1 Introduction

The early Cambrian recorded substantial changes in global ocean geochemical conditions and biological features compared with late Ediacaran (Knoll and Carroll 1999; Kimura and Watanabe 2001; Wille et al. 2008): a rapid large-scale transgression occurred during the early Cambrian resulted in the global ocean transforming to an anoxic environment from an oxic environment in the late Ediacaran (Fike et al. 2006; Jiang et al. 2009; Babcock et al. 2015); a biological event known as the ‘Cambrian Explosion’ happened represented by the abundance and species of fossils increasing abruptly (Brasier 1992; Marshall 2006). Based on the geochemical data, the early Cambrian ocean in the Upper Yangtze was strongly stratified and stagnant, with euxinic bottom water (Pi et al. 2013; Feng et al. 2014; Jin et al. 2016) and multiple periods of hydrothermal activities in local areas (Steiner et al. 2001; Pašava et al. 2008; Li et al. 2015).

The lower Cambrian black shale (LCBS) has generally been considered as depositing in an anoxic–euxinic environment in the early Cambrian global transgression (Guo et al. 2007; Lehmann et al. 2007; Liu et al. 2016a, b; Zhang et al. 2017a, b) and it is a high-quality source rock (Hu et al. 2015; Song et al. 2015; Wu et al. 2016; Liu et al. 2017a, b; Zhang et al. 2018) similar to typical black shale in North America, characterized by wide distribution, large thickness (100–400 m), high TOC (2%–10%), which means LCBS has geological advantages for shale gas accumulation (Dong et al. 2009; Liu et al. 2009; Wang et al. 2009a, b; Xiao et al. 2015; Zhang et al. 2015). The LCBS in Upper Yangtze has been well analyzed, focusing on sedimentary environment assessment (Zhu et al. 2003, 2006; Wang et al. 2015a), redox conditions (Guo et al. 2007; Lehmann et al. 2007; Jiang et al. 2009; Wang et al. 2012, 2015b; Xu et al. 2012; Och et al. 2013) and hydrothermal activity (Lott et al. 1999; Steiner et al. 2001; Jiang et al. 2006, 2007; Chen et al. 2009). However, the controlling factors of TOC in the LCBS are still unclear, especially the control from multiple conditions in the sedimentary process.

The southeastern margin of the Upper Yangtze (SMUY) is a promising area in shale gas exploration. This was the important depositional center of black shale during the early Cambrian, with thicker black shale deposited (80–120 m) and larger TOC values (1.5%–20%) than other regions. This study selected three locations (Taozichong, Longbizui and Yanbei) in SMUY, located in deep shelf, slope and deep basin environments, respectively, in the early Cambrian (see locations in Fig. 1).

Fig. 1
figure 1

a Lithofacies paleogeographic map in Upper Yangtze during the early Cambrian, from Steiner et al. (2001) and Luo (2014); b profile of sedimentary model of lower Cambrian in Upper Yangtze from Pašava et al. (2008)

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Furthermore, black shale samples in the lower Cambrian were collected from these 3 locations for geochemical tests to reveal the relationship between geochemical data and TOC. Confirming controlling factors of TOC could provide important theoretical and practical significance for shale gas exploration.

2 Geological setting

The South China Platform was formed by the collision of Yangtze Platform and Cathaysia Platform during the early Neoproterozoic Sibao Orogeny (ca. 1.0 Ga) (Li et al. 2002). Then the South China Platform evolved from a rift basin into a passive continental margin basin at ca. 750 Ma–690 Ma, when it separated from the Rodinia supercontinent during its breakup (Li et al. 1995; Wang and Li 2003).

The Upper Yangtze changed into a deep muddy shelf system in the early Cambrian from a carbonate platform in late Neoproterozoic as a result of large-scale transgression, which could be divided, from west to east, into six sedimentary facies: (1) platform, (2) extension zone, (3) shallow shelf, (4) deep shelf, (5) slope, (6) deep basin (Zhu et al. 2003; Liu et al. 2013, 2016a, b, 2017a, b) (Fig. 1a). At the beginning of the early Cambrian, the relative sea level in the Yangtze Platform was low with high energy, so anoxic water infiltrated to the euphotic zone during transgression. In this stage, it deposited black chert and black siliceous shale in SMUY under the influence of two short periods of siliceous hydrothermal activities (Fig. 1b). During the early Niutitang Stage, the Upper Yangtze became quiet and stratified, when the relative sea level reached its maximum flooding surface, and the chemocline migrated to the euphotic zone. Afterward to the end of the Niutitang Stage, the reducing conditions in the bottom water weakened due to the sea-level falling, while sulfuration appeared in the deep basin only (Xu et al. 2012; Li et al. 2015; Wang et al. 2015b). At the end of the early Cambrian, black shale deposited only in SMUY when the main part of the Upper Yangtze changed to a carbonate platform.

Taozichong area located in Guizhou Province was in a deep shelf environment during the early Cambrian (Fig. 1a); the lower Cambrian strata (overlying the Dengying Formation dolomite unconformably on top of the Ediacaran) are divided into the Taozichong and Niutitang Formations (Fig. 1b). Taozichong Formation is composed of phosphorite (2 m) at the bottom, phosphorous and dolomitic cherts intercalated with thin biological phosphorite with abundant algae microfossils and small shell fossils (Fig. 2a). There was Qingzhen Fauna, consisted of hyoliths, analogous Ediacaran Fauna, Monoplacophora, bradoriid and sponges, found in argillaceous siltstone and silty mudstone upon the siliceous phosphorite (Yang et al. 2002). The lower member of the Niutitang Formation is composed of black siliceous shale (42 m), with simple trace fossils and sponge spicules on the bottom. The upper member of the Niutitang Formation is composed of black carbonaceous shale, dark-gray shale containing trilobites, and siltstone (Fig. 2a).

Fig. 2
figure 2

Stratigraphic column and variation trends of TOC for lower Cambrian black shale in research sections (Fm.: Formation; Edi.: Ediacaran; DY Fm.: Dengying Formation)

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Longbizui area, located in Hunan Province, was in a slope environment in the early Cambrian (Fig. 1a); the lower Cambrian strata (overlying the Doushantuo Formation conformably) are divided into the Liuchapo and Niutitang Formations (Fig. 1b). The Liuchapo Formation is composed of dark-gray thin-medium chert intercalated by thin siliceous shale.

The lower member of the Niutitang Formation is composed of phosphoric siliceous shale with phosphorite interlayers. The upper member of the Niutitang Formation is composed of thick black carbonaceous shale and mudstone (Fig. 2b). Fossils are rare in the Liuchapo Formation and the lower member of the Niutitang Formation, but sponges and sponge spicules which possibly belong to the Protospongiidae sp are common in the upper member of the Niutitang Formation (Wang et al. 2012) (Fig. 2b).

Yanbei area, located in Hunan Province, was in a deep basin environment in the early Cambrian (Fig. 1a); the lower Cambrian strata (overlying the Doushantuo Formation conformably) are divided into the Liuchapo and Niutitang Formations (Fig. 1b). The Liuchapo Formation is composed of black chert, siliceous shale and carbonaceous shale. The lower member of the Niutitang Formation is composed of black carbonaceous shale, with some chert layers occurring at the base. The upper member of the Niutitang Formation is composed of dark-gray carbonaceous shale and mudstone intercalated by black coal beds, and gray marl at the top (Fig. 2c). Benthic fossils are rare, indicating a deep basin environment in the early Cambrian.

3 Materials and methods

We studied the LCBS from three locations introduced above in SMUY. The fresh rock samples were ground into powders to test TOC (total of 140 samples), mineral components (total of 47 samples), element contents (total of 30 samples) and molecular biomarkers. TOCs were analyzed with a CS-200 Carbon Sulfur Analyzer, and mineral components were examined with a D8 ADVANCE XRD Diffractometer, at the Key Laboratory of Hydrocarbon Accumulation, SINOPEC. The mineral components are presented in Table 1, and TOC results are shown in Fig. 2.

Table 1 Mineral components from XRD
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Element contents were analyzed by X-ray fluorescence (major elements) and inductively coupled plasma mass spectrometry (ICP-MS) (trace elements) at the laboratory of Beijing Research Institute of Uranium Geology, CNNC (China National Nuclear Corporation). Saturated hydrocarbon gas was analyzed by gas chromatography based on standard GB/T 18340.5-2010 at the Key Laboratory of Hydrocarbon Accumulation, SINOPEC (Table 2).

Table 2 Geochemical parameters of saturated hydrocarbons in the Niutitang Formation
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Eu/Eu*, representing Eu enrichment anomaly, was calculated as suggested by Dulski (1994):

$${\text{Eu}}/{\text{Eu}}^{*} = \, (3 \times {\text{Eu}}_{\text{N}} )/ \, (2 \times {\text{Sm}}_{\text{N}} + {\text{Tb}}_{\text{N}} )$$

where XN refers to normalized concentration against PAAS (post-Archean Australian shale) (Taylor and McLennan 1985). The results of Eu/Eu* are presented in Table 3.

Table 3 Calculated results of geochemical proxies
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Only biogenic elements can indicate primary productivity (Brumsack 2006), so the content of terrigenous elements should be deducted when calculating primary productivity using geochemical elements. Much previous research shows that aluminum (Al) can be used to represent the terrigenous constituent, since Al in silicate minerals is largely immobile during diagenesis as a main component of crustal rocks (Saito et al. 1992). Mobio and Babio, representing biogenetic origin molybdenum and barium, respectively, have been calculated in the formula:

$$w\left( {X_{\text{bio}} } \right) \, = w\left( {X_{\text{sample}} } \right) \, - w\left( {{\text{Al}}_{\text{sample}} } \right) \, \times \, \left[ {w\left( X \right)/w\left( {\text{Al}} \right)} \right]_{\text{N}}$$

where w(Xbio) represents the mass fraction of biogenic element X; w(Xsample) represents mass fraction of element X in sample; [w(X)/w(Al)]N represents mass fraction of X and Al in PAAS. The results of Mobio and Babio are presented in Table 3.

4 Results and discussion

4.1 Relationship between TOC and mineral components

Mineral components of the lower Cambrian strata in SMUY are dominated by quartz, followed by clay minerals, with a little feldspar, carbonate, pyrite and anhydrite (Table 1). The SMUY was located in a deep shelf–deep basin environment during the early Cambrian (Steiner et al. 2001; Zhu et al. 2003) far from the western ancient land (Fig. 1), where the depositional interface was under wave base or storm-wave base. Therefore, clay minerals deposited through a long transportation by wind and waves, in this case the content of clay minerals was mainly controlled by offshore distance. Europium (Eu) shows a positive anomaly generally in oceanic hydrothermal deposition (Ruhlin and Owen 1986; Douville et al. 1999), in which Eu/Eu* > 1 represents a positive anomaly. The formula for Eu/Eu* is discussed in Sect. 3.

The SMUY experienced multi-period hydrothermal activities in the early Cambrian, resulting in Eu/Eu* > 1 for all the samples from Taozichong (except TZC-8-1 and TZC-13), and most of the samples in the Liuchapo Formation from Longbizui and Yanbei (Table 3). The quartz in the strata affected by hydrothermal activities was hydrothermal origin without any obvious linear relation with TOC. Therefore, the relationship between TOC and the content either of biogenic quartz or of clay minerals in the samples from the Niutitang Formation in Longbizui and Yanbei would be discussed mainly in this article. The microlitic quartz in black shale was derived mainly from the opals in sponge spicules and radiolarians (Bowker 2003), the content of which was far more than the terrestrial quartz in such as Niutitang black siliceous shale (Loucks and Ruppel 2007). As suggested by Rowe et al. (2008), ratios of Si/Al located above Si/Al in illite (fitted by contents of Si and Al in the Barnett shale) represent biogenic excess Si contained in shale. The majority of Si/Al in the Niutitang Formation has been located in silica excess region, which means biogenic quartz dominated in the Niutitang shale (Fig. 3). It shows an obvious positive relation between TOC and content of quartz (Fig. 4a, c) and an obvious negative relation between TOC and content of clay minerals in the Niutitang shale from these two locations (Fig. 4b, d). That is to say, more quartz and less clay minerals accompany higher TOC (Fig. 5). Because more quartz represents larger biomass in the Niutitang shale, the deep shelf and deep basin environments, quartz-rich and clay-mineral-poor, provided a favorable living environment for marine organisms (i.e., bacteria, algae, radiolarian, sponge, bradoriid and eodiscid). In general, the TOC is positively correlated with the content of quartz and negatively correlated with the content of clay minerals, thus the deep shelf–deep basin environment favored the enrichment of organic matter.

Fig. 3
figure 3

Cross-plot of Si and Al contents in lower Cambrian shale

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Fig. 4
figure 4

Correlation diagram between TOC and mineral components (a, b Longbizui; c, d Yanbei. Red points are from the Liuchapo Formation, blue points are from the Niutitang Formation. Dotted lines represent trend lines)

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Fig. 5
figure 5

Correlation diagram between TOC and the content of quartz and clay minerals (green points are from Longbizui; purple points are from Yanbei. The arrow line represents the trend)

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4.2 Relationship between TOC and primary productivity

Primary productivity is the velocity of energy fixation by paleo-marine organisms during the energy cycle, i.e., the amount of organic matter generation per unit area and per unit time. Previous research shows that molybdenum (Mo) deposits in the form of stable sulfide through combination of Mo(Ox, S4−x)2−(x = 0–3) and sulfur-rich organic molecules (Tribovillard et al. 2004) or/and pyrites (Vorlicek et al. 2004) under anoxic conditions with participation of organic matter. This kind of combination is irreversible (Bostick et al. 2003), which would lead to more Mo deposited with more organic carbon availability. In bathyal–abyssal regions where primary productivity is high, both the barium (Ba) flux in sea water and the content of barite in deposition are high, with a 30% preserving rate of biologic Ba (Dymond et al. 1992; Paytan et al. 1996), thus the relationship between Ba and primary productivity in surface water could be established (Francois et al. 1995). The calculating formula of Mobio and Babio is discussed in Sect. 3. As a crucial nutrient element (Howarth 1988), phosphorus (P) in deposits is contained in dead plankton falling on the water–sediment interface; for this reason, the content of P can reflect primary productivity largely in geological epochs (Tyrrell 1999). In general, low molecular weight n-alkanes were sourced from plankton, algae or bacteria, while high molecular weight n-alkanes were source from advanced plants (Clark and Blumer 1967).

It shows an obvious positive relationship between TOC and Mobio, Babio and P in the samples from Taozichong and Longbizui (Fig. 6). Likewise, it shows an obvious positive relationship between TOC and Mobio and P in the samples from Yanbei (Fig. 6g, i); however, the relationship between TOC and Babio in this location shows a weaker relationship than the other two locations (Fig. 6h). It is because the Yanbei area was located in a deep basin environment during the early Cambrian which was deeper than the other two areas with sulfide bottom water containing free H2S. In this case, sulfate could be reduced by sulfur-reducing bacteria resulting in the BaSO4 crystals dissolving (Dymond et al. 1992), which could weaken the relativity between TOC and Babio in Yanbei. As it shows an obvious positive relationship between TOC and primary productivity in the LCBS from these three locations, primary productivity could provide the basis for organic accumulations in black shale (Pedersen and Calvert 1990), and organic carbon formed by primary productivity could still remain after a series of complex processes, e.g., deposition and burial. Alkane distribution in studied samples are nC18–nC25, and odd–even priority (OEP) values are close to 1 (0.73–1.13) (Table 2), which suggest organic matter was from marine plankton, e.g., algae and bacteria.

Fig. 6
figure 6

Correlation diagram between TOC and indicators of primary productivity (a, b, c Taozichong; d, e, f Longbizui; g, h, i Yanbei. Dotted lines represent correlation trends)

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4.3 Relationship between TOC and redox conditions

Pristane (Pr) and phytane (Ph) can be used as paleo-redox indicator: Ph has obvious advantage in a (strong) anoxic depositional environment, while Pr abundance is advantaged in a weak anoxic or oxic environment (Peters and Moldowan 1991). The ratios of trace metals, such as U/Th and Ni/Co, can be used to evaluate redox conditions. Uranium (U) usually dissolves in oxic seawater as U6+, while it could be absorbed easily by organic particles as U4+ in anoxic seawater (Algeo and Maynard 2004). In contrast to the U, thorium (Th) is concentrated in weathering-resistant minerals as a constituent of heavy minerals and clay minerals. Th does not migrate easily in a low-temperature environment. Therefore, ratios of U/Th can represent redox conditions in seawater, as Jones and Manning (1994) suggested that U/Th < 0.75 indicates oxic conditions, 0.75 < U/Th < 1.25 indicates dysoxic conditions, U/Th > 1.25 indicates anoxic conditions. Nickel (Ni) and cobalt (Co) usually exist in pyrite, and a higher ratio of Ni/Co indicates stronger reducing conditions. Jones and Manning (1994) suggested that Ni/Co < 5 indicates oxic conditions, 5 < Ni/Co < 7 indicates dysoxic conditions, Ni/Co > 7 indicates anoxic conditions.

In this study, Pr/Ph values change from 0.18 to 0.63 (Table 2), indicating strong-normal anoxic environment in LCBS. The average values of U/Th and Ni/Co are 1.66 and 9.80 (Taozichong), 3.54 and 11.84 (Longbizui), 3.97 and 13.32 (Yanbei), respectively, which shows the reducing conditions are Taozichong < Longbizui < Yanbei (Table 4). Almost all of the geochemical parameters show dysoxic and anoxic bottom water covered the sedimentary interface of black shale in SMUY (Fig. 7a). Moreover, a weak positive relationship existing between TOC and ratio of U/Th and of Ni/Co can be observed (Fig. 7b, c). In consequence, LCBS deposited under dysoxic–anoxic bottom water, which was beneficial for organic preservation, while the effect of reducing intensity on organic accumulation is quite weak.

Table 4 Statistical table of average TOC and redox level in three locations
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Fig. 7
figure 7

Correlation diagram between TOC and redox indicators (the ordinate scale is log of 10)

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4.4 Main factors affecting organic accumulation

The development of marine black shale is caused by multiple factors of sedimentary conditions. In addition, marine black shale would deposit only when each of the factors reaches a favorable condition, rather than emphasizing the influence from just one of them. Normally, organic accumulation needs two main conditions: (1) favorable preservation, which means bottom water should be dysoxic anoxic (Jenkyns 2010; Sun et al. 2016); (2) high primary productivity, which means abundant plankton living in surface water (Pedersen and Calvert 1990).

During the early Cambrian, the Yangtze Platform was located between 30° and 60° north latitude in the subtropical dry zone controlled by subtropical highs, where broad equatorial current divergence developed in the oceanic bottom, transformed into eutrophic upwelling at the continental margin like SMUY (Xia et al. 2015; Tang et al. 2017; Yeasmin et al. 2017). In consequence, the species and number of organisms in the early Cambrian increased rapidly in quartz-rich and clay-mineral-poor deep shelf–slope–basin environments, resulting in massive quantities of organic matter settling down and being buried, which is represented by high values from geochemical parameters of primary productivity (Mobio, Babio and P) and the obvious positive relationship between these parameters and TOC. On the other hand, the LCBS deposited in dysoxic–anoxic bottom water, which was beneficial for preservation of organic matter generated by productivity in surface water. However, an extremely weak positive relativity between the geochemical parameters of redox (Ni/Co and U/Th) and TOC could be found, which might mean an adequate source of organic matter diminished the importance of organic preservation, especially under dysoxic–anoxic bottom water.

In total, three main points can be drawn to explain the relationship between data and the factors affecting organic accumulation: (1) quartz-rich and clay-mineral-poor deep shelf–slope–basin environments provided a favorable environment for living organisms; (2) high primary productivity provided the foundation for organic matter generation; (3) redox levels impact slightly on the content of organic matter under high productivity and dysoxic–anoxic condition.

5 Conclusions

In conclusion, content of organic matter in the lower Cambrian black shale in SMUY is controlled by the following factors:

  1. (1)

    Quartz in samples of the Niutitang Formation in Longbizui and Yanbei was of biogenic origin indicated by excess Si content, and there is a positive correlation between the content of quartz and TOC and a negative correlation between the content of clay minerals and TOC. A deep shelf–deep basin environment with abundant marine organisms and featuring quartz-rich and clay-poor lithofacies provided a favorable environment for the living organisms.

  2. (2)

    Primary productivity in these three locations, represented by contents of Mobio, Babio and P, was positively correlated to TOC in the lower Cambrian black shale. Organic geochemical data suggest organic matter was sourced from plankton. High primary productivity provided a basis for organic matter generation, and organic carbon formed by primary productivity could survive after a series of complex processes.

  3. (3)

    Geochemical parameters of redox (Pr/Ph, Ni/Co and U/Th) suggest the lower Cambrian black shale deposited in dysoxic–anoxic bottom water, which was beneficial for preservation of organic matter generated by productivity in surface water. However, the weak positive relativity between the TOC and ratios of Ni/Co and U/Th means redox conditions impact only slightly on the content of organic matter under high productivity and dysoxic–anoxic condition.