Abstract
Taking the layered argillaceous limestone in the upper Es4 in the Dongying Sag as the research object, the geochemical analysis of major, trace, and rare earth elements (REEs) established the response relationship between REE distribution characteristics and sedimentary diagenesis. The average values of total light REE ()/total heavy REE () of micrite calcite and argillaceous laminae are 6.75 and 4.06, respectively. The LREEs and HREEs are differentiated, consistent with the distribution pattern of REEs in the crust. Th and U elements are more enriched in the sediments in the lacustrine sedimentary environment than in the diagenetic calcite veins. In primary sediments (argillaceous clay and micrite calcite laminae), LREEs are more enriched, HREEs are depleted, and Eu shows positive anomaly-enrichment characteristics. The LREEs and HREEs of the sparry calcite veins are lower than those of the original sediment argillaceous clay and micrite calcite, showing characteristics of a negative anomaly depletion. Sparry calcite veins originate from diagenetic fluid crystallization and precipitation and have the characteristics of low Th and U, evident positive anomalies of Sr and Eu, and substantial depletion of La. The distribution patterns of REEs within the four components of the laminated argillaceous sparry limestone reflect the order of REE distribution from primary sediment laminae (argillaceous clay and micrite calcite) to diagenetic laminae (calcite veins). Compared with the North American shale, the four components of the contact surface between the argillaceous and bright crystalline laminae, the micrite calcite, the calcite veins, and the argillaceous laminae all showed weak negative anomalies and positive anomalies. The fractionation degree between LREEs and HREEs reflected by and is in descending order: the interface between the argillaceous lamina and sparry calcite lamina, micritic calcite, calcite vein, and argillaceous lamina. The argillaceous laminar material has the characteristics of basalt REEs, indicating that the terrestrial debris and argillaceous lacustrine shale in the upper Es4 member of the Niuzhuang subsag are primarily derived from the basic extrusive rocks of the Qingtuozi bulge. REE differentiation is most noticeable at the interface between the argillaceous lamina and calcite vein, proving the directionality of REE differentiation from the original sedimentary lamina to the diagenetic lamina. Shale in the study area is primarily deposited below the redox interface of water at a certain depth, and the deposition rate is stable and slow, providing good conditions for preserving organic matter.
1. Introduction
The chemical properties of rare earth elements (REEs) are highly stable. They are challenging to differentiate and have a high degree of homogeneity during the parent rock’s weathering denudation, transportation by flowing water, and sedimentary diagenesis [1–7]. REE chemical properties are widely used in many studies, such as indicating provenance, determining the depositional environment, analyzing the deposition rate, and revealing the relationship between depositional deposits, geological events, and crustal evolution [8–11].
By analyzing the changes in REE abundance in hydrothermal water, researchers found differences between REE and trace element geochemical characteristics in springs and outcrop rocks of the geothermal system [2, 4–6]. These experiments and results provide a better method for analyzing diagenetic veins in sedimentary rocks. Continental lake basin sediments include detrital materials and organic matter from the mechanical input of rivers, organic matter from algae in lake water, biochemically formed carbonate minerals, and other evaporite minerals. The terrestrial input minerals are crucial to determining the total REE () content in lake water. Therefore, the REE distribution model of mud shale implies the REE distribution model of parent rock, which can be used as an index to indicate the parent rock in the provenance area [3, 12–14]. The primary sedimentary carriers of REEs include detrital particles, clay, biochemical crystalline carbonate, organic matter, and other components. Typically, three factors control REE enrichment: the REE content in ancient water, the material components suspended in the water body (for example, clay minerals have the advantage of large specific surface area and adsorb REEs easier than other components), and the slower the paleodeposition rate, the longer the time for sediments to adsorb REEs in paleowater bodies. In ancient water bodies dominated by clay minerals and organic matter, REEs were adsorbed by clay and complexed with organic matter for a sufficient time; therefore, the deposited argillaceous laminae typically have a high degree of REE differentiation and evident depletion and enrichment of light REEs (LREE) and heavy REEs (HREEs), respectively.
The Dongying Sag is a structural unit with the highest hydrocarbon abundance in the Jiyang Depression, and exploration has found that the Paleogene shale oil resources are enormous, especially the shale of Es4 [15–18]. However, studies have focused on the organic geochemistry of source rocks [19–21], and a few studies on the inorganic geochemistry and formation environment have been conducted [19, 22]. The sedimentary changes in shale laminae are closely related to changes in the paleoclimate and paleoenvironment, and the difference in the primary material components in different layers indirectly implies the changes in the paleoclimate and paleoenvironment in the corresponding geological historical period [23–26]. The analysis of the distribution characteristics of REEs in fine-grained sediments revealed the laws of fossil water, paleoprovenance, and paleoenvironmental transformations and assisted in discovering similar high-quality source rocks and shale oil and gas exploration.
This study analyzed the REE partition characteristics and the sedimentary diagenetic response of laminated argillaceous limestone from the perspective of inorganic geochemistry through sampling analysis. The study revealed the REE geochemical characteristics of high-quality source rocks in the upper Es4 in the Nx55 well area and discussed their source and formation environments. The REE distribution pattern proves that the material components of mud shale come from various habitats. Understanding the results helps study the formation and deposition environment of fine-grained sediments and enrich the exploration theory of continental shale oil reservoirs.
2. Geological Background
The Dongying Sag is a secondary structural unit of the Jiyang Depression. The sandstone and argillaceous shale deposited in the Paleogene strata in the sag are more than 5000 m thick [15–18]. With the successful drilling and testing of Nx55 and FYP1 high-yield shale oil (Figure 1), it is announced that the shale oil exploration in the Dongying Sag has made a breakthrough and has become a vital oil and gas replacement position. The wells Nx55 and N55-x1 drilling test oil layers are the pure upper source rocks in the upper part of Es4, which are fine-grained sedimentary argillaceous shale formed in the brackish water environment [17, 18, 27].
3. Lithologic Types and Characteristics of the Upper Es4 in Well Nx55
3.1. Rock Types
Well Nx55 belongs to the Niuzhuang subsag and contains fine-grained shale oil reservoirs in the upper submember of the Es4 during drilling. Four lithologies are superimposed from bottom to top: massive mudstone, lamellar-layered (including) silty mudstone, lamellar (including) silty-micrite limy mudstone, and lamellar argillaceous bright crystal limestone by the analysis results of the core (3783.55–3790.63 m). The optimal lithology of the oil storage and production interval is laminar argillaceous splenite (Figure 2).
3.1.1. Layered Argillaceous Bright Crystal Limestone
Organic-rich argillaceous laminae, part of the calcite columnar fibrous structure, and part of the microcrystalline-coarse-crystalline structure are distributed in a laminar shape. A small amount of dolomite in a fine powder crystal structure is distributed in strips. Organic matter infiltrated most of the mud, and the scale structure is observed locally and distributed along the layers. Microfaults are occasionally observed. A small amount of calcite intercrystalline pores are observed, and organic matter is filled in the pores.
3.1.2. Layered (Containing) Powder: Micrite Lime Mudstone
Organic-rich/containing argillaceous laminae of a calcite micrite structure are distributed in strips and lenses. The argillaceous component has a scaly structure,distributing along the layer.A small amount of argillaceous content is mixed with organic matter with unclear structure. See synsedimentary microfaults.
3.1.3. Laminated-Layered (Inclusive) Silty Mudstone
The argillaceous laminae and silty-bearing argillaceous laminae are primarily argillaceous in mineral composition, with some fine-silt-grade quartz and feldspar debris, with a muddy scale structure. The terrigenous detrital is scattered in the mud, showing lamellar and layered enrichment.
3.1.4. Massive Mudstone
The mineral composition is primarily muddy, some calcite micrite structures are dispersed in the mudstone, and fine-silt-grade quartz and feldspar debris are occasionally observed. It has a muddy scale structure and local organic matter infiltration, and terrigenous debris is dispersed in mud.
Using Chaoqing and Qingan’s [28] classification method on carbonate and terrigenous clastic mixed sedimentary rocks of the Middle Devonian Qujing Formation in Qujing, Yunnan, a three-end-member map of clay, terrigenous clast, and carbonate was established [27] (Figure 3). The sample data from the upper member of Es4 (Ny1, Fy1, Ly1, and Nx55) are projected from the shale coring wells drilled in the corresponding sag to the three-end-member diagram (Figure 3). The samples from Nx55 are distributed in carbonate fine-grained diamictite and clastic mudstone-like areas.
3.2. Characteristics of Sedimentary Diagenesis
3.2.1. Laminated Argillaceous Sparry Limestone
The rock and mineral analysis shows that the laminated argillaceous sparry limestone primarily comprises two microcrystalline-fine grain calcite laminae between the bright and dark phases (Figures 2(a)–2(c)), and the laminae are rich in organic matter and asphaltene. One type of sparry calcite has good transparency. The intercrystals are in a bar-like arrangement, the intercrystal pores are in the form of microslits, some laminae have intermediate unhealed joints, the surrounding calcite has a clear corrosion edge, and the joints are filled with asphaltene. Such calcite belongs to the origin of calcite veins [27, 29]. Another type of calcite has poor light transmittance, brown luster, irregular accumulation of calcite grains into laminae, developed intergranular dissolution pores, and is filled with asphaltene crude oil. This calcite belongs to the primary microcrystalline calcite. After the deposition, the recrystallization diagenesis occurs, the crystal is reorganized, and the organic matter adsorbed or precipitated between the calcite crystals is squeezed. During the hydrocarbon generation process, the residual organic matter flows with the hydrocarbons, forming the appearance of black asphaltenes and residual organic matter in a lamellar distribution.
Under fluorescent illumination, the residual organic matter emits brown fluorescence, and the oily bitumen filled with calcite veins emits blue light. The asphaltene asphalt filled in the intercrystalline pores, dissolution pores, and microcracks of recrystallized calcite hardly fluoresce due to mixed residual organic and argillaceous matter. Under the cathodoluminescence irradiation, the calcites on the dissolution edge of the recrystallized calcite crystals and both sides of the dissolution cracks emit orange light, whereas the calcite veins comprising comb-nodal calcites emit brownish yellow light. Therefore, the dissolution of recrystallized calcite is earlier than the formation time of calcite veins, and the rod-like calcite is formed after hydrocarbon charging. If the formation time of calcite veins is earlier than the generation and expulsion of hydrocarbon by organic matter, organic acids will corrode it and form corrosion pores. The speculated process is as follows: the hydrocarbon generation pressure of organic matter increases the pore fluid pressure in shale, opening the bedding joints. The higher the pore fluid pressure, the more extensive the bedding fracture’s extension range and the wider the fracture width. After filling the fluid, the bedding joints are layered or lamellar. When the overpressure fluid’s pressure is low, the bedding fractures are opened only in the microregion and are lens-like or discontinuous after filling with fluid. This could be the reason for the layered, lamellar, and lenticular output of sparry calcite veins found in core observations and rock thin-section microscopy.
After calcite dissolution in mud shale, the Ca2+ and Mg2+ ion concentrations in pore water are bound to increase. Under compaction, pore water is either discharged into the surrounding rock with better porosity and permeability with mudstone compaction water or stays in the mud shale to form overpressure and floods between microfractures. In the transformation of formation temperature, pressure conditions, and acid-base environments, crystallization precipitation occurs, and calcite veins are formed. The calcite veins formed in this case lack intergranular dissolution pores and fractures. Because the calcite veins originate from the mineral precipitation of pore water in the shale, the metal ion composition of the micrite calcite parent rock is not much different; therefore, the overall cathodoluminescence characteristics are consistent. Only in the microregion with well-developed dissolution pores and fractures will the Mn2+ and Fe2+ ratios increase significantly, corresponding to brighter orange-yellow cathodoluminescence.
3.2.2. Layered (Containing) Powder-Mud Crystalline Gray Mudstone
The argillaceous laminae contain silt, and the argillaceous components are mostly laminae with thickness from 10 μm to 300 μm (Figures 2(d)–2(f)). The micrite calcite composition is in laminae with thicknesses from 20 μm to 200 μm, which are locally disturbed to form wrap-like and grain-like structures. The micrite calcite aggregates (micrite ash strips) often develop staggered and folded deformation, and the micrite ash strips are filled with argillaceous and organic matter. This microdeformation structure arises from the influence of water flow during deposition, and the hydrodynamic conditions for forming the grain-like encapsulated structure are stronger than that of the horizontal bedding. The cathodoluminescence characteristics are the same, indicating that the lamellar micrite strips and the limestone grains have the same material source. The micrite gray bar has weak fluorescence, and the muddy substance is rich in organic matter and emits brownish yellow light.
3.2.3. Layered-Layered (including) Silty Mudstone
The content of terrigenous debris in laminar-layered (containing) silty mudstone can reach 10%–20% (Figures 2(g)–2(i)), and some terrigenous debris has reached the mud level, and the composition of silty sand primarily comprises quartz and feldspar and some debris. Under the irradiation of cathode rays, the whole emits variegated light. Micritic calcite is mixed in muddy and silty laminae and emits dark orange-red light. The fluorescence characteristics are not bright, and the overall luminescence is dim.
3.2.4. Massive Mudstone
Massive mudstone primarily comprises clay minerals and organic matter, with some micrite calcite and pyrite. The micrite calcite is dispersed and distributed and emits orange-orange light under the irradiation of cathode rays. Mud adsorbs organic matter with darker fluorescence and emits yellow-green under cathode ray irradiation (Figures 2(j)–2(l)).
4. Test Methods and Analysis Results
4.1. Testing Methods
Determining the major and trace elements and REEs was completed in the Sinopec Key Laboratory of Shale Oil and Gas Exploration and Development. An Agilent 7900 laser ablation (LA) inductively coupled plasma (ICP) mass spectrometer (MS) was used, equipped with an NWR193UC LA system and a 193 ArF excimer laser produced by the Newwave Company. In this test, the laser beam spot is 30 μm, the ablation depth is 20–30 μm, the laser pulse is 10 Hz, and the energy is 34–40 mJ. The gas was used as the carrier gas, and high-purity Ar gas was used as the compensation gas. The standard reference material NIST610 developed by the National Institutes of Standards and Technology of the United States was used as the external standard. The analytical accuracy of each REE is within 5% and rarely more than 10%.
4.2. Analysis Results of Major and Trace Elements
Significant differences occur in the distribution characteristics of major and trace elements among the components of the laminar argillaceous splenite (Figures 4 and 5). The performance is as follows: clay minerals are rich in Al and Si elements because they primarily comprise aluminosilicates. The Al and Si element contents in sparry calcite veins are the lowest, indicating that calcite veins are purer than micrite calcite. Sr is enriched in the interface between the argillaceous and bright crystal laminae, and the sparry calcite veins are enriched in Sr (Figure 5), reflecting the characteristics of Sr’s active participation in fluid diagenesis. The lowest K element content appears in the sparry calcite veins, the interface between argillaceous and bright crystal laminae, and the micritic calcite. Each component’s Mg and S contents had little difference (Figure 5). Ca primarily comes from carbonate minerals, sparry calcite veins, and mud crystal calcite contains high Ca contents (Figure 5). Mn and Fe are the most abundant in the argillaceous lamina. The range of Fe and Cu contents in the sparry calcite veins is the broadest and most varied (Figure 5). The sparry calcite veins have positive Sr anomalies and negative Pb, Th, and U anomalies. The order of the Th and U contents from high to low is (Table 1) argillaceous laminae, micritic calcite, the interface between argillaceous and bright crystal laminae, and sparry calcite veins, reflecting the redistribution order of Th and U elements in original sediments during compaction and diagenesis.
Compared with micrite calcite and argillaceous laminae, sparry calcite veins are characterized by high Ca, Na, low K, and Mg and extremely low Th and U (Figure 5), reflecting that Th and U do not easily enter the formation fluid with the characteristics of highly incompatible elements. In each component, the Th/U value of micritic calcite was the highest (1.00–1.38; av. ), followed by micritic calcite (1.00–1.20; av. ), sparry calcite vein (0.75–1.27; av. ), and the interface between argillaceous and sparry calcite vein (0.97–1.11; av. ). The Th/U value of each component above is less than that of the crustal clay rock (shale) (3.43).
Comparing the Th and U contents of the four components (Table 1), the Th and U contents in micrite calcite and argillaceous lamina are the highest, significantly higher than those at the interface between sparry calcite veins and argillaceous lamina, indicating that the original sediments precipitated in lacustrine sedimentary environments are more enriched in Th and U than diagenetic minerals. Therefore, Th and U can be used as identification markers of diagenetic events.
4.3. Analysis Results of REEs
According to the normalized data of laminated argillaceous sparry limestone, North American shale [30], compared with the marine shale of North America, the shale of Well N55-x1 deposited in the saline environment of the continental lake basin, the primary sediment argillaceous clay and micrite calcite laminae are more enriched in LREEs. However, the HREEs are depleted, and Eu shows positive abnormal enrichment characteristics. The LREEs at the interface between the argillaceous laminae and the sparry calcite laminae with active diagenetic fluids have positive and negative anomalies; however, the LREEs and HREEs of the sparry calcite veins show characteristics of negative anomaly depletion. The REE distribution patterns in the four components of laminated argillaceous sparry limestone reflect the order of distribution from primary sedimentary laminae (argillaceous clay and micrite calcite) to diagenetic laminae (calcite veins) (Figure 6).
Figure 1 shows that the LREE fractionation is clear, but the HREE fractionation is unclear. The LREE and HREE fractionations of the micritic calcite layer are the strongest, that of the sparry calcite layer the weakest, that of the argillaceous layer are moderate, and the REEs are the most enriched. The interface between argillaceous and sparry calcite laminae has dual characteristics (Figure 6 and Table 2). The rare earth chondrite curve characteristics of sparry calcite veins show a low partition ratio. The highest La and Ce elements are lower than 50, the contact surface between calcite veins and argillaceous laminae is La (50–100) and Ce (40–80), and Eu is abnormal in individual samples. In the muddy laminae, La is 0–80 and Ce is 0–85. The La of micrite calcite is 150–300 and Ce is 120–250.
5. Discussion
5.1. Sedimentary Environment
The distribution characteristics of REEs in primary sediments in sedimentary rocks can partly reflect the redox conditions of ancient water bodies during deposition. When the water is in an oxidative environment, Ce3+ is easily oxidized to Ce4+, decreasing the concentration. However, the Ce3+ concentration in anoxic water is higher [13]. All lanthanides have similar chemical properties and enrichment laws. Elderfield and Greaves and Elderfield and Pagett [33, 34] used the formula to represent the positive and negative anomalies of sedimentary Ce, used to reflect the changes in water redox conditions. It is defined that , Ce is abnormal, and the paleowater corresponds to an anoxic environment. is a Ce negative anomaly, and the ancient water body corresponds to an oxidative environment.
The ICP-MS results of layered argillaceous bright crystal limestone samples in the upper Es4 in Well N55-x1 show that the average value of argillaceous is −0.05, the average value of micritic calcite is −0.03, and the average value of the interface between argillaceous and bright crystal laminae is −0.04, greater than −0.1. Ce is a positive anomaly and shows a reducing environment. The average value of sparry calcite veins is −0.16, less than 0.1 of the diagenetic veins. Although does not indicate a redox environment, it reflects that the differentiation pattern of Ce3+ during diagenesis is different from that of original sediments.
During the burial of lacustrine shale in the study area, calcite veins arise from the chemical precipitation of high-salinity fluid in overpressure microfractures. During the water-rock exchange between high-salinity fluid and clay and micritic calcite minerals, Th and U do not readily dissolve in the formation water, resulting in the loss of Th and U elements in the calcite veins. Th and U can be used as immobile elements to monitor hydrothermal alterations [35, 36]. In fine-grained sedimentary rocks, such as lacustrine shale, the characteristics of major and trace elements dominate their migration in a closed reductive diagenetic environment. Major elements, such as Na, Ca, Mg, and Mn, easily transfer from sedimentary components to diagenetic calcite veins, whereas the transfer capability of Th and U is limited (Figure 5). This phenomenon is consistent with the general law of water-rock interactions in geothermal systems under the framework of carbon dioxide storage [37].
5.2. Deposition Rate
Studies have found that due to the differences in electricity prices, adsorption capacity, and other properties of different REEs, REEs are differentiated in during environmental changes, manifested in the separation between LREEs and HREEs, Ce and Eu, and other elements in sediments [33]. Fine clastic suspended solids, such as clay in lake water, are typical hosts of organic matter and REEs, whereas organic matter and clay minerals are the most robust adsorption media. Therefore, the degree of REE differentiation responds to the rate of sedimentary particle decline. Tenger et al. [38] and Yang et al. [39] used to describe the deposition rate of marine and lacustrine sediments.
The REE curve gradient can better reflect the differentiation of REEs in the North American shale-normalized REE partition diagram (Figure 6). The more prominent the gradient, the slower the deposition rate. The ICP-MS analysis of laminated argillaceous sparry limestone samples from N55-x1 shows that the values of the four components deviate significantly. The argillaceous values are 6.29–12.42 (av. ), and the values of micrite calcite are 6.60–34.94 (av. 20.06). However, the deposition rate of micrite calcite lamina is much lower than that of the argillaceous lamina, indicating that micrite calcite lamina is formed in cleaner still water without terrigenous debris and argillaceous input, and the deposited organic matter is primarily derived from algae in the lake water. Due to the strong influence of diagenetic fluid, the value of the interface between the sparry calcite vein, argillaceous lamina, and sparry lamina does not represent the deposition rate.
The La/Ce values in seawater are typically greater than 1 and in sediments less than 1, indicating thermal influence during deposition [1]. The La/Ce value range of laminated argillaceous sparry limestone in the study area is 0.99–4.96 (av. ), belonging to the typical lacustrine salinization sedimentary environment. The primary factor affecting the difference in the REE differentiation degree in lacustrine sediments comes from the suspension time of fine-grained sediments in the ancient lake water [7, 38]. The REE differentiation degree of calcite veins primarily comes from the content of REE redifferentiation in formation fluids.
5.3. Sources of Sedimentary Substances
The upper crust is characterized by LREE enrichment, HREE stability, and negative Eu anomalies [3, 13, 14]. The discrimination marks are that the LREE/HREE ratio is small, no anomalies, and the provenance tends to basic rocks. The LREE/HREE ratio is high, is a negative anomaly, and the material source tends to acid rocks [40]. The average values of of micrite calcite and argillaceous lamina in N55-x1 laminated argillaceous sparry limestone are 6.75 and 4.06 (Table 2), respectively. The difference between LREEs and HREEs is clear, and it is significantly higher than the average value of the continental upper crust (146.4 μg/g), close to the average value of North American shale (173.2 μg/g).
Continental lacustrine shale deposits belong to fine-grained sediments. The differentiation characteristics of LREEs and HREEs inherit the rare earth characteristics of source material components, and the source materials come from denudation areas; therefore, they are similar to the differentiation of LREEs and HREEs in the upper crust. The LREE/HREE ratio of mud was 2.49/5.37 (av. 4.06), and the value range was 0.73–2.55 (av. 1.14), which is inclined to basic rocks. The LREE/HREE ratio of micrite calcite was 3.32/9.70 (av. 6.75), and the range was 0.84–1.01 (av. 0.89). According to the diagram, the parent material of layered argillaceous sparry limestone has REE characteristics of basalt (Figure 7), supporting the understanding that the upper submember of Es4 in the Niuzhuang subsag is mainly derived from the basic extrusive rocks of the Qingtuozi uplift [26, 42]. The sparry calcite veins have evident positive Eu anomalies, reflecting that Eu can easily enter diagenetic fluid, enriching the diagenetic secondary minerals and the REE differentiation. Through the calcite vein formation experiment, Perry and Gysi [5] found that calcite-fluid partition coefficients depend on the ionic radius of the REE and vary as a function of initial REE concentrations. The fits indicate that the strain-induced Ca2+ substitution in the calcite structure controls the LREE partitioning, whereas the HREEs deviate from these trends. This finding correlates well with our research on diagenetic evolution reflected by REE differentiation (Figure 8).
5.4. Diagenetic Evolution Reflected by REE Differentiation
REE distribution in marlite calcite veins, argillaceous, the interface between marlite laminae, and micritic calcite reflects the ability of various minerals or mineral assemblages to differentiate REEs (Figure 8). On the one hand, acidic fluids, such as organic acids associated with hydrocarbon generation, dissolve carbonate minerals to form dissolution pores. However, during tectonic stress and abnormal fluid pressure initiating bedding fractures or generating interlayer microfractures, some pore water with higher and higher salinity enters the fractured system, calcite precipitation and crystallization occur, and sparry calcite veins are formed. The formation fluids with high salinity from the argillaceous lamina and micrite calcite lamellar mudstone carry REE. During the formation of calcite veins, REE fractionation occurs. Because REE ions are easily adsorbed and enriched by clay, the value of the interface between the argillaceous and splendid laminae is close to that of the splendid calcite laminae but slightly lower than that of the argillaceous laminae. REEs are weak alkaline elements, and the alkaline environment of sparry calcite precipitation is conducive to REE enrichment; therefore, the value of sparry calcite veins is higher than that of argillaceous and micrite calcite laminae.
The more active the fluid in the mud shale is, the stronger the diagenesis. The value range of sparry calcite is 0.95–4.07 (av. 2.06; Table 2), the differentiation is unclear, and it has positive anomaly and enrichment characteristics. The value range of the argillaceous laminae is 0.73–2.55, frequently characterized by weak negative anomaly depletion. The value range of the interface between the argillaceous and bright crystalline laminae is 0.78–1.03 (av. 0.88), and the range of micrite calcite is 0.84–1.01 (av. 0.89). All have weak negative anomaly loss characteristics. The value range of sparry calcite is 0.81–1.27 (av. 0.94), and that of argillaceous laminae is 0.86–1.02 (av. 0.93). The value range of the interface between the argillaceous and bright crystalline laminae is 0.97–1.05 (av. 1.01), and that of micrite calcite is 0.91–1.07 (av. 1.02), both of which have weak deficit-normal characteristics.
The at two points at the interface between the argillaceous and bright crystalline laminae are much higher than 7.50, and there are two points at which the micrite calcite is slightly higher than 7.50. The REE distribution reflects the LREE enrichment and the HREE depletion (Figure 6).
and represent the gradients of REEs in a chondrite-normalized plot. The average value of at the interface between the muddy and bright crystal laminae is 82.94, and the average value of is 11.30, the highest average among the four components. The micrite calcite is second, followed by the calcite veins, and the average values of and of the argillaceous laminae are the lowest. The average values of the LREEs and HREEs at the interface between the argillaceous and bright crystalline lamina are the most differentiated, and the other components in descending order are micrite calcite, calcite veins, and argillaceous laminae.
and reflect the differentiation between LREEs and HREEs. Each component’s average values of and reflect the degree of fractionation from strong to weak (4.71 interface between argillaceous grain layer and bright grain layer; 7.17 (Table 2), calcite 3.98; 3.05, calcite vein 3.81; 3.10, argillaceous lamina 3.10; 1.86). The interface between the argillaceous and bright crystal layers, micritic calcite, calcite vein, and argillaceous layer indicates that the greater the degree of separation of LREEs and HREEs is, the higher the degree of fractionation corresponding to REEs. The data show that the LREEs in the rock samples are differentiated, whereas the HREEs are not. The LREEs at the contact surface between argillaceous clay and sparry calcite laminae are the most significantly differentiated. The micritic calcite lamina is the second, consistent with the direction of REE differentiation from argillaceous clay and micritic calcite laminae to the calcite vein. From the REE distribution curve (Figure 6), compared with NASC, the REEs of the laminated argillaceous sparry limestone in the study area are deficient. The REE curve characteristics of the calcite vein and argillaceous clay lamina are flat, indicating that the LREE and the HREE fractionation is unclear. The interface between argillaceous clay and sparry calcite laminae has a higher LREE fractionation degree, whereas the HREE fractionation degree is low.
The LREEs and HREEs of the sparry calcite veins have a positive correlation power function relationship (), indicating that the calcite formed by the chemical precipitation of the formation fluid has a clear REE distribution pattern. The LREE and HREE at the interface between the argillaceous and bright-grained laminae have a positive correlation power function relationship (), indicating the REE distribution pattern of the bright-grained calcite veins (Figure 8). However, the LREE and HREE of argillaceous and micrite calcite do not correlate, both originate from primary sedimentary materials, and the REEs in argillaceous come from clay flocculation and algal adsorption. The REEs in micritic calcite come from biochemical carbonate crystallization differentiation. During the burial and diagenesis of fine-grained sediments, the compaction, dehydration, and mineral transformation differentiate the REEs in sedimentary laminae. The LREEs and HREEs in the original sediments are unrelated, attributed to the large mixing of REEs in the lake sedimentary period. The diagenetic fluid carries the dissolved LREEs and HREEs and crystallizes to form calcite veins. The positive correlation power function relationship between the LREEs and HREEs reflects the process of the diagenetic fluid crystallizing into sparry calcite. The LREEs and HREEs have a specific partition coefficient, and their contents are slightly lower than that of original sediment argillaceous and micrite calcite.
The interface between the argillaceous laminae and the sparry calcite veins shows the same LREE and HREE distribution characteristics (Figure 8). The La/Ca–Yb/La scatter plot (Figure 8) shows that the original sedimentary micrite calcite differs significantly from the diagenetic carbonate (calcite vein) and the sparry calcite differs significantly from the micrite calcite. The latter has a lower La/Ca value and a higher Yb/La value, indicating the significant REE differentiation during the diagenetic fluid crystallization into calcite veins. However, the La content of calcite veins is 0.14– (av. ), that of micrite calcite is 40.48– (av. ), and that of mud is 53.95– (av. ). The La element in the calcite veins only accounts for 4.96% of the argillaceous and micrite calcite, indicating that the La compound has a weak ability to migrate and transform, and it is challenging to enter the diagenetic fluid from argillaceous clay and micrite calcite. This finding supports Bhati et al.’s [12] suggestion that sedimentation and diagenesis play a weak role in changing REE contents in sediments.
Compared with the respective average values of and for North American shale (0.70; 1.11), the interface between argillaceous and bright crystalline laminae (0.88; 1.02), mirctic calcite (0.89; 1.02), calcite veins (2.06; 0.94), and muddy laminae (1.14; 0.93) showed positive abnormalities, the calcite vein showed a significant positive abnormality, and showed a weak negative abnormality (Table 2).
The values of sparry calcite are 0.95–4.07, and the (0.81–1.27) has the smallest value. The range of (0.73–2.55) and (0.86–1.02) of argillaceous laminae is double low. The (0.78–1.03) of the interface between the argillaceous and bright crystalline laminae is the smallest, and the (0.97–1.05) value is close to 1. The (0.84–1.01) and (0.91–1.07) of micrite calcite, the primary distribution interval, and the contact surface are consistent (argillaceous and bright crystalline laminae).
The and of the interface between the argillaceous and bright crystalline laminae are positively correlated with a power function (Figure 8) because the interface is prone to bedding fractures and beneficial to formation fluid migration and plays a vital role in differentiating REEs.
reflects the differentiation degree of LREEs and HREEs and is the distribution curve gradient in the laminated argillaceous sparry limestone, North American shale-normalized REE partition diagram, reflecting the curve’s inclination degree. reflects the differentiation degree between LREEs, and the larger the ratio, the more enriched the LREEs are. reflects the differentiation degree among HREEs, and the smaller the ratio, the more enriched the HREEs are.
In the laminated argillaceous sparry limestone, the of the sparry calcite veins are 2.64–23.40 (av. 13.00), indicating moderate LREE enrichment, and the is 2.11–7.02 (av. 3.81), indicating a moderate LREE difference. The ranged from 0.86 to 6.26 (av. 3.06), indicating a moderate HREE difference (Figure 9).
The of the mud is 6.29–12.42 (av. 8.73), indicating low LREE enrichment, the is 2.40–3.47 (av. 3.10), indicating the worst LREE differentiation, and ranged from 1.23 to 2.38 (av. 1.87), and the difference of HREE was the worst (Figure 9).
The of the interface between the argillaceous and bright crystalline laminae ranges from 7.33 to 159.10 (av. 82.94), indicating that LREEs are the most enriched, is 3.07–6.27 (av. 4.72), indicating that the LREEs had the highest differentiation, and was 1.90–11.27 (av. 7.17), indicating that the HREEs had the highest differentiation.
The of micrite calcite ranges from 6.60 to 34.94 (av. 20.06), indicating moderate LREE enrichment, ranges from 2.27 to 5.90 (av. 3.98), indicating moderate LREE differentiation, and is 1.80 to 4.68 (av. 3.05), indicating moderate HREE differentiation (Figure 9).
The sparry calcite veins originate from formation fluid precipitation and have the lowest content, lower than that of argillaceous clay and micrite calcite. The three indexes (, , and ) of the interface between the muddy and bright crystal laminae are the highest (Figure 9), indicating that the interface is favorable for LREE and HREE differentiation and enrichment. The three indexes of the argillaceous laminae are the lowest, indicating the redistribution of REEs and the migration of formation fluids, actively participating in the diagenesis processes, such as calcite crystallization. REE differentiation and reenrichment occurred, resulting in today’s argillaceous laminae’s lowest differentiation and enrichment parameters. The three index parameters of the sparry calcite veins are slightly lower than those of micrite calcite, indicating that the parent material forming the calcite veins is from the anions and cations that entered the formation water through the dissolution of the original micrite calcite, resulting in close LREE and HREE differentiation and enrichment characteristics.
According to the standardized data of laminated argillaceous sparry limestone chondrite [30], the mud shale of Well N55-x1 in the saline environment of the continental lake basin shows evident REE differentiation characteristics. The primary sediment argillaceous clay and micrite calcite laminae are more enriched in LREEs, whereas HREEs are depleted, and Eu shows positive abnormal enrichment. The LREEs at the interface between the argillaceous and bright crystal laminae vary considerably. The LREEs in the sparry calcite veins are moderately enriched, and the HREEs are moderately depleted. The REE distribution patterns in the laminated argillaceous sparry limestone also reflect the distribution sequence from primary sedimentary laminae (argillaceous clay and micrite calcite) to diagenetic laminae (calcite veins) (Figure 10).
6. Conclusions
(1)Evident differences occur in the distribution characteristics of major and trace elements among the components of the laminar argillaceous splenite. Sr is enriched in the interface between sparry calcite veins, argillaceous laminae, and bright crystalline laminae, reflecting that Sr is active in fluid diagenesis. Mn and Fe are the most abundant in the argillaceous laminae, indicating that they are primarily from terrigenous material input. The sparry calcite veins show positive Sr anomalies and negative Pb, Th, and U anomalies. The Th and U contents in the argillaceous laminae, micrite calcite, the interface between argillaceous and bright crystalline laminae, and the sparry calcite veins are from high to low, indicating the redistribution sequence of Th and U in the original sediments during sedimentary diagenesis(2)The primary sediment argillaceous clay and micrite calcite laminae are moderately LREE enriched, whereas HREEs are depleted, and Eu shows positive abnormal enrichment characteristics. The LREEs at the interface between the argillaceous and bright crystal laminae, where the diagenetic fluid is active, show positive and negative anomalies. The LREEs and HREEs of the sparry calcite veins show negative anomaly depletion characteristics. The REE distribution patterns in the four components of laminated argillaceous sparry limestone reflect the distribution order from primary sedimentary laminae (argillaceous clay and micrite calcite) to diagenetic laminae (calcite veins)(3)The REE characteristics of layered argillaceous bright crystalline limestone belong to a typically closed reduction environment conducive to organic matter preservation. The sparry calcite veins have positive Eu anomalies, and the LREE and HREE contents are slightly lower than that of the original sediment argillaceous clay and micrite calcite. Diagenesis distinctly affects the sparry calcite veins, and their REE differentiation patterns differ from those of the original sediments(4)The fractionation of rare earth elements may come from the products of hydrothermal alteration or other physical and chemical processes. The REE fractionation of sedimentary rocks is mainly controlled by the nature of sediment primary rocks and late diagenesis. The fractionation of rare earth elements in lacustrine fine-grained sediments is mainly affected by the adsorption and removal of settling particles and their dissolution and decomposition after sedimentation. In the process of burial diagenesis, REE fractionation takes place between different components of sediments and formation water. The of micrite calcite and argillaceous laminae show LREE and HREE differentiation characteristics, and the parent material has the REE characteristics of basalt. The fractionation degree between LREEs and HREEs reflected by and is in descending order of the interface between argillaceous and bright crystalline laminae, micrite calcite, calcite veins, and argillaceous laminae. The greater the LREE and HREE differentiation degrees, the higher the fractionation degree corresponding to REEs is
Data Availability
Contact the author for the data.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported by the Natural Science Foundation of China (Grant Nos. 42172153 and 41802172) and the National Science Technology Major Project, P. R. China (Grant No. 2017ZX05049-004).
Supplementary Materials
Experimental analysis data of the macroelement, trace, and rare earth elements of the shale sample with the drilling coring depth of 3350.60 m, which corresponds to the shale microregion points in Figure 4. The concentration unit of various elements is 10−6 or ppm. (Supplementary Materials)