Abstract
The characteristics of shale gas enrichment conditions at different depositional positions of organic-rich shale in the Niutitang Formation of the Lower Cambrian of the Upper Yangtze in South China vary greatly. The study of pyrite provides a basis for the restoration of the ancient environment and a reference for the prediction of organic-rich shale. In this paper, the organic-rich shale of the Cambrian Niutitang Formation in the Cengong area is analyzed by means of the optical microscope, scanning electron microscope observation, carbon and sulfur analysis, X-ray diffraction whole rock mineral analysis, sulfur isotope test, and image analysis. The morphology and distribution characteristics, genetic mechanism, water column sedimentary environment, and influence of pyrite on the preservation conditions of organic matter are discussed. This study shows that the upper, middle, and lower sections of the Niutitang Formation are rich in pyrite (framboid, euhedral pyrite, subhedral pyrite, etc.). Meanwhile, the sulfur isotopic composition of pyrite (δ34Spy) shows a tight correlation with the framboid size distribution throughout the shale deposits of the Niutang Formation, and the average particle size (9.6 μm; 6.8 μm; 5.3 μm) and distribution range of framboids (2.7–28.1 μm; 2.9–15.8 μm; 1.5–13.7 μm) in the upper, middle, and lower sections show a downward trend. In contrast, the sulfur isotopic composition of pyrite shows a tendency to become heavier from above and below (mean = 0.25‰ to 5.64‰). Together with the covariant mode of pyrite trace elements (such as Mo, U, V, Co, Ni, etc.), the results showed significant differences in the oxygen levels in the water column. They show that the transgression led to long-term anoxic sulfide conditions in the lower water column of the Niutitang Formation. In addition, the main and trace elements in pyrite jointly indicated that there was hydrothermal action at the bottom of the Niutitang Formation, which led to the destruction of the preservation environment of organic matter and the decrease of TOC, which can also explain the reason why the TOC content in the middle part (6.59%) was higher than that in the lower part (4.29%). Finally, the water column became an oxic-dysoxic condition due to the decline of sea level, and the TOC content decreased (1.79%).
1. Introduction
Black shale is a sedimentary assemblage of black fine-grained argillaceous rocks formed in an anoxic environment with sedimentological, paleoecological, and geochemical characteristics.1 At present, the surge in global demand for energy forces countries to pay attention to the exploration and development of unconventional energy. As an effective oil and gas reservoir, organic shale is of great significance to the exploration and development of unconventional reservoirs and has attracted extensive attention.2 China’s marine shale is widely distributed, mainly in the Yangtze platform, the Sichuan basin, and its surrounding areas, and the Tarim basin, where the Upper Ordovician Wufeng Formation - Lower Silurian Longmaxi Formation and the Lower Cambrian are the two most favorable strata for exploration and development.3 North Guizhou experienced multistage tectonic movement, resulting in the development of deep and large faults and complex surface structural conditions. Due to the lack of sufficient theoretical support and technical advantages, the current shale gas exploration results are not ideal. Compared with the Wufeng-Longmaxi Formation, there is less research and exploration on the shale sedimentary environment and organic matter enrichment mechanism the of Niutitang Formation. The main research contents are geological characteristics, reservoir forming conditions, fracture development characteristics, resource evaluation, and favorable area optimization.4−6 The research idea also continues the traditional method of source rock, and there is little research on the evolution of the sedimentary environment and the reconstruction of redox conditions of the paleomarine environment. The source rocks of the Yangtze platform in southern China have the characteristics of high maturity, which is directly related to the long-term thermal evolution and strong tectonic movement in geological history.1 These strong evolutions result in more impurities mixed in biomarker indicators.7 Therefore, elemental geochemistry is widely used in the study of the sedimentary environment of organic-rich shale.8
The ocean stores the most sulfur resources on earth,9 in which sulfate is the main occurrence form of sulfur in seawater, and its isotopic composition is mainly controlled by the products of continental weathering imported through rivers.10 Driven by organic matter mineralization, seawater sulfate is transformed into sulfide and enters marine sediments.11 Sulfur isotope is an important indicator for understanding the evolution and sulfur cycle of various sulfur substances in the formation. In recent years, scholars at home and abroad have shown that the geochemical cycle and fractionation of sulfur play an irreplaceable role in the formation of pyrite. By measuring the composition of sulfur isotopes in pyrite, we can not only understand the source and genesis of pyrite but also provide a basis for the medium conditions of the sedimentary environment. However, sediments are often affected by geological factors such as biological disturbance, crustal movement, and event deposition, which often lead to the overlapping or deletion of oxidation zones and the formation of an unsteady sedimentary diagenetic environment.12 In the sediment, sulfate reduction occurs in the anoxic region, which is lower than the depth of oxygen depletion and generally lower than the depth of reduction of oxidized metal phases (such as Mn4+and Fe3+oxides).10,11 Therefore, disturbed authigenic pyrite is the main mineral form of sulfide in marine sediments, and its formation process is usually accompanied by diagenetic processes such as organic matter mineralization, iron oxide reduction, and sulfate reduction, and its morphological characteristics,13 framboids size,14 and sulfur isotope15,16 have strong indicative significance for the reconstruction of the sedimentary environment.
As an important fine-grained mineral that affects the quality of shale gas reservoirs, pyrite has attracted great attention from scholars all over the world in recent years.9,10,12 The particle size and distribution pattern of pyrite in ancient sediments provide an effective measurement method for reducing the redox conditions of the paleoenvironment. Framboid, as the main research object, is applied in many paleoenvironment studies.17−19 The formation mechanism is relatively simple. After burial, it will not be affected by diagenesis and later weathering and can still maintain its original shape and size.17 At present, the main research methods include the box whisker diagram,20,21 maximum framboid diameter (MFD),17 and binary graphic method.14 The maximum particle size method is to distinguish the redox conditions of the sedimentary environment by counting the maximum particle size in framboids. When the maximum particle size <18 μm, the indicated deposition condition may be a sulfide environment; when the maximum particle size >18 μm, the sedimentary conditions may be a nonsulfide environment.14,19
Bacterial sulfate reduction (BSR) is the basic process connecting the biochemical cycle of oxygen, carbon, and sulfur on the earth. It is the main way of remineralization of organic matter in marine and terrestrial sediments.11 The measured sulfur isotope characteristics of sulfate or sulfide phase can be used to explain the ancient sulfur cycle. Therefore, when interpreting the stable sulfur isotope data in the geological record, it is necessary to understand the sedimentary environment of the sample formation, the connection with the open ocean, and the subsequent diagenetic history.12 Pyrite typically forms below a redox cline. The presence of oxygen typically removes Fe as Fe(III) and oxidizes hydrogen sulfide to sulfate rapidly. With the participation of sulfate-reducing bacteria, BSR activity will lead to the fractionation of stable isotopes 34S and 32S, but due to the complex reaction process, the sulfur isotope value of pyrite will also have a large range distribution.23 Sulfur isotope fractionation will occur in the process of each sulfur element participation, and the degree of sulfur isotope fractionation will also be greatly related to the water environment, organic matter, active iron, and other factors at that time. When Wilkin14 studied pyrite in modern Black Sea sediments, he found that the content of framboid pyrite in sediment cores gradually increased from bottom to top, while the proportion of euhedral and heteromorphic pyrite decreased. The δ34Spyr value also changed from 10 ‰ to −20 ‰ or even −40 ‰. Analysis of sulfur isotope changes in pyrite by SIMS shows that the variation range of δ34Spyr value is −41.6 ‰ to +114.8 ‰, and the difference between the maximum value and the minimum value is 156 ‰. Framboids in deep sediments have a higher δ34Spyr value than framboids in shallow sediments.24 Previous studies reported that the sulfur of pyrite in the sedimentary environment mainly comes from BSR,25 but for the change of sulfate content in the environment, the sulfur fractionation caused by BSR reaction is gradual and framboid will continuously absorb the sulfur in the environment during the formation process. At the same time, the changes in sulfur isotope fractionation in the environment will also be recorded with the growth of the framboid.
Large et al.26 proposed the same principle as that of hydrothermal pyrite to trace the chemical changes of ore fluid. Sedimentary pyrite contains the history of marine trace elements. The content of trace elements in black shale, such as Mo, U, and V, has been proven to be used to determine the redox conditions of the paleoenvironment.27,28 Huerta-Diaz and Morse30 use partial digestion to dissolve pyrite and then analyze the generated solution to determine which specific trace elements come from pyrite. However, this technology cannot be applied to ancient rocks because it cannot make pyrite of different generations selective. In addition, Martin31 found that the leaching reagent in the experimental process will release metal elements from other sulfides (such as sphalerite), resulting in an abnormal concentration of trace elements in pyrite. The content of trace elements in the pyrite particles of syngenetic and diagenetic processes was measured.29 However, the trace elements in pyrite will be well preserved in the process of secondary action, so the study on the characteristics of trace elements in sedimentary pyrite can well reflect the redox conditions of the ancient ocean. Large et al.29 found that the composition of pyrite in early diagenetic cores is still preserved in the syngenetic facies of these samples after hydrothermal and metamorphism. Because the extraction technology of pyrite from traditional samples cannot distinguish sedimentary pyrite, hydrothermal pyrite, and metamorphic pyrite, LA-ICP-MS technology can better distinguish and measure the content of trace elements in three kinds of pyrite. The LA-ICP-MS technology utilizes a narrow diameter (approximately 20 μm). The laser beam can minimize the probability of analyzing other sulfides and mineral inclusions and can analyze a wider range of elements than conventional analysis techniques. In addition, by examining the LA-ICP-MS output diagram, it can be distinguished by what form trace elements appear in the pyrite lattice (such as solid solution, nano micro inclusion, or mineral micro inclusion).18 More specifically, a consistent iron trace element ratio distribution usually indicates that trace elements are contained in the lattice or as uniformly distributed nano inclusions, The sharp change in iron trace element ratio indicates that trace elements appear in pyrite as inclusions.2
During the Early Cambrian period, Northern Guizhou was in the transition zone between the Yangtze shelf and the marginal sea of the south of the Yangtze River. After experiencing a series of regional faults extending in the northeast and the northeast, due to the melting of glaciers, the sea level rose relatively, and the transgressive area of the Yangtze gradually expanded, forming a wide range of black shale of the lower Cambrian Niutitang Formation in the Southern Yangtze region. In studying and analyzing the sedimentary environment of the black shale at the bottom of the lower Cambrian in northern Guizhou, Li32 pointed out that the ratio of V/(Ni + V) and U/Th was high, the ratio of Co/Ni was low, and the enrichment of trace elements such as Mo, Sb, U, and V occurred. It was concluded that the black shale of the lower Cambrian in northern Guizhou was formed in a reduced open marine environment and was affected by the deep hydrothermal activity of the rising ocean current.6,32 For the Early Cambrian period of the Yangtze platform, predecessors mainly focused on the sedimentary transition surface from late Ediacaran to Early Cambrian. At present, for the black shale of Niutitang Formation in northern Guizhou, the reports of using the characteristics of pyrite to study the sedimentary environment are relatively weak.
Therefore, by combining the morphological characteristics of pyrite and the relationship between the content of sedimentary pyrite geochemical characteristics (such as trace elements or sulfur isotopes) and the changes in depositional conditions, the redox conditions of the depositional environment, the influence of hydrothermal activity, etc. can be further constrained, which is important for predicting the hydrocarbon potential of shales. In this paper, based on previous studies and combined with the complete CY-1 well samples, we mainly studied the morphological and geochemical characteristics of pyrite, reconstructed the redox conditions of the paleoenvironment of the Lower Cambrian Niutitang Formation in northern Guizhou, and explored the influence of hydrothermal activities as well as changes in the sedimentary environment on the preservation conditions of organic matter.
2. Geological Setting
Frequent tectonic movements lead to the complex structural distribution pattern of the Yangtze platform, while the formation of complex and multidirectional tectonic deformation areas (NS, NNE, NE, etc.) in northern Guizhou is also controlled by the tectonic movements in the whole geological history period (Figure 1). From the late Mesoproterozoic to the Cenozoic period, it mainly experienced Wuling movement, Guangxi movement, Yanshan movement, Himalayan movement, and neotectonics movement.4 The shale of the Niutitang Formation shows the structural evolution characteristics of “slight uplift in the early stage, relatively stable in the long term and strong transformation in the later stage”. In the early stage of sedimentation, the basin was uplifted but accepted stable sedimentation, forming the shale of the Niutitang Formation buried at a depth of 3000 m.32 That is, the lower Cambrian Niutitang Formation reached a maximum burial depth of 3500–4000m at the end of the Ordovician period. The study area was affected by the hard block of the Sichuan Basin in the east and south. The restriction in the south came from the east–west structure, and the obstruction in the east came from the north–south structure. This effect led to the trough fold in the NE structure of the block, which is characterized by the transverse arrangement of S-shaped bending and the reverse fault of high-angle strike-slip, The structural direction is mainly NE-NNE. The large-scale folds and faults in this period laid the foundation for the development of the current geological structure and landform and provided conditions for the distribution, enrichment, and preservation of shale gas.3
Figure 1.
Paleoenvironmental map of the South China Craton in the early Cambrian period. Reprinted with permission from ref (58). Copyright 2013 Elsevier Ltd.
A set of gray-black and black organic-rich shale is developed at the bottom of the Niutitang Formation, and the middle and upper parts are mainly developed with calcareous mudstone and limestone, as well as spongy bone needles, spongy bodies, bacteria, algae, and other organisms.33 Chen et al.34 have shown that the black shale in northern Guizhou has a high degree of thermal evolution and is in the mature over the mature stage. Its depositional environment of strong reduction and euxinia in marine environments and the organic carbon isotope has shifted by 4.3%, indicating that anoxic events have occurred in this period.35 The active tectonic movement of the Yangtze platform is often accompanied by a certain degree of volcanic hydrothermal activity. This sudden and sustained hydrothermal activity can change the properties of a water column over a long time. Li et al.32 thoroughly discussed the sedimentary environmental conditions and source tectonic background of the black shale of the Niutitang Formation in the study area after systematically sampling the black shale of the lower Cambrian in northern Guizhou and analyzing its trace element and rare earth geochemistry. They believed that the source rocks of the black organic-rich shale had many genetic properties such as granite, sedimentary rock, and basalt, and the tectonic background was not only dominated by the passive continental margin but due to the influence of deep hydrothermal solution, it also shows the structural background characteristics of a continental island arc.35
3. Samples and Methods
In this study, 50 shale core samples were collected from CY-1 well in Guizhou Province. These samples were from the Niutitang Formation of the Cambrian, and all samples were black gray shale. All samples are well preserved and have not experienced weathering and oxidation. All samples were collected from the same well to ensure that they experienced similar continuous geological processes. A large number of sedimentary environment analyses were carried out on 48 continuous borehole samples in the CY-1 well, such as analyses of the total organic carbon content (TOC), total sulfur content (TS), pyrite sulfur isotopes (δ34Spy), LA-ICP-MS analyses, and pyrite morphology analyses. Before analysis, thin slices of about 3–5 mm were cut from the surface of the rock sample to reduce the effect of sample oxidation.
3.1. Optical Microscopy and FE-SEM Imaging
The mineralogical analyses were carried out at the Mineralogy Laboratory of China University of Geoscience (Beijing) using a ZEISS Axioscope 40 petrological microscope. The scanning electron microscope (SEM) analyses were carried out at the Microstructure Laboratory for Energy Materials of China University of Petroleum (Beijing) using a HITACHI SU8010 SEM at a working distance of 15 mm, voltage of 20 kV, current of 10 nA, and signal intensity of 5000 cps. Before the sample was observed and imaged, a Buehler Automet 250 polishing machine in the DGS sample preparation laboratory was used for cleaning and polishing preparation.36 Due to the unique crystal morphology and high atomic number of pyrite crystals, they show bright colors in the backscattered electron mode of SEM. Next, NIH software Fiji was used to process and analyze SEM images. Finally, the size of the framboid pyrite is measured. Because it is difficult to ensure that the size of each framboid is measured through the axis, the measured diameter of framboid pyrite is generally smaller than the actual diameter. Wilkin et al.14 studied and found that the deviation of this method is small (usually less than 10%), so it can ensure the practicability of this method.
3.2. Total Organic Carbon and Total Sulfur Concentrations
For the analysis of total organic carbon (TOC) and total S (TS) concentrations, 40 samples were collected from the CY-1 well. In order to avoid core weathering polluting the sample, each sample was cut from the inside of the core. The samples were tested and analyzed at the State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum (Beijing). All of the selected samples were crushed into powder and then screened with a 20-mesh sieve. 10% hydrogen chloride (HCl) was added to the obtained powder sample to eliminate the influence of inorganic carbon in the sample. Distilled water was then used to remove the residual HCl in the sample. Finally, TOC and TS analysis were performed on all samples using LECO CS-230; see Chen et al.37 The analytical accuracy of TOC and TS is greater than 0.03%.
3.3. Sulfur Isotope Analysis of Bulk-Rock Pyrite
Sulfur isotope analysis of pyrite (δ34Spy) was performed by the Analytical Laboratory of the Beijing Nuclear Industry Geological Analysis and Testing and Research Center in 40 samples of the Niutitang shale. The pyrite sulfur isotopes were analyzed using the chromium reduction method.38 In order to eliminate the influence of acid volatile sulfide (AVS) on the experimental results, an appropriate amount of powder sample was mixed with 4.8 mol/L HCl at 75 °C for more than 2 h. Then the residual solution was mixed with bromide water until the solution was colorless. Then 15 mL of 10% barium chloride solution was added to the reaction mixture for about 1 h, and the reaction mixture was allowed to sit for about 12 h. The released hydrogen sulfide gas was soaked into the mixed solution of 2% silver nitrate solution and 10% ammonium hydroxide, filtered, and precipitated by silver sulfide (Ag2S) δ34Spy analysis. Finally, the solution was filtered with a 25 mm filter and the precipitated silver sulfide wsa placed in the oven for one night. The analytical precision of the device is limited to ±0.2‰ for δ34S, and the δ34S was shown as per mil (‰) relative to the Vienna Canyon Diablo Troilite (VCDT) standard and was calibrated using the international standards: IAEA-S-1 (δ34S: −0.30‰), IAEA-S-2 (δ34S: 22.64‰), and IAEA-S-3 (δ34S: −32.06‰).
3.4. Analysis of Element Characteristics of Pyrite
The major elements (Fe, S) in pyrite were analyzed by a Shimadzu EPMA-1600 electron probe in the State Key Laboratory of deposit geochemistry, Chinese Academy of Sciences. Test conditions: beam spot 10 μm. The current is 10 nA and the accelerating voltage is 25 kV. The standard sample is from the National Bureau of standards.
Trace element concentrations in magnetite were determined by LA-ICP-MS on polished thick sections at the Key Laboratory of Regional Geology and Mineralization, Hebei Geo University. The instrument coupled a quadrupole ICP-MS (THERMO-ICAP RQ, Thermo Scientific) and a 193 nm ArF Excimer laser (Resolution-LR, Australian Scientific Instruments) with a Laurin Technic S155 sample chamber and GeoStarμ GIS software. For the present work, the laser spot size was set to 29 μm for most analyses to trace sulfur from the ore-forming material effectively because selecting a laser beam with a small diameter avoids mixed signals from different sulfides that result in mixed values of the trace elements. The laser energy density was set at 3 J/cm2, and the repetition rate was at 8 Hz. The timing procedure of the laser sampling was 10 s blank, 40 s sampling ablation, and 20 s sample-chamber flushing after the ablation. The ablated material was carried into the ICP-MS using a high-purity helium gas stream with a flux of 0.6 L/min. The entire laser path had an Ar flow (0.8 L/min) to ensure energy stability.
4. Results
4.1. Pyrite Morphology
Pyrite is widely distributed in the shale of the Niutitang Formation in CY-1 well. According to the formation characteristics, pyrite can be divided into sedimentary type, diagenetic metamorphic type, and hydrothermal type. Specifically, the main morphological characteristics of sedimentary pyrite are framboid pyrite and diagenetic metamorphic pyrite. The main morphological characteristics are euhedral or semieuhedral pyrite, and the main forms of hydrothermal pyrite are vein pyrite or colloidal pyrite.
Pyrite in the upper part of the Niutitang Formation is mainly framboid pyrite (Figure 2a) and a small amount of euhedral crystalline pyrite (Figure 2b). Under the scanning electron microscope, pyrite exists in the shale matrix in the form of a framboid aggregate (Figure 2c), and the quantity is large. It is unevenly distributed on the surface of the sample. It is common that a large amount of pyrite is concentrated in a certain area of the sample (Figure 2d). Most framboid pyrites have clear contours, clear internal microcrystalline particles, and obvious boundaries (Figure 2e). Organic matter filling can be seen between microcrystalline particles, and many organic matter pores (Figure 2f) can be seen on the filled organic matter. The morphology of this type of pyrite is usually changed by diagenesis to varying degrees, which is characterized by fewer nano intergranular pores and the increase of microcrystalline size, or the appearance of overgrown rings (Figure 2b). With the continuous evolution of framboid pyrite spheres, filled framboid pyrite can be formed, which is characterized by a smooth and uniform shape and no obvious pore structure on the surface. In addition, a small number of euhedral crystals of pyrite can be seen. The shape of the euhedral crystals is usually octahedron and pentahedron. Some euhedral crystals gather with each other to form aggregates (Figure 2e), and the euhedral pyrite crystals are distributed in pits in the cracks.
Figure 2.
SEM observation of pyrite in shale of Niutitang Formation of CY-1 well. (A) Normally developed framboids, (Sample N1-1). (B) Oversized framboid is associated with euhedral pyrite, in which oversized framboid has different grain sizes, and the peripheral grains are large and disordered, (Sample N1-3). (C) Framboids growing along fracture pores, (Sample N1-4). (D) The single crystal of euhedral granular pyrite has regular morphology and coexists with framboids, (Sample N1-6). (E) Nodular infilled pyrite is composed of core part and edge part. The nodular shape is generally spherical or ellipsoidal. It coexists with euhedral pyrite in pores, and the pyrite crystal shape is regular and complete, (Sample N1-7). (F) Part of the microcrystalline gap of normal framboid is filled, and the grain size of pyrite in the same sample varies greatly (Sample N1-9).
Framboid pyrite developed in the middle and lower segment of Niutitang Formation has both single framboid microcrystals and its aggregates. Some euhedral pyrites have uneven crystal size but closely arranged structures (Figure 3a), and their distribution is mainly controlled by pores. Some framboid pyrites form oversized lump pyrites (Figure 3b) due to late diagenesis. Anhedral pyrite with uneven crystal size is relatively developed, and its aggregates are usually vein or colloidal (Figure 3d). Part of the euhedral pyrite crystal is wrapped by silica, the solution pores in the crystal are filled with silica (Figure 3e), the solution pores are filled with silica, and part of the euhedral pyrite is associated with framboid pyrite. Shale core photos show that pyrite is unevenly distributed and developed and accumulated to a certain extent to form a network or belt. Under some thin sections, it is observed that belt or debris aggregation (Figure 3f) is the main mode of existence.
Figure 3.
SEM photomicrographs of pyrite morphology. (A) Euhedral pyrite grows along pores and fractures (Sample N2-2). (B) The filled framboid shows the wrapped growth of grains in the later filling process. The ellipsoidal pyrite has a smooth surface, ellipsoid and smooth surface, which cannot be distinguished from the crystal form and obviously has epigenetic properties (Sample N2-5). (C) The microcrystalline particles in the normally developed polyframboid show a variety of forms, including cube, octahedron, pentagonal dodecahedron, and so on (Sample N2-8). (D)Anhedral-subhedral pyrite lump is similar to pentagonal, but the edges of pentagonal are obviously damaged, which is the result of typical epigenetic action (Sample N3-1). (E) Pyrite is mostly distributed in fractures, and its shape changes greatly under the control of the fault surface or structural fractures (Sample N3-5). (F) Broken pyrite filled with mud (Sample N3-8).
4.2. Size Distribution of Pyrite Framboids
Polishing and SEM analyses were carried out on 32 samples, of which 21 were rich in framboid pyrite. In addition, only part of the framboid pyrite or no pyrite was found in the other 11 samples, which greatly reduced the statistical significance. Therefore, such samples were not included in the statistical range. The diameter of filled framboid pyrite was slightly larger than that of unfilled framboid pyrite, but the difference was negligible; it has no significant effect on the overall statistical results of framboid pyrite. Wignall and Newton39 pointed out that Max Framboid Diameter (MFD) is an important indicator indicating the redox conditions of water column. They believe that the MFD of framboid pyrite formed under oxic-dysoxic conditions is usually greater than 20 μm. The MFD of framboid pyrite deposited in an anoxic environment is generally less than 20 μm. Through detailed observation and statistics (Table 1), we found that the maximum particle size of 9 pyrite samples in the upper section of the Niutitang Formation is greater than 20 μm (N1-1, N1-3, N1-4, N1-6, N1-8, N1-9, and N1-10 respectively). The maximum particle size of some samples (N1-4, N1-6) is greater than 25 μm, and the maximum particle size of other pyrite samples is less than 20 μm. This shows that the middle and upper sections of the Niutitang Formation may be in the process of transformation from a sulfide environment to a nonsulfide environment. From the statistical results of framboid pyrite particle size in the middle and lower sections of the Niutitang Formation, the maximum particle size is less than 18 μm. It shows that the samples in the middle and lower sections were formed in a relatively anoxic environment.
Table 1. Statistics of Particle Size Distribution of Framboid Pyrite in Black Shale of Niutitang Formation of the CY-1 Well.
| Sample No. | Depth (μm) | Median (μm) | Mean (μm) | MFD (μm) | Min (μm) | Standard deviation | Skewness | No. |
|---|---|---|---|---|---|---|---|---|
| N1-1 | 1401.49 | 8.9 | 9.7 | 21.2 | 3.8 | 7.2 | 3.4 | 89 |
| N1-3 | 1403.38 | 9.9 | 9.8 | 21.3 | 3.9 | 5.7 | 1.9 | 135 |
| N1-4 | 1407.56 | 9.1 | 10.9 | 28.1 | 3.5 | 7.5 | 3.2 | 96 |
| N1-5 | 1410.83 | 8.6 | 8.9 | 19.5 | 4.2 | 5.2 | 2.8 | 93 |
| N1-6 | 1412.94 | 9.8 | 9.9 | 20.5 | 3.1 | 6.7 | 3.1 | 119 |
| N1-8 | 1416.42 | 11.4 | 10.1 | 24.2 | 4.5 | 5.9 | 1.7 | 126 |
| N1-9 | 1417.86 | 12.5 | 9.5 | 22.1 | 7.1 | 6.1 | 2.6 | 94 |
| N1-10 | 1419.43 | 9.3 | 9.3 | 17.4 | 2.7 | 5.8 | 2.5 | 114 |
| N2-1 | 1421.92 | 9.1 | 8.8 | 15.8 | 3.6 | 3.7 | 2.1 | 92 |
| N2-2 | 1424.37 | 8.3 | 7.9 | 13.8 | 3.1 | 1.9 | 1.5 | 67 |
| N2-3 | 1426.47 | 7.7 | 8.5 | 11.3 | 4.6 | 1.8 | 2.6 | 117 |
| N2-4 | 1428.63 | 8.9 | 9.3 | 15.1 | 4.7 | 3.7 | 1.6 | 86 |
| N2-5 | 1430.61 | 8.3 | 8.4 | 14.5 | 2.9 | 6.1 | 1.8 | 81 |
| N2-6 | 1431.89 | 7.6 | 7.9 | 15.7 | 4.3 | 4.4 | 2.3 | 83 |
| N2-8 | 1435.16 | 9.2 | 4.5 | 13.6 | 3.5 | 5.1 | 3.2 | 95 |
| N2-9 | 1437.72 | 7.2 | 7.4 | 14.5 | 3.7 | 5.5 | 2.9 | 93 |
| N2-10 | 1439.24 | 7.1 | 6.9 | 11.9 | 3.4 | 2.4 | 1.9 | 62 |
| N3-2 | 1442.91 | 5.8 | 5.3 | 12.5 | 1.5 | 1.6 | 1.3 | 79 |
| N3-3 | 1444.18 | 5.6 | 4.4 | 11.2 | 2.6 | 2.7 | 2.2 | 63 |
| N3-5 | 1447.25 | 5.4 | 6.4 | 13.7 | 2.9 | 2.9 | 2.8 | 61 |
| N3-7 | 1450.13 | 4.3 | 4.6 | 9.1 | 2.6 | 2.7 | 2.3 | 78 |
The box-and-whisker plot method was used to study the grain size distribution pattern of framboid in the shale of the Niututang Formation (Figure 4). The vertical line in the middle of the box plot represents the median value of framboid pyrite in statistical grain size, and the horizontal line represents the distribution range of framboid pyrite grain size. The particle size of framboid in the upper Niutitang Formation is large and the particle size distribution range is wide, ranging from 2.7 to 28.1 μm (mean 9.6 μm). The sample with the widest particle size distribution is N1-4, and the MFD is 28.1 μm. The minimum size is 3.5 μm. The diameter distribution range of framboid in the middle Niutitang Formation is narrow, with a range of 2.9–15.8 μm. (mean 6.8 μm). N2-6 has the widest particle size distribution range, and the MFD is 15.7 μm. The minimum size is 4.3 μm. The diameter distribution range of framboid in the lower Niutitang Formation is 1.5–13.7 μm. (mean 5.3 μm).
Figure 4.
Pyrite framboids box-and-whisker plots of the Niutitang Shale, the CY-1 core.
4.3. Geochemical Characteristics
The total organic carbon (TOC), total sulfur (TS), TOC\TS, and pyrite sulfur isotope content of the shales of the Cambrian Niutitang Formation in well CY-1 are shown in Table 2. The organic carbon and total sulfur change similarly, with higher TOC values in the middle section of the Niutitang Formation, followed by the lower section and smaller in the upper section. The TOC of the upper section of the Niutitang Formation (1400–1419m) ranged from 0.76% to 2.77% (mean 1.79%). The TOC of the middle section of the Niutitang Formation (1419–1438 m) ranged from 4.84% to 8.5% (mean 6.59%). The TOC of the lower section of the Niutitang Formation (1438–1454 m) ranged from 1.24% to 6.85% (mean 4.29%). The value of TS in the three submember of Niutitang Formation is 1.32%–4.97% (mean 3.04%); 1.61%–6.62% (mean 3.64%); and 1.37%–5.48% (mean 3.07%). The sulfur isotopic compositions of pyrites (δ34Spy) also change regularly in the vertical direction. The δ34Spy value in the upper Niutitang Formation is −5.64‰–6.86 ‰ (mean 0.25‰); The δ34Spy value in the middle is −7.76 ‰ −17.55 ‰ (mean 9.57 ‰); The δ34Spy value in the lower part is −3.12‰ to 9.24‰ (mean 1.71 ‰).
Table 2. Total Organic Carbon (TOC), Total Sulfur (TS), TOC/TS Ratios, and Sulfur Isotopic Compositions in Pyrites (δ34Spy) of the Niutitang Shale.
| Sample | TOC | TS | TOC/TS | δ34Spy |
|---|---|---|---|---|
| N1-1 | 1.46689 | 3.75126 | 0.42735 | 0.25 |
| N1-2 | 1.60743 | 3.42028 | 0.470085 | –4.91951 |
| N1-3 | 0.796499 | 2.29153 | 0.34188 | –1.71645 |
| N1-4 | 1.54531 | 2.80407 | 0.555556 | –2.7494 |
| N1-5 | 2.7741 | 4.97864 | 0.598291 | –5.6462 |
| N1-6 | 2.73928 | 1.32172 | 2.05128 | 1.28364 |
| N1-7 | 0.763246 | 3.04553 | 0.17094 | 6.8643 |
| N1-8 | 2.45977 | 2.93286 | 0.897436 | 4.23842 |
| N1-9 | 1.97133 | 2.44346 | 0.897436 | 4.78208 |
| N1-10 | 1.87376 | 3.45012 | 0.598291 | 2.93636 |
| N2-1 | 4.8433 | 1.61459 | 2.90598 | 17.5572 |
| N2-2 | 6.62139 | 2.92113 | 2.30769 | 17.0507 |
| N2-3 | 6.04668 | 2.81789 | 2.13675 | 9.10417 |
| N2-4 | 4.91797 | 4.83969 | 1.06838 | 16.0277 |
| N2-5 | 7.49569 | 6.6281 | 1.11111 | 13.1227 |
| N2-6 | 6.44948 | 2.4332 | 2.64957 | 5.17624 |
| N2-7 | 5.17175 | 4.69781 | 0.940171 | 3.06684 |
| N2-8 | 7.67073 | 2.63078 | 2.90598 | 13.1707 |
| N2-9 | 8.5077 | 3.46865 | 2.47863 | 9.20928 |
| N2-10 | 8.25266 | 4.39186 | 1.88034 | –7.76466 |
| N3-1 | 3.73906 | 2.47853 | 1.45299 | 9.24462 |
| N3-2 | 4.4336 | 2.06881 | 2.13675 | 4.47673 |
| N3-3 | 6.8529 | 3.06982 | 2.26496 | 3.432 |
| N3-4 | 4.39125 | 2.10703 | 2.05128 | 7.697 |
| N3-5 | 3.11541 | 2.87747 | 1.11111 | 0.007853 |
| N3-6 | 5.78317 | 1.37293 | 4.2735 | –1.5751 |
| N3-7 | 6.38674 | 3.86717 | 1.53846 | –0.22955 |
| N3-8 | 1.24698 | 2.74219 | 0.34188 | –2.06983 |
| N3-9 | 5.09019 | 5.48396 | 0.940171 | –3.12181 |
| N3-10 | 1.91988 | 4.67488 | 0.384615 | –0.71431 |
In order to explore the sedimentary environment of Niutitang Formation shale from the perspective of trace element composition of pyrite, the occurrence characteristics of trace elements in 33 pyrite samples were analyzed. The test results are shown in Table 3. The relatively enriched trace elements include Co, Ni, Cu, Zn, Mo, U, V, Al, Ag, and other elements. The contents of redox-sensitive elements such as Mo, U, and V in the upper part of the Niutitang Formation are, respectively, 1.07–7.12 × 10–6 ppm (average = 3.85 × 10–6 ppm); 0.61–7.55 × 10–6 ppm (average = 2.81 × 10–6 ppm); and 2.61–30.585 × 10–6 ppm (average = 2.605 × 10–6 ppm). The contents in the middle section of Niutitang Formation are, respectively, 4.04–17.838 × 10–6 ppm (average = 12.025 × 10–6 ppm); 3.2347–106.771 × 10–6 ppm (average = 42.2135 × 10–6 ppm); and 38.57–826.636 × 10–6 ppm (average = 322.841 × 10–6 ppm); The contents in the lower part of Niutitang Formation are, respectively, 8.837–126.619 × 10–6 ppm (average = 49.058 × 10–6 ppm); 5.649–978.342 × 10–6 ppm (average = 214.943 × 10–6 ppm); and 26.334–3825.23 × 10–6 ppm (average = 974.179 × 10–6 ppm).
Table 3. Results of the LA–ICP–MS Analysis of Selected Pyrites from the Niutitang Formation (10–6).
| Mo (ppm) | U (ppm) | V (ppm) | Al (ppm) | Co (ppm) | Ni (ppm) | As (ppm) | Cu (ppm) | Se (ppm) | Mn (ppm) | Cr (ppm) | Zn (ppm) | Ag (ppm) | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| NTT1 | 1.007 | 0.617 | 8.534 | 47.108 | 12.049 | 98.973 | 36.115 | 158.435 | 15.430 | 427.736 | 25.830 | 8.282 | 0.233 |
| 2.204 | 4.483 | 20.110 | 58.039 | 15.033 | 122.444 | 34.258 | 140.911 | 31.129 | 244.459 | 18.620 | 14.121 | 0.809 | |
| 2.857 | 2.326 | 8.022 | 61.742 | 77.257 | 86.537 | 19.257 | 67.230 | 18.031 | 149.879 | 12.010 | 208.502 | 0.713 | |
| 3.948 | 3.564 | 20.179 | 77.034 | 16.105 | 141.725 | 28.877 | 103.606 | 10.847 | 343.272 | 34.630 | 9.268 | 1.825 | |
| 4.738 | 1.040 | 2.605 | 70.293 | 75.528 | 325.109 | 22.417 | 53.739 | 26.637 | 232.593 | 27.740 | 3.097 | 1.077 | |
| 3.401 | 1.899 | 13.024 | 48.712 | 30.072 | 419.854 | 26.415 | 116.537 | 17.093 | 393.040 | 22.100 | 5.471 | 2.252 | |
| 6.170 | 1.808 | 30.585 | 66.645 | 91.458 | 1232.290 | 72.716 | 67.331 | 44.852 | 10.586 | 3.960 | 13.468 | 0.632 | |
| 7.123 | 7.560 | 27.390 | 76.299 | 79.045 | 701.336 | 57.734 | 34.833 | 28.671 | 19.079 | 2.015 | 5.635 | 0.408 | |
| 5.321 | 5.905 | 9.925 | 52.377 | 58.539 | 496.936 | 36.562 | 64.719 | 92.796 | 22.827 | 5.882 | 5.438 | 0.133 | |
| 3.487 | 1.681 | 6.470 | 27.645 | 160.486 | 313.893 | 37.923 | 72.545 | 93.308 | 24.216 | 5.947 | 14.191 | 0.205 | |
| 4.286 | 2.948 | 20.551 | 32.359 | 63.031 | 437.832 | 32.692 | 1654.525 | 86.369 | 22.674 | 4.942 | 23.493 | 0.184 | |
| 4.193 | 1.743 | 4.078 | 30.412 | 121.214 | 756.253 | 38.973 | 2478.990 | 77.003 | 43.150 | 8.781 | 70.728 | 0.234 | |
| 1.442 | 0.840 | 2.946 | 10.034 | 65.450 | 599.508 | 68.297 | 64.811 | 92.745 | 19.121 | 7.347 | 12.092 | 0.169 | |
| NTT2 | 4.124 | 31.117 | 78.901 | 51.819 | 24.135 | 359.687 | 15.356 | 275.496 | 4.064 | 107.635 | 0.284 | 30.348 | 0.379 |
| 5.301 | 53.548 | 38.571 | 49.396 | 97.115 | 103.120 | 21.490 | 6442.520 | 35.599 | 5.857 | 0.507 | 906.054 | 0.198 | |
| 9.141 | 106.772 | 731.551 | 50.642 | 561.329 | 251.424 | 14.922 | 305.106 | 104.810 | 2536.540 | 1.771 | 2.650 | 2.444 | |
| 14.885 | 33.765 | 826.636 | 52.193 | 934.531 | 576.729 | 10.278 | 117.611 | 31.307 | 378.501 | 31.155 | 4.142 | 1.972 | |
| 16.396 | 51.632 | 349.240 | 51.607 | 104.722 | 2271.120 | 15.472 | 112.671 | 52.420 | 134.845 | 4.604 | 21.091 | 4.533 | |
| 9.594 | 22.993 | 47.258 | 26.899 | 733.192 | 2871.490 | 18.225 | 129.240 | 60.003 | 329.271 | 16.274 | 19.752 | 6.045 | |
| 14.986 | 60.993 | 459.102 | 30.281 | 402.463 | 694.379 | 28.252 | 9232.890 | 21.873 | 2.249 | 3.604 | 9.146 | 0.067 | |
| 16.052 | 24.865 | 370.887 | 27.750 | 254.528 | 97.763 | 11.684 | 9811.920 | 24.516 | 0.487 | 0.242 | 144.980 | 0.114 | |
| 17.834 | 3.237 | 93.416 | 27.930 | 548.013 | 1374.320 | 41.609 | 4557.510 | 46.415 | 23.554 | 2.745 | 6.905 | 1.767 | |
| NTT3 | 11.928 | 98.243 | 338.050 | 65.464 | 619.447 | 719.075 | 13.359 | 5981.400 | 31.706 | 5.493 | 5.901 | 4.280 | 0.530 |
| 56.322 | 145.440 | 2202.636 | 250.872 | 74.282 | 1854.497 | 21.265 | 6999.580 | 39.250 | 54.473 | 5.256 | 1.668 | 10.546 | |
| 21.015 | 165.158 | 1078.060 | 62.317 | 75.518 | 329.902 | 14.571 | 376.680 | 46.444 | 960.438 | 4.301 | 47.248 | 1.902 | |
| 126.619 | 513.921 | 3825.230 | 278.752 | 89.215 | 419.195 | 23.452 | 568.787 | 36.432 | 433.317 | 3.196 | 12.701 | 0.391 | |
| 122.748 | 978.342 | 2175.341 | 268.875 | 0.435 | 1.913 | 4.285 | 22.849 | 79.102 | 1897.850 | 0.483 | 5.548 | 0.003 | |
| 39.533 | 177.631 | 622.948 | 69.916 | 1.905 | 6.689 | 2.529 | 27.993 | 38.806 | 9.921 | 7.227 | 10.156 | 0.018 | |
| 18.346 | 14.034 | 26.334 | 23.819 | 0.789 | 8.251 | 1.314 | 0.070 | 39.900 | 3.127 | 0.018 | 108.351 | 0.002 | |
| 24.709 | 51.981 | 149.798 | 27.098 | 2.127 | 39.454 | 22.717 | 1.200 | 53.885 | 4.212 | 0.239 | 46.181 | 0.002 | |
| 8.837 | 13.252 | 28.977 | 9.480 | 2.337 | 7.504 | 0.482 | 0.052 | 8.011 | 0.003 | 0.138 | 19.683 | 0.029 | |
| 35.286 | 5.650 | 177.917 | 33.773 | 3.517 | 86.484 | 3.178 | 0.021 | 10.137 | 0.034 | 0.005 | 3.965 | 0.188 | |
| 74.302 | 200.727 | 90.682 | 65.155 | 0.002 | 0.056 | 1.203 | 0.269 | 2.203 | 0.675 | 0.396 | 1.671 | 0.719 |
5. Discussion
5.1. The Redox Conditions Were Reconstructed by Pyrite Morphology and Framboid Size Distribution
Pyrite (FeS2) is the most common sulfide mineral in the earth’s crust. It can be produced not only in the hydrothermal environment but also in a low-temperature diagenetic environment. Pyrite formed in a low-temperature diagenetic environment is a characteristic mineral rich in organic deposition and an important index for restoring the sedimentary environment.22 The morphology of authigenic minerals is related to their formation conditions. Therefore, the study of sedimentary pyrite morphology can provide useful information about the sedimentary environment and early diagenetic process.14 For example, Goldhaber and Kaplan39 suggested that pyrite morphology is related to the type of reaction that generates pyrite; i.e., framboid pyrite is formed by slow transformation of greigite, whereas euhedral pyrite is formed by direct precipitation from solution. However, Schoonen and Barnes showed that euhedral pyrite does not form directly from a homogeneous solution but rather from the transformation of amorphous FeS generated by the initial supersaturation of the solution. Although the mechanism of formation of specific pyrite forms is still debated, it is at least certain that different pyrite forms have different formation pathways.
Using FE-SEM, we can distinguish framboid pyrite, euhedral pyrite, and anhedral pyrite in shale. There are many styles of pyrite crystals in Niutitang Formation shale in cengong area, Guizhou. As shown in Figure 2d, most framboid pyrites are similar in shape and size. Because the growth of framboids is very sensitive to environmental factors, scholars usually use the size of framboid pyrite to limit the redox conditions of the paleomarine environment.14,25,40Figure 2B shows the growth of framboid pyrite inclusions. Similarly, in this sample, framboid microcrystalline particles of different sizes reflect the different growth time of microcrystals. In framboid bodies, the larger hexahedral microcrystals indicate a long growth time in sediments (Figure 2e). These diagenetic framboid are usually formed in sediments under an oxic-dysoxic water column.22 In addition, framboid pyrite aggregates, filled pyrite, and overgrown framboid bodies are also formed in sediments of early diagenesis.41
Some samples have very similar textures (Figure 3a), which indicates the local enrichment of pyrite. The quartz cement and shale connect the pores to retain the original framboid pyrite structure to the greatest extent and avoid the change of later action. The space and shape between framboid pyrite particles show that they have no trace of mechanical compaction. Polyframboids crystals40 (Figure 3c) are also found in the samples. This type of pyrite has been reported by many scholars all over the world,44,45 indicating that the growth rate is slow in the later stage of early diagenesis,22 and finally formed in the later stage of diagenesis. Framboid pyrite crystals are found in the middle section; one is formed in fine-grained sediments, and the other is formed in voids with organic matter. Euhedral pyrite crystals are very common in the middle and lower section (Figure 3a). The combination of framboid pyrite and euhedral pyrite was found in the shale samples of the Niutitang Formation (Figure 3b), suggesting the formation mode of evolution from framboid pyrite to euhedral crystals.41 Although sulfide is very common in the sedimentary environment, observation of the samples shows that the reactive iron from the rhodochrosite around the siliceous shale at the bottom of the Niutitang Formation and the iron ions from the volcanic hydrothermal action lead to high reactive iron during the formation of the shale at the bottom of the Niutitang Formation, while the occurrence of hydrothermal activity also locally changes the redox conditions of the pore water and a large amount of Fe and Mn elements enter the water column and the anoxic reduction environment rich in H2S was formed at the bottom. The iron minerals surrounding pyrite are more indirectly indicative of the reduction of pore water during the diagenesis of the diagenetic shale (Figure 3f). In the middle and lower siliceous shales of the Niutitang Formation, a large amount of anhedral pyrite with uneven crystal size was also found. Anhedral pyrite was found to have undergone recrystallization or alteration during a magmatic hydrothermal intrusion as indicated by accountable remnant structures and metamorphic colloidal structures developed, with obvious recrystallization and obvious hydrothermal accounting (Figure 3e), distributed in the bottom siliceous shale in the form of dip or star, or interspersed in the early calcite pores in the form of fine veins. Only from the morphology and distribution of the minerals themselves, part of the anhedral pyrite may be euhedral pyrite or framboid pyrite through the late deformation recrystallization–hydrothermal superposition account, a small amount of anhedral pyrite has the crystal shape of euhedral pyrite, and a small amount of sphalerite and chalcopyrite is seen growing along its edge or crevice, and the phenomenon of framboids wrapped inside can be seen locally, so it is tentatively inferred that anhedral pyrite may be the product of the sedimentary period and the product of hydrothermal activity.
Pyrite is formed by the reaction of hydrogen sulfide produced by bacterial sulfate reduction with Fe ions in seawater.22 In the oxidation anoxic water column, when the redox boundary is above or below the sediment water interface, the pyrite formed in the sediment tends to have a large particle size (>10 μm).42 On the contrary, due to the movement of redox interface to the upper part of water column, the oxygen content around syngenetic pyrite decreases. In this case, framboid pyrite formed in a suboxic-anoxic environment usually has a small size (<5.7 μm).22 Therefore, the size distribution of framboid pyrite has been widely used to indicate the redox conditions of the water column.14,42,43 The box-and-whisker diagram of the upper shale of the Niutitang Formation (N1-4) has a larger maximum framboid pyrite grain size distribution (maximum >25 μm) and a smaller minimum framboid pyrite grain size (3.5 μm), with a decreasing trend. The wider size distribution (4.5–24.2 μm) and larger intermediate size (11.4 μm) in the upper part of the Niutitang Formation suggest that the water column is in the oxic-dysoxic transition stage; in the middle part of the Niutitang Formation (N2-5), the pyrite particle size exhibits a very narrow size range (2.9–14.5 μm) with a small median value (8.3 μm), indicating that the deposition process may be in intermittent anoxic conditions. In addition, in the middle and lower part of the Niutitang Formation, some samples have similarly narrow size distribution (N2-8, N2-10) and small median values (6.9 μm), with the largest grain size being only 13.6 μm, and the deposited environment is in an euxinic condition. According to the above grain size characteristics of framboid pyrite in the Niutitang Formation, the oxic-dysoxic, anoxic, and euxinic conditions were classified.
Table 1 shows that the average size difference between the two groups of samples in the middle section (N2 submember) and the lower section (N3 submember) of the Niutitang Formation is narrow, and their sedimentary environment is similar, but the MFD of the samples in the middle section of Niutitang Formation is large, indicating that there are differences in redox conditions between the two groups of samples. Wignall and Newton17 pointed out that MFD is an important indicator indicating the redox conditions of pore water. They believe that the MFD of framboid pyrite formed under the condition of oxic-dysoxic is usually greater than 20 μm. The MFD of framboid pyrite deposited in euxinic environment is generally less than 20 μm. Through detailed observation and statistics, we found that the MFD of the upper part of Niutitang Formation was greater than 21 μm. Some samples (N1-4, N1-6) have MFD greater than 25 μm. The MFD of the samples in the middle section of Niutitang Formation was less than 15 μm. This shows that the samples in the top part of Niutitang Formation (N1-1, N1-10) are formed at the oxic-dysoxic interface, and some samples in the middle part of Niutitang Formation are formed in an euxinic environment.
The redox conditions of the water column in the process of shale deposition of Niutitang Formation can be divided in more detail through the relationship diagram between the average value and standard deviation17 (Figure 5). In oxic conditions, framboid pyrite is usually less developed or undeveloped.46 The dotted line in the figure separates euxinic–anoxic and dysoxic–oxic conditions. Figure 5 shows that there are obvious differences between each submember of the Niutitang Formation. Except for individual samples, pyrite in the upper section of the Niutitang Formation is formed in a dysoxic–oxic environment (occasionally interrupted by oxic conditions) (N1-7, N1-10, N2-3). The sample distribution is closer to the euxinic–anoxic interface, which indicates that the oxygen consumption of bottom water is more serious than that of the upper section. Anoxic conditions interrupted by euxinic conditions occur at the middle and lower members of the Niutitang Formation. This is basically consistent with the environment indicated by the data in the box whisker diagram and MFD. Although the size parameters of some samples in the middle and lower section of the Niutitang Formation are close to the range of the dysoxic–oxic environment, they are also very close to the threshold of the anoxic environment and the average particle size of some samples is 7.4–9.2 μm, which is much coarser than the framboid pyrite formed in the modern anoxic or euxinic environment, so the local samples may be formed in the dysoxic–oxic environment.
Figure 5.
(A) Relationship between average particle size and skewness of particle size of framboid in the Niutitang Formation. (B) Relationship between standard deviation of particle size and skewness of particle size of framboid in the Niutitang Formation.
Through the multiple analysis of the particle size distribution of framboid pyrite in the three subsections of Niutitang Formation, it is concluded that the upper section of Niutitang Formation is in dysoxic–oxic environment, the middle siliceous shale is mainly formed in anoxic reduction environment, and the redox conditions in the lower part are more reductive than those in the middle and upper part. However, the effect of each index of framboid pyrite particle size distribution on indicating the redox conditions of water may be limited. For example, this method cannot judge the hypoxic environment (oxygen content <0 mL/L) and iron hypoxic environment (oxygen content <0.2 mL/L). Therefore, it is necessary to combine other indicators to indicate the redox conditions of the paleoenvironment.
The TOC of Niutitang Formation samples show strong spatial and stratigraphic variability. Zuo et al. reported the phenomenon of local high TOC in Niutitang Formation. In the samples of the Niutitang Formation of CY-1 well, the highest TOC is 7.78%. The TOC value mainly reflects the original accumulation of organic matter, which is affected by factors such as paleoproductivity, organic matter preservation conditions, and terrigenous input.47 By observing the TOC of the middle section (N2-2, N2-6) of the Niutitang Formation and the particle size of framboid pyrite, it can be found that the middle and lower section (1418–1440 m) of the Niutitang Formation are rich in framboid pyrite, and the average size of framboid pyrite is small (about 8.4 μm). The size distribution range is relatively narrow, which corresponds to the higher TOC. The small particle size and relatively narrow distribution range of framboid pyrite indicate that the water column environment is in an anoxic or euxinic redox state during the deposition process, which can provide good preservation conditions for organic matter. Many euhedral pyrites are found in the middle and upper members of the Niutitang Formation, which are usually formed in diagenesis. It can be found from Table 2 that the TOC is significantly smaller than that of the middle and lower members, which indicates that the reduction state of its sedimentary environment is significantly smaller than that of the middle and lower members. The larger particle size and distribution of framboid pyrite and the larger range of size distribution in the upper part of Niutitang Formation, as well as many euhedral pyrites, jointly indicate that the upper part of Niutitang Formation is in an oxic-dysoxic environment. Due to the continuous fluctuation of sea level, the diagenetic environment is constantly changing, and the hydrodynamic conditions are relatively strong, which is not suitable for the preservation of organic matter, which is consistent with the phenomenon of low TOC in the upper part of Niutitang Formation.
5.2. Depositional Conditions Constrained by δ34Spy Records
Sulfate reduction often occurs in a reduction environment with relatively rich organic matter and oxygen deficiency.48 The formation of sulfide can be controlled by the supply of organic matter, the concentration of iron ions participating in the reaction, the rate of deposition, and the supply of sulfate in the system. Compared with sulfate concentration, the concentration of organic matter that can be metabolized by bacteria has a greater impact on sulfate reduction.38,49 The above factors also control the effect of sulfur isotope fractionation in the process of sulfate conversion to sulfide.42,50
The lithology, TOC, TS, and δ34Spy of CY-1 well are shown in Figure 6. According to the trend of sulfur isotope values of pyrite, overall, there is a significant negative deviation in the upper part of the Niutitang Formation and a significant positive deviation in the middle and lower part of the Niutitang Formation. The sulfur isotope composition of pyrite in the high TOC section of the middle and lower Niutitang Formation is relatively heavy, and its δ34S value continuously drifted positive from −7.4 ‰ to +16.8 ‰. Sulfate ions in modern seawater have a uniform and stable sulfur isotope composition and their δ34S value is +21 ‰. However, in the Early Cambrian period, the content of sulfate ion in seawater was low, and its δ34S value was relatively high, about +32 ‰; especially in the transition period between Ediacaran and Cambrian, sulfate ions in seawater δ34S values may be as high as +40 ‰.51 This is mainly because in the whole Cryptozoic period, the earth’s atmosphere was basically in a reducing environment, and the low atmospheric oxygen content made pyrite stable in the continental crust, which could not be oxidized into soluble sulfate ions and imported into the ocean with the river. If we assume that in the Early Cambrian organic shale deposition period the sulfate ion in the sea of Cengong area. If the δ34S value is +30 ‰ to +40 ‰, the sulfur isotope fractionation range caused by sulfate-reducing bacteria in this organic matter rich section is only −47‰ to −13‰, which is far lower than the fractionation range under the condition of a sufficient supply of sulfate ions and organic carbon. Considering that TOC content in this organic rich section is very high, pyrite particles are about 5–7 μm, and seawater is under reduction anoxic conditions (closed water column), the lack of sulfate ions may be the main reason for this low sulfur isotope fractionation range. The continuous heaviness of sulfur isotopes within the organic matter-rich section may be due to the continuous loss of 32S from the limited reservoir of sulfate ions in seawater under long-term sulfate reduction, and therefore the sulfur isotopic composition of sulfate ions in seawater is also continuously heavier. The continuous weight increase of sulfur isotope in the organic-rich section may be due to the continuous loss of 32S in the limited sulfate ion reservoir in seawater under long-term sulfate reduction, so the sulfur isotope composition of sulfate ion in seawater also keeps getting heavier. The depletion of sulfate ions in seawater and the weighting of its sulfur isotopic composition combine to cause a continuous positive shift in pyrite sulfur isotope values within this organic-rich section. In the late stage of organic-rich mudstone deposition of the Cambrian Niutitang Formation, i.e., the top of the high TOC section (burial depth of about 1420m), the δ34S value of pyrite was 17‰, and it can be assumed that the sulfate ions in seawater have been depleted at this time and the residual very trace sulfate further enriches 34S, resulting in the formation of pyrite with significantly heavier δ34S values. Above this limit, the organic matter content of the shale in the upper part of the Niutitang Formation continues to decline (1400–1420 m), and the TOC continues to decline from about 6.3% to about 1.9%, while the δ34S value of pyrite continues to drift negatively from +17.3‰ to −2.3‰.
Figure 6.
Profiles for TOC, TS, ratios of TOC/TS, and δ34Spy of the Niutitang Shale, the CY-1 core.
Therefore, the continued negative drift of δ34S values likely corresponds to the resupply of sulfate ions in seawater. Since sulfate ions in the previous sulfide–anoxic ocean system were likely largely depleted, the replenishment of enriched 32S sulfate ions may be closely related to atmospheric oxidation. The increased atmospheric oxygen content at the end of the Cambrian would have caused the oxidation of pyrite that had accumulated on the continental crust, and the formation of sulfate ions enriched in light sulfur isotopic composition (32S) and transported to the marine system via rivers, resulting in an increase in sulfate ion concentration in the ocean. After the light sulfur isotope-enriched sulfate is added to the seawater system, sulfate reduction is initiated again, but because the bottom seawater is the oxidizing condition, sulfate bacterial reduction occurs in the pore water below the water column-marine sediment interface, resulting in a negative drift of pyrite sulfur isotope values.
5.3. Using the Major and Trace Elements of Pyrite to Restrict the Sedimentation Processes
Mo is usually present in pyrite as nanoinclusions or pyrite structures. In highly reducing waters (mostly sulfide environments), Mo often precipitates rapidly as sulfides. It has been suggested that the concentration of H2S in seawater is the main influencing factor controlling elemental Mo.52 Shale pyrite in Niutitang Formation is rich in Mo, U, and V elements, which are normalized with Al elements. It is found that there is a different covariant relationship between the enrichment degree of trace elements in pyrite in different layers of Niutitang Formation shale and TOC. As shown in Figure 7a, there is a weak correlation between the content of Mo/Al and TOC in shale pyrite in the upper section of Niutitang Formation. Compared with this, Figure 7b shows a good correlation between Mo/Al and TOC in shale pyrite in the middle and lower section of Niutitang Formation, indicating that the sedimentary environment in the middle and lower section of Niutitang Formation is anoxic and sulfide environment. When the H2S concentration in water is greater than the critical value (11 μmol/L, which is also known as the starting switch value “APS”), and its corresponding pore water sulfide concentration is 100 μmol/L, then MoS42–, MoO42–, and inert Mo will be activated. At this time, Mo element precipitates in the form of Mo–S or adsorbed on particles, so that Mo in the water is completely removed.27 However, when the H2S concentration in pore water is greater than 0.1 μmol/L but lower than APS, Mo may precipitate mainly in the form of Mo–Fe–S, resulting in a strong covariance between Mo/Al and TOC content in shale pyrite in the middle and lower sections of Niutitang Formation. In anoxic sulfide-biased environments, the action of H2S leads to the precipitation of U and V elements mainly in the form of sulfides and other forms from sediment waters and at the sediment-water interface,27 resulting in a weak correlation between them and TOC content.
Figure 7.
Plots of Mo/Al, U/Al, V/Al, and TOC contents of pyrite in shale of the Niutitang Formation. (A) Plots of Mo/Al and TOC contents of pyrite in NTT I shale. (B) Plots of Mo/Al and TOC contents of pyrite in NTT II, III shale. (C) Plots of V/Al and TOC contents of pyrite in NTT I shale. (D) Plots of V/Al and TOC contents of pyrite in NTT II, III shale. (E) Plots of U/Al and TOC contents of pyrite in NTT I shale. (F) Plots of U/Al and TOC contents of pyrite in NTT II, III shale.
Siliceous shale is a common sediment in hydrothermal activity, so it is widely considered by scholars as petrological evidence of hydrothermal activity. Hydrothermal activity often carries many mineralized elements, and the sedimentation will cause the elemental content in the sediment to be abnormal and distributed radially in all directions with the hot water spillway as the center of the abnormality. Elemental anomalies are an important feature of the geochemistry of hot water sedimentation, which is an important sign to distinguish hot water sedimentation from normal sedimentation and to identify the cause of hot water sedimentation. Irregular heteromorphic pyrite and large particle euhedral pyrite growing along the fissure (>15 μm) are found in the middle and lower member of Niutitang Formation or vein pyrite has rough surface and irregular shape, which is obviously affected by later transformation. Combined with previous experimental observations,53 comparing pyrite rapidly formed at lower temperatures (<150°c) with pyrite slowly formed in hydrothermal fluid at higher temperatures (>200°c), it is found that pyrite rapidly formed at low temperatures is mostly fine-grained and framboid shaped, and most of the pyrite formed in the hydrothermal fluid is coarse-grained. When hydrothermal pyrite is metamorphosed, due to the changes in the physical and chemical conditions of the sedimentary environment during metamorphism, the newly formed pyrite usually undergoes recrystallization or continues to grow covered with older pyrite. The relatively developed cubic pyrite single crystal (Figure 3e) in the lower part symbolizes pyrite formed under high oxygen fugacity, low sulfur fugacity, low-temperature, or high-temperature environment, while the pentagonal dodecahedral pyrite single crystal symbolizes pyrite formed under high sulfur fugacity and medium temperature environment, it has shown that local hydrothermal action occurred during the sedimentation of the middle and lower members of Niutitang Formation. McKibben et.al showed that sulfate-reducing bacteria were more active at 200–400 m above the hydrothermal activity zone, while the biological productivity was 1–3 times higher than that at the normal seawater surface, so there was a significant promotion of sulfate-reducing bacteria in organic matter enrichment.54 Hydrothermal action was frequent in the early Cambrian in the study area due to the tensional standing effect between the Huaxia and Yangzi platforms.55 Hydrothermal action changed the redox conditions and biological productivity of the sedimentary environment and created an anoxic environment favorable for organic matter preservation, which had a significant impact on organic matter enrichment.53
TOC content is easy to change under the influence of diagenesis, biogenesis, and changes in redox conditions of the sedimentary environment.27 By judging the relationship between Co/Zn and TOC, the lower TOC value in the upper member of Niutitang Formation may be related to the increase of oxygen content in sedimentary water and the weak hydrothermal activity, while the organic matter enrichment horizon in the middle member of Niutitang Formation corresponds to the occurrence period of hydrothermal activity (Figure 8). The higher TOC value in the middle of the Niutitang Formation shows the higher paleo-productivity, and the TOC content and the corresponding hydrothermal sensitive elements peaked, but then TOC showed a significant downward trend. The reason for analysis may be that the intense hydrothermal action brought about severe sea-level transgressive changes and strong seafloor upwelling currents, which promoted the enrichment of organic matter in the ocean. The appropriate hydrothermal deposition environment is suitable for the growth and development of aquatic organisms, improved paleo-ocean productivity, and provided an anoxic reduction environment for the production and preservation of organic matter, but too strong hydrothermal action will destroy the submarine ecosystem, it can also explain the sudden drop of TOC in the lower member of Niutitang Formation.
Figure 8.
(A) Relationship between Co/Zn and TOC content of pyrite in NTT I shale. (B) Relationship between Co/Zn and TOC content of pyrite in NTT II, III.
After studying the shale interlayer and chert in the Yangqiao section of Guizhou Province, some scholars found that the shale horizon was mainly deposited in the period of weak hydrothermal activity and the relatively developed chert interval was mainly deposited in the period of strong hydrothermal activity. Hydrothermal activities in the Early Cambrian led to the enrichment of elements (such as V, U, Mo, etc.) that are very sensitive to redox conditions in a sedimentary environment. V also presents different valence states under different redox environments, usually V5+ under oxidizing conditions and dissolved in seawater; under reducing conditions, V5+ will be reduced to V4+ and combined with organic matter and enriched in the sediment. Mo is also less enriched under oxidizing conditions and more enriched under reducing conditions. Mo enrichment is mainly related to the concentration of iron sulfide and H2S in the water column, and the decomposition of organic matter results in strong sulfate reduction, with the resulting H2S reacting further with molybdate to form thiomolybdate, which is then preserved together with iron sulfide and undecomposed organic matter.59 Comparing the V, U, and Mo elements in shale pyrite of Niutitang Formation in CY-1 well with the trace elements in the typical hydrothermal environment in the Atlantic ridge (as shown in Figure 9), it can be found that the V, U, and Mo elements in shale pyrite of the middle and lower members of Niutitang Formation (NTT II, III) show poor correlation (Figure 9c,d), which is similar to the correlation of various trace elements in the typical hydrothermal environment of the Atlantic ridge. In contrast, the trace elements in the upper segment of Niutitang Formation (NTT I) showed good correlation (Figure 9a,b). The enrichment mode of V, U, Mo elements in shale pyrite in the middle and lower members of Niutitang Formation is similar to that of trace elements in the typical hydrothermal environment of the Atlantic ridge (Figure 9e,f), indicating that the middle and lower members of Niutitang Formation experienced hydrothermal activities during the deposition process, while the hydrothermal activities occurred in the upper members of Niutitang Formation were less or not occurred. The special crystal structure and element composition of pyrite determine that only some foreign elements can replace Fe and S into the lattice of pyrite. For example, Co, Ni, As, Se, Te, etc. can replace anions and cations in the form of isomorphism, thus changing the cell parameters of pyrite and causing changes in the properties of pyrite. Trace elements in the process of mineral crystallization can record many factors such as the composition of ore-forming fluid and the changes of physical and chemical conditions. Therefore, through the distribution law of trace elements in pyrite, we can study the mineral genesis of pyrite and the physical and chemical conditions during crystallization. After studying the trace elements in pyrites of different origins, Bajwah Z U56 concluded that the ratio of Co/Ni of pyrite formed by volcanism is 5–50 because its Co value is high (average 480 × 10–6), low Ni value (average 100 × 10–6), the ratio of Co/Ni of hydrothermal pyrite (formation temperature is about >200 °C) is 1.15–5, and the ratio of Co/Ni of pyrite formed by sedimentation at low temperature is usually <1. Through the Co/Ni of pyrite (Figure 10), it is found that the pyrite in the upper member of Niutitang Formation mainly falls into the sedimentary area, the pyrite in the middle member is within the scope of hydrothermal genesis, and the intersection of hydrothermal and sedimentation, and the pyrite in the lower segment has many genetic types, but most of them fall into the scope of sedimentary genesis. In addition, unlike the currently widely considered magmatic hydrothermal-associated pyrite with high Co and Ni contents, the Co and Ni content distribution of pyrite in the shales of the Niutitang Formation is relatively low, with most points having low Co and Ni contents of pyrite. The Co/Ni ratio diagram shows that the upper member of Niutitang Formation was deposited in the normal marine environment, some pyrites in the middle member were distributed in the normal marine sedimentary hydrothermal sedimentary environment, indicating that the middle member of Niutitang Formation was affected by submarine hydrothermal during the deposition process, and the elements in pyrites in the lower member of Niutitang Formation were distributed in the hydrothermal environment, indicating that this segment was strongly affected by hydrothermal sedimentation during the deposition process.
Figure 9.
Correlation between the redox metal elements Mo, U, and V of pyrite in the shale of the Niutitang Formation and the metal elements Mo, U, and V in the hydrothermal plume particles of the Atlantic Ridge. (A) Correlations between U and Mo of pyrite in NTT I shale. (B) Correlations between V and Mo of pyrite in NTT I shale. (C) Correlations between U and Mo of pyrite in NTT II, III shale. (D) Correlations between V and Mo of pyrite in NTT II, III shale. (E) Correlations between U and Mo in the hydrothermal plume particles of the Atlantic Ridge. (F) Correlations between V and Mo in the hydrothermal plume particles of the Atlantic Ridge.
Figure 10.
Ratio of Co and Ni of pyrite in shale of the Niutitang Formation.
By analyzing the sulfur isotope values of black shale in Niutitang Formation, it is found that there are obvious differences in the numerical distribution of three different types of pyrite identified by scanning electron microscope in the upper, middle, and lower submembers. From Figure 11, there are two geneses of pyrite in the black shale of the Niutitang Formation; due to the difference in their formation environment, the upper and middle members of pyrite in the Niutitang Formation were formed in closed and anoxic sedimentary, while the lower section of the Niutitang Formation was influenced by late hydrothermal action and the 32S supply was relatively abundant and its sulfur isotope values showed a negative feature. The redox-sensitive elements and sulfur isotopes together indicate that the reduction of the environment in the lower part of the Niutitang Formation is stronger than that in the upper part of the Niutitang Formation, and even reaches the sulfide state. This led to the reduction of oxygen escape in the lower and middle water column and the stratification of the redox state of seawater, which eventually led to the lower section of the Niutitang Formation having lower sulfate concentrations than the upper and middle sections.
Figure 11.
Crossplots of the δFe and δS of pyrite in shale of the Niutitang Formation.
The elemental deviation of Fe or S in pyrite samples occurs during sedimentary diagenesis. δFe or δS can be calculated by the degree of deviation from the theoretical value of elemental Fe or elemental S content of pyrite (46.55% for Fe mass fraction and 53.45% for S, eqs 1 and 2) which indicates the degree of deviation of the number of elements and the degree of deviation of the mass, and thus analyzes the genesis. It is expressed by the folowing formula where Fe and S represent mass fractions (%):
 |
1 |
 |
2 |
The distribution of δFe and δS quadrants are as follows: the third quadrant of sulfur-deficient elemental iron-deficient elemental pyrite is distributed in the volcanic hydrothermal pyrite area, and the offset value variation range is within 5%; the second quadrant is distributed in the subvolcanic hydrothermal pyrite, and the offset value variation range is within 5%; the cross-quadrant distribution is more concentrated in the magmatic-hydrothermal pyrite, and its offset value variation range is larger, and there are more than 5% of the value range The mean values of δFe and δS are mainly distributed in the third quadrant, but the range of variation of offset values is up to 20% or more, and the distribution is more scattered, showing serious Fe deficiency and Fe deficiency characteristics. According to the calculation results of the study area (Figure 11), the complex genesis of subhedral pyrite in the middle part of the Niutitang Formation mainly falls in the second, third, and fourth quadrants, and the range of variation is large, which belongs to subvolcanic hydrothermal genesis and magmatic-hydrothermal genesis.
Pyrite in the shale of the upper member of the Niutitang Formation is mainly characterized by anhedral pyrite and late diagenetic framboid, which has a significant symbiotic relationship with biology. This kind of pyrite has the lowest Co/Ni ratio and is in the variation range of sedimentary genesis, showing the characteristics of sulfur-rich and iron deficient, reflecting that the sedimentary environment is relatively open and the influence of heating fluid is relatively shallow. It is mainly biogenic, and the typical sedimentary characteristics are obvious. The structural characteristics of pyrite in the shale in the middle of Niutitang Formation are mainly a layered structure, block structure and disseminated structure, and occasionally brecciated structure, and stockwork structure. It is mainly euhedral pyrite and quasi-contemporaneous framboid pyrite. The measured distribution range of Co/Ni ratio is 2.3–5, indicating that it is greatly affected by volcanism, and the Co/Ni ratio is the largest, and the falling point of δFe and δS is in the magmatic-hydrothermal area, indicating that its formation is deeply affected by mantle-derived hydrothermal solution. The structure of pyrite in the lower member of Niutitang Formation is a banded structure and vein structure, mainly colloidal pyrite. The Co/Ni ratio of this kind of pyrite is in the middle, generally between I and II, which is less affected by volcanism. The falling point of δFe and δS is in the subvolcanic hydrothermal area, showing the characteristics between the endogenous exhalative deposition and the normal deposition of pyrite. To sum up, it is inferred that the shale in the lower member of Niutitang Formation was affected by weak hydrothermal action during the deposition process, and the middle member experienced strong hydrothermal activity during the deposition process, while the hydrothermal activity in the upper member was not obvious. Similarly, Gao et al.57 also found the hydrothermal process of the Early Cambrian after comparing rare earth elements in some sections of southern China.
5.4. A Conceptual Environmental Model for the Niutitang Formation Shale
The Precambrian-Early Cambrian transitional period, as a key transition in geological history, was characterized by large-scale anoxic, iron-rich, and silicon-rich ocean chemistry inherited from the Early Cambrian paleo-ocean.15 There were deep major fractures and accompanying hydrothermal activities on the slope zone of the passive continental margin at the southeastern margin of the Yangzi platform, and the nutrient-rich hydrothermal fluid in the deep crust provided energy and nutrients essential for life, which promoted the increase of biological population and enhanced the paleo productivity level of the Niutitang Formation shales.
Figure 12 illustrates the conceptual model of shale deposition of Niutitang Formation in Cengong area. At the initial stage of shale deposition of Niutitang Formation (Figure 12a), large-scale transgression made the sea level rise rapidly, the seawater accommodation space increased, and the original productivity was relatively small. The water column in northern Guizhou was in an undercompensated state, and the study area was in a deep-water sedimentation environment. Under the influence of weak hydrothermal effects, microorganisms (mainly marine algae, marine plankton, and fungi) proliferate in the true light layer of surface seawater, and paleontological productivity is gradually improved. However, the enhanced hydrodynamic conditions at the bottom of seawater due to the early Cambrian Sea erosion and the generation of destructive factors such as water column enrichment and depletion of oxygen destroyed the organic matter preservation conditions. The development of framboid pyrite with small particle size and narrow particle size distribution, trace elements in pyrite, and sulfur isotopes in the lower part of Niutitang Formation together indicate the anoxic bottom water conditions that existed during the deposition at the bottom of the Niutitang formation.
Figure 12.
Development of bottom water redox conditions in northern Guizhou related to different pyrite forms in Niutitang Formation shale. (A) Early stage of shale deposition of the Niutitang Formation. (B) Middle stage of shale deposition of the Niutitang Formation. (C) Late stage of shale deposition of the Niutitang Formation.
During the middle sedimentary period of the Niutitang Formation (Figure 12b), the redox conditions of the marine waters were dominated by stagnation, accompanied by the culmination of pan-continental rifting and a significant increase in hydrothermal activity, with the corresponding higher productivity level providing sufficient material sources for the enrichment of organic matter and increasing the reduction potential of the marine waters. At the same time, sea invasion leads to deep water shelters, slopes, and even some shallow water shelters in a more reductive anoxic deep water environment (euxinic condition), the narrower distribution of framboid pyrite grain size and smaller average value also confirm this view, this more reductive sulfide environment also provides good preservation conditions for the enrichment of organic matter, which is conducive to the distribution and development of algae, and the restricted water column in the sedimentary environment leads to the connectivity with the surrounding seawater The restricted water column in the sedimentary environment leads to poor connectivity with the surrounding seawater, and the oxygen efficiency in the water column decreases, reducing the decomposition and consumption of organic matter. As a result, the organic matter accumulated heavily during this period, and the TOC value was high at about 7.6 wt % and up to 8.2 wt %. In addition, sea erosion and hydrothermal action brought a large amount of sulfate to stimulate the BSR reaction, resulting in relatively light sulfur isotope values of shale pyrite in the middle and lower sections of the Niutitang Formation.
The sea level of the Niutitang Formation gradually declined in the late sedimentary period (Figure 12c), the oxidized seawater expanded, and the original deep-water shelf environment changed into an oxygen-rich shallow-water shelf area. As a result, the organic matter preservation environment was destroyed and the level of paleo-productivity decreased significantly. The organic carbon content of the shales in the upper part of the Niutitang Formation gradually decreases, and the grain size of framboid pyrite is widely distributed and has a large average value. At the same time, the sea level decline leads to the decrease of sulfate concentration in the sedimentary environment, the δ34S value of pore water increases, and the 34S enrichment in sulfides becomes increasingly high.
6. Conclusions
By analyzing the morphological characteristics of pyrite, trace elements and sulfur isotopic composition in the black shale of the Lower Cambrian Niutitang Formation, the redox conditions of the depositional environment of the important hydrocarbon source rocks in northern Guizhou were reconstructed.
In the process of sedimentation at the bottom of Niutitang Formation, the seawater environment is suboxic–anoxic due to marine transgression. It can be proved by the characteristics of framboid with small particle size and narrow distribution range in the lower part of Niutitang Formation, as well as the characteristics of trace elements and δ34Spy. This condition promotes the enrichment and preservation of organic matter. However, due to the hydrothermal activity, the anoxic degree of the deep-water shelf environment is deepened, and the influence of the hydrothermal activity from the bottom to the top is significantly weakened, and the top is close to the state of normal seawater. In the reduced deep-water continental shelf environment, the injection of reducing hydrothermal fluid deepens the degree of anoxic environment, makes the bottom water column more conducive to the preservation of organic matter, has a positive impact on the improvement of paleoproductivity, and brings favorable conditions for the formation of high-quality organic source rocks. However, long-term hydrothermal action will destroy the preservation of organic matter, so hydrothermal activity may have a dual impact on the supply and preservation of organic matter. In addition, the water body at the bottom of the upper section of the Niutitang Formation is in an oxygen-poor environment due to the decline of sea level. This view is confirmed by the characteristics of strawberry pyrite with large particle size and wide distribution, pyrite trace elements and heavier sulfur isotope composition. Finally, the preservation environment of organic matter was destroyed (TOC decreased).
Acknowledgments
We acknowledge the National Natural Science Foundation of China (Grant Nos. 41927801 and 42002156); Science and Technology Project of Hebei Education Department (Grant No. QN2021027), Natural Science Foundation of Hebei Province of China (Grant No. D2021403015).
Author Present Address
‡ Exploration Department, PetroChina Southwest Oil & Gasfield Company, Chengdu 610041, China
The authors declare no competing financial interest.
References
- Birkett T. The Petrography and Geochemistry of Komatite Flows from the Albitibi Greenstone Belt and A Model for Their Formation-Comments. Lithos. 1987, 20, 181–185. 10.1016/0024-4937(87)90006-5. [DOI] [Google Scholar]
- Loucks R. G.; Ruppel S. C. Mississippian Barnett shale: lithofacies and depositional setting of a deep-water shale-gas succession in the Fort Worth basin, Texas. AAPG (Am. Assoc. Pet. Geol.) Bull. 2007, 91 (4), 579–601. 10.1306/11020606059. [DOI] [Google Scholar]
- Zhang J.-C.; Huo Z.-P.; Tan X.; Han S.-B.. Shale gas geology in China: Shanghai, China; East China University of Science and Technology Press, 2017; p 263
- Cheng M.; Li C.; Zhou L.; Algeo T. J.; Zhang F.-F.; Romaniello S.; Jin C.-S.; Lei L.-D.; Feng L.-J.; Jiang S.-Y. Marine Mo biogeochemistry in the context of dynamically euxinic mid-depth waters: A case study of the lower Cambrian Niutitang shales, South China. Geochim. Cosmochim. Acta 2016, 183, 79–93. 10.1016/j.gca.2016.03.035. [DOI] [Google Scholar]
- Guo Q.-J.; Deng Y.; Hippler D.; Franz G.; Zhang J.-M. REE and trace element patterns from organic-rich rocks of the Ediacaran-Cambrian transitional interval. Gondwana Res. 2016, 36, 81–94. 10.1016/j.gr.2016.03.012. [DOI] [Google Scholar]
- Pi D.-H.; Liu C.-Q.; Shields-Zhou G. A.; Jiang S.-Y. Trace and rare earth element geochemistry of black shale and kerogen in the early Cambrian Niutitang Formation in Guizhou province, South China: Constraints for redox environments and origin of metal enrichments. Precambrian Res. 2013, 225 (1), 218–229. 10.1016/j.precamres.2011.07.004. [DOI] [Google Scholar]
- Murphy A. E.; Sageman B. B.; Hollander D. J.; Lyons T. W.; Carlton E. E. Black shale deposition and faunal overturn in the Devonian Appalachian Basin: Clastic starvation, seasonal water-column mixing, and efficient biolimiting nutrient recycling. Paleoceanography. 2000, 15 (3), 280–291. 10.1029/1999PA000445. [DOI] [Google Scholar]
- Rimmer S. M. Geochemical paleoredox indicators in Devonian–Mississippian black shales, Central Appalachian Basin (USA). Chem. Geol. 2004, 206 (3–4), 373–391. 10.1016/j.chemgeo.2003.12.029. [DOI] [Google Scholar]
- Wu Z.-J.; Ren D.-Z.; Zhou H.-Y. Anaerobic Oxidation of Methane (AOM) in Marine Sediments and Its Effect on Inorganic Sulfur Cycle. Advances in Earth Science 2013, 28 (07), 765–773. 10.11867/j.issn.1001-8166.2013.07.0765. [DOI] [Google Scholar]
- Fike D. A.; Bradley A. S.; Rose C. V. Rethinking the ancient sulfur cycle. Annu. Rev. Earth Planet. Sci. 2015, 43, 593–622. 10.1146/annurev-earth-060313-054802. [DOI] [Google Scholar]
- Jo̷rgensen B. B.; Kasten S. Sulfur cycling and methane oxidation. Mar. Geochem. 2006, 271–309. 10.1007/3-540-32144-6_8. [DOI] [Google Scholar]
- Aller R. C.; Madrid V.; Chistoserdov A.; Aller J. Y.; Heilbrun C. Unsteady diagenetic processes and sulfur biogeochemistry in tropical deltaic muds: Implications for oceanic isotope cycles and the sedimentary record. Geochim. Cosmochim. Acta 2010, 74 (16), 4671–4692. 10.1016/j.gca.2010.05.008. [DOI] [Google Scholar]
- Rickard D. Sedimentary pyrite framboid size-frequency distributions: A meta-analysis. Palaeogeogr., Palaeoclimatol., Palaeoecol. 2019, 522, 62–75. 10.1016/j.palaeo.2019.03.010. [DOI] [Google Scholar]
- Wilkin R. T.; Barnes H. L.; Brantley S. L. The size distribution of framboidal pyrite in modern sediments: An indicator of redox conditions. Geochim. Cosmochim. Acta 1996, 60 (20), 3897–3912. 10.1016/0016-7037(96)00209-8. [DOI] [Google Scholar]
- Canfield D. E.; Farquhar J.; Zerkle A. L. High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology. 2010, 38 (5), 415–418. 10.1130/G30723.1. [DOI] [Google Scholar]
- Shi W.; Li C.; Luo G.-M.; Huang J.-H.; Algeo T. J.; Jin C.-S.; Zhang Z.-H.; Cheng M. Sulfur isotope evidence for transient marine-shelf oxidation during the Ediacaran Shuram Excursion. Geology. 2018, 46 (3), 267–270. 10.1130/G39663.1. [DOI] [Google Scholar]
- Wignall P. B.; Newton R.; Brookfield M. E. Pyrite framboid evidence for oxygen poor deposition during the Permian-Triassic crisis in Kashmir. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2005, 216, 183–188. 10.1016/j.palaeo.2004.10.009. [DOI] [Google Scholar]
- Roberts A. Magnetic mineral diagenesis. Earth Sci. 2015, 151, 1–47. 10.1016/j.earscirev.2015.09.010. [DOI] [Google Scholar]
- Wang L.; Shi X.-Y.; Jiang G.-P. Pyrite morphology and redox fluctuations recorded in the Ediacaran Doushantuo formation. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2012, 333–334, 218–227. 10.1016/j.palaeo.2012.03.033. [DOI] [Google Scholar]
- Wei H.-Y.; Wei X.-M.; Qiu Z.; Song H.-Y.; Shi G. Redox conditions across the G–L boundary in south China: evidence from pyrite morphology and sulfur isotopic compositions. Chem. Geol. 2016, 440, 1–14. 10.1016/j.chemgeo.2016.07.009. [DOI] [Google Scholar]
- Wignall P. B.; Newton R. Pyrite framboid diameter as a measure of oxygen deficiency in ancient mudrocks. Am. J. Sci. 1998, 298, 537–552. 10.2475/ajs.298.7.537. [DOI] [Google Scholar]
- Raiswell R.; Berner R. A. Pyrite formation in euxinic and semieuxinic sediments. Am. J. Sci. 1985, 285 (8), 710–724. 10.2475/ajs.285.8.710. [DOI] [Google Scholar]
- Riciputi L. R.; Cole D. R.; Machel H. G. Sulfide formation in reservoir carbonates of the Devonian Nisku Formation, Alberta, Canada: An ion microprobe study. Geochim. Cosmochim. Acta 1996, 60 (2), 325–336. 10.1016/0016-7037(96)83133-4. [DOI] [Google Scholar]
- Lin Z.-Y.; Sun X. M.; Peckmann J.; Lu Y.; Xu L.; Strauss H.; Zhou H.-Y.; Gong J.-L.; Lu H.-F.; Teichert B. M. A. How sulfate-driven an aerobic oxidation of methane affects the sulfur isotopic composition of pyrite: A SIMS study from the South China Sea. Chem. Geol. 2016, 440, 26–41. 10.1016/j.chemgeo.2016.07.007. [DOI] [Google Scholar]
- Sawlowicz Z.Framboids: From their origin to application. Pr. Mineral. 2000, 88, 1–80. [Google Scholar]
- Large R. R.; Halpin J. A.; Danyushevsky L. V.; Maslennikov V. V.; Bull S. W.; Long J. A.; Gregory D. D.; Lounejeva E.; Lyons T. W.; Sack P. J.; McGoldrick P. J.; Calver C. R. Trace element content of sedimentary pyrite as a new proxy for deep-time ocean-atmosphere evolution. Earth Planet. Sci. Lett. 2014, 389, 209–220. 10.1016/j.epsl.2013.12.020. [DOI] [Google Scholar]
- Tribovillard N.; Algeo T. J.; Lyons T.; Riboulleau A. Trace metals as paleoredox and paleoproductivity proxies: An update. Chem. Geol. 2006, 232, 12–32. 10.1016/j.chemgeo.2006.02.012. [DOI] [Google Scholar]
- Scott C.; Lyons T.; Bekker A.; Shen Y.; Poulton S. W.; Chu X.; Anbar A. D. Tracing the stepwise oxygenation of the Proterozoic Ocean. Nature 2008, 452, 456–459. 10.1038/nature06811. [DOI] [PubMed] [Google Scholar]
- Large R. R.; Maslennikov V. V.; Robert F.; Danyushevsky L. V.; Chang Z.-S. Multistage sedimentary and metamorphic origin of pyrite and gold in the giant Sukhoi Log deposit, Lena gold province. Russia. Econ. Geol. 2007, 102, 1233–1267. 10.2113/gsecongeo.102.7.1233. [DOI] [Google Scholar]
- Huerta-Diaz M. A.; Morse J. W. A quantitative method for determination of trace metal concentrations in sedimentary pyrite. Mar. Chem. 1990, 29, 119–144. 10.1016/0304-4203(90)90009-2. [DOI] [Google Scholar]
- Wignall P. B.; Newton R.; Brookfield M. E. Pyrite framboid evidence for oxygen poor deposition during the Permian-Triassic crisis in Kashmir. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2005, 216, 183–188. 10.1016/j.palaeo.2004.10.009. [DOI] [Google Scholar]
- Li J.; Yu B.-S.; Guo F. Analysis of the Sedimentary Environmental Conditions and the Tectonic Background of the Source Area of the Black Shale at the Bottom of the Lower Cambrian in Northern Guizhou. Journal of Sedimentology. 2013, 31 (1), 20–31. [Google Scholar]
- Wang L.-B.; Jiu K.; Zeng W.-T.; Fu J.-L.; Zhao S. Characteristics of Lower Cambrian marine black shale in the upper Yangtze and northern Guizhou region and evaluation of shale gas prospect. Acta Petrologica Sinica. 2013, 29 (9), 307–322. [Google Scholar]
- Chen L.; Zhong H.; Hu R.-Z.; Xiao J.-F. Changes of organic carbon isotopic composition of early Cambrian black shale in Human Guizhou area and their significance. Mineralogy and Petrology 2006, 26 (1), 81–85. [Google Scholar]
- Chang C.; Hu W.-X.; Fu Q.; Cao J.; Wang X.-L.; Yao S.-P. Characterization of trace elements and carbon isotopes across the Ediacaran-Cambrian boundary in Anhui Province, South China: Implications for stratigraphy and paleoenvironment reconstruction. Asian J. Earth Sci. 2016, 125, 58–70. 10.1016/j.jseaes.2016.05.014. [DOI] [Google Scholar]
- Schindelin J.; Arganda-Carreras I.; Frise E.; Kaynig V.; Longair M.; Pietzsch T.; Preibisch S.; Rueden C.; Saalfeld S.; Schmid B.; Tinevez J. Y.; White D. J.; Hartenstein V.; Eliceiri K.; Tomancak P.; Cardona A.; Liceiri K.; Tomancak P. Fiji: an open-source platform for biological image analysis. Nat. Methods 2012, 9, 676–682. 10.1038/nmeth.2019. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Chen G.; Gang W.-Z.; Tang H.-Z.; Gao G.; Wang N.; Liu L.; Yang S.-R.; Wang Y. Astronomical cycles and variations in sediment accumulation rate of the terrestrial lower Cretaceous Xiagou Formation from the Jiuquan Basin, NW China. Cretac. Res. 2020, 109, 104156. 10.1016/j.cretres.2019.06.002. [DOI] [Google Scholar]
- Canfield D. E.; Raiswell R.; Westrich J. T.; Reaves C. M.; Berner R. A. The use of chromium reduction in the analysis of reduced inorganic sulfur in sediments and shales. Chem. Geol. 1986, 54, 149–155. 10.1016/0009-2541(86)90078-1. [DOI] [Google Scholar]
- Goldhaber M. B.; Kaplan I. R.. The Sulfur Cycle. In The Sea; Goldberg E. D., Ed.; Wiley, 1974; Vol. 5, Marine Chemistry, pp 569–655. [Google Scholar]
- Love L. G. Early diagenetic polyframboidal pyrite, primary and redeposited, from the Wenlockian Denbigh Grit Group, Conway, North Wales. U.K. J. Sediment. Res. 1971, 41, 41. 10.1306/74D723EC-2B21-11D7-8648000102C1865D. [DOI] [Google Scholar]
- Sawlowicz Z. Pyrite framboids and their development: a new conceptual mechanism. Geol. Rundsch. 1993, 82, 148–156. 10.1007/BF00563277. [DOI] [Google Scholar]
- Wilkin R. T.; Arthur M. A. Variations in pyrite texture, sulfur isotope composition, and iron systematics in the Black Sea: Evidence for Late Pleistocene to Holocene excursions of the O2-H2S redox transition. Geochim. Cosmochim. Acta 2001, 65 (9), 1399–1416. 10.1016/S0016-7037(01)00552-X. [DOI] [Google Scholar]
- Bond D.; Wignall P. B.; Racki G. Extent and duration of marine anoxia during the Frasnian–Famennian (late Devonian) mass extinction in Poland, Germany, Austria and France. Geol. Mag. 2004, 141, 173–193. 10.1017/S0016756804008866. [DOI] [Google Scholar]
- Hu S.-Y.; Evans K.; Fisher L.; Rempel K.; Craw D.; Evans N. J.; Cumberland S.; Robert A.; Grice K. Associations between sulfides, carbonaceous material, gold and other trace elements in polyframboids: implications for the source of orogenic gold deposits, Otago Schist, New Zealand. Geochim. Cosmochim. Acta 2016, 180, 197–213. 10.1016/j.gca.2016.02.021. [DOI] [Google Scholar]
- Schieber J.; Baird G. On the origin and significance of pyrite spheres in Devonian black shales of North America. J. Sediment. Res. 2001, 71, 155–166. 10.1306/051600710155. [DOI] [Google Scholar]
- Takahashi S.; Yamasaki S.; Ogama K.; Kaiho K.; Tsuchiya N. Redox conditions in the end-early Triassic Panthalassa. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2015, 432, 15–28. 10.1016/j.palaeo.2015.04.018. [DOI] [Google Scholar]
- Passey Q. R.; Bohacs K. M.; Esch W. L.; Klimentidis R. E.; Sinha S. K. My source rock is now my reservoir - geologic and petrophysical characterization of shale-gas reservoirs. Search Discovery 2012, 80231, 47. [Google Scholar]
- Jo̷rgensen B. B. Mineralization of organic matter in the sea bed-the role of sulfate reduction. Nature. 1982, 296, 643–645. 10.1038/296643a0. [DOI] [Google Scholar]
- Lyons T. W.; Anbar A. D.; Severmann S.; Scott C.; Gill B. C. Tracking euxinia in the ancient ocean: a multiproxy perspective and proterozoic case study. Annu. Rev. Earth Planet Sci. 2009, 37, 507–534. 10.1146/annurev.earth.36.031207.124233. [DOI] [Google Scholar]
- Kaplan I. R.; Rittenberg S. C. Microbiological fractionation of sulfur isotopes. J. Gen. Microbiol. 1964, 34, 195–212. 10.1099/00221287-34-2-195. [DOI] [PubMed] [Google Scholar]
- Goldberg T.; Poulton S. W.; Strauss H. Sulphur and oxygen isotope signatures of the late Neoproterozoic to early Cambrian sulfate, Yangtze Platform, China: Diagenetic constraints and seawater evolution. Precambrian Res. 2005, 137 (3–4), 223–241. 10.1016/j.precamres.2005.03.003. [DOI] [Google Scholar]
- Vorlicek T. P.; Kahn M. D.; Kasuya Y.; Helz G. R. Capture of molybdenum in pyrite-forming sediments: role of ligand-induced reduction by polysulfides. Geochim. Cosmochim. Acta 2004, 68, 547–556. 10.1016/S0016-7037(03)00444-7. [DOI] [Google Scholar]
- Heiligmann M.; Williams-Jones A. E.; Clark J. R. The role of sulfate-sulfide-oxide-silicate equilibria in the metamorphism of hydrothermal alteration at the Hemlo gold deposit. Ontario. Econ. Geol. 2008, 103, 335–351. 10.2113/gsecongeo.103.2.335. [DOI] [Google Scholar]
- McKibben M. A.; Williams A. E.; Hall G. E. M. Solubility and transport of platinum group elements and Au in saline hydrothermal fluids. Constraints from the geothermal brine data. Econ. Geol. 1990, 85 (8), 1926–1934. 10.2113/gsecongeo.85.8.1926. [DOI] [Google Scholar]
- Sun X.; Chen J.-F.; Liu W.-H.; Zhang S.; Wang D.-R. Hydrothermal venting on the sea-floor and formation of organic-rich sediments-evidence from the Neoproterozoic Xiamaling Formation, North China. Geol. Rev. 2003, 49 (6), 588–595. [Google Scholar]
- Bajwah Z. U.; Seccombe P. K.; Offler R. Trace element distribution, Co: Ni ratios and genesis of the Big Cadia iron-copper deposit, New South Wales. Australia. Miner. Deposita. 1987, 22, 292–300. 10.1007/BF00204522. [DOI] [Google Scholar]
- Gao P.; He Z.-L.; Li S.-J.; Lash G. G.; Li B.-Y.; Huang B.-Y.; Yan D.-T. Volcanic and hydrothermal activities recorded in phosphate nodules from the Lower Cambrian Niutitang Formation black shales in South China. Palaeogeogr., Palaeoclimatol., Palaeoecol. 2018, 505, 381–397. 10.1016/j.palaeo.2018.06.019. [DOI] [Google Scholar]
- Och L. M.; Shields-Zhou G. A.; Poulton S. W.; Manning C.; Thirlwall M. F.; Li D.; Chen X.; Ling H.-F.; Osborn T.; Cremonese L. Redox changes in Early Cambrian black shales at Xiaotan section, Yunnan Province, South China. Precambrian Res. 2013, 225, 166–189. 10.1016/j.precamres.2011.10.005. [DOI] [Google Scholar]
- Helz G. R.; Miller C. V.; Charnock J. M.; Mosselmans J. F. W.; Pattrick R. A. D.; Garner C. D.; Vaughan D. J. Mechanism of molybdenum removal from the sea and its concentration in black shales: EXAFS evidence. Geochim. Cosmochim. Acta 1996, 60, 3631–3642. 10.1016/0016-7037(96)00195-0. [DOI] [Google Scholar]