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Essay

Characteristics and Genesis of Collophane in Organic-Rich Shale of Chang 7 Member in Ordos Basin, North China

by
Yu Zhang
1,
Chaocheng Dai
1,*,
Congsheng Bian
2,
Bin Bai
2 and
Xingfu Jiang
1
1
School of Earth Sciences, East China University of Technology, Nanchang 330013, China
2
The Research Institute of Petroleum Exploration and Development, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(12), 1184; https://doi.org/10.3390/min14121184
Submission received: 22 October 2024 / Revised: 18 November 2024 / Accepted: 19 November 2024 / Published: 21 November 2024
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

:
(1) Background: The Ordos Basin is one of the sedimentary basins in China that is richest in oil and gas resources. The Chang 7 member of the Yanchang Formation is a set of organic-rich shale, abundant in collophane. (2) Methods: The observation and analysis of rock thin sections, combined with major elements, trace elements, electron probes, and other technical means, the characteristics and genesis mechanism of collophane in the organic-rich shale of the Chang 7 member of the Yanchang Formation in the Ordos Basin were studied. (3) Results: Collophane are divided into oolitic collophane, red-yellow aggregate collophane, and apatite-containing crystalline collophane; the main chemical compositions of the collophane were CaO, P2O5, FeO, Al2O3, and MgO. (4) Conclusions: Phosphorus elements of collophane in the organic-rich shale of the Chang 7 member of the Ordos continental lake basin are mainly derived from the nutrients carried by the volcanic ash sediments around the basin and the hydrothermal fluid at the bottom of the lake. The formation of collophane is divided into two periods: during the sedimentary period, the phosphorus released by the aerobic decomposition of phytoplankton to the mineralization and degradation of organic matter, and the death of phosphorus-rich organisms is preserved in the sediment by adsorption and complexation with iron oxides and then combined with calcium and fluoride plasma to form collophane; during the early diagenesis process, collophane underwent recrystallization, forming a colloidal, cryptocrystalline, and microcrystalline apatite assemblage.

1. Introduction

Collophane is widely developed in organic-rich shale, and has attracted attention because it contains a redox environment and primary productivity information. Collophane is a microcrystalline apatite deposited by biological and biochemical processes, generally lenticular and rich in organic matter [1]. The main chemical compositions of collophane are CaO, P2O5, Al2O3, and SiO2 [2]. Most of the collophane occur in marine phosphate block rocks, and the previous research results on the source and circulation of phosphorus in marine origin phosphate block rocks and the mineralization mechanism of phosphate rocks are abundant. The geochemical cycle of phosphorus in marine phosphate block rocks mainly includes land weathering, river transport, marine biological utilization, marine internal circulation, and seabed sedimentation and burial [3]. The sources of phosphorus in marine sediments mainly include upwelling, terrigenous material input, and biopolyphosphorus, and deep-sea sources [4], hot water sources [5], and mantle sources [6] can also be considered. The causes of the mineralization of collophane proposed by the predecessors include biological origin [7], chemical origin [8], mechanical re-enrichment [9], upwelling genesis [10], etc.
However, there are occasional exceptions, and a large amount of collophane has been found in the organic-rich shale interval enriched in the Chang 7 member of the Ordos continental lake basin. Some scholars have proposed that collophane is a marker of the development of high organic carbon shale [11], and its formation is related to volcanic activity and lake bottom hydrothermal activity [12,13]. The Ordos Basin is one of the sedimentary basins with the richest oil and gas resources, and it is also an important base for oil and gas production in China [14]. The organic-rich shale of the Chang 7 member of the Ordos Basin is the most important source rock in the Mesozoic oil-bearing system of the Ordos Basin, which provides a sufficient oil source for the oil and gas accumulation of the Yanchang Formation. Through thin section observation, the author found that collophane developed in the organic-rich shale of the Chang 7 member of the Yanchang Formation in the Ordos Basin, and the microscopic characteristics of different types of collophane were quite different. At present, the source, circulation, and genesis mechanism of phosphorus in the collophane developed in the organic-rich shale of the Chang 7 member of the Yanchang Formation in the Ordos Basin are not clear, and the specific relationship between the formation of collophane and biological and organic matter is not clear. This study focused on the collophane in the organic-rich shale of the Chang 7 member of the Yanchang Formation in the Ordos Basin, and clarified the source, cycle and genesis mechanism of phosphorus in the collophane through rock thin section analysis, major- and trace element analysis, and electron probe analysis, and revealed the relationship between the formation of collophane and organic-rich shale, which provided some enlightenment for the oil and gas enrichment mechanism.

2. Materials and Methods

The Ordos Basin, located in the western part of the North China Platform, is a large multi-cyclic craton basin with an area of about 25 × 104 km2. The regional tectonic dynamic background of the basin periphery is complex, with the Pacific tectonic system in the east, the Helan Mountain Liupan Mountain tectonic system in the west, the Qilian Qinling orogenic system in the south, and the Tianshan Xingmeng fold tectonic system in the north [12,15] (Figure 1a). According to the geological characteristics of the middle and shallow structures in the basin, and considering the deep structure and basement properties, the Ordos Basin is divided into six tectonic units: the Jinxi flexure fold belt, the western thrust belt, the uplift of the Weibei, the northern Yimeng uplift, the Tianhuan depression, and the Yishan slope (Figure 1b) [16,17]. And the study area is mainly located in the southeast of the Yishan slope and the southern part of the Tianhuan depression (Figure 1b) [18,19]. There are 10 intervals (oil formation) developed from bottom to top in the Yanchang Formation (Figure 1c) [17], among which the organic-rich shale of the Chang 7 member oil formation is the most important source rock in the Mesozoic in the Ordos Basin, with high TOC content [20]. During the Chang 7 sedimentary period, the largest lacvial flooding event in the Late Triassic, the strong tectonic activity caused the basin basement to subside rapidly, and rapid lactodian transgression occurred, forming a large-scale deep-water sediment and forming a set of high-quality source rocks with large thickness and high organic matter content [21]. The objective of this study is to extend the collophane developed in the organic-rich shale of the Chang 7 member of the Yanchang Formation.
In this study, three shale oil wells in the southwestern part of the Ordos Basin, including well Z40, H317, and B522 (Figure 1b), were observed, and 67 rock samples were collected. This paper divides it into five categories: organic-rich shale, massive mudstone, silty mudstone, fine sandstone, and tuff, according to factors such as color, mineralogical and petrological characteristics, and sedimentary structures.
Among them, 28 organic-rich shale samples were ground and the microstructure characteristics were observed with Germany Zeiss AxioImager.M2m polarizing microscope; major- and trace element analysis of 68 samples were performed at the State Key Laboratory of Nuclear Resources and Environment at East China University of Technology, with the analysis instrument being the ICP-AES Amercia Thermo Scientific iCAP 7000 series. Ten organic-rich shale samples were used for electron probe analysis of the composition of collophane, using the instrument Japan Jxa-8100 at East China University of Technology, with a voltage of 15 Kv, a current of 20 nA, and a beam spot of 5 microns.

3. Results

3.1. Mineralogical Characteristics of Collophane

Collophane in the organic-rich shale is relatively developed (Figure 2), and most of the bedding is developed in the organic grain layer. Collophane is a microcrystalline apatite deposited by biological and biochemical processes, which is generally lenticular and rich in organic matter. Due to the influence of diagenesis, some collophane have undergone recrystallization, resulting in the formation of various and unevenly sized apatite crystals within or at the edges (Figure 2g–i). Based on the morphological characteristics of collophane in the study area, three types could be recognized and described as follows. First type is oolitic collophane (Figure 2a–c), it has colors of brownish-brown, it is mostly cryptocrystalline and distributes individually with no internal structure, often surrounded by a thick, elongated, fishbone-like layer. Second type is a reddish-yellow aggregate collophane characterized by a relatively glossy surface (Figure 2d–f) mixing with reddish-yellow and orange-yellow collophane that disorderly fill the interior of dark red collophane. Third type is apatite-bearing collophane (Figure 2g–i), constituting the main body of cryptocrystalline collophane; in addition, apatite crystals could formed due to recrystallization within or at the edges. The apatite crystals mainly appear in the form of polygonal flakes or columns with variable sizes, with diameters ranging from a few to several tens of micrometers, accounting for 10% to 30% of the collophane.
The main chemical compositions of collophane were CaO, P2O5, FeO, Al2O3, and MgO (Table 1), and the mass fraction of CaO was 14.72%~53.77%, and the mass fraction of P2O5 was 11.69%~36.14%. The presence of calcium and phosphorus in the collophane should be related to phosphate deposition.

3.2. Major Elements

The major elements in organic-rich shales not only reflect the material sources and phosphorite formation environments, but also record the elemental composition of the ancient lake basin during the phosphorite-forming period. From the major element data of the Chang 7 member organic-rich shales in the Ordos Basin (Table 2), it can be seen that the major elements in the organic-rich shales are dominated by SiO2, with a distribution range of 28.71%~58.04%, and its average content reaches 42%. Additionally, the contents of Al2O3 and TFe2O3 are relatively high, with Al2O3 ranging from 5.88% to 19.51% with an average of 11.98%; and TFe2O3 ranging from 5.30% to 19.45% with an average of 9.81%. The contents of SiO2, Al2O3, and TFe2O3 constitute the main body of the major elements. The contents of K2O, CaO, Na2O, and MgO are lower, with average values of 3.58%, 1.91%, 1.37%, and 1.08%, respectively; while P2O5, TiO2, and MnO have the lowest contents in organic-rich shales, generally below 1%.
It is generally believed that Al and Ti elements primarily originate from terrigenous input [26,27,28]. Therefore, correlation diagrams are created between selected major elements and the Al2O3 content (Figure 3). From the diagram, it can be observed that there is a positive correlation between the SiO2, K2O, MgO, TiO2, Na2O, and the Al2O3 in the Chang 7 organic-rich shale. Among them, SiO2, MgO, and TiO2 have a better correlation with Al2O3 (especially TiO2), indicating that they all originate from terrigenous input. However, the correlation between K2O and Al2O3 is poor, indicating that in addition to most of the terrigenous inputs, some is non-terrigenous contamination. Unlike the aforementioned elements, P2O5, CaO, and TFe2O3 exhibits a negative correlation with Al2O3. Although their correlations are not strong, it can still indicate that the sources of P, Ca, and Fe are different from Al, primarily non-terrigenous. Additionally, there is no significant correlation between MnO and Al2O3, which may suggest that they have contributions from both terrigenous and non-terrigenous detrital sources.
Previous studies have shown that the uranium (U) content in the Chang 7 Member organic-rich shale, is anomalously high, and it was attributed to volcanic ash deposition or hydrothermal activity at the bottom of the lake [1,12,29,30]. The contents of TFe2O3 and P2O5 are positively correlated with U (Figure 4), suggesting that P and Fe have a similar source to the U, originating from volcanic ash deposition or contributions from hydrothermal activity at the bottom of the lake.
The average values of the major elements from different lithofacies samples in the study area were calculated and compared with the PAAS (Post-Archean Australian Shale) major element values (data from reference [31]), which are depicted in Figure 5a. It can be visually observed from the figure that the P2O5 content is significantly higher than that of PAAS, which may be related to volcanic eruptions activities around the Ordos Basin [32]. Additionally, the TFe2O3 and CaO contents are also higher than those of PAAS, which aligns with the characteristics of high pyrite and collophane content in organic-rich shale. Compared to other lithofacies, organic-rich shale exhibits high P and Fe characteristics (Figure 5a).

3.3. Trace Elements

In this study, trace elements in organic-rich shales were normalized (Table 3) and comparative diagrams of trace elements in different lithofacies were made (Figure 5b). From Table 3 and Figure 5b, it can be seen that, compared to PAAS, organic-rich shales are enriched in such elements as Mo, U, Ga, Cu, Zn, V, and Sr, in contrast, Cr, Co, Ni, Pb, Th, Zr, and Nb are relatively depleted.

4. Discussion

4.1. Sources of Phosphorus

Phosphorus is the ultimate factor limiting changes in the level of biological primary productivity [34]. Phosphorus can cycle through the hydrosphere, lithosphere, and biosphere, and the process of delivering phosphorus to lakes is primarily controlled by wind transport and surface runoff, with phosphorus entering lakes mainly relying on biological processes for cycling [35,36,37]. In the Ordos terrestrial lake basin, phosphorus sources generating from terrestrial and aerial. Terrestrial sources refer to phosphorus being dissolved in lake water and subsequently entered into the basin while aerial sources refer to phosphorus depositing in the lake basin from the air, contributing volcanic ejecta and cosmic materials. Cosmic materials are too scarce to be considered [38]. In the Chang 7 section of organic-rich shale, which is enriched in collophane, there are many laminated tuff sedimentary deposits, which provide a possible source of phosphorus. To verify this possibility, the major elements of the Chang 7 organic-rich shale and the tuff within it in the study area were analyzed, and data on fresh volcanic compositions from different regions of the world, with properties similar to the tuff in the study area, were statistically analyzed (Table 4).
The statistical results show that, compared to North American shale (with an average P2O5 content of 0.13%) [39], the P2O5 contents of organic-rich shale in the study area ranges from 0.20% to 2.18%, with an average value of 0.66%. The organic-rich shale in the Chang 7 member exhibits a high P content. However, the P2O5 contents of tuff in the Chang 7 member range from 0.02% to 0.32%, with an average value of 0.10%. This is significantly lower than the P2O5 contents of fresh volcanic ash (with an average value of 0.27%), exhibiting a low P content. These differences in P2O5 contents indicate that, when volcanic ash was deposited, the P element was transferred from the volcanic ash into the lake basin water, and, through biogeochemical cycles, it was preserved in the subsequently formed organic-rich shale.
Liu et al. (2023) [11] found that the Chang 7 interval in the Ordos Basin exhibited extremely high primary productivity, with the lake basin being in a hyper-eutrophic state, closely related to volcanic activity. The rich tuff laminae and volcanic ash deposits record the occurrence of high-frequency volcanic activities during this period, with the main type of volcanic eruptions being Plinian, which are characterized by high intensity and large energy. When volcanic ash was deposited, the life-essential elements (such as P and Fe) in it underwent hydrolysis, enhancing the nutrient supply rate of the water body and directly providing nutrients to the primary producers in the lake basin, promoting the flourishing of organisms and increasing the lake’s primary productivity [40]. After the death of organisms, the P contained in their bodies would be deposited along with the organisms in the form of organic phosphorus, and, during the initial stages of diagenesis, organic phosphorus would be released as organic matter mineralized and preserved in organic-rich shale by adsorbing onto the surface of iron oxides or forming authigenic phosphate minerals (collophane) for precipitation, resulting in organic-rich shale showing high P2O5 and TFe2O3 contents. With further research, Zhang et al. (2010) [13] and Qiu et al. (2011) [12] discovered hydrothermal activities on the lake bottom during the Chang 7 period and believed that these activities might also have provided P elements to the Ordos lake basin. In summary, collophane enrichment in the organic-rich shale of the Ordos lake basin mainly originates from the volcanic ash deposits brought by volcanic eruptions around the basin and the hydrothermal deposits on the lake bottom, with the enrichment being closely related to biological processes.

4.2. The Cycle of Phosphorus

An important site of the phosphorus geochemical cycle in is the sediment water interface, where phosphorus migration and transformation occur frequently. The sediment water interface is a crucial site for the geochemical cycling of phosphorus in the Ordos continental lake basin, where phosphorus frequently migrates and transforms at this interface.
The organic matter composition, microbial, and iron content of the sediment all affect the interfacial phosphorus migration. The organic matter in the sediment contains fatty acids, humus, and proteins, which mainly come from the deposition of algae, animal, and plant residues in the lake. As an important driving force of the phosphorus cycle in lakes [41], organic matter mineralization can affect the adsorption rate and adsorption amount of phosphorus to varying degrees, such as, humus can form a glue film on the surface of inorganic matter and slow down the migration rate of phosphorus; fulvic acid competes with phosphate for adsorption and affects phosphorus migration [42]. In addition, organic matter can also form complexes with metals such as Fe and Al to promote phosphorus adsorption, and the content of organic matter is positively correlated with the release of phosphorus from sediments [43]. As an important part of lake ecology, microorganisms promote the release of phosphorus from sediments. On the one hand, microbial mineralization decomposes organic phosphorus, and, on the other hand, organic acids secreted by microbial metabolism reduce the pH of the microenvironment [44], and both partial acids and partial alkalis are conducive to the release of phosphorus from sediments [45].
Iron phosphorus coupling is the main mechanism affecting phosphorus migration, the reduction of iron leads to the release of phosphorus. Since the Fe (II)-Fe (III) cycle mainly occurs near the redox interface, and the cycling of P is closely related to that of Fe, the mineralization of P enrichment also mainly occurs near the redox interface [46]. During the deposition period of the Chang 7 organic-rich shale in the Ordos Basin, the redox conditions of the lake basin bottom water were primarily oxidizing-suboxic environments, with periodic reducing events [46]. Under the oxidizing-suboxic bottom water conditions, the redox interface is located within the sediments, and the P elements released by microbial degradation of organic matter are enriched in the pore water, which is conducive to the preservation of P [47]. When the sediment is rich in organic matter, the aerobic environment is more conducive to the mineralization and decomposition of phosphorus-containing organic matter and promotes the release of phosphorus. In anaerobic or anoxic environments, Fe3+ is easily reduced to Fe2+, which promotes the release of phosphorus or Fe-P adsorbed by Fe (OH)3 [48].
Dissolved inorganic phosphorus (DIP) in the overlying water is partially transported to the lake bottom through the photosynthesis of phytoplankton, where it is synthesized into organic matter and buried at the sediment water interface; another portion is absorbed by zooplankton for reproduction and growth, resulting in a large number of phosphorus-rich organisms. In the early stages of diagenetic evolution, the mineralization and degradation of organic matter depend on various terminal electron acceptor processes (such as aerobic respiration, sulfate reduction, and iron oxide reduction), which convert organically bound phosphorus in the organic matter into inorganic phosphate ions (PO43−), entering the pore water of the sediments. Some of these inorganic phosphate ions co-precipitate with iron ions or are adsorbed onto sediment particles, remaining in the pore water of the sediments; others are released back into the overlying water under suitable conditions. After the death of phosphorus-rich organisms, phosphorus contained in their bodies is released by bacterial aerobic decomposition. Some of this phosphorus re-enters the overlying water and is utilized by organisms, while some is adsorbed onto metal oxides (iron oxide) and enters the lake bottom sediments. These processes result in the enrichment and preservation of phosphorus in the pore water of the sediments [49].

4.3. The Formation Mechanism of Collophane

The monomer and aggregate morphology of apatite minerals is associated with the primary sedimentary state and diagenetic changes. The collophane in the organic-rich shale of the Chang7 member of the Yanchang Formation in the Ordos Basin is different in size and morphology, the colloidal aggregate type of collophane reflects the characteristics of the primary sedimentation of collophane [50], and the sedimentation is mainly chemical; however, the apatite crystals can be seen in the apatite-containing crystalline gelatite with an orderly arrangement around the collophane, indicating that the sedimentary environment of phosphate at this time is stable, so that it can be continuously and stably crystallized and precipitated, which may be formed by the precipitation of phosphate in the pore water in the early stage of diagenesis [51]. The primary sedimentary collophane undergoes aging and recrystallization during the early diagenesis to form colloidal, cryptocrystalline, and microcrystalline apatite assemblages. In addition, the trace elements such as Ga, Cu, Zn, and V were enriched in the trace element composition of organic-rich shale, indicating that organisms interacted with phosphate during the phosphate deposition period in the collophane, and the organisms absorbed phosphorus to promote their own growth and reproduction, and the life activities of organisms provided conditions for phosphate deposition. This formation process is primarily influenced by biomass and redox conditions.
During the formation period of the organic-rich shale in the Chang 7 Member, the tectonic activity was strong, and the southern part of the basin was in a tensile stress environment, and the rapid extensional settlement led to the rapid expansion and subsidence of the lake basin, which made the deep lake environment widely developed, and the basement fault provided a favorable channel for hydrothermal fluids. Generally, the stronger the intensity of sub-lacustrine hydrothermal activities, the more enriched Cu and Zn elements are in the sediments. Therefore, the enrichment of Cu and Zn elements in organic-rich shales may be related to sub-lacustrine hydrothermal activities (Figure 6). Frequent volcanic eruptions and earthquakes and other tectonic events may cause the sediment to collapse and instability in the slope zone, resulting in the underwater gravity flow transporting a large number of fine-grained sediments to the deep lake, and, due to the instability of the volcanic ash components, the components deposited at the edge of the lake basin and in the lake basin will decompose, providing a large number of nutrients (N, P, Fe, Ba, etc.) leading to the abnormal flourishing of phytoplankton, thereby increasing the primary productivity of organic matter [52]. Since the P and Fe elements provided by the volcanic ash sediments are involved in phytoplankton metabolism, which are then entered into the sediments through biogeochemical cycles along with the dead phytoplankton, and preserved by the formation of autogenous minerals such as pyrite and collophane, the Chang7 organic-rich shale is characterized by high P2O5 and TFe2O3 contents (Figure 6).
The formation of collophane is divided into two stages: the first stage is the accumulation and deposition of phosphorus. During the sedimentary period, a large amount of phosphorus in the volcanic ash sediment was hydrolyzed in the overlying water, resulting in the precipitation of supersaturated phosphorus in oxygen-rich water due to chemical and bio-chemical processes, some of which were synthesized by phytoplankton through photosynthesis and were adsorbed and deposited in organic matter, and some of which were absorbed by plankton and multiplied. The second stage is the re-enrichment of previously deposited phosphorus. The phosphorus released by the mineralization and degradation of organic matter by microorganisms is preserved in the sediment through adsorption and complexation with iron oxides; after the death of P-rich organisms, phosphorus is released by aerobic decomposition by bacteria, and part of the phosphorus is adsorbed on the surface of iron oxide into the sediment at the bottom of the lake, both of these increase the concentration of phosphorus in the pore water of the sediment, so that the original deposited phosphorus is enriched in the sediment, and then the inorganic phosphate ions combine with calcium and fluoride ions to form collophane (Figure 7b). During the early diagenesis, collophane underwent aging and recrystallization, forming colloidal, cryptocrystalline, and microcrystalline apatite assemblages (Figure 2g–i).

5. Conclusions

A large number of collophane are developed in the organic-rich shale of the Chang 7 member of the Yanchang Formation in the Ordos Basin, and the collophane are mainly divided into oolitic collophane, red-yellow aggregate collophane, and apatite-containing crystalline collophane. The phosphorus elements of collophane in the organic-rich shale of the Chang 7 member of the Ordos continental lake basin are mainly derived from the nutrients carried by the volcanic ash sediments around the basin and the hydrothermal fluid at the bottom of the lake. The formation of collophane is divided into two stages: the first stage is the accumulation and deposition of phosphorus. During the sedimentary period, a large amount of phosphorus in volcanic ash sediments was hydrolyzed, resulting in the precipitation of supersaturated phosphorus in oxygen-rich water bodies due to chemical and bio-chemical processes, and some of them were synthesized by phytoplankton and deposited in organic matter, and some were absorbed by plankton and multiplied; the second stage is the re-enrichment of previously deposited phosphorus. The mineralization and degradation of organic matter by microorganisms and the phosphorus released by the aerobic decomposition of phosphorus-rich organisms by bacteria participate in the iron cycle at the redox interface and are re-enriched in the sediment pore water, which then combines inorganic phosphate ions with calcium and fluoride ions to form collophane. During the early diagenesis process, collophane underwent recrystallization, forming a colloidal, cryptocrystalline, and microcrystalline apatite assemblage. The formation of collophane indicates the high productivity of the lake basin, and the phosphorus element in collophane mainly comes from the event-action deposition of the Chang 7 sedimentary period in the Ordos Basin, so the study of the source, enrichment, and genetic mechanism of phosphorus is helpful to understand the sedimentary environment of organic-rich shale an d the relationship between the formation of organic-rich shale and organisms, and is conducive to further exploring the formation of organic-rich shale and the exploration and development of unconventional oil and gas resources.

Author Contributions

Conceptualization, C.D.; methodology, C.D.; software, Y.Z. and C.D.; validation, Y.Z.; formal analysis, Y.Z.; investigation, Y.Z.; resources, Y.Z.; data curation, C.B. and B.B.; writing—original draft preparation, Y.Z.; writing—review and editing, Y.Z.; visualization, Y.Z.; supervision, C.D. and X.J.; project administration, B.B. and C.B.; funding acquisition, B.B. and C.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data is unavailable due to privacy or ethical restrictions.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Regional geological map of the Ordos Basin (modified from [22]); (b) Distribution map of sedimentary facies in the Chang 7 member of the Ordos Basin, modified from [23]; (c) Comprehensive strata log diagram of the Yanchang Formation in the Ordos Basin [24,25].
Figure 1. (a) Regional geological map of the Ordos Basin (modified from [22]); (b) Distribution map of sedimentary facies in the Chang 7 member of the Ordos Basin, modified from [23]; (c) Comprehensive strata log diagram of the Yanchang Formation in the Ordos Basin [24,25].
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Minerals 14 01184 g002
Figure 2. (a–c) Oolitic collophane, brownish-brown, in individual strip form, distributed along bedding, with surface development of crack patterns, cryptocrystalline, surrounded by a fishbone-like shell, under plane-polarized light; (d–f) Red-yellow aggregate collophane, red-yellow mixed, and dark red collophane filled with large amounts of orange-yellow collophane, single polarized light; (g–i) Apatite crystal type collophane, in the edge or interior of cryptocrystalline collophane, forms lamellar and columnar apatite crystals under plane-polarized light.
Figure 2. (a–c) Oolitic collophane, brownish-brown, in individual strip form, distributed along bedding, with surface development of crack patterns, cryptocrystalline, surrounded by a fishbone-like shell, under plane-polarized light; (d–f) Red-yellow aggregate collophane, red-yellow mixed, and dark red collophane filled with large amounts of orange-yellow collophane, single polarized light; (g–i) Apatite crystal type collophane, in the edge or interior of cryptocrystalline collophane, forms lamellar and columnar apatite crystals under plane-polarized light.
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Minerals 14 01184 g003
Figure 3. Relationship diagram of the oxide content of major elements in the organic-rich shale of the Chang 7 member, Ordos Basin, with respect to Al2O3 content.
Figure 3. Relationship diagram of the oxide content of major elements in the organic-rich shale of the Chang 7 member, Ordos Basin, with respect to Al2O3 content.
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Figure 4. Relationship diagram of TFe2O3 and P2O5 content with U content in organic-rich shale of Chang 7 member, Ordos Basin.
Figure 4. Relationship diagram of TFe2O3 and P2O5 content with U content in organic-rich shale of Chang 7 member, Ordos Basin.
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Minerals 14 01184 g005
Figure 5. (a) Comparison of major element contents in different lithofacies of the study area with PAAS major element contents; (b) Trace element PAAS normalization diagram for different lithofacies of the Chang 7 member in the Ordos Basin.
Figure 5. (a) Comparison of major element contents in different lithofacies of the study area with PAAS major element contents; (b) Trace element PAAS normalization diagram for different lithofacies of the Chang 7 member in the Ordos Basin.
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Minerals 14 01184 g006
Figure 6. Evolution map of major and trace elements in the organic-rich shale of the Chang 7 member of the Ordos Basin (Well Z40).
Figure 6. Evolution map of major and trace elements in the organic-rich shale of the Chang 7 member of the Ordos Basin (Well Z40).
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Minerals 14 01184 g007
Figure 7. (a) Depositional pattern of organic-rich shale in the Chang 7 member of the Yanchang Formation in the southern Ordos Basin (modified from [53]); (b) Formation of collophane.
Figure 7. (a) Depositional pattern of organic-rich shale in the Chang 7 member of the Yanchang Formation in the southern Ordos Basin (modified from [53]); (b) Formation of collophane.
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Table 1. Analysis results of electron probe of collophane.
Table 1. Analysis results of electron probe of collophane.
Sample Number CaOP2O5FeOAl2O3MgONa2OMnOClElse
Z40-3A-248.5333.810.30.040.100.720.350.0316.12
Z40-6A-348.6433.590.720.380.100.460.130.0515.93
Z40-9C-145.3336.140.270.030.140.150.040.8317.07
Z40-14A-349.1533.850.500.030.101.060.220.0515.04
Z40-16B-247.4535.520.730.090.120.900.230.0514.91
Z40-19B-234.8627.200.200.250.060.850.110.0336.44
Z40-24A-239.71 28.560.431.350.080.420.120.0629.27
Z40-49A-344.2732.370.710.060.060.480.030.0821.94
Z40-55B-128.2825.620.150.280.030.080.020.0245.52
Table 2. Analytical results for major elements (%) of Chang 7 member organic-rich shales in Ordos Basin.
Table 2. Analytical results for major elements (%) of Chang 7 member organic-rich shales in Ordos Basin.
Sample Number and LithologySiO2Al2O3TFe2O3Na2OK2OCaOMgOP2O5TiO2MnO
Z40-347.9316.087.110.894.691.250.980.530.540.46
Z40-651.0313.776.721.082.282.081.290.390.360.16
Z40-953.6315.325.882.892.043.891.310.240.440.18
Z40-1443.5812.5110.271.292.871.900.611.110.360.07
Z40-1633.898.8512.920.981.902.220.440.850.320.08
Z40-1840.089.2310.550.612.670.510.470.240.280.06
Z40-1929.617.7010.700.332.000.420.320.200.230.03
Z40-2442.0112.1311.030.963.320.580.800.370.390.06
Z40-5528.718.9219.451.422.730.910.850.530.400.06
H317-938.9514.888.901.713.921.541.520.790.700.14
H317-1243.7713.379.801.305.432.901.281.960.620.04
H317-1453.3919.515.631.625.961.222.720.200.900.07
H317-1553.6515.768.241.595.771.152.430.440.910.06
H317-1758.0215.275.301.656.121.111.740.220.690.06
H317-1858.0411.217.381.753.571.380.860.590.480.05
H317-2033.4014.0214.671.144.141.311.590.700.600.17
B522-942.139.558.872.212.014.611.462.180.580.08
B522-1039.7711.207.731.952.261.151.150.500.470.05
B522-1344.689.777.202.292.392.420.660.680.350.07
B522-1646.338.707.951.404.171.040.600.520.300.13
B522-1947.537.159.510.972.431.060.600.520.320.13
B522-2128.715.8811.220.602.572.000.551.040.220.16
B522-2433.708.1511.610.952.362.330.490.790.400.07
B522-2734.6410.2510.440.655.128.450.440.370.510.23
B522-3143.8113.9311.572.045.631.031.000.570.580.10
B522-4042.9815.077.811.283.962.081.290.820.440.05
B522-42-141.1614.6713.861.403.961.231.490.530.620.03
B522-4538.3812.7112.251.353.901.661.250.650.490.04
AVERAGE42.6311.989.811.373.581.911.080.660.480.10
Table 3. Trace element data and normalization of organic-rich shale in the Chang 7 member, Ordos Basin.
Table 3. Trace element data and normalization of organic-rich shale in the Chang 7 member, Ordos Basin.
TracePAAS/ppmMean of Sample Analysis Data Data/ppmStandardization (Sample/PAAS)
Ba650487.10.75
V1501961.31
Cr11053.50.49
Co2318.10.79
Ni5526.40.48
Cu50961.92
Zn85118.91.4
Ga20213.910.69
Pb202.70.14
Th14.68.90.61
Sr200225.61.13
Zr21075.70.36
Nb187.10.39
Y2724.30.9
U3.140.513.06
Be—7—
Mo17877.98
Sc16130.81
Note: The PAAS value is based on the literature [33].
Table 4. Homogeneous comparison table of major elements in tuff, Organic-rich shale, fresh volcanic ash, and North American shale from Ordos Basin research area.
Table 4. Homogeneous comparison table of major elements in tuff, Organic-rich shale, fresh volcanic ash, and North American shale from Ordos Basin research area.
LithologyMajor Element Content (%)
SiO2TiO2Al2O3TFe2O3MgOCaONa2OK2OP2O5
North American Shale64.80.716.95.662.863.631.143.970.13
Chang 7 Organic-rich Shale42.630.4811.989.811.911.911.373.580.66
Chang 7 Tuff42.090.310.672.021.173.431.811.790.1
Fresh volcanic ash62.110.6515.985.61.984.814.321.830.27
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Zhang, Y.; Dai, C.; Bian, C.; Bai, B.; Jiang, X. Characteristics and Genesis of Collophane in Organic-Rich Shale of Chang 7 Member in Ordos Basin, North China. Minerals 2024, 14, 1184. https://doi.org/10.3390/min14121184

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Zhang Y, Dai C, Bian C, Bai B, Jiang X. Characteristics and Genesis of Collophane in Organic-Rich Shale of Chang 7 Member in Ordos Basin, North China. Minerals. 2024; 14(12):1184. https://doi.org/10.3390/min14121184

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Zhang, Yu, Chaocheng Dai, Congsheng Bian, Bin Bai, and Xingfu Jiang. 2024. "Characteristics and Genesis of Collophane in Organic-Rich Shale of Chang 7 Member in Ordos Basin, North China" Minerals 14, no. 12: 1184. https://doi.org/10.3390/min14121184

APA Style

Zhang, Y., Dai, C., Bian, C., Bai, B., & Jiang, X. (2024). Characteristics and Genesis of Collophane in Organic-Rich Shale of Chang 7 Member in Ordos Basin, North China. Minerals, 14(12), 1184. https://doi.org/10.3390/min14121184

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