Abstract
A significant deposition of black shales occurred during the Mesoproterozoic Oxygenation Event (MOE). In order to investigate the hydrocarbon generation potential and organic matter enrichment mechanism of these shale deposits, we studied the Xiamaling Formation shale in the North China region as a representative sample of the Mesoproterozoic shale. The research involved organic petrology, organic geochemistry, mineralogy, and elemental geochemistry. The following observations were made: (1) The depositional environment of the Xiamaling Formation shale can be categorized as either oxic or anoxic, with the former having shallow depositional waters and high deposition rates, while the latter has deeper depositional waters and slower deposition rates. (2) Anoxic shales exhibited significantly better hydrocarbon generation potential compared to shales deposited in oxic environments, although the latter still demonstrated high hydrocarbon generation potential. (3) Shales deposited in anoxic environments displayed higher paleoproductivity compared to those deposited in oxic environments. The high deposition rate in oxic environments slowed the decomposition and mineralization of organic matter, leading to the formation of high-quality shales. In contrast, the strong paleoproductivity, along with favorable preservation conditions, accounted for the high hydrocarbon potential of anoxic shales.
Article Highlights
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(1)
Two types of shales with different genesis were deposited under the MOE.
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(2)
Shales deposited in oxic environments have good hydrocarbon potential.
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(3)
Shales formed in anoxic environments are higher quality than those in oxic environments.
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(4)
High deposition rates are key to forming high quality shales in oxic environments.
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1 Introduction
The Early Archean (2.45–2.10 Ga) and Late Neoproterozoic (0.8–0.54 Ga) are two well-established key oxygenation events (Cloud 1968; Farquhar et al. 2007; Tomitani et al. 2006; Berner et al. 2007; Och and Shields-Zhou 2012; Zhang et al. 2022b, c). The degree of atmospheric oxidation between these two events is a subject of intense research (Large et al. 2019; Scott et al. 2014; Cole et al. 2020). Holland et al. (1989) and Babechuk et al. (2015) suggested that the atmospheric oxygen content (AOC) after 1.85 Ga was about 1% of present atmospheric oxygen levels (PAL), while Kasting (1993) believes the AOC was similar to PAL after 1.85 Ga. In contrast, Cole et al. (2016) suggested that the AOC was less than 1% PAL during 1.8–0.8 Ga, whereas Zhang et al. (2016) suggested that the AOC during deposition of the Xiamaling Formation was 4-8% PAL. Recent studies indicate that the AOC was above 4% PAL during three periods: 1.59–1.56 Ga, 1.44–1.43 Ga, and 1.4–1.36 Ga (Miao et al. 2018; Wang et al. 2021; Zhang et al. 2016, 2017, 2019, 2022b, c; Shang et al. 2019; Xie et al. 2022; He et al. 2022). Therefore, Zhang et al. (2022b, c) named the high AOC during the 1.59–1.36 Ga period as the Mesoproterozoic Oxygenation Event (MOE).
As shale oil and gas resources are explored and developed, fine-grained sediments have become a research focus. Understanding sedimentary environments and terrestrial input is crucial for analyzing and reconstructing organic matter enrichment (Li et al. 2020; Miao et al. 2021a). Extensive research on shale has examined sedimentary environments and organic matter enrichment mechanisms. High paleoproductivity and reducing environments are considered essential for forming shale with high organic matter abundance (Arthur and Sageman 1994; Liu et al. 2022; Pedersen and Calvert, 1990; Sageman et al. 2003; Mort et al. 2007; Wei et al. 2012). However, the Mesoproterozoic period witnessed the formation of several high-quality shales globally, such as the Xiamaling Formation in China, the Velkerri Formation in Australia, and the Kaltasy Formation in Russia. These shales have high total organic carbon (TOC) and favorable source rock potential (Su et al. 2010; Zhang et al. 2015; Wang et al. 2017b; Mitchell et al. 2021; Lyu et al. 2021; Xie et al. 2022; He et al. 2022; Sergeev et al. 2016; Canfield et al. 2020). Despite extensive research, little is known about formation mechanisms and organic matter enrichment of these high-quality shales formed in oxygenated Mesoproterozoic environments (Wang et al. 2018, 2021; Zhang et al. 2016, 2017, 2019, 2022; Zhao et al. 2019; Canfield et al. 2018; Lyu and Liu 2022). The Xiamaling Formation shale, deposited over 1.4 Ga, has relatively low thermal maturity, leading to preserved organic matter. This provides an opportunity to investigate hydrocarbon potential and formation of high-quality Mesoproterozoic shales (Luo et al. 2014).
Regarding the Xiamaling Formation shale, Wang et al. (2016) analyzed the heterogeneity characteristics. Liu et al. (2018) analyzed the depositional environments in the Beijing area. Zhang et al. (2018) analyzed depositional environments in the Jingxi Depression and discussed the exploration and development potential of shale gas in the region. Xiao et al. (2022) analyzed organic matter biomarker characteristics and dominant biological assemblages during shale deposition. Wu et al. (2022) found differences in water depth during deposition. Despite research, understanding is limited regarding hydrocarbon generation and organic matter enrichment mechanisms for the Xiamaling Formation shale.
Based on this, we conducted a comprehensive study utilizing organic petrology, mineralogy, geochemistry, and previous research on the Xiamaling Formation shale from outcrops. We analyzed hydrocarbon potential and sedimentary environments of Mesoproterozoic shales represented by the Xiamaling Formation. Furthermore, we aimed to explore formation mechanisms of high-quality shales during the Mesoproterozoic oxygenation event.
2 Geologic setting
The Yanshan-Liaohe Fault Zone, located in the northern part of the North China Platform, is an active tectonic unit structure on the North China Craton (Fig. 1a, b). Extending in an east-west direction, it is considered the oldest oil and gas-bearing structural zone in China (Wang et al. 2004). The Yanshan-Liaohe Fault Zone formed during the breakup of the Columbia supercontinent and convergence of the Rodinia supercontinent. It underwent three stages: early rift development, mid-rift expansion, and late rift stability (Hao et al. 1990; Zhao and Li 1997). The Yanshan-Liaohe Fault Zone can be divided into seven sub-structural units (five depressions and two uplifts) from west to east, including the Xuanlong Depression, Mihuai Uplift, Jingxi Depression, Jibei Depression, Liaoxi Depression, Shanhaiguan Uplift, and Jidong Depression (Luo et al. 2014; Wu et al. 2022; Fig. 1c).
In this study, we focus on the Xiamaling Formation, which consists of four sections from bottom to top. The first section mainly comprises gray-green siltstone and yellowish iron-rich mudstone. The second section contains interbedded gray and green siltstone with locally occurring black to dark gray mudstone. The third section consists mainly of black shale and carbonaceous shale, with greenish-gray pyroxene beds found in some locations. The fourth section consists mainly of sandstone and variegated mud shale (Fig. 2).
3 Samples and experiments
3.1 Samples
In this study, we collected eight black shale samples from the third section of the Xiamaling Formation in the ZJS profile located in the Xuanlong Depression of the Yanshan region (Figs. 1c and 2). The thickness of the Xiamaling Formation shale at the sampling point was 7.3 m. We collected samples at 0.9 m intervals, resulting in eight total samples numbered XML-1 to XML-8 from top to bottom. During sampling, we avoided weathered sections by collecting 10 cm below the surface. The samples were divided into two portions. The first portion was used to create thin sections by mixing with epoxy resin and triethanolamine for microscopic observation. The second portion was crushed for total organic carbon (TOC) analysis, Rock-Eval pyrolysis, mineralogy, and elemental geochemistry.
Stratigraphic column, sampling strata and profile photos of the Xiamaling Formation(modified after Wang et al. 2017b)
3.2 Experiments
Organic petrology and vitrinite reflectance (Ro) were carried out at the Key Laboratory of Unconventional Oil and Gas of China National Petroleum Corporation. For organic petrology experiments, we used the Axioskop polarizing microscope produced by Zeiss, and for fluorescence observation, we used the Olympus IX73 fluorescence microscope with ultraviolet light as the excitation source. Since the Xiamaling Formation shale lacks vitrinite components, we obtained vitrinite-like maceral reflectance (Rb) for Ro calculations using the Ro–Rb conversion relationship (Luo et al. 2021). We measured Rb over 100 times per sample, and took the average.
Total Organic Carbon (TOC) testing was carried out at the Geological Engineering Experimental Base of Guizhou Coalfield using an ACS744 carbon-sulfur analyzer (Miao et al. 2021a). Samples were crushed and soaked in the diluted hydrochloric acid (HCl) for 48 h, and 2 g of dried sample was used for TOC testing. We conducted at least three TOC measurements per sample, and took the average. Rock-Eval pyrolysis was done at the State Key Laboratory of Oil and Gas Resource and Exploration using a YQ-VII rock pyrolysis instrument. The instrument was stabilized for 30 min before analysis, with ± 1% error.
X-ray diffraction (XRD) analysis was carried out at the Geological Engineering Experimental Base of Guizhou Coalfield using a Ultima IV X-ray diffractometer (Rigaku Corporation, Japan). Prior to testing, the samples were ground to 200 mesh and pressed into 20 × 14 × 1.5 mm glass slides to determine bulk mineralogy. Clay minerals were isolated by sedimentation before analysis. The powder samples were added to distilled water and thoroughly stirred before allowing the suspension to settle for 6 h. The supernatant was then carefully removed and the settled clay minerals were collected for further analysis.
Elemental analysis was conducted at Oriental oil Laboratory Company, using a RIGAKU ZSX Priums X-ray fluorescence spectrometer (Rigaku Corporation). Prior to testing, 1.5 g of shale powder was mixed with Li2B4O7, LiBO2, and LiBr, and then melted and pressed into a crucible. The instrument’s error range was approximately 1–5%. The ICP-MS analysis was conducted using a Thermo-iCAP6300 spectrometer. Prior to testing, 1 g of the sample was mixed with concentrated nitric acid and hydrofluoric acid, and underwent 48 h of heat treatment at 195 °C in a sealed oven. The instrument’s error range was approximately 3% (Miao et al. 2023).
4 Results
4.1 Organic petrology
The Xiamaling Formation shale contains scattered organic matter appearing grayish white and black. Under the microscope, identified organic matter types include vitrinite-like macerals (Fig. 3b, c, g, h, I, g, n, o), chlorella (Fig. 3a), red algae (Fig. 3d–f), and siphonophycus solidium (Fig. 3k–i) exhibiting fluorescence. These components appeared yellow or dark yellow under fluorescence microscope (Fig. 3), indicating algal-dominated organic matter in the Xiamaling Formation shale. The organic matter was likely Type I or Type II.
Vitrinite-like maceral reflectance (Rb) was measured and converted to Ro using the formula by Luo et al. (2021) (Table 1). Ro values for the Xiamaling Formation shale ranged from 0.84 to 0.90 (mean = 0.86), indicating oil generation stage (Miao et al. 2021a). The observed Ro was higher than the XHY area (Wu et al. 2022), but lower than the Jingxi Depression and Liaoxi Depression (Zhang et al. 2018; Liu et al. 2019).
Morphological pictures of organic matter in the Xiamaling Formation shale: a XML-1, fluorescent photo of algae; b XML-2, vitrinite-like maceral; c XML-2, fluorescent photo of vitrinite-like maceral; d XML-3, red algae; e XML-3, red algae; f XML-3, fluorescent photo of red algae; g XML-4, vitrinite-like maceral; h XML-4, fluorescent photo of vitrinite-like maceral; i XML-5, vitrinite-like maceral; j XML-5, fluorescent photo of vitrinite-like maceral; k XML-6, Oscillatoriopsis obtusa and Siphonophycus septatum; l XML-7, Siphonophycus solidium; m XML-7, fluorescent photo of Siphonophycus solidium; n XML-8, vitrinite-like maceral; o XML-8, fluorescent photo of vitrinite-like maceral
4.2 Organic geochemistry
TOC and Rock-Eval pyrolysis results are presented in Table 1. TOC in the ZJS profile of Xiamaling Formation shale ranged from 0.82 to 3.95% (with a mean value of 1.85%), which was lower than other areas but similar to the Velkerri Formation in Australia (Cox et al. 2016). However, there were no significant differences for Tmax (430–442 °C, mean value of 434 °C) and HI (339.95–558 mgHC/g, mean value of 421.73 mgHC/g) compared to other areas (Wu et al. 2022; Song et al. 2021; Xiao et al. 2022; Wang 2021; Zhang et al. 2018; Liu et al. 2019). TOC, S2, HI, and Tmax indicated overall good hydrocarbon generation potential, with most samples being excellent source rocks and some very good (Fig. 4a). The organic matter was predominantly Type II, with minor Type II-III (Fig. 4b).
Organic geochemical characteristics of the Xiamaling Formation shale: a Correlation between TOC and S2 indicating kerogen type; b Correlation between Tmax and HI indicating kerogen type
4.3 Mineralogy
The Xiamaling Formation shale mainly comprises quartz, clay, and plagioclase minerals. Quartz was found to be the most abundant mineral, ranging from 60.43 to 68.66% (mean value = 65.52%), followed by clay minerals and then plagioclase (Table 2; Fig. 5a). Clays consist mainly of kaolinite, illite, and chlorite, with illite being the most abundant, and kaolinite and chlorite in similar quantities (Fig. 5b). Illite/smectite mixed layers were only found in samples XML-1 and XML-8.
Additionally, lithofacies play a crucial role in understanding the origin, depositional environment, and rock properties (Glaser et al. 2014). Lithofacies types were classified using a ternary diagram of mineral compositions. Results show that the Xiamaling Formation shale is characterized by siliceous and argillaceous shale (Fig. 6). Argillaceous shale was derived from the Xiamaling Formation shale in the Jingxi depression (Zhang et al. 2018).
Mineralogical characteristics of the Xiamaling Formation shale: a Whole-rock mineral composition; b Clay mineral composition
Classification of shale facies in the Xiamaling Formation (modified after Glaser et al. 2014)
4.4 Element Geochemistry
4.4.1 Major element
The SiO2 content in the Xiamaling Formation shale of the ZJS section was highest among major elements, ranging from 75.39 to 81.4% (mean = 77.51%), comparable to other Xiamaling Formation shales in the Yan-Liao fault zone (mean = 70.41%, N = 86) but higher than the Velkerri Formation in Australia (Yang 2013; Wang 2014; Wu et al. 2022; Zhang et al. 2022b, c). The Al2O3 content ranged from 7.75 to 10.58%, with a mean value of 8.89%. The Fe2O3 content ranged from 2.62 to 3.23% (mean = 3.16%). The K2O content ranged from 1.85 to 2.59% (mean = 2.11%), and the MgO content ranged from 1.62 to 2.10%, with a mean value of 1.89%. Other major elements were less than 1% (Table 3).
Additionally, the chemical alteration index (CIA) of the Xiamaling Formation shale in the ZJS section ranged from 76.66 to 78.16 (mean = 77.31), slightly higher than other Xiamaling Formation shales (N = 42, mean = 72.75) and the Velkerri Formation (N = 34, mean = 70.29). The P/Ti in the Xiamaling Formation shale was in the range of 0.16–0.35 (mean = 0.26), lower than other blocks (N = 79, 5.8) but much higher than the Velkerri Formation in Australia (N = 7, 0.07). These findings suggests that the Xiamaling Formation shales have much higher paleoproductivity relative to the Velkerri Formation shales in Australia (Yang 2013; Wang 2014; Zhang et al. 2022b, c; Kidder and Erwin 2001).
4.4.2 Trace elements
The distribution of trace elements in the Xiamaling Formation shale in the ZJS section is shown in Table 4. Standardized against Post-Archean Australian Shale (PAAS) following Taylor and McLennan (1985), the trace elements, namely, Sc (1.06), Ni (1.05), Zn (1.06), Ga (1.14), Th (1.11), U (1.04), and Hf (1.26) in the Xiamaling Formation shale show enrichment, whereas others show no enrichment (Fig. 7).
PAAS standardized multi-element diagrams of Xiamaling Formation shale
The CaO content showed a significantly weak correlation with the Sr and Cr content in the Xiamaoling Formation shale under the ZJS profile (R2Cr = 0.0322, R2Sr = 0.0196), indicating limited carbonate rock influence on Sr and Cr. The ratios of V/Cr, Ni/Co, U/Th, Sr/Ba, and Mo/TOC were 1.35 (1.03–1.78), 2.51 (2.08–2.85), 0.2 (0.16–0.26), 0.06 (0.05–0.06), and 0.29 (0.23–0.55), respectively, much lower than other areas and the Velkerri Formation, Australia (Yang 2013; Wang 2014; Wu et al. 2022; Rafiei and Kennedy 2019). Rb/Ga ratio of 6.84 (5.73–7.33), indicates deposition under a warm, humid paleoclimate.
4.4.3 Rare earth element
Table 5 displays the distribution of rare earth elements (∑REE) in the Xiamaling Formation shale in the ZJS section. Total rare earth elements ∑REE ranged from 89.17 to 111.55 ppm, with an average of 104.6 ppm. The LREE distribution range was 81.2 to 100.2 ppm, with an average of 93.86 ppm, while the HREE ranged from 9.83 to 13.29 ppm (average = 10.74 ppm). After normalization to PAAS, most samples showed no significant REE differentiation, except XML-5 with Eu depletion and enrichment in other REEs (Fig. 8).
Rare earth element diagram of the Xiamaling Formation shale under PAAS normalization
5 Discussion
Analyzing high-quality marine shales from the Mesoproterozoic and reconstructing their sedimentary environments are important aspects of geological research. This study examined the sedimentary environment and formation process of Mesoproterozoic marine shale using organic petrology, organic geochemistry, mineralogy, and elemental geochemistry.
5.1 Sedimentary environment reconstruction
5.1.1 Paleoclimate
Alkali metals such as Ca, Na, and K in sedimentary rocks readily migrate from feldspars to form clay minerals, increasing the Al2O3 ratio during weathering. Thus, the chemical weathering index of alteration (CIA) quantifies chemical weathering in source areas for paleoclimate reconstruction (Nesbitt and Young,1984). CIA values for the Xiamaling Formation shale range from 65.47 to 78.81, with an average of 73.48. On the other hand, the Velkerri Formation has CIA from 61 to 79.49, with an average of 70.29. This indicates that the Mesoproterozoic shales could only be deposited under a warm and humid paleoclimate with a moderate degree of chemical weathering (Fig. 9; Nesbitt and Young 1984).
Chemical alteration index of Mesoproterozoic shale (modified from Nesbitt and Young 1984)
Additionally, Rb (rubidium) and Ga (gallium) distributions on the Earth’s surface depend upon climatic conditions. In arid environments, the reduced moisture content on the surface due to evaporation increases the Rb/Ga ratio. Conversely, in humid environments, the abundant moisture increases the surface’s moisture content, thereby decreasing the Rb/Ga ratio (Wang et al. 2017b). Therefore, the Rb/Ga ratio can be used to reconstruct ancient climates. In our case, the Rb/Ga ratio of the Xiamaling Formation shale was 6.84 (5.73–7.33), indicating warm, humid conditions during Xiamaling Formation deposition.
5.1.2 Paleo-redox condition
Transition metal elements, such as Mo, V, Cr, U, Co, and Ni, are sensitive to changes in redox conditions. Analyzing the degree of enrichment as well as ratios of these elements can reconstruct ancient redox environments (Hatch and Leventhal 1992; Rosenthal et al. 1995; Tribovillard et al. 2006). However, U, V, and Th can be influenced by input from continental sources (Tribovillard et al. 1994). The lack of U, V, and Th correlations with Al2O3 in the Xiamaling Formation suggests their use for reconstructing the Mesoproterozoic shale environment.
Mo(EF) and U(EF) were significantly lower than other regions (Yang 2013; Wu et al. 2022; Zhang et al. 2022b, c), indicating differences in sedimentary environments between ZJS section and other areas. The ZJS section is characterized by an oxidizing environment, while most other areas exhibited dyoxic and anoxic conditions (Fig. 10a). The Xiamaling and Velkerri Formation shales have Mo/TOC contents to the Sannich Inlet but differ from the Cariaco Basin and Black Sea (Fig. 10b). Hence, the Mesoproterozoic-era shale sediments were mainly deposited in open sea basins dominated by anoxic conditions.
U/Th, V/Cr, and Ni/Co ratios are important indicators for reconstructing the ancient redox environment of shales (Hatch and Leventhal 1992; Rosenthal et al. 1995; Tribovillard et al. 2006). U/Th ratios indicates that the deposition of Xiamaling Formation shale primarily occurred in oxidizing and dyoxic environments, while V/Cr and Ni/Co ratios indicate that only the Xiamaling Formation shale in the ZJS profile was deposited in oxidizing environment, while other areas mainly had anoxic environments (Yang 2013). This aligns with the moderate chemical weathering of the Xiamaling Formation shale (Cao et al. 2021).
Moreover, electron microscope observations revealed the presence of aerobic organisms, such as red algae, in the Xiamaling Formation shale in the ZJS profile, which further confirms the oxic deposition of the Xiamaling Formation shale (Fig. 3a, d, e). In summary, distinct oxic and anoxic sedimentary environments existed for the Xiamaling Formation, with the Mesoproterozoic Oxygenation Event influencing shale deposition.
Oxidation-reduction environment during the Xiamaling Formation shale deposition: a EF(Mo)-EF(U) diagram (Algeo and Tribovillardt, 2009); b Mo-TOC diagram (Algeo and Lyons 2006); c V/Cr vs. U/Th; d V/Cr vs. Ni/Co
5.1.3 Paleosalinity and paleowater depth
The solubility of compounds containing Sr is higher than those containing Ba, and as the salinity increases, Sr/Ba can indicate paleosalinity during shale deposition (Chen et al. 1997; Remírez and Algeo 2020; Wei and Algeo 2020). The overall low Sr/Ba ratio of 0.15 (0.05–1.06) points to low paleosalinity during Xiamaling Formation deposition. Detrital silicate Ga depletes in seawater compared to freshwater, a useful indicator of changes in paleosalinity during shale deposition (Chen et al. 1997; Remírez and Algeo 2020; Wei and Algeo 2020). The Ga content of Xiamaling Formation shale ranged from 7.98 to 17.3 (Mean = 13.24), indicating that the water body during deposition was mainly brackish water (Fig. 11a).
Additionally, Sr transports more readily than Ba to deep oceans. Thus, Sr/Ba ratio can indicate changes in paleodepth during the sedimentary period (Xu et al. 2017). The Sr/Ba of Xiamaling Formation shale was significantly lower than other areas, indicating a shallower water body during sedimentation. Furthermore, Ce/Ce* decreases with increasing water depth (Wang et al. 2018). Nevertheless, the Xiamaling Formation shales in the ZJS section present higher Ce/Ce* (0.89–0.92, mean = 0.91) than other areas (0.69–0.92, mean = 0.81), also indicates shallower ZJS waters (Fig. 11b).
Moreover, the correlation between the redox environment of the water and paleowater depth was found significant (Fig. 11c,d), indicating that the atmosphere was still oxidizing during the sedimentation of the third section of the Xiamaling Formation. However, due to the limited range of the oxidizing environment, the Xiamaling Formation shales deposited in other areas were below the minimum oxygen zone (Wang et al. 2021).
Paleosalinity and paleodepth of water during shale deposition in the Xiamaling Formation: a Sr/Ba vs. Ga; b Ce/Ce* vs. Sr/Ba; c Sr/Ba vs. V/Cr; d Ce/Ce* vs. V/Cr
5.1.4 Provenance
Provenance is a comprehensive method for analyzing the parent rock types and locations based on rock composition, elucidating basin evolution (McLennan et al. 1993; Kröner et al. 1985). The transportation of sediment from the provenance area to the depositional area is influenced by various geological processes, and it is important to consider sorting and sedimentary recycling effects when analyzing provenance (Miao et al. 2022; Zhu et al. 2021). Zr, Th, and Sc exhibit a relative stability during geological processes (weathering and transportation), and are therefore often used as indicators of parent rock type. The composition of the Xiamaling and the Velkerri Formation shales is primarily controlled by their parent rocks and has not undergone significant sedimentary sorting and recycling. The Xiamaling Formation shale resembles felsic volcanic rocks in the upper crust, while the Velkerri Formation shale reflects andesite compositions (Fig. 12a).
Rare earth elements have high stability in rocks, and the variability in rare earth elements among different parent rock types is significant. Therefore, rare earth elements also used for identifying parent rock types (Basu et al. 2016; Hu et al. 2021; Floyd and Leveridge 1987; Wronkiewicz and Condie 1987; Allègre and Minster 1978). The La/Th-Hf diagram (Fig. 12b) and the La/Sc-Co/Th diagram (Fig. 12c) reveal differences in the parent rock types of the Xiamaling and the Velkerri Formation shales. The parent rock of the Xiamaling Formation shale mainly consisted of felsic provenance in the upper crust, while the Velkerri Formation shale has characteristics of felsic and basic mixture provenance, as well as andesitic island arc provenance from the lower crust. These results are consistent with the TiO2 and Zr diagram (Fig. 12d). The La/Yb and ∑REE diagram (Fig. 12e) shows that all data points of the Xiamaling and Velkerri Formation shales are located in the sedimentary rock area. In conclusion, despite sharing the same sedimentary basin (Mitchell et al. 2021), the formations have distinct provenances and transport pathways.
The parent rock types of the Mesoproterozoic shales: a Th/Sc vs. Zr/Sc (based on the modification by Floyd and Leveridge 1987); b La/Th vs. Hf (based on the modification by Floyd and Leveridge 1987); c Co/Th vs. La/Sc (based on the modification by Wronkiewicz and Condie 1987); d TiO2 vs. Zr; e La/Yb vs. ∑REE (based on the modification by Allègre and Minster 1978)
Before the deposition of the Xiamaling Formation shale, two ancient Yan-Liao rift uplifts existed: the Inner Mongolia uplift northward and the Shanhaiguan uplift eastward (Cui et al. 1979; Liu et al. 2014, 2018). The Shanhaiguan uplift has granite and gneiss developed in the Archean and Early Proterozoic basement, but the area has long been subjected to erosion and weathering (Liu et al. 2014, 2018). On the other hand, the basement of the Inner Mongolia uplift in the Archean and Early Proterozoic is mainly composed of diorite and granodiorite, providing abundant felsic materials for the Xiamaling Formation shale (Zhang 2004; Li et al. 2010; Zhao et al. 2010). Therefore, the Inner Mongolia uplift was the provenance area of the Xiamaling Formation shale.
5.1.5 Terrigenous debris input and sedimentation rate
Due to stability, Ti, Al, and Si indicate the terrigenous clastic flux input in sedimentary rocks (Hatch and Leventhal 1992; Algeo and Maynard 2004). However, there are multiple sources of Si, including biogenic, terrigenous clastic input, and hydrothermal (Kidder and Erwin 2001). Zr, usually derived from the heavy mineral zircon, is used to discriminate the SiO2 source in Zr-Si diagrams (Wright et al. 2010). A positive correlation between Si and Zr content indicates non-biogenic origin, whereas negative correlation indicates biogenic origin of SiO2 (Wright et al. 2010; Zhang et al. 2021). The Si and Zr contents in the Xiamaling Formation shale exhibited weak negative correlation (Fig. 13a), coupled with strong SiO2 relationships to Al2O3 and TiO2 (Fig. 13b, c). This indicated that Si has both biogenic and terrigenous clastic sources, but the main source is terrigenous clastic input. Ti/Al is the most common indicator of terrigenous clastic input flux. The Ti/Al ratio in the Xiamaling Formation shale ranged from 0.04 to 0.06 (mean = 0.05), and did not show significant differences between oxidizing and reducing environments.
The source of Si in the Xiamaling Formation shale: a Si vs. Zr; b Al2O3 vs. SiO2; c TiO2 vs. SiO2
In sedimentary environments, REEs are associated with detritus or suspended particles, and their residence time in the water column leads variable REE fractionation (Murray et al. 1991). Therefore, REE fractionation and LaN/YbN in sedimentary rocks can be used to infer water body sedimentation rates. After PAAS normalization, the REE fractionation in the Xiamaling Formation shale of the ZJS section was not significant, with LaN/YbN ratios ranging from 0.66 to 1.09 (mean = 0.92). In contrast, LaN/YbN ratios in the Xiamaling Formation shale from other areas ranged from 0.41 to 1.63 (mean = 0.57), while Velkerri Formation shale range from 0.32 to 0.47 with a mean of 0.41 (Yang 2013; Wu et al. 2022; Zhang et al. 2022b, c). This indicates the fastest sedimentation rate for ZJS Xiamaling shales and the slowest rate for Velkerri shales.
5.2 Differences in hydrocarbon generation potential between shale types
TOC is a significant indicator for evaluating the OM enrichment in the source rocks, with higher TOC indicating stronger hydrocarbon generation potential (Van and D.W., 1961; Tissot and Welte 1984; Peters and Cassa 1994; Miao et al. 2021a, 2022). Based on the sedimentary environment, Xiamaling shales comprise oxic and anoxic types. In anoxic shales, TOC ranges from 0.07 to 15.46% (mean = 3.68%) (Fig. 14a), while the TOC for anoxic environment was in the range of 0.09–23.02% (mean = 4.44%) (Fig. 14b). Of oxic shales, 42.86% was classified as excellent source rock, and 32.14% was classified as poor source rock (Fig. 14c, He et al. 2022). Comparatively, 70.19% of anoxic shales was classified as excellent source rock, and only 6.73% was classified as poor source rock (Fig. 14d). Anoxic Xiamaling shales clearly show superior hydrocarbon potential over oxic shales, indicating more favorable high-quality shale deposition in anoxic settings. However, oxic environments can also generate quality shales.
Hydrocarbon generation potential evaluation of the Xiamaling Formation shale (Partial data of the Xiamaling Formation shale are sourced from Wu et al. 2022; Song et al. 2021; Xiao et al. 2022; Wang 2021; Zhang et al. 2018; Liu et al. 2019; He et al. 2022; Yang 2013; Wang 2014): TOC frequency distribution histogram in the Xiamaling Formation shale deposited in (a) an oxidizing environment; and b an anoxic environment; Hydrocarbon generation potential evaluation of the Xiamaling Formation shale deposited in (c) an oxidizing environment; and d an anoxic environment
5.3 Organic-rich shale formation under the MOE
Previous studies have suggested that high productivity and preservation conditions primarily impact the formation of organic-rich shale (Arthur and Sageman 1994; Liu et al. 2022; Padersen and Calvert 1990; Sageman et al. 2003; Mort et al. 2007; Wei et al. 2012). However, recent research show organic matter enrichment is an extremely complex process involving multiple factors (Fu et al. 2023; Zhang et al. 2023; Nie et al. 2023). Here, we examine enrichment mechanisms for MOE shales.
5.3.1 Control of TOC by paleoproductivity
The abundance of primary organic matter in sedimentary rocks is determined by paleoproductivity (Algeo and Ingall 2007; Ross and Bustin 2009; Schoepfer et al. 2015)d Mo, Si, Ba, Zn, and Cu are commonly used indicators to evaluate the paleoproductivity magnitude. P is both a pivotal nutrient for biological growth and an essential component of organisms, and its content indicates the magnitude of paleoproductivity (Kidder and Erwin 2001). However, only biogenic Si and Ba indicate the magnitude of paleoproductivity, as Si and Ba have multiple sources. The calculation formula of biogenic Si and Ba can be found in the research of Saito et al. (1992). Zn and Cu are only applicable for evaluating changes in paleoproductivity in sulfidic and reducing environments (Wei et al. 2012; Algeo and Ingall 2007).
The Xiamaling Formation shale has a P/Ti range of 0.016 to 26.64 (average = 4.35), Babio range of -152-342 ppm (mean = 98.41 ppm), and Mo content range of 0.34–51.2, with an average value of 9.73 (Yang 2013; Wang 2014; Wu et al. 2022; He et al. 2022). P/Ti showed weak positive correlation with both Babio and Mo (Fig. 15a,b). In addition, P/Ti, Babio, and Mo were lower in oxic environments than in anoxic environments (Fig. 15c,d,e), indicating higher anoxic paleoproductivity. This partially explains higher anoxic shale hydrocarbon generation potential. The P/Ti, Babio, and Mo of the Xiamaling Formation shale showed weak positive correlations with TOC (Fig. 15f,g,h,i), demonstrating that paleoproductivity influences organic matter abundance, but does not primarily control TOC.
Paleoproductivity characteristics of the Xiamaling Formation shale: a Babio vs. P/Ti; b Mo vs. P/Ti; c P/Ti vs. V/Cr; d Babio vs. V/Cr; e Mo vs. V/Cr; f Babio vs. TOC; g, h P/Ti vs. TOC; i Mo vs. TOC
5.3.2 Control of TOC by oxidation-reduction
Redox environments play an important role in TOC enrichment. Under oxidizing conditions, oxygen reacts with carbon in organic matter to form carbon dioxide, leading to organic matter decomposition and mineralization, which restricts its preservation (Padersen and Calvert 1990; Arthur and Sageman 1994; Sageman et al. 2003; Mort et al. 2007). However, the V/Cr and Ni/Co ratios in the Xiamaling Formation shale showed no clear relevance to the TOC content (Fig. 16a,b), indicating that redox environments can influence the TOC content, but are not the primary controlling factors.
The relationship between paleo-redox environments and TOC a V/Cr vs. TOC; b Ni/Co vs. TOC
5.3.3 Control of TOC by terrestrial debris
Terrigenous debris is another factor that influences the organic matter enrichment in sediment. It provides a source of organic material (Tissot and Welte 1984; Peters and Cassa 1994; Hu et al. 2017; Bao and Chen 2011; Meyers 1997; Yang and Chen 1997), and contains a large amount of nutrients and soil microorganisms, which accumulate and enrich in sediment (Tissot and Welte 1984; Yang and Chen 1997; Bao and Chen 2011). Fine particle size and smooth shapes provide a larger surface area and better physical protection for the organic matter enrichment, thereby slowing down the organic matter decomposition rate (Suess 1973; Tyson 1995; Hedges and Oades 1997). However, high inputs of terrigenous sediments can dilute the organic matter in sediment, resulting in decreased TOC content (Tyson 2001; Betts and Holland 1990). Therefore, terrigenous debris can significantly impact the organic matter enrichment in sediment. In the Xiamaling Formation shale, the TOC content had a weak negative correlation with Ti/Al, indicating the dilution effect of the terrigenous debris input on the organic matter abundance (Fig. 17).
The relationship between the TOC of the Xiamaling Formation shale and the input of terrigenous debris (a) and sedimentation rate (b)
5.3.4 Control of TOC by sedimentation rate
The organic matter enrichment in sediments is significantly influenced by the sedimentation rate, with a higher sedimentation rate limiting the oxidative decomposition of organic matter, and promoting its enrichment.
Sedimentation rate is associated with TOC content in ancient marine sediments (Ding et al. 2021; Cao et al. 2018; Tyson 2001; Betts and Holland 1990; Muller and Suess 1979). Rapid sedimentation rates also increase the organic matter input and the relative content in sediments (Tyson 2001; Betts and Holland 1990). However, above critical thresholds, the decomposition rate of organic matter can exceed its enrichment rate, leading to a decrease in organic matter content (Ding et al. 2021; Tyson 2001). Therefore, an appropriate sedimentation rate is essential for organic matter enrichment.
Moreover, the critical values for diluting organic matter induced by sedimentation rate differ in oxidative and anoxic environments (Tyson 2001; Betts and Holland 1990). Organic matter is prone to decomposition and mineralization in oxidative environments, so rapid sedimentation benefits the organic matter preservation. The oxic shales were deposited at higher rates than for anoxic shales, and the oxic shales exhibited a lower paleoproductivity than anoxic shales, suggesting that rapid deposition primarily enabled oxic high-quality shale formation.
5.3.5 Organic matter enrichment patterns in the MOE
The Xiamaling Formation comprises oxic and anoxic shales with similar terrigenous inputs but differing enrichment processes. In oxidizing environments, sufficient oxygen provides conditions for the presence of oxygenic algae, so eukaryotic organisms such as red algae appear during this period (Zhang et al. 2016; Wang et al. 2018; Miao et al. 2021a; Song et al. 2021; Xiao et al. 2022). However, these eukaryotic organisms contributed little or no productivity (Song et al. 2021; Xiao et al. 2022), resulting in relatively low paleoproductivity. Although oxidizing environments promote the decomposition and mineralization of organic matter, the high sedimentation rate during this period slowed down the speed of organic matter decomposition and mineralization, thus allowing the formation of high-quality shales under oxidizing conditions with low productivity (Fig. 18).
In anoxic environments, anaerobic bacteria, such as sulphide bacteria, proliferate, resulting in higher paleoproductivity, while anoxic conditions prevent organic matter decomposition and mineralization. Although the organic matter abundance was somewhat diluted by terrestrial clastic input, the combination of high paleoproductivity and favorable preservation conditions allowed the Xiamaling Formation shales to retain high hydrocarbon production potential (Fig. 18).
Patterns of shale organic matter enrichment under MOE
6 Conclusions
Insights into the Mesoproterozoic Xiamaling Formation shale have been obtained through organic petrological, organic geochemical, mineralogical and elemental geochemical analyses, revealed:
-
(1)
Based on environment, the Xiamaling Formation shale comprises oxic and anoxic types, with the former having shallow, rapid deposition and the latter deeper, slower deposition.
-
(2)
Anoxic shales showed superior hydrocarbon potential over oxic shales, although the latter also has high potential.
-
(3)
Shales deposited in anoxic environments exhibited higher paleoproductivity than oxic shales. Rapid deposition in oxic settings reduced decomposition, forming quality shales. In contrast, the strong paleoproductivity and favorable preservation conditions in anoxic environments ensure their high hydrocarbon potential.
Availability of data and materials
Data will be made available on request.
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This study completed with the support of the Innovative Research Group Project of the National Natural Science Foundation of China (Grant nos. 41872135 and 42072151).
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MH. analyzed the original data of the manuscript, conceived the structure of the manuscript, and completed the first draft. JZ.X.revised the first draft and provided fund support. TX.L. and DZ.provided the original data and ideas of the first draft, revised the first draft, and provided fund support. ZC.J. provided the idea of the first draft and revised it. LZ.K. provides test samples and shale physical properties. SY.G. participated in the collation of manuscript original data and the production of drawings. All authors contributed to the review of the manuscript.
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Miao, H., Jiang, Z., Tang, X. et al. Hydrocarbon generation potential and organic matter accumulation patterns in organic-rich shale during the mesoproterozoic oxygenation event: evidence from the Xiamaling formation shale. Geomech. Geophys. Geo-energ. Geo-resour. 9, 134 (2023). https://doi.org/10.1007/s40948-023-00668-3
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DOI: https://doi.org/10.1007/s40948-023-00668-3