (2018) Euxinia caused the late Ordovician extinction: Evidence from pyrite morphology and pyritic sulfur isotopic composition in the Yangtze area, south China. Palaeogeography, Palaeoclimatology, Palaeoecology 511(12): 1–11.

53.

Zou

Zhu

Chen

, et al. (2019) Organic-matter-rich shales of China. Earth-Science Reviews 189(2): 51–78.