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Article

Boron Enrichment in Salinized Lacustrine Organic-Rich Shale of the Paleogene Biyang Depression, East China: Occurrence and Geological Controlling Factors

by
Yu Song
1,2,3,*,
Paerzhana Paerhati
3,
Shilin Xu
1,2,4,
Bo Gao
1,2,
Shu Jiang
3,*,
Shuifu Li
3,
Yuchen Wang
3 and
Hecun Lv
3
1
State Key Laboratory of Shale Oil and Gas Enrihment Mechanisms and Efficient Development, Beijing 100083, China
2
Sinopec Key Laboratory of Shale Oil/Gas Exploration and Production Technology, Beijing 100083, China
3
Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China
4
Sinopec Petroleum Exploration and Production Research Institute, Beijing 100083, China
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(9), 904; https://doi.org/10.3390/min14090904
Submission received: 30 June 2024 / Revised: 29 August 2024 / Accepted: 29 August 2024 / Published: 3 September 2024
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

:
Although boron (B) is widely applied as a paleosalinity indicator for ancient lakes, the occurrence and geological controls of B enrichment in salinized lacustrine organic-rich shale (SLORS) are poorly understood. This study addresses this issue by comparing the mineral and element compositions of high-boron shale (HBS) and low-boron shale (LBS) from the Paleogene Biyang Depression, using integrated XRD, XRF, and ICP-MS analyses. The mineral composition of HBS is dominated by illite, whereas LBS primarily consists of albite; both are of detrital origin. Compared to the element composition of UCC, HBS is extremely enriched in Mo and W, whereas LBS is extremely enriched in W and U. Boron is positively correlated with Al2O3 and negatively correlated with Na2O, suggesting that B primarily occurs in illite. An enhanced extent of chemical weathering prevailed during the deposition of HBS, providing a greater supply of illite to the basin. Higher pH levels and greater reduction during HBS deposition encouraged illite absorption of B, ultimately leading to B enrichment in shale. Our findings suggest that pH and redox conditions, as well as the mineral compositions of shale, should be fully considered during the application of B and related ratios as paleosalinity indicators.

1. Introduction

Boron (Symbol B) is the lightest element of the boron group (Group 13) in the periodic table and has an atomic number of 5. Unlike many other elements, B is synthesized entirely by cosmic ray spallation and supernovae, not by stellar nucleosynthesis [1]. This unique origin results in relatively low B concentrations in the Earth’s crust, with values around 8 ppm in the lower continental crust (LCC) and 15 ppm in the upper continental crust [2]. In the crust, B primarily exists as borates, which are readily soluble and enter the hydrological system as boric acid [3]. Seawater typically contains higher and more uniform B concentrations than in freshwater [4]. However, elevated B levels can also be found in some freshwater systems, often associated with geothermal activity, connate waters from older marine formations, or evaporite deposits [5].
When the concentration of B available during the crystallization of primary minerals is insufficient to form an independent B mineral, such as tourmaline, B will substitute for the primary element in the mineral [6]. Experiments have shown that B is more readily adsorbed onto layered silicates, such as mica group and clay minerals, compared to other minerals [6]. Additionally, organic matter and Fe-/Mn-oxyhydroxides can also serve as adsorption substrates for B [7]. The uptake of B by minerals is a chemical complexation process, influenced by multiple factors such as salinity, pH, temperature, and the crystallinity and surface area of minerals [8]. Although adsorption patterns are well-studied for some elements, such as V [9], the competitive adsorption of B on different minerals (e.g., clay minerals and Fe-/Mn-oxyhydroxides) remains poorly understood.
Previous research has indicated that variable B concentrations are present in salinized lacustrine organic-rich shale (SLORS) of the Paleogene Biyang Depression, providing an excellent example to investigate the occurrence and geological controlling factors of B enrichment in these shales [10]. Despite this potential, little research has been conducted on B enrichment in the Biyang Depression, although B and related ratios, such as B/Ga, have been used to infer paleosalinity [11]. In this study, SLORS with variable B concentrations from the Paleogene Biyang Depression were selected to conduct X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), and inductively coupled plasma-mass spectrometry (ICP-MS) analyses in order to determine their mineral and element compositions. This study aims to better understand the processes that lead to B enrichment in shale, which can shed light on paleoenvironmental reconstruction during lacustrine shale deposition. The results not only contribute to the geological understanding of the Biyang Depression but also provide insights that may be applicable to similar geological settings worldwide.

2. Geological Setting

The Biyang Depression is situated in the eastern part of the Nanxiang Basin in East China (Figure 1A,B). Encompassing an area of approximately 1000 km2, the depression is divided into three structural units: the northern slope belt, the central deep sag, and the southern steep slope belt (Figure 1C). The basin fill, which is up to 8000 m, predominantly consists of Paleogene strata overlying various basement units, including granites, felsic-intermediate volcanic rocks, and metamorphic rocks dating from the Proterozoic to the Paleozoic (Figure 1D).
The structural evolution of the Biyang Depression during the Paleogene is categorized into four episodes (Figure 2) [12]. Episode I (66.2–54.2 Ma) marks the early initial rifting stage, characterized by the deposition of the Paleocene–Eocene Yuhuangding Formation (Ey). The Ey Formation is primarily composed of coarse-grained sandstone interbedded with thin mudstone layers, indicative of fluvial facies. Episode II (54.2–49.6 Ma), the late initial rifting stage, coincides with the deposition of the Eocene Dacangfang Formation (Ed), which developed under semi-saline lake environments and comprises coarse- and medium-grained sandstone interbedded with mudstone and trona.
The maximum rifting stage of the depression (Episode III, 49.6–32.99 Ma) aligns with the deposition of the third (Eh3) and second members (Eh2) of the Eocene–Oligocene Hetaoyuan Formation. The Eh3 member consists of interbedded grey shale and carbonate rocks, which is indicative of deep-to-semi-deep saline lake environments. The Eh2 member is characterized by light-grey shale, carbonate rock, and thin fine-grained sandstone layers, suggesting a shallow lake environment. Nahcolite is found in the upper Eh3 and lower Eh2 members, representing the largest economically viable accumulation of nahcolite in Asia.
Episode IV, the late rifting stage, corresponds to the deposition of the first member of the Hetaoyuan Formation (Eh1) and the Oligocene Liaozhuang Formation (El). The Eh1 member is characterized by light-grey shale interbedded with medium- and fine-grained sandstone, indicating a shore-shallow lake environment. The El Formation consists of fine-grained sandstone, shale, and gypsum, representing fluvial facies and saline lake environments.

3. Materials and Methods

A total of 27 core samples were collected from two exploration wells, designated as Well BY and Well B3, located in the Southeastern Biyang Depression (Figure 1C). The samples were taken from Unit III (Eh33) in Well BY and Units IV–V (Eh34−5) in Well B3. The sampling depths are illustrated in Figure 2, and the lithologies of the samples include shale, silty shale, and muddy siltstone. Comprehensive analysis, including X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), and inductively coupled plasma-mass spectrometry (ICP-MS), was conducted on all samples to determine their mineralogical and elemental compositions. Additionally, a subset of these samples was selected for scanning electron microscopy (SEM) to reveal mineral morphology.
XRD analysis was performed using a Bruker AXS D80-Focus instrument. For this analysis, the powdered samples were dispersed in a dilute sodium phosphate solution using a sonic probe, producing random whole-rock mounts. The clay fractions were isolated as particles smaller than 4 μm equivalent spherical diameter (ESD) from the suspension and were subsequently collected on silver membrane filters. For SEM analysis, samples were coated with gold and observed using an FEI Quanta 650 FEG SEM instrument.
A Shimadzu XRF-1800 equipment was employed to determine the contents of major-element oxides in the samples. Powdered samples (less than 200 mesh) were heated to 815 ℃ in a muffle furnace to quantify the loss on ignition (LOI). The residuals were subsequently combined with lithium tetraborate and pressed into powder mounts for XRF analysis. Trace element concentrations were measured using a Thermo Fischer Scientific Element XR high-resolution ICP-MS. Prior to analysis, powdered samples (less than 200 mesh) underwent pretreatment with a mixture of 2 mL of HNO3 (65% v/v) and 5 mL of HF (40% v/v). For the determination of boron concentrations, the samples were first weighed into a nickel crucible, sodium hydroxide flux was added, and the mixture was thoroughly combined before melting at a high temperature. After cooling, the melt was dissolved in deionized water and diluted to 100 mL. An equal volume of HCl was then added, mixed thoroughly, and analyzed using the ICP-MS. The final analysis results were obtained after correcting for spectral interference between elements [4]. Analytical reliability was ensured through the use of standard reference materials (e.g., AGV-2, BHVO-2 [13]) and by conducting duplicate analyses on selected samples. Total organic carbon (TOC) contents of studied samples have been provided in [10], ranging from 1.1 wt. % to 7.6 wt. % in Well BY and 0.1 wt. % to 2.3 wt. % in Well B3.

4. Results

4.1. Boron Concentrations

Boron (B) concentrations in the Well BY range from 44 to 464 ppm, whereas in Well B3, they range from 10 to 30 ppm (Figure 3). The Bsample/BUCC ratios (upper continental crust; data from [2]) range from 2.9 to 31.0 in Well BY and from 0.7 to 2.0 in Well B3. Based on the B concentrations, the samples are categorized into two types: samples with high B concentrations (151–464 ppm) and high Bsample/BUCC ratios (10.1–31.0) are classified as high-boron shale (HBS), whereas samples with low B concentrations (10–67 ppm) and low Bsample/BUCC ratios (0.7–4.4) are classified as low-boron shale (LBS). The B concentrations of the studied shale are generally higher than those of other lacustrine shales in China; for example, the shale of Upper Triassic Chang 7 Member (B: 20–31 ppm) in Ordos Basin (North China), and the Paleogene brackish to marine shale (B: 14–70 ppm) in Bohai Bay Basin (East China) [4].

4.2. Mineral Compositions of HBS and LBS

Illite, averaging 31 wt.%, is the predominant mineral in HBS (Figure 4), followed by quartz (21 wt.%), albite (20 wt.%), calcite (11 wt.%), dolomite (8 wt.%), and pyrite (6 wt.%). Minor proportions of montmorillonite and gypsum were also identified via XRD analysis. In contrast, LBS primarily consists of albite (52 wt.%), dolomite (20 wt.%), illite (11 wt.%), and K-feldspar (6 wt.%), with minor proportions of quartz, chlorite, and pyrite. Quartz grains, typically 2–5 μm in size with a sub-rounded shape, suggest a detrital origin (Figure 5A). The rough surfaces of quartz grains indicate significant collisions during long-distance transport. Albite and K-feldspar, measuring 5–10 μm and 5–20 μm, respectively, predominantly exhibit sub-angular-to-sub-rounded shapes, suggesting detrital origins with short-distance transport (Figure 5B,C). Calcite is 1–5 μm in size with a sub-rounded shape (Figure 5B), whereas dolomite, measuring 5–10 μm, occurs in a rhombohedron shape (Figure 5D). Illite shows layered crystal structures and occurs as very fine-grained (<4 μm) platy particles, believed to be weathering products of parent rocks (Figure 5C–E). Gypsum, occasionally found in samples, ranges from 1–10 μm in size, suggesting precipitation in saline lacustrine environments (Figure 5F).

4.3. Elemental Compositions of HBS and LBS

Compared to the UCC, HBS is enriched in Al2O3, Fe2O3, MgO, CaO, K2O, P2O5, and MnO, and it is similar to or depleted in SiO2, Na2O, and TiO2 (Figure 6A). In contrast, LBS is enriched in Fe2O3, MgO, CaO, Na2O, P2O5, and MnO but depleted in SiO2, Al2O3, K2O, and TiO2 (Figure 6B). The enrichment of Na2O in LBS is associated with high percentages of albite (Figure 4).
Most trace elements in HBS are more enriched compared to the UCC, with the exception of Zr, Sn, and Hf (Figure 6A). Among the trace elements, Mo and W are extremely enriched, exhibiting sample/UCC ratios greater than 10 in HBS (Figure 6A). Conversely, trace elements including Be, Ge, Rb, Zr, Sn, Cs, Hf, and Tl are depleted in LBS relative to the UCC, while other elements are more enriched (Figure 6B). Notably, W and U are particularly enriched in LBS, with sample/UCC ratios exceeding 10.
The concentrations of rare Earth elements (REE) in HBS range from 215 to 587 ppm, with an average of 374 ppm, which are higher than those in LBS, which range from 116 to 301 ppm, with an average of 201 ppm. Light rare Earth elements (LREE: La-Eu) are more enriched compared to heavy rare Earth elements (HREE: Gd-Lu), with LREE/HREE ratios varying from 10 to 37 in HBS and from 3 to 16 in LBS. The UCC-normalized REE patterns of both HBS and LBS are characterized by sloping LREE and flat HREE trends, accompanied by weak Eu anomalies (Figure 7). Additionally, the variance of HREE in LBS is greater than that in HBS.

5. Discussion

5.1. Occurrence of Boron in Lacustrine Shale

The B concentrations show significant vertical variation, with high concentrations in the middle part of the studied intervals from Well BY, reaching up to 464 ppm (Figure 8). A positive correlation between B and Al2O3 in all shale (r = 0.74) suggests that B primarily occurs in Al-bearing minerals (Figure 9A). Furthermore, the negative correlation between B and Na2O (r = −0.81) indicates that B is mainly associated with clay minerals, rather than albite (Figure 9B). High-correlation coefficients are also observed between B and trace elements such as Be, Ga, Ge, Sn, Cs, and Pb, all of which show r > 0.80 according to Pearson correlation coefficients (Figure 9C–H).
Previous studies have indicated that B primarily occurs in clay, shale, and glauconite rocks, with B concentrations ranging from 25 to 800 ppm (clay and shale) and 350 to 2000 ppm (glauconite rocks), which are generally higher than those in other sedimentary rocks such as sandstone (5–70 ppm), limestone (2–95 ppm), and dolostone (10–70 ppm) [6]. Layered silicates, including minerals from the mica group and clay minerals, are the primary hosts of B in sediments [6]. Illite and glauconite contain the highest B concentrations (100 to several thousand ppm), followed by muscovite (10–500 ppm), paragonite (50–250 ppm), and montmorillonite (5–200 ppm), whereas other layered silicates generally contain relatively low B concentrations (<50 ppm). Additionally, B concentrations in framework silicates, such as quartz and feldspars, are commonly lower than those in layered silicates. Based on the differences in mineral compositions between HBS and LBS, illite is considered the primary host of B in HBS, followed by montmorillonite.

5.2. Controlling Factors of Boron Enrichment in Lacustrine Shale

Rimstidt et al. identified five factors that control element abundances in shale: (1) weathering, transport, and deposition; (2) early diagenesis; (3) burial diagenesis; (4) exhumation; and (5) weathering [14]. Studies have revealed that no major uplifts during the burial history of the Eh3 shale in representative wells from the central deep sag and the southern steep slope belt of the Biyang Depression [15,16]. Additionally, the index of compositional variability (ICV = (Fe2O3 + K2O + Na2O + CaO + MgO + MnO + TiO2)/Al2O3) is used to determine the mineralogical maturity of sediments [17]. ICV values greater than 1.0 generally indicate “immature” sediments that have only experienced a single weathering cycle, whereas values less than 1.0 represent “mature” sediments that have undergone multiple processes of chemical weathering, erosion, and redeposition. Both HBS and LBS exhibit ICV values greater than 1.0, indicating that the influence of both exhumation and weathering (the fifth controlling factor) on B concentrations in the studied shale can be ruled out.
Based on previous studies, the vitrinite reflectance of sampled intervals from Well BY and Well B3 is 0.59% and 0.88%, respectively, both corresponding to early oil window maturity [18]. Although a minor proportion of chlorite is found in LBS from Well B3, illite remains the predominant clay mineral in both wells (Figure 4). Therefore, we speculate that the influence of clay mineral transformations during diagenesis on B enrichment in shale is minimal.

5.2.1. Provenance and Tectonic Setting

Trace elements such as La, Th, Zr, Hf, Sc, and Co are widely used to infer sediment provenance due to their immobility during post-depositional processes [19,20,21,22]. Th, Zr, and La are typically more enriched in granites and felsic volcanic rocks, whereas Sc and Co are more enriched in andesites and basalts [23]. Therefore, Th/Sc, Zr/Sc, Co/Th, and La/Sc ratios are employed to distinguish between felsic and mafic volcanic rocks in this study. In cross-plots of Th/Sc versus Zr/Sc and Co/Th versus La/Sc, both HBS and LBS plot near points representing granites and felsic volcanic rocks, indicating a primary derivation from more felsic volcanic rocks (Figure 10A,B). This conclusion is further supported by granular felsic volcanic lithic fragments in the samples (Figure 11A,B). The presence of polycrystalline quartz, characterized by more than five elongated irregular crystals with crenulated inter-crystalline boundaries, suggests a metamorphic origin (Figure 11C,D) [24]. Therefore, both HBS and LBS are primarily derived from granites, felsic volcanic rocks, and metamorphic rocks.
Element concentrations and ratios of siliciclastic sediments vary across different tectonic settings. Thus, these parameters can be used to distinguish the tectonic settings of siliciclastic sediments [25]. In this study, the cross-plot of DF1 versus DF2 (representing discriminant-function-based major elements) and the ternary plot of La-Th-Sc are used to infer the tectonic setting of parent rocks of HBS and LBS [25,26]. HBS and LBS samples are scattered across areas representing three main tectonic settings: continental rift, island or continental arc, and collision, in the DF1 versus DF2 cross-plot (Figure 10C). In the ternary plot of La-Th-Sc, the samples are primarily located in areas indicative of active continental margin/passive margin and continental arc settings (Figure 10D). The results suggest a complex tectonic setting for the parent rocks of HBS and LBS, involving continental rifting, subduction, continental arc, and collision processes.
The Biyang Depression is situated within the Qinling–Tongbai–Dabie–Sulu orogenic belt between the North China Block and the South China Block. The basement, serving as the parent rock for HBS and LBS, likely experienced the complete Wilson Cycle [27,28]. This aligns with the tectonic settings inferred from the element concentrations and ratios of HBS and LBS.

5.2.2. Chemical Weathering

Chemical weathering is primarily associated with the loss of alkaline metals from silicates and the enrichment of insoluble elements. Consequently, the relative ratios of immobile-to-mobile elements are widely used to infer the degree of chemical weathering in geological periods [29,30]. The chemical index of alteration (CIA) and weathering (CIW) are widely applied proxies to reveal the degree of chemical weathering [31]. The CIA value is calculated as CIA = Al2O3/(Al2O3 + K2O + Na2O + CaO*) × 100, where Al2O3, K2O, and Na2O represent the molar amounts of these element oxides, and CaO* = min (CaO-10/3 × P2O5, Na2O) [32]. The CIW value, calculated as CIW = Al2O3/(Al2O3 + Na2O + CaO*) × 100, is also applied in this study to reduce the influence of K enrichment during diagenesis on the degree of chemical weathering.
The CIA and CIW values of HBS range from 54 to 75 (avg. 65) and 60 to 87 (avg. 74), respectively, which are higher than those of LBS (CIA: 35–64, avg. 41; CIW: 38–81, avg. 46) (Figure 12). A positive correlation between CIA and CIW (r2 = 0.99) suggests a minimal effect of K enrichment during diagenesis on the chemical-weathering parameters. The results indicate an enhanced degree of chemical weathering during the deposition of HBS, likely associated with increased precipitation [33]. The elevated supply of weathering products via drainage likely contributed to the enrichment of B in lacustrine shale, as B silicates are generally stable during weathering [6].

5.2.3. Water Chemistry

Boron concentrations in seawater are inversely correlated with dissolved oxygen and positively correlated with pH [34]. Therefore, this study examines the redox conditions and pH of paleo-lake water during shale deposition. The enrichment factors of U (UEF = (U/Al)sample/(U/Al)UCC) and Mo (MoEF = (Mo/Al)sample/(Mo/Al)UCC), along with the TOC/P molar ratio, are used to infer the redox conditions. Uranium and Mo are more enriched in oxygen-depleted conditions than in oxic conditions. Therefore, high values of UEF and MoEF indicate more reducing conditions [35]. As P concentrations in water are primarily associated with P burial efficiency and/or enhanced P recycling influenced by redox conditions, TOC/P molar ratios are significantly negatively correlated with dissolved oxygen [36].
The UEF and MoEF values are generally higher in HBS (UEF: 1.1–6.2; MoEF: 4.2–46.8) than those in LBS (UEF: 0.5–5.9, except for three abnormally high values > 10; MoEF: 1.5–25.4), suggesting more reducing conditions during the deposition of HBS (Figure 13). This conclusion is further supported by comparatively high molar ratios of TOC/P in HBS (21–141, avg. 69; LBS: 1–270, avg. 45).
To our knowledge, there are no valid parameters for assessing the pH of paleo-lake water before the Neogene. In this study, the percentage of syngenetic gypsum is used to infer the pH of the paleo-lake during shale deposition. HBS, with relatively high percentages of gypsum, generally contains lower percentages of B-bearing minerals (e.g., illite; Figure 4). Since gypsum tends to form under low pH conditions [37], these results suggest that high pH is favorable for B enrichment in shale. Additionally, salinity likely influences the B concentrations in shale, as higher concentrations are generally found in brackish to saline shale compared to freshwater shale [4].

5.3. Implications for B as a Paleosalinity Indicator

Boron concentrations and related ratios, such as B/Ga ratios, are widely used as paleosalinity indicators in geological periods [4]. This is due to the nearly uniform B-to-salinity ratio found in modern oceans, including the Baltic Sea, North Pacific, and North Atlantic oceans [34,38]. Boron can be absorbed by minerals such as illite and mica under all salinity conditions, though this effect is more pronounced in seawater than in freshwater [4]. However, this absorption process is influenced by both pH and redox conditions. For instance, B is absorbed into clay minerals by displacing H2O and OH- from surface sites on a-Al2O3, a process promoted by higher pH conditions [39]. Additionally, the type of adsorption substrates can influence B absorption. Experiments have shown that mica or illite can uptake water-insoluble B from solution, and other phases such as Fe- and Mn-oxyhydroxides may also absorb B, as indicated by strong B-Fe correlations in sediments [6,7]. In summary, B concentrations in ancient sediments like shale are controlled by multiple factors, including salinity, pH, water temperature, and the type of absorption substrates. Therefore, paleo-water conditions, especially pH and redox conditions, along with the mineral compositions of sediments, should be assessed before using B concentrations and related ratios (e.g., B/Ga) to determine paleosalinity. Additionally, the influence of volcanic activity and hydrothermal fluids on the enrichment of boron (B) in shale should be considered, although these factors have minimal impact on the samples in this study. Further research should also be focused on the influence of interactions between clay minerals and organic matter on the B enrichment in organic-rich shale [40].

6. Conclusions

Boron concentrations are up to 464 ppm in the salinized lacustrine organic-rich shale (SLORS) of the Paleogene Biyang Depression, East China. In this study, shale characterized by high-boron concentrations (151–464 ppm) and high Bsample/BUCC ratios (10.1–31.0) are termed high-boron shale (HBS), whereas shale with low B concentrations (10–67 ppm) and low Bsample/BUCC ratios (0.7–4.4) are classified as low-boron shale (LBS). The mineral and elemental compositions of both HBS and LBS were determined using X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), and inductively coupled plasma-mass spectrometry (ICP-MS) analyses to reveal the occurrence and geological controlling factors of B enrichment in SLORS.
Illite is the predominant mineral in HBS, followed by quartz, albite, and calcite. In contrast, LBS is dominated by albite, followed by dolomite, illite, and K-feldspar. Compared to the element composition of the upper continental crust (UCC), HBS is extremely enriched (sample/UCC > 10) in Mo and W, whereas LBS is extremely enriched in W and U. Ligh rare Earth elements (LREE) are more enriched compared to heavy rare Earth elements (HREE), and both HBS and LBS exhibit weak Eu anomalies. Pearson correlation coefficients suggest that B is positively correlated with Al2O3 (r = 0.74) and negatively correlated with Na2O (r = −0.81), indicating that B primarily occurs in illite, followed by montmorillonite. Additionally, there are obvious positive correlations between B and trace elements, including Be, Ga, Ge, Sn, Cs, and Pb (r > 0.80 for all correlations). Both HBS and LBS are derived from granite, felsic volcanic rocks, and metamorphic rocks, as indicated by trace element ratios (Th/Sc, Zr/Sc, Co/Th, and La/Sc), and the presence of corresponding lithic fragments. These parent rocks formed under a complex tectonic setting involving continental rift, subduction, continental arc, and collisions are consistent with the geotectonic background of the Biyang Depression. The chemical index of alteration/weathering (CIA/CIW) reveals an elevated degree of chemical weathering during the deposition of HBS, which provided a greater supply of B-bearing weathering products and contributed to B enrichment in shale. More reducing and higher pH conditions predominated during the deposition of HBS, as indicated by redox condition parameters (MoEF, UEF, and TOC/P) and the percentage of syngenetic gypsum.
Our results indicate that applying B and related ratios as paleosalinity indicators should be approached with caution. Paleo-water conditions, especially pH and redox conditions, as well as the mineral compositions of sediments, should be thoroughly assessed beforehand. Further research is needed to understand the influence of exhumation and weathering on B concentrations in SLORS.

Author Contributions

Conceptualization, writing—original draft preparation, Y.S.; writing—review and editing, P.P.; funding acquisition, S.X.; project administration, S.J.; funding acquisition, B.G.; funding acquisition, methodology, S.L.; investigation, Y.W.; investigation, H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Open funding project of Sinopec Key Laboratory of Shale Oil/Gas Exploration and Production Technology (Reconstruction of biological community structure of organic-rich shale in saline lake basins and its quantitative indication of paleoproductivity), and National Natural Science Foundation of China (Grant Nos. 42072174; 42073067; 41902139).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Acknowledgments

We appreciate the valuable comments and suggestions from the editor and two anonymous reviewers, which significantly improve the quality of the manuscript. Henan Oilfield Company (SINOPEC) gratefully appreciates the permission of sampling.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (A) Location of the Nanxiang Basin in China. (B) Location of the Biyang Depression within the Nanxiang Basin. (C) Division of structural units in the Biyang Depression. (D) Profiles crossing the Biyang Depression. Location of profiles are illustrated in (C) (A-A’ correspond to the cross-section profiles A-A’ in (C), B-B’ correspond to the cross-section profiles B-B’ in (C)).
Figure 1. (A) Location of the Nanxiang Basin in China. (B) Location of the Biyang Depression within the Nanxiang Basin. (C) Division of structural units in the Biyang Depression. (D) Profiles crossing the Biyang Depression. Location of profiles are illustrated in (C) (A-A’ correspond to the cross-section profiles A-A’ in (C), B-B’ correspond to the cross-section profiles B-B’ in (C)).
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Figure 2. Stratigraphic section, lithological assemblage, sedimentary facies, and structural evolution of the Late Cretaceous to Neogene in the Biyang Depression, as well as the sampling depth of wells BY and B3. Locations of the wells are provided in Figure 1C.
Figure 2. Stratigraphic section, lithological assemblage, sedimentary facies, and structural evolution of the Late Cretaceous to Neogene in the Biyang Depression, as well as the sampling depth of wells BY and B3. Locations of the wells are provided in Figure 1C.
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Figure 3. Variations of boron (B) concentrations and Bsample/BUCC (upper continental crust) ratios across Wells BY and B3. (Gray rectangular for boron concentrations, Red square for Bsample/BUCC ratios).
Figure 3. Variations of boron (B) concentrations and Bsample/BUCC (upper continental crust) ratios across Wells BY and B3. (Gray rectangular for boron concentrations, Red square for Bsample/BUCC ratios).
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Figure 4. Mineral compositions of high-boron shale and low-boron shale.
Figure 4. Mineral compositions of high-boron shale and low-boron shale.
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Figure 5. SEM photos of representative minerals: (A) Quartz. (B) Albite and calcite. (C) K-feldspar and illte. (D) Dolomite and illite. (E) Illite. (F) Gypsum.
Figure 5. SEM photos of representative minerals: (A) Quartz. (B) Albite and calcite. (C) K-feldspar and illte. (D) Dolomite and illite. (E) Illite. (F) Gypsum.
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Figure 6. Major-element oxides and trace element concentrations of shale versus those in upper continental crust (UCC): (A) High-boron shale. (B) Low-boron shale. (Black square for the maximum value, gray circle for the average value, triangle for the minimum value).
Figure 6. Major-element oxides and trace element concentrations of shale versus those in upper continental crust (UCC): (A) High-boron shale. (B) Low-boron shale. (Black square for the maximum value, gray circle for the average value, triangle for the minimum value).
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Figure 7. Rare Earth-element concentrations of shale versus those in upper continental crust (UCC): (A) High-boron shale. (B) Low-boron shale. (Black square for the maximum value, gray circle for the average value, triangle for the minimum value).
Figure 7. Rare Earth-element concentrations of shale versus those in upper continental crust (UCC): (A) High-boron shale. (B) Low-boron shale. (Black square for the maximum value, gray circle for the average value, triangle for the minimum value).
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Figure 8. Vertical variations of trace elements exhibiting obvious correlations with B, including Al2O3, Na2O, Be, Ga, Ge, Sn, Cs, and Pb.
Figure 8. Vertical variations of trace elements exhibiting obvious correlations with B, including Al2O3, Na2O, Be, Ga, Ge, Sn, Cs, and Pb.
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Figure 9. Cross-plots of (A) Al2O3 versus B, (B) Na2O versus B, (C) Be versus B, (D) Ga versus B, (E) Ge versus B, (F) Sn versus B, (G) Cs versus B, (H) Pb versus B.
Figure 9. Cross-plots of (A) Al2O3 versus B, (B) Na2O versus B, (C) Be versus B, (D) Ga versus B, (E) Ge versus B, (F) Sn versus B, (G) Cs versus B, (H) Pb versus B.
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Figure 10. Cross-plots of (A) Th/Sc versus Zr/Sc, (B) Co/Th versus La/Sc. (C) DF1 versus DF2. DF1 = (0.608 × ln(TiO2/SiO2)adj) + (−1.854 × ln(Al2O3/SiO2)adj) + (0.299 × ln(Fe2O3/SiO2)adj) + (−0.550 × ln(MnO/SiO2)adj) + (0.120 × ln(MgO/SiO2)adj) + (0.194 × ln(CaO/SiO2)adj) + (−1.510 × ln(Na2O/SiO2)adj) + (1.941 × ln(K2O/SiO2)adj) + (0.003 × ln(P2O5/SiO2)adj) − 0.294; DF2 = (−0.554 × ln(TiO2/SiO2)adj) + (−0.995 × ln(Al2O3/SiO2)adj) + (1.765 × ln(Fe2O3/SiO2)adj) + (−1.391 × ln(MnO/SiO2)adj) + (−1.034 × ln(MgO/SiO2)adj) + (0.225 × ln(CaO/SiO2)adj) + (0.713 × ln(Na2O/SiO2)adj) + (0.330 × ln(K2O/SiO2)adj) + (0.637 × ln(P2O5/SiO2)adj) − 3.631. The subscript adj refers to the major-element oxide values obtained after volatile-free adjustment of the 10 major-elements to 100 wt.%. (D) Ternary plot of La-Th-Sc.
Figure 10. Cross-plots of (A) Th/Sc versus Zr/Sc, (B) Co/Th versus La/Sc. (C) DF1 versus DF2. DF1 = (0.608 × ln(TiO2/SiO2)adj) + (−1.854 × ln(Al2O3/SiO2)adj) + (0.299 × ln(Fe2O3/SiO2)adj) + (−0.550 × ln(MnO/SiO2)adj) + (0.120 × ln(MgO/SiO2)adj) + (0.194 × ln(CaO/SiO2)adj) + (−1.510 × ln(Na2O/SiO2)adj) + (1.941 × ln(K2O/SiO2)adj) + (0.003 × ln(P2O5/SiO2)adj) − 0.294; DF2 = (−0.554 × ln(TiO2/SiO2)adj) + (−0.995 × ln(Al2O3/SiO2)adj) + (1.765 × ln(Fe2O3/SiO2)adj) + (−1.391 × ln(MnO/SiO2)adj) + (−1.034 × ln(MgO/SiO2)adj) + (0.225 × ln(CaO/SiO2)adj) + (0.713 × ln(Na2O/SiO2)adj) + (0.330 × ln(K2O/SiO2)adj) + (0.637 × ln(P2O5/SiO2)adj) − 3.631. The subscript adj refers to the major-element oxide values obtained after volatile-free adjustment of the 10 major-elements to 100 wt.%. (D) Ternary plot of La-Th-Sc.
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Figure 11. Representative photos of lithic fragments under the microscope. (A) Felsic volcanic rock fragment (Lv) under transmitted light. Kf = K-feldspar. Qm = monocrystalline quartz. (B) The same field of (A) under crossed polarized light. (C) Polycrystalline quartz (Qp) fragment under transmitted light. (D) The same field of (C) under crossed polarized light.
Figure 11. Representative photos of lithic fragments under the microscope. (A) Felsic volcanic rock fragment (Lv) under transmitted light. Kf = K-feldspar. Qm = monocrystalline quartz. (B) The same field of (A) under crossed polarized light. (C) Polycrystalline quartz (Qp) fragment under transmitted light. (D) The same field of (C) under crossed polarized light.
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Figure 12. Ternary plot of Al2O3-CaO* + Na2O-K2O. CaO* = min (CaO-10/3 × P2O5, Na2O). CIA = chemical weathering of index. UCC = upper continental crust.
Figure 12. Ternary plot of Al2O3-CaO* + Na2O-K2O. CaO* = min (CaO-10/3 × P2O5, Na2O). CIA = chemical weathering of index. UCC = upper continental crust.
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Figure 13. Cross-plot of the enrichment factors U (UEF = (U/Al)sample/(U/Al)UCC) and Mo (MoEF = (Mo/Al)sample/(Mo/Al)UCC).
Figure 13. Cross-plot of the enrichment factors U (UEF = (U/Al)sample/(U/Al)UCC) and Mo (MoEF = (Mo/Al)sample/(Mo/Al)UCC).
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Song, Y.; Paerhati, P.; Xu, S.; Gao, B.; Jiang, S.; Li, S.; Wang, Y.; Lv, H. Boron Enrichment in Salinized Lacustrine Organic-Rich Shale of the Paleogene Biyang Depression, East China: Occurrence and Geological Controlling Factors. Minerals 2024, 14, 904. https://doi.org/10.3390/min14090904

AMA Style

Song Y, Paerhati P, Xu S, Gao B, Jiang S, Li S, Wang Y, Lv H. Boron Enrichment in Salinized Lacustrine Organic-Rich Shale of the Paleogene Biyang Depression, East China: Occurrence and Geological Controlling Factors. Minerals. 2024; 14(9):904. https://doi.org/10.3390/min14090904

Chicago/Turabian Style

Song, Yu, Paerzhana Paerhati, Shilin Xu, Bo Gao, Shu Jiang, Shuifu Li, Yuchen Wang, and Hecun Lv. 2024. "Boron Enrichment in Salinized Lacustrine Organic-Rich Shale of the Paleogene Biyang Depression, East China: Occurrence and Geological Controlling Factors" Minerals 14, no. 9: 904. https://doi.org/10.3390/min14090904

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