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Article

Quartz Origins and Paleoenvironmental Controls on Organic Matter Accumulation of Marine Shale in the Ordovician Wulalike Formation, Northwestern Ordos Basin, China: Significance for Shale Gas Exploration

by
Yanni Zhang
1,
Rongxi Li
1,*,
Shengli Xi
2,
Jianwen Gao
3,
Lei Chen
4,*,
Hexin Huang
1,*,
Bangsheng Zhao
1 and
Ahmed Khaled
1
1
School of Earth Science and Resources, Chang’an University, Xi’an 710054, China
2
PetroChina Changqing Oilfield Company, Xi’an 710018, China
3
Research Institute of Exploration and Development, PetroChina Changqing Oilfield Company, Xi’an 710018, China
4
School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China
*
Authors to whom correspondence should be addressed.
Energies 2023, 16(14), 5278; https://doi.org/10.3390/en16145278
Submission received: 20 June 2023 / Revised: 5 July 2023 / Accepted: 6 July 2023 / Published: 10 July 2023
(This article belongs to the Special Issue Formation and Distribution of Conventional and Unconventional Oil and Gas)
Browse Figures
Versions Notes

Abstract

:
The Ordovician Wulalike marine siliceous shale is a notable hydrocarbon source rock in the Northwestern Ordos Basin. However, the causes of quartz and organic matter enrichment are still a mystery to experts. In this study, the organic geochemistry (maceral compositions, Rbitu, and TOC) and elemental geochemistry (major and trace elements) with mineralogy (XRD) and petrography were jointly acquired to systematically investigate the quartz origins and the paleoenvironment, and the main controlling factors for organic matter enrichment in the Wulalike shale. The results show that the organic matter is type I kerogen with low TOC concentrations (average 0.51%), and that the thermal evolution has reached mature and high mature stages (mean Requ is 1.08%). Three types of quartz are developed in Wulalike shale: biogenic quartz (average 63%) is the most dominant, followed by clastic quartz (average 31%) and microcrystalline quartz (average 6%). Babio and Ba/Al values indicate the low paleoproductivity, which is the primary cause for the low TOC found throughout the area. Redox indexes show the anoxic or dysoxic to oxic conditions from the bottom to the top of the section. Paleoclimate and paleowater depth proxies also changed from the bottom to the top. Various paleoenvironments and sedimentological evidence show that Wulalike shale went through the changing environments, and the early sedimentary environments were conducive to organic matter enrichment, leading to a relatively high TOC. The paleoproductivity and preservation conditions have an impact on organic matter enrichment. Based on the results of biogenic quartz distribution and sedimentary environments, it is considered that the bottom of the Wulalike Formation is the most favorable for shale gas exploration and development.

1. Introduction

Shale gas exploitation went through a significant progress over the last decade and has emerged as the most important driver of the global gas supply. The Lower Paleozoic Wufeng–Longmaxi Formation in the Sichuan Basin, South China, has become important for the future increase in gas reserves and production in China [1]. The Middle–Upper Ordovician Wulalike organic-rich marine shale in the northwestern margin of the Ordos Basin, North China, has not received much attention as it is believed to form a non-commercial shale gas characterized by a high permeability, low porosity, low strata pressure, and low total organic carbon (TOC) concentration [2,3,4]. However, shale gas has been gradually discovered in several exploratory wells in the Wulalike shale recently, e.g., the gas production of the Zhong4 well and Zhongping1 well, reaching up to 4.18 × 104 m3/day and 6.42 × 104 m3/day, respectively. This is the first time obtaining shale gas in the Lower Paleozoic of the Ordos Basin, which has aroused great interest in researchers. Meanwhile, shale oil was recently discovered in the Wulalike Formation in the southwestern margin of the Ordos Basin [4], which proves that the Wulalike Formation has development potential and is expected to become one of the main relay targets for unconventional oil and gas resources in China in the future. Gas-bearing layers in the Sichuan Basin are mainly distributed in the quartz-rich organic-rich shales in the lower part of the Longmaxi Formation [5]. In contrast, the Wulalike shale has a high content of quartz (more than 45%) and numerous siliceous organisms, but a low TOC concentration (averaging 0.49%) [3,6].
Quartz is one of the most prevalent minerals in shale (usually greater than 30%) [6,7], and it is not only positively connected with TOC content [8,9], but it is also a major determinant of shale reservoir quality [10,11]. It is derived primarily from four sources: terrigenous clastic quartz, Si-bearing hydrothermal fluids input, conversion of smectite to illite formed quartz (S-I quartz), and biogenic quartz. Furthermore, scholars [10,12,13,14] discovered that when analyzing the brittleness of shale, it is not enough to merely consider the volume content of the quartz, but that it is also necessary to clarify the origins of the quartz, as different sources contribute differently to the reservoir attributes [12]. Biogenic quartz has a greater impact on the brittleness of shale than S-I quartz and terrigenous clastic quartz, and can easily produce more fractures during hydraulic fracturing [10,14,15,16]. Furthermore, it is worth noting that biogenic quartz protects reservoir pores from destruction, but S-I quartz does not [14,17]. Biogenic quartz is formed by dissolution and recrystallization of siliceous plankton, such as radiolarians, sponge spicules, and algae [6,14]. Siliceous plankton is not only a source of high-quality hydrocarbon parent rock, but it also has an interior cavity that can help preserve organic matter (OM) by providing an OM-hosted space, which is linked to OM enrichment [14]. The OM is the prerequisite for shale gas formation and its sedimentary environment is directly related to hydrocarbon generation. The factors controlling OM enrichment can be summarized into two categories: OM input factors (e.g., paleoproductivity, paleoclimate, and terrigenous detrital input) and preservation condition factors (e.g., redox conditions, paleowater depth, and sedimentation rates) [17,18,19,20]. Three modes of OM enrichment have been proposed: (1) the high productivity model [9]; (2) the anoxic preservation model [21,22]; and (3) superposition of the two models above [23,24]. The deposition of the OM in shales in different sedimentary environments is controlled by various factors. For example, both the marine shale of the Longmaxi Formation and the North American shale have high productivity and good preservation conditions [7,25]. The continental shale of the Yanchang Formation was controlled by high productivity [26]. Moreover, the marine–continental transitional shale of the Shanxi Formation was affected by terrigenous clastic input and redox conditions [27]. Thus, it can be seen that understanding the quartz origins and sedimentary environment can not only predict the high-quality reservoirs, but also provide valuable guidance for the generation and evaluation of shale gas.
In this study, an array of analyses including TOC content, bitumen reflectance (Rbitu), organic maceral, mineral composition (XRD), as well as major and trace element concentrations, along with petrographic observations, were jointly acquired for the outcrop profile of the Wulalike shale in the northwest margin of the Ordos Basin. The purpose of this study is: (1) to clarify the OM characteristics and shale lithofacies type; (2) to identify different quartz sources and calculate their contents; (3) to elaborate on the sedimentary environments and their effects on OM enrichment; and (4) to reconstruct the accumulation model of OM. This study is expected to provide some basis for further shale gas exploration in the northwestern margin of the Ordos basin.

2. Geological Setting

Located in the west of the North China Craton (NCC), the Ordos Basin is a significant hydrocarbon-bearing basin in China, with an area of about 25 × 104 km2 [4,28] (Figure 1A). The basin includes six sub-tectonic units: Tianhuan Sag, Yimeng Uplift, Western Thrust Zone, Jinxi Fold, Yishan Slope, and Weibei Uplift (Figure 1B), and has undergone multi-stage tectonic evolution [29,30,31]. Since the early Ordovician, it was affected by the closure of the Qinling sea and Qilian sea; thus, the sedimentary characteristics in the Ordos Basin are significantly different from those in North China [32]. In the mid–late Ordovician period, influenced by the expansion of the Helan trough, the main part of the Ordos Basin began to uplift, while the western and southern margins subsided substantially, forming an L-shaped marginal sea and depositing a suite of deep water facies shale [33]. The study area is situated in the Zhuozishan district in the northwestern margin of the Ordos Basin, and the Ordovician is composed of the Sandaokan Formation, Zhuozishan Formation, Kelimoli Formation, Wulalike Formation, and Lashizhong Formation in ascending order [6] (Figure 2A). The Wulalike Formation is the target layer characterized by the grey–black bearing graptolite shale and a small amount of limestone interlayers [34], which is distributed in the western and southern margins of the basin, with the thickness ranging from 20 to 120 m and the burial depths varying from 1500 to 4700 m, as well as a low TOC contents [3,4,5,6].

3. Samples and Methods

To verify the integrity and representativeness of the sample distribution, the entire stratigraphic section of the Wulalike Formation in the Zhuozishan area was surveyed and sampled (Figure 1B and Figure 2B). All the samples were selected from the fresh, non-weathered surface. Thirty-two samples were chosen for TOC, major, and trace elements; 21 were prepared for XRD; and 9 samples for organic maceral compositions and Rbitu. Major and trace elements were measured in the Key Laboratory of Western Mineral Resources and Geological Engineering Ministry of Education, Chang’an University. TOC, XRD, maceral compositions, and Rbitu were conducted in the Key Laboratory of Oil and Gas Resources, Ministry of Education, Yangtze University.
For the major elements analysis, 0.5 g of powdered sample (200 mesh), 0.3 g of ammonium nitrate (oxidant), and 5 g of anhydrous lithium tetraborate (co-solvent) were weighed and mixed evenly. Next, 0.5 g of potassium permanganate release agent was added to the crucible. The mixed sample was placed in a crucible and heated at 1100 °C for 6 min in a molten furnace; the molten mixture was then poured out to form a translucent sheet after cooling. The elements were tested using the Shimadzu XRF-1800 spectrometer, and the relative error of detection was less than 5%. Another 1 g of sample was heated to 1000 °C in the molten furnace, and when cooled to room temperature, the sample was re-weighed to calculate the loss on ignition.
For the trace elements test, 50 mg of the 200 mesh powdered sample was firstly weighed in a solution tank, and 0.5 mL of HNO3 and 1 mL of HF were subsequently added. The mixture sample was heated in an oven at 190 °C for 48 h. After cooling, it was taken out and steamed in an electric heating plate at 140 °C to dry. Next, 2.5 mL of 40% HNO3 was added and heated in the cylinder liner at 140 °C for 4 h to dissolve the sample again. After re-melting, the solution was diluted to 50 g with 2% HNO3 in a polyethylene vial for analysis. The American Thermoelectric X-7 inductively coupled plasma mass spectrometer was used for testing. The relative error was less than 5%.
For the TOC analysis, 0.1 g of each 200 mesh sample was firstly weighed. The samples were thoroughly immersed in 10% diluted hydrochloric acid to eliminate inorganic carbon and dried at 70 °C. After treatment, the samples were washed with distilled water several times to eliminate any remaining acid. The pretreatment samples were thoroughly burnt in oxygen at 1200 °C to convert all organic carbon into carbon dioxide. The Leco CS-400 carbon analyzer was used to measure the volume of CO2 to estimate the TOC content with an analytical uncertainty of less than 0.1%.
The 200 mesh samples were mixed with ethanol for XRD analysis and then evenly coated onto slides. After the ethanol was evaporated, the samples were radiated using a PW1730 diffractometer with an operating voltage of 40 KV and a current of 100 mA. The minerals were identified based on the diffractograms using the Origin 8.5 software. Precipitation was used to separate the clay minerals, and then X-ray diffraction was used to determine the relative amount of each type of mineral. The analytical uncertainties were less than 5%.
To determine the microscopic components and Rbitu, the sample was cut into blocks of 2 cm × 2 cm along the vertical plane, placed in a fixed grinding tool, and poured into the epoxy resin for cold curing. After complete curing, automatic grinding and polishing machines and alumina polishing solutions were used for grinding and polishing. After observation under a 20× dry objective lens, the polished surface had no stains and scratches with a clear boundary between components. After the sample was completely dried, the macerals were identified under oil immersion reflected light and fluorescence conditions using a Leica DM4500 microscope with a 50 times objective. The Leica DM4500P polarizing microscope with the MPS200 photometer was used to measure the Rbitu.

4. Results

4.1. Petrographic Features

The Wulalike Formation is dominated by grey and black shale, with a weathered and brittle surface in the outcrop (Figure 3A). Graptolites can be observed on a bedding plane of shale, showing vein-like, short linear, and grid-like shapes, with sizes ranging from 20 μm to several centimeters (Figure 3B,C). Horizontal bedding is common in shale (Figure 3D), indicating a deep water sedimentary environment.

4.2. Quartz Characteristics

Three kinds of quartz were recognized in the Wulalike shale under an optical microscope: biological skeletal quartz, microcrystalline quartz, and terrigenous quartz. Biological skeletal quartz refers to the quartz with biomorphic forms, mainly including radiolarians and sponge spicules. Radiolarian is marked by the central capsule and radial spines with spherical to ellipsoidal forms (Figure 4A,B). The capsules have a diameter of 50–300 μm filled with quartz and OM (Figure 4A,B). Sponge spicules have uniaxial and branching shapes with a length of 100 to 800 μm (Figure 4C,D); some of them only have spicules fragments due to poor preservation (Figure 4E). Microcrystalline quartz with an amorphous form appeared as siliceous cementation, which is the most common variety found in shale, a majority of which co-existed with OM (Figure 4F,G) and a few with clay minerals (Figure 4H). Terrigenous quartz is floating and dispersed throughout the shale matrix and is rarely in contact with other quartz grains (Figure 4I). The grains of terrigenous quartz are sub-circular to sub-angular in shape, and 20 to 80 μm in size (Figure 4I). It is worth mentioning that biological skeleton quartz is often found near the bottom of the Wulalike Formation. On the other hand, terrigenous quartz is mostly found at the top of the Wulalike Formation.

4.3. OM Features

The TOC is the most critical index to assess the quality of source rocks [34,36]. The TOC content of the Wulalike shale is generally low, varying from 0.14% to 0.90%, with an average of 0.51% (Table 1). In addition, the TOC pattern displays a decreasing trend from the bottom to the top of the Wulalike Formation (Figure 5).
All the organic macerals of the samples are composed of sapropelinite, mainly including telalginite, lamalginite, and bituminite (Table 1). This study used the following formula to estimate the Type Index (TI) [27]:
TI = (100 × A + 50 × B − 75 × C − 100 × D)/100
where A, B, C, and D correspond with sapropelinite, liptinite, vitrinite, and inertinite, respectively [27].
The TI ranges from <0, 0–40, 40–80, and >80 represent type III, type II2, type II1, and type I, respectively [37]. All the TI values of the Wulalike shale are 100, indicating type Ⅰ kerogen (Table 1).
Vitrinite reflectance (Ro) is a common indicator of OM maturity [25]. Ro needs to be replaced by Rbitu to evaluate the OM maturity due to a lack of vitrinite in the Lower Paleozoic source rocks. The conversion formula is as follows [25]:
Requ = 0.618 × Rbitu + 0.400
where Requ is equivalent to vitrinite reflectance, and Rbitu is bitumen reflectance.
The Requ ranges from 0.89% to 1.40%, with an average of 1.08% (Table 1), suggesting that the thermal evolution of the Wulalike shale in the study area has reached the mature to high mature stages.

4.4. Mineralogical Compositions and Lithotypes

The Wulalike shale consists of quartz, clay, K-feldspar, plagioclase, calcite, and dolomite (Table 2, Figure 6). Quartz and clay minerals are the most abundant minerals in all samples, ranging from 47.1 to 81.6% (averaging 69.5%) and 14.8 to 33.1% (averaging 22.8%). Clay minerals are dominated by illite and illite/smectite, with an average content of 46.05% and 37.48%, respectively (Table 2, Figure 6). Evidently, the Wulalike Formation’s quartz content decreases from the bottom to the top, with the largest concentration at the bottom (Figure 5).
Shale can be categorized into four kinds based on the concentration of the minerals: siliceous shale (SM), argillaceous shale (AM), calcareous shale (CM), and mixed shale [38] (Figure 7). According to the ternary diagram, the Wulalike shale belongs to the siliceous shale (Figure 7).

4.5. Elemental Geochemistry

4.5.1. Major Elements

The contents of the major elements are shown in Table 3. SiO2 is the dominant element in samples, varying from 50.21% to 85.51% (averaging 74.38%), which is significantly higher than the Upper Continental Crust (UCC) values [39]. The Al2O3 content ranges from 5.46% to 12.60%, with an average of 8.63%, which is lower than the UCC [39]. The other elements, including TiO2, TFe2O3, CaO, P2O5, K2O, Na2O, MnO, and MgO, have a relatively low content. The SiO2 content shows a trend of gradual decline from the bottom to the top of the Wulalike Formation, while Al2O3 is the opposite (Figure 5). The variation of TiO2 and CaO was consistent with that of Al2O3, and their contents gradually increased (Figure 7). When normalized to the UCC, most elements are in depletion, such as TiO2, MgO, TFe2O3, K2O, and P2O5, in which Al2O3, MnO, and Na2O are strongly depleted (Table 3, Figure 8A).

4.5.2. Trace Elements

The contents of the selected trace elements are shown in Table 4. When normalized to the UCC, the elements Cr, Co, Sr, Zr, and Ba are strongly depleted, while Cu and U are slightly enriched (Table 4, Figure 8B). Sc, V, Ni, Zn, Rb, Pb, and Th are close to the UCC (Table 4, Figure 8B) [39].

5. Discussion

5.1. Origins of Quartz

The input of Si-bearing hydrothermal fluids into the ocean is an important source of quartz [40]. In marine sediments, the major elements Al, Fe, and Mn are generally not affected by diagenesis and weathering. Therefore, they are effective indicators for exploring the quartz origin [40]. The Al-Fe-Mn triangular plot shows that the Wulalike shale was not influenced by hydrothermal fluids (Figure 9A). According to Marchig et al. [41], the plotted data points for the shale samples are away from the modern hydrothermal trend line in the Cr-Zr scatter diagram (Figure 9B), indicating that hydrothermal fluids are negligible. Several geochemical criteria, such as Al/(Al+Fe+Mn) < 0.35, Fe/Ti > 20, and (Fe+Mn) × Ti > 25(±5), can also be used to determine the quartz origin, implying that hydrothermal fluids have no effect [42]. The average values of the above parameters are 0.67, 9.38, and 0.61, respectively (Table 5), implying that hydrothermal fluids are unlikely.
The element Ti usually represents the degree of terrestrial detrital influx [27]. The correlation diagram between TiO2 and SiO2 shows a conspicuous negative relationship (R2 is 0.84) (Figure 10A), indicating that terrigenous quartz is not a primary source of silica. Since zirconium (Zr) usually occurs in zircon, it is an effective substitute for terrigenous input [8,43]. A significant negative trend (R2 is 0.84) between SiO2 versus Zr of the samples suggests that clastic quartz was not the main contributor of silica (Figure 10B).
Rowe et al. [44] documented that the area above the illite Si/Al line represents a greater abundance of biogenic silica in the cross-plot of Si and Al (Figure 11). All the samples fall above the illite Si/Al line (Figure 11), similar to the Wufeng–Longmaxi shale and Barnett shale, implying that the quartz is mainly of biogenic origin. In addition, pure marine biogenic siliceous rocks tend to have values of Al/(Al+Fe+Mn) around 0.60 [45] and Si/(Si+Al+Fe+Ca) values generally higher than 0.85 [40]; the values in this study are 0.67 and 0.79, respectively, indicating that the quartz in the Wulalike shale predominantly originated from a biogenic source, and that other sources also contributed to quartz. Microscopic observation showed that the Wulalike shale is enriched in siliceous microorganism fossils, i.e., radiolarians and sponge spicules, which is credible evidence for biogenic quartz (Figure 4A–E).
Another origin of silica is the transformation of clay minerals, which formed the microcrystalline quartz that co-existed with the clay minerals [47,48]. The type of quartz that commonly appears as siliceous cement can be observed under the microscope in the Wulalike shale (Figure 4H). The reaction formula is:
Smectite + K+ → Illite + SiO2 + H2O
The temperature at which conversion occurs is generally between 60 to 100 °C [49,50]. The Wulalike shale has reached mature to high mature stages (average Requ is 1.08%) (Table 1), suggesting that the reaction temperature has been reached. The clay minerals of Wulalike shale are dominated by illite and illite/smectite, and smectite contents are almost 0 (Table 2), indicating that smectite has been transformed into illite.
As discussed before, the quartz in the Wulalike Formation is formed from three quartz forms: terrigenous clastic quartz, biogenic quartz, and S-I quartz. Terrigenous detrital quartz transported to the basin is the product of parent rocks’ physical and chemical weathering, and is primarily floating in the shale matrix. Biogenic quartz refers to the dissolution and reprecipitation of siliceous organisms. Previous research has revealed that opal-A, an amorphous unstable mineral (SiO2∙nH2O), represents the initial constitution of siliceous organisms. As the diagenesis progresses, opal-A is converted into opal-CT and eventually to microcrystalline quartz [6,14]. Most siliceous organisms are dissolved due to the unsaturated silica in seawater [51]. The remaining biological skeletal quartz makes up a minor proportion of biogenic quartz, most of which is microcrystalline quartz; this is why microcrystalline quartz is significantly concentrated in siliceous organisms. Therefore, biogenic quartz, in this paper, includes biological skeletal quartz and a portion of microcrystalline quartz. The other part of microcrystalline quartz generated by S-I quartz is commonly surrounded by clay minerals and dispersed throughout shale matrix, acting as cement (Figure 4H).

5.2. Contents of Different Origins of Quartz

Excess Si is defined as Si unrelated to detrital Si and can be computed by subtracting Si connected to terrigenous detrital Al from total Si. The calculation formula is as follows [52]:
Excess Si = Sisample − Alsample × (Si/Al)background
where Sisample and Alsample denote the concentrations of Si and Al in the sample, respectively; (Si/Al)background denotes the average Si/Al ratio in the shale, and 3.11 is used as the background value [40].
According to the law of conservation of elements, the Si derived from S-I quartz is originally related to terrigenous detrital Al. Due to the exclusion of bearing-Si hydrothermal fluids in this study, the excess Si can only be biogenic and calculated by the Formula (4) above.
The S-I reaction equation is as follows [49]:
1.308{(Al3.15Mg0.85)(Si8.00)O20(OH)4(Na0.85)2H2O [smectite, 1101.1 g]
+ (0.06Fe2O3 + 0.56K2O + 0.02CaO)}
→[(Al4.12Fe0.1Mg0.56)(Si7.17)O20(OH)4(K1.47Na0.01Ca0.03)] [illite, 787.2 g]
+ 3.29SiO2 [197.7 g] + 0.56Na2O + 0.55MgO + 3.23H2O
Converting clay minerals releases the SiO2 content (S-I SiO2), which may be determined using the Formula (5). The complete analysis and calculation process are presented in Yang et al. [13], and are not mentioned in this study. Finally, the terrigenous SiO2 content can be calculated by removing biogenic SiO2 and S-I SiO2 from the overall SiO2 content. Thus, the percentages of SiO2 from three different origins have been calculated (Table 6). The Wulalike shale samples contain an average of 62.52% biogenic SiO2, 31.01% terrigenous SiO2, and 6.46% S-I SiO2, indicating that biogenic quartz is the predominant source, subsidiary from terrigenous quartz, and S-I quartz may nearly be ignored.

5.3. Paleoenvironments

5.3.1. Paleoproductivity

The total amount of OM generated per unit area and time is referred to as paleoproductivity [24]. In general, high productivity contributes to the formation of source rocks. Previous research has shown that siliceous plankton contributed to significant productivity; thus, biogenic SiO2 can indicate paleoproductivity [8,53]. As discussed in Section 5.2, the contents of biogenic SiO2 ranged from 14.88% to 68.53% (with an average of 48.48%) in the Wulalike Formation (Table 6), and are primarily developed at the bottom of the Formation and have an obvious tendency to decrease at the top, which is consistent with the petrographic observations (Figure 12).
Barite (BaSO4) formed in the decomposition of plankton was well preserved and is more stable than OM; therefore, the content of Barium (Ba) can be referred to as paleoproductivity [27,54]. The calculation formula of biogenic Ba (Babio) is as follows [27]:
Babio = Basample − Alsample × (Ba/Al)detrital
where Basample and Alsample represent the content of Ba and Al in the sample, and (Ba/Al)detrital represents the average Ba/Al value of the crustal rocks.
(Ba/Al)detrital ranges from 0.0032 to 0.0046, and 0.0039 were commonly selected for the Babio calculation [43]. The Babio ranges < 200 μg/g, 200–1000 μg/g, and >1000 μg/g indicate low, intermediate, and high paleoproductivity, respectively [54]. The value of Babio in the Wulalike shale ranges from 30.06 μg/g to 181.82 μg/g (averaging 86.60 μg/g), indicating a lean paleoproductivity. Meanwhile, the content of Babio decreases gradually at the top of the Wulalike Formation. Additionally, Ba/Al can also reconstruct paleoproductivity [55]. Core samples from Central California (CCAL), USA, with Ba/Al values of 100–120, suggest that they were deposited under high paleoproductivity conditions [56]. The ratios of Ba/Al in the Wulalike shale vary from 44.57 to 81.56, with an average of 58.82, indicating low paleoproductivity. Babio and Ba/Al parameters indicate low paleoproductivity during the Wulalike stage. Furthermore, the paleoproductivity fell steadily at the top of the section.

5.3.2. Paleoclimate

Paleoclimate affects the weathering of parent rocks and the OM accumulation of source rocks [57]. Sr/Cu values can be used to assess the paleoclimate [58]. Generally, Sr/Cu values of 1–5 and >5 indicate humid and arid climates [58]. The Sr/Cu values range from 1.02 to 9.13 and increase gradually from the bottom to the top of the Wulalike Formation (Table 5, Figure 12), suggesting that the climate changed from humid to arid. The increase in Rb/Sr indicates the transition of the paleoclimate from relative drought to humidity, while the decrease in Rb/Sr is on the contrary [9]. The Rb/Sr ratios decrease gradually from the bottom to the top of the Wulalike Formation (Figure 12), revealing that the paleoclimate changed from relative humidity to drought. The ratio of MgO/CaO can reflect the paleoclimate [57]. The increase in the MgO/CaO ratio represents a warmer and humid paleoclimate. The MgO/CaO ratio is higher at the bottom of the Wulalike Formation and is lower at the top (Figure 12), indicating that the climate has changed from relative humidity to drought.
The above proxies of Sr/Cu, Rb/Sr, and MgO/CaO indicate that the paleoclimate was mainly warm and humid at the bottom of the Wulalike Formation, gradually turning to relatively arid at the top.

5.3.3. Redox Conditions

The redox condition is crucial for the preservation of OM. Generally, the anoxic condition is conducive to the preservation of OM [59]. Trace elements and their derived parameters can reflect the redox condition, such as V/Cr, δU, and Ni/Co [27,60].
Vanadium (V), not affected by diagenesis in sediments, is enriched in an anoxic environment [61]. Cr is preferentially gathered into an organic complex under a reducing atmosphere; therefore, V/Cr can be used as an ideal redox indicator. V/Cr < 2, 2.00–4.25, >4.25 indicate oxic, dysoxic, and anoxic conditions, respectively [27]. The V/Cr ratios of samples ranged from 1.24 to 3.59 (Table 5). Furthermore, the higher V/Cr ratios are largely found near the bottom of the Wulalike Formation and gradually decreased upward (Figure 12), indicating that the shale was deposited in anoxic conditions early on and steadily transitioned to oxic needs later on.
In addition, δU (δU = 2U/(U + Th/3)) is a conventional parameter to judge the sedimentary environment, with δU > 1 and <1 corresponding to an anoxic and oxic environment [62]. The δU values of the Wulalike shale vary between 0.78 and 1.22 (Table 5), suggesting an unstable redox environment, and are varied from anoxic to oxic conditions from the bottom to the top of the Wulalike Formation (Figure 12).
The Ni/Co ratio is another credible index for assessing redox conditions [60]. The specific criteria are shown in Table 7. The Ni/Co ratios range from 2.67 to 14.00, offering a changing trend from an anoxic condition at the bottom to an oxic state at the top of the Wulalike Formation (Table 5, Figure 12).
The above geochemical parameters, V/Cr, δU, and Ni/Co, show that the deposition environment has changed. The bottom of the Wulalike Formation is in an anoxic or dysoxic condition, and it gradually changes to the oxic state when it reaches the top.

5.3.4. Paleowater Depth

The sediment grain size (coarse or fine) can determine the paleowater depth by reflecting the hydrodynamic conditions [63]. Zr and Rubidium (Rb) are enriched in coastal and fine-grained sediments, respectively [63]; thus, Zr/Rb can reflect the paleowater depth. Zr/Al and Rb/K can also indicate the paleowater depth based on a similar principle, and the ratios of Zr/Rb and Zr/Al will increase as the water depth deepend, while the Rb/K ratio is the opposite [64]. The Zr/Rb and Zr/Al ratios are relatively low at the bottom of the Wulalike Formation and gradually increase going upward (Figure 12), suggesting that the water depth becomes shallower from the bottom to the top. The Rb/K ratio has the opposite trend compared with Zr/Rb and Zr/Al (Figure 12), indicating that the water depth becomes shallower from the bottom to the top.
Graptolites generally live in a deeper water environment [65]. The horizontal bedding and abundant graptolites developed at the bottom of the Wulalike Formation and a few at the top suggest that the water depth has changed (Figure 3). Therefore, it can be concluded that the water depth became shallower from the early to late stage of the Wulalike Formation.

5.4. Controlling Factors for Low TOC

The enrichment of OM is a complex process controlled by multiple factors. Generally, the flourishing growth of plankton indicates high paleoproductivity, and then promotes enrichment of the OM [17]. Biogenic SiO2, Babio, and Ba/Al with TOC show certain positive relationships (R2 = 0.41, 0.33, and 0.33, respectively) (Figure 13A–C), indicating that paleoproductivity plays a primary controlling factor for OM enrichment. However, the low paleoproductivity resulted in a low TOC content during the Wulalike deposition.
In addition, the Sr/Cu is negatively correlated with TOC (R2 = 0.42) (Figure 13D), revealing that the paleoclimate contributed to the accumulation of OM. The positive correlation between Rb/Sr, MgO/CaO, and TOC further supports this view (R2 = 0.22 and 0.11, respectively) (Figure 13E,F). The warm and humid climate in the early stage of the Wulalike Formation accelerated weathering of the parent rocks. It facilitated more terrestrial nutrients entering the ocean, which are beneficial to the reproduction of marine organisms. In the late period, the arid climate and weakened weathering decreased the terrigenous nutrients input, resulting in the lean TOC.
The redox condition is a crucial factor in promoting the preservation of OM [21]. Multiple geochemistry proxies show that the Wulalike shale was deposited in anoxic to oxic environments. There are certain positive correlations in the V/Cr, δU, and Ni/Co versus TOC (R2 = 0.28, 0.24, and 0.28, respectively) (Figure 14A–C), indicating that OM accumulation was also controlled by the redox condition. Moreover, the shale formed under anoxic or dysoxic conditions has a relatively higher TOC content at the bottom of the Wulalike Formation, indicating that anoxic or dysoxic is conductive to OM preservation.
Meanwhile, the water depth parameters of Zr/Rb and Zr/Al present clear negative correlations with TOC, as well as a positive correlation of Rb/K with TOC (R2 = 0.39, 0.42, and 0.45, respectively) (Figure 14D–F). These all suggest that water depth is a pivotal factor controlling OM enrichment. It could be due to the ocean’s increased reducibility in deep water, making more space accessible for the preservation of OM.
Based on the above discussion, we can conclude that the Wulalike marine shale is a typical superposition model of productivity and preservation conditions and can establish the OM accumulation mechanism (Figure 15). Under the influence of the Caledonian movement in the Middle–Late Ordovician period, the tectonic rift was developed in the west of the Ordos Basin, forming the deep water basin facies deposits found in the Wulalike Formation. Simultaneously, the eastern part of the basin was uplifted and denuded. Ma et al. [33] documented that a rapid transgression occurred, resulting in the maximum flooding surface in the early stage of Wulalike; after that, the sea level declined slowly, followed by the water depth becoming shallow. The water depth variation is consistent with the geochemical indexes (Zr/Rb, Zr/Al, and Rb/K) and petrographic observation results. Influenced by the transgression, on the one hand, the rising sea level led to the transformation of biota from benthos to plankton. The copious nutrients transported by the upwelling encouraged plankton blooming, providing a sufficient material basis for OM enrichment. However, as the sea depth increased rapidly, the water column changed into a dysoxic or anoxic environment, reducing OM decomposition. The hydrodynamic conditions weakened, allowing fine sediments, such as clay and organic matter, to precipitate, depositing the organic-rich shale of the Wulalike Formation. At the same time, the lipid components in the OM are preferentially preserved and enriched under the activity of anaerobic bacteria, tending the OM towards sapropel (type I kerogen), which has a better hydrocarbon generation potential. Zhai et al. [66] and Chen et al. [67] believed that the high water level under the transgressive system tract is favorable for OM enrichment. Thus, the early sedimentary environments of the Wulalike Formation are most conducive to organic accumulation (Figure 15A).
On the other hand, the late sedimentary settings changed and were characterized by shallow water, decreasing paleoproductivity, and arid climates or oxic conditions, all of which were detrimental to the production and preservation of OM (Figure 15B). As a result, the next exploration target should be aimed at the bottom of the Wulalike Formation on the Ordos Basin’s western margin.

6. Conclusions

The OM characteristics, shale lithofacies type, origins of quartz, paleoenvironments, and OM enrichment mechanisms of the Ordovician Wulalike shale near the northwestern fringe of the Ordos Basin are studied in detail in this research.
  • The Wulalike shale is characterized by a low TOC and type Ⅰ kerogen; the thermal evolution has reached the mature to highly mature stages; and the lithotype is siliceous shale.
  • The origins of quartz in Wulalike shale are biogenic quartz (average 63%), terrigenous clastic quartz (average 31%), and S-I quartz (only 6%).
  • The dominating factors affecting OM accumulation are paleoproductivity and redox conditions; in addition, paleoclimate and paleowater depth have a certain contribution.
  • The OM enrichment mechanism is the typical superposition model of productivity and preservation conditions. Based on the results of biogenic quartz distribution and sedimentary environments, it is considered that the shale at the bottom of the Wulalike Formation is conducive to exploration, and the next exploration target should focus on this interval.

Author Contributions

Conceptualization, Y.Z.; methodology, Y.Z. and R.L.; software, Y.Z.; validation, L.C., H.H. and B.Z.; writing—original draft, Y.Z.; supervision, R.L. and H.H.; investigation, S.X., J.G. and A.K.; resources, L.C. and B.Z.; All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (No. 42273064), National Natural Science Foundation of China (No. 42202160), Shaanxi’s Natural Science Basic Research Program (No. 2021JQ226), Natural Science Foundation of Shandong Province (No. ZR2021QD057).

Data Availability Statement

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Map of major tectonic plates in China. NCC: North China Craton, SCC: South China Craton, TC: Tarim Craton, QB: Qaidam Block, QM: Qiangtang Massif. (B) Structural sketch of the Ordos basin and its adjacent areas. Modified from reference [29].
Figure 1. (A) Map of major tectonic plates in China. NCC: North China Craton, SCC: South China Craton, TC: Tarim Craton, QB: Qaidam Block, QM: Qiangtang Massif. (B) Structural sketch of the Ordos basin and its adjacent areas. Modified from reference [29].
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Figure 2. (A) Ordovician stratigraphic histogram, northwest of the Ordos Basin. (B) Regional geological sketch map of the Zhuozishan area. Modified from reference [35].
Figure 2. (A) Ordovician stratigraphic histogram, northwest of the Ordos Basin. (B) Regional geological sketch map of the Zhuozishan area. Modified from reference [35].
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Figure 3. Characteristics of the Wulalike Formation shale. (A) The weathered surface of shale developed foliation. (B) Graptolite fossils, occurring on the bedding plane of shale. (C) Fragment of graptolite under the microscope in plane polarized light. (D) Shale with vertical fractures and horizontal bedding.
Figure 3. Characteristics of the Wulalike Formation shale. (A) The weathered surface of shale developed foliation. (B) Graptolite fossils, occurring on the bedding plane of shale. (C) Fragment of graptolite under the microscope in plane polarized light. (D) Shale with vertical fractures and horizontal bedding.
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Energies 16 05278 g004 550
Figure 4. Microscopic characteristics of quartz. (A) Radiolarian, showing poor preservation. Cross-polarized light. (B) Radiolarian with the complete morphology. Plane polarized light. (C,D) are the forked sponge spicules. (C) and (D) are plane polarized light and cross-polarized light, respectively. (E) Sponge spicule fragment. Plane polarized light. Microcrystalline quartz with an amorphous form is symbiotic with OM (F) and algae (G). Plane polarized light. (H) Microcrystalline quartz coexists with clay minerals. Plane-polarized light. (I) Terrigenous quartz, characterized by a subangular–subcircular shape, was mainly distributed at the top of the Wulalike Formation. Plane polarized light.
Figure 4. Microscopic characteristics of quartz. (A) Radiolarian, showing poor preservation. Cross-polarized light. (B) Radiolarian with the complete morphology. Plane polarized light. (C,D) are the forked sponge spicules. (C) and (D) are plane polarized light and cross-polarized light, respectively. (E) Sponge spicule fragment. Plane polarized light. Microcrystalline quartz with an amorphous form is symbiotic with OM (F) and algae (G). Plane polarized light. (H) Microcrystalline quartz coexists with clay minerals. Plane-polarized light. (I) Terrigenous quartz, characterized by a subangular–subcircular shape, was mainly distributed at the top of the Wulalike Formation. Plane polarized light.
Energies 16 05278 g004
Energies 16 05278 g005 550
Figure 5. Vertical content distribution of the selected minerals and major elements.
Figure 5. Vertical content distribution of the selected minerals and major elements.
Energies 16 05278 g005
Energies 16 05278 g006 550
Figure 6. Composition of the minerals and clay in the Wulalike shale samples.
Figure 6. Composition of the minerals and clay in the Wulalike shale samples.
Energies 16 05278 g006
Energies 16 05278 g007 550
Figure 7. Lithology discrimination map of shale. The base plot is from reference [38].
Figure 7. Lithology discrimination map of shale. The base plot is from reference [38].
Energies 16 05278 g007
Energies 16 05278 g008 550
Figure 8. Normalized diagram of (A) the major elements and (B) the trace elements of the shale samples.
Figure 8. Normalized diagram of (A) the major elements and (B) the trace elements of the shale samples.
Energies 16 05278 g008
Energies 16 05278 g009 550
Figure 9. Discriminate diagram of the siliceous origins. (A) All samples fall into the non-hydrothermal zone. Base polt is from reference [40], (B) All samples are far from the modern hydrothermal trend line. The base polt is from reference [41].
Figure 9. Discriminate diagram of the siliceous origins. (A) All samples fall into the non-hydrothermal zone. Base polt is from reference [40], (B) All samples are far from the modern hydrothermal trend line. The base polt is from reference [41].
Energies 16 05278 g009
Energies 16 05278 g010 550
Figure 10. Correlation diagrams of SiO2 with TiO2 (A) and Zr (B), respectively.
Figure 10. Correlation diagrams of SiO2 with TiO2 (A) and Zr (B), respectively.
Energies 16 05278 g010
Energies 16 05278 g011 550
Figure 11. Correlation diagram of Al and Si with the data of Barnett shale and Wufeng–Longmaxi shale from reference [44] and reference [46], respectively.
Figure 11. Correlation diagram of Al and Si with the data of Barnett shale and Wufeng–Longmaxi shale from reference [44] and reference [46], respectively.
Energies 16 05278 g011
Energies 16 05278 g012 550
Figure 12. Stratigraphic distribution of the TOC and other geochemical parameters of the Wulalike shale.
Figure 12. Stratigraphic distribution of the TOC and other geochemical parameters of the Wulalike shale.
Energies 16 05278 g012
Energies 16 05278 g013 550
Figure 13. Parameters of OM inputs. (A–C) Plots of paleoproductivity parameters; biogenic SiO2, biogenic Ba, and Ba/Al showing significant positive correlations with the TOC content. (D–F) Plots of paleoclimate parameters with TOC content, showing a negative correlation of TOC content with Sr/Cu and positive correlations with Rb/Sr and MgO/CaO.
Figure 13. Parameters of OM inputs. (A–C) Plots of paleoproductivity parameters; biogenic SiO2, biogenic Ba, and Ba/Al showing significant positive correlations with the TOC content. (D–F) Plots of paleoclimate parameters with TOC content, showing a negative correlation of TOC content with Sr/Cu and positive correlations with Rb/Sr and MgO/CaO.
Energies 16 05278 g013
Energies 16 05278 g014 550
Figure 14. Parameters of preservation conditions. (A–C) Plots of redox parameters V/Cr, δU, and Ni/Co with TOC content, respectively, indicating certain positive correlations. (D–F) Plots of paleowater depth parameters with TOC content, showing obvious negative correlations of Zr/Rb and Zr/Al and a positive correlation of Rb/K with TOC content.
Figure 14. Parameters of preservation conditions. (A–C) Plots of redox parameters V/Cr, δU, and Ni/Co with TOC content, respectively, indicating certain positive correlations. (D–F) Plots of paleowater depth parameters with TOC content, showing obvious negative correlations of Zr/Rb and Zr/Al and a positive correlation of Rb/K with TOC content.
Energies 16 05278 g014
Energies 16 05278 g015 550
Figure 15. Diagram of the organic matter enrichment mechanism of the Wulalike shale. (A,B) are the early and late stage of the sedimentary paleoenvironments, respectively.
Figure 15. Diagram of the organic matter enrichment mechanism of the Wulalike shale. (A,B) are the early and late stage of the sedimentary paleoenvironments, respectively.
Energies 16 05278 g015
Table
Table 1. TOC, Rbitu, and maceral composition of the OM in the shale samples.
Table 1. TOC, Rbitu, and maceral composition of the OM in the shale samples.
SamplesTOC (%)Rbitu (%)Requ (%)Sapropelinite (%)Liptinite (%)Vitrinite (%)Inertinite (%)Kerogen Type
SD02-10.900.910.96100.00///Ⅰ
SD02-20.69///////
SD02-30.440.920.97100.00///Ⅰ
SD03-10.28///////
SD03-20.71///////
SD03-30.440.920.97100.00///Ⅰ
SD03-40.31///////
SD03-50.540.790.89100.00///Ⅰ
SD03-60.81///////
SD03-70.721.101.08100.00///Ⅰ
SD03-80.751.591.38100.00///Ⅰ
SD03-90.75///////
SD03-100.781.201.14100.00///Ⅰ
SD03-110.61///////
SD03-120.481.621.40100.00///Ⅰ
SD03-130.55///////
SD03-140.50///////
SD03-150.60///////
SD03-160.52///////
SD03-170.36///////
SD03-180.41///////
SD03-190.500.860.93100.00///Ⅰ
SD03-200.46///////
SZ01-10.83///////
SZ01-20.25///////
SZ01-30.70///////
SZ01-40.64///////
SZ01-50.21///////
SZ01-60.19///////
SZ02-10.14///////
SZ02-20.18///////
SZ02-30.16///////
Table
Table 2. XRD mineralogy results of the shale samples.
Table 2. XRD mineralogy results of the shale samples.
SamplesMineral Composition (%)Clay Mineral Relative Content (%)
QtzK-FeldPlagCalcDoloClayI/SICSKI/S Ratio
SD02-179.11.70.81.1/17.3305014/65
SD02-281.61.40.7//16.345469//5
SD02-379.81.5///18.8345610//10
SD03-280.81.4///17.764297//10
SD03-381.31.40.60.7/16682111//10
SD03-479.31.3/0.3/19.251436//5
SD03-5831.31//14.8592219//10
SD03-673.91.60.9//23.66124141/10
SD03-774.41.51.30.8/2234613/25
SD03-1065.71.21.47.5/24.3424211/55
SD03-1172.21.51.10.8/24.3553312//10
SD03-1377.61.30.8//20.35526181/5
SD03-1673.71.60.80.9/23.1404110/95
SD03-1976.81.20.70.4/20.9533314//5
SD03-2073.91.40.8//23.8523117//5
SZ01-259.41.62.8/3.133.1315013/65
SZ01-451.6/214.11.231.2433815/410
SZ01-651.3/1.523.2/23.94727251/5
SZ02-149.31.12.217.4/30413517/75
SZ02-247.20.82.721.5/27.8224726/55
SZ02-347.11.3220/29.6403221/75
Avg.69.51.21.15.20.222.81637.513.90.12.46.7
Note: Qtz = Quartz; K-feld = K-feldspar; Plag = Plagioclase; Calc = Calcite; Dolo = Dolomite; I/S = Illite/Smectite; I = Illite; C = Chlorite; S = Smectite; K = Kaolinite.
Table
Table 3. Major element content (wt%) in shale samples.
Table 3. Major element content (wt%) in shale samples.
SamplesSiO2TiO2Al2O3TFe2O3MnOMgOCaONa2OK2OP2O5LOITotal
SD02-180.500.348.072.060.011.050.540.082.750.054.0399.48
SD02-284.520.296.311.850.010.850.260.302.070.043.2899.78
SD02-382.910.297.072.100.011.020.260.022.290.043.2099.21
SD03-182.770.297.352.070.010.950.350.272.300.063.85100.27
SD03-284.020.296.892.100.011.030.260.392.210.053.41100.66
SD03-384.770.286.531.920.010.610.200.131.940.103.1899.67
SD03-482.720.327.382.970.010.880.200.122.210.053.38100.24
SD03-585.510.255.462.310.020.700.180.141.550.043.0099.16
SD03-682.120.296.702.940.011.180.270.361.940.043.8299.67
SD03-780.480.357.902.570.011.080.260.222.520.043.9899.41
SD03-879.900.358.362.950.011.360.310.462.490.054.09100.33
SD03-980.150.368.142.860.011.140.280.122.410.054.1899.70
SD03-1080.280.378.532.840.011.240.270.332.490.064.09100.51
SD03-1180.010.358.002.990.011.340.290.182.360.063.8699.45
SD03-1279.740.378.423.060.021.530.350.182.530.074.05100.32
SD03-1381.250.337.453.130.021.460.270.362.140.053.62100.08
SD03-1477.340.459.763.320.021.790.290.442.850.074.07100.40
SD03-1573.870.5010.614.760.011.660.360.193.320.084.72100.08
SD03-1679.190.378.163.390.011.560.270.112.450.073.7699.34
SD03-1778.680.388.474.000.021.660.250.292.530.073.73100.08
SD03-1877.370.418.743.860.011.500.260.222.640.063.9699.03
SD03-1981.070.317.332.930.011.470.240.612.210.063.4399.67
SD03-2080.740.347.632.850.011.440.230.092.350.063.4299.16
SZ01-168.580.5611.884.780.022.621.010.503.560.086.0699.65
SZ01-265.990.6612.604.910.032.482.660.453.600.136.0699.57
SZ01-364.030.5410.984.400.022.235.100.663.240.098.2399.52
SZ01-458.250.5210.204.110.032.259.640.672.960.0911.55100.27
SZ01-555.320.528.323.350.041.8413.720.392.470.1613.7099.83
SZ01-655.230.549.083.490.032.0213.230.272.650.1613.42100.12
SZ02-152.300.6111.784.400.032.5711.610.203.310.1312.9799.91
SZ02-250.210.6310.584.630.042.4814.090.352.810.1614.43100.41
SZ02-350.300.6711.394.530.042.5613.270.473.170.1314.05100.58
Avg.74.380.418.633.260.021.5532.000.302.570.085.8399.86
UCC a66.000.6415.405.040.102.483.593.272.800.15--
a Date cited from reference [39].
Table
Table 4. Trace element content (ppm) in the shale samples.
Table 4. Trace element content (ppm) in the shale samples.
SamplesScVCrCoNiCuZnRbSrZrBaPbThU
SD02-16.26145.7340.562.2221.7132.81112.9111.1681.4650.4348.4418.358.43.66
SD02-25.56124.8434.92.0623.2633.47156.0781.250.244.99230.3415.167.583.71
SD02-35.94124.8235.891.9945.3732.59179.0581.8740.1247.17225.312.528.124
SD03-16.15104.7233.891.9627.4926.44119.4583.7140.3345.24212.4225.588.174.18
SD03-26.07105.1135.073.6248.5422.1366.8482.0549.9745.26241.5817.097.823.57
SD03-35.5993.3231.851.1910.2527.4930.4778.036045.53216.7839.67.373.89
SD03-46.45123.7638.412.821.0532.1886.4386.01107.5649.9240.6526.248.53.45
SD03-54.9111.4734.781.5418.3425.878265.4182.4539.23180.9116.366.333.3
SD03-65.99108.6438.121.7819.1624.8330.877.7540.8449.94200.4316.67.342.98
SD03-77.07115.3442.282.0622.9924.6447.18101.8244.2267.96254.7619.89.533.81
SD03-87.87125.8744.442.4631.4427.8648.8103.746.3255.65268.0117.8210.14.04
SD03-97.24117.9542.212.2130.5129.1353.7198.9247.0857.87256.8418.859.24.03
SD03-108.09123.7848.22.4232.1332.8155.1114.2353.6366.47268.1715.7110.224.23
SD03-117.45119.5542.682.3426.929.4858.38106.8946.4859.83254.43138.753.33
SD03-128.14113.4650.542.7327.823352.97115.5448.2269.46266.0311.6110.384.06
SD03-137.45122.543.243.7526.2634.5143.5997.740.9355.27225.6216.348.563.39
SD03-149.929856.275.1924.9736.5843.51124.0343.1672.93322.4110.9211.723.68
SD03-159.71136.5261.178.2231.246.9842.11145.5347.7784.34349.8625.7814.134.55
SD03-167.8130.6344.764.5121.5732.6331.27107.4638.960.09282.215.910.533.89
SD03-178.1106.8548.935.1826.236.3731.88111.5939.4763.89268.4824.4510.14.06
SD03-188.7112.4553.385.2722.4337.0333.22119.5272.4468.8525030.6310.914.31
SD03-197.35107.7243.922.9621.1328.8445.5997.6143.4757.25243.047.868.283
SD03-207.53108.2242.913.1722.6734.0742.39106.9644.6659.11277.9910.979.643.22
SZ01-111.51134.1966.196.4535.6340.5878.12163.4750.05103.11360.9328.1115.335.85
SZ01-213.23115.8574.088.5333.2240.1777.92172.8577.76121.94331.6214.215.545.3
SZ01-311.4123.963.668.0932.7744.8667.27152.4160.4790.07326.2621.2114.634.76
SZ01-411.01130.6757.287.4124.3539.450.05131.75240.9987.1240.6617.8811.834.03
SZ01-510.0683.255.757.5821.226.0869.3113.43237.1888.04223.611.910.593.19
SZ01-611.4291.6164.617.4123.2526.4178.44136.57241.1999.87239.369.6311.63.36
SZ02-112.37100.6271.158.8630.1225.3885.54165.71228.02131.33287.4712.5313.283.77
SZ02-212.8287.8270.8410.4127.7635.5267.2141.9268.1134.41252.0811.6413.632.88
SZ02-313.0596.8172.939.6629.5232.8564.85155.56253.99129.35297.1914.714.233.29
Avg.8.51113.9449.534.5626.9132.2866.64113.5195.8671.93263.8717.7810.393.84
UCC a13.610785174425711123501905501710.72.8
a Date cited from reference [39].
Table
Table 5. Geochemical indexes of the shale samples.
Table 5. Geochemical indexes of the shale samples.
SamplesAl/(Al+Fe+Mn)Si/(Si+Al+Fe+Ca)(Fe+Mn) × TiFe/TiBabio *Ba/Al *Sr/CuMgO/CaORb/SrV/CrδUNi/CoZr/RbZr/AlRb/K
SD02-10.750.860.307.07181.8281.562.481.941.363.591.139.760.450.00360.0216
SD02-20.720.890.237.44100.0668.951.503.271.623.581.1911.300.550.00230.0185
SD02-30.720.880.268.4579.3360.191.233.922.043.481.1912.000.580.00240.0168
SD03-10.730.870.258.3360.6754.595.312.710.603.091.2114.000.540.00130.0179
SD03-20.710.880.268.4599.3266.232.263.961.643.001.1613.410.550.00350.0196
SD03-30.720.890.238.0081.9562.711.783.051.022.931.238.580.580.00190.0179
SD03-40.650.860.4010.8388.2761.593.344.400.803.221.107.510.580.00160.0171
SD03-50.640.900.2410.7868.1862.593.193.890.793.211.2211.880.600.00170.0178
SD03-60.630.870.3611.8362.1056.511.644.371.902.851.1010.770.640.00250.0178
SD03-70.700.860.388.5791.6460.911.794.152.302.731.0911.180.670.00290.0190
SD03-80.680.850.449.8395.4060.561.664.392.242.831.0912.770.540.00200.0180
SD03-90.680.850.439.2788.7759.601.624.072.102.791.1413.780.590.00220.0177
SD03-100.690.850.448.9592.0559.381.634.592.132.571.1113.260.580.00450.0190
SD03-110.670.850.449.9789.2560.071.584.622.302.801.0711.510.560.00190.0206
SD03-120.670.840.489.6592.1859.681.464.372.402.241.0810.200.600.00500.0187
SD03-130.640.860.4411.0771.8057.201.195.412.392.831.096.990.570.00220.0192
SD03-140.690.820.638.61120.9062.401.186.172.871.740.974.810.590.00560.0181
SD03-150.630.791.0011.11130.7962.291.024.613.052.230.983.800.580.00730.0168
SD03-160.640.840.5310.69113.7265.321.195.782.762.921.054.780.560.00620.0174
SD03-170.610.830.6412.2893.6059.871.096.642.832.181.095.060.570.01080.0183
SD03-180.630.830.6710.9897.0359.971.965.771.652.111.084.260.580.01580.0172
SD03-190.650.860.3811.0391.7062.631.516.132.252.451.047.130.590.00240.0200
SD03-200.670.860.419.78120.4568.821.316.262.392.521.007.150.550.00490.0188
SZ01-10.650.761.139.96115.6457.391.232.593.271.661.075.520.630.00280.0181
SZ01-20.660.721.378.6871.4749.711.940.932.221.561.013.890.710.01090.0156
SZ01-30.650.701.009.5199.5656.133.580.440.951.950.994.050.590.00630.0158
SZ01-40.650.640.909.2230.0644.576.120.230.552.281.013.290.660.00970.0159
SZ01-50.650.610.747.5251.8150.769.090.130.481.490.952.800.780.02530.0132
SZ01-60.660.610.807.5451.8949.799.130.150.571.420.933.140.730.06700.0153
SZ02-10.670.581.148.4244.2546.098.980.220.731.410.923.400.790.03570.0148
SZ02-20.630.551.248.5733.6445.017.550.180.531.240.782.670.950.04490.0140
SZ02-30.650.561.297.8962.0249.297.730.190.611.330.823.060.830.07150.0139
Avg.0.670.790.619.3886.6058.823.073.421.732.451.067.740.620.01150.0175
* The units of Ba/Al and Babio are ppm/% and ppm, respectively.
Table
Table 6. Quartz percentage of different origins.
Table 6. Quartz percentage of different origins.
SamplesSiO2Biogenic SiO2S-I SiO2Terrigenous SiO2Percentage of Biogenic SiO2 (%)Percentage of S-I SiO2 (%)Percentage of Terrigenous SiO2 (%)
SD02-180.555.43.4121.6968.824.2426.94
SD02-284.5264.93.6315.9976.784.318.92
SD02-382.9160.924.0917.973.484.9321.59
SD03-284.0262.593.8517.5874.54.5820.92
SD03-384.7764.463.317.0176.043.920.06
SD03-482.7259.774.4118.5472.255.3322.42
SD03-585.5168.532.7914.1980.143.2616.59
SD03-682.1261.284.6816.1674.635.6919.68
SD03-780.4855.915.1519.4169.476.4124.12
SD03-1080.2853.75521.5366.966.2326.82
SD03-1180.0155.135.0319.8568.96.2924.8
SD03-1381.2558.083.9919.1871.484.9123.61
SD03-1679.1953.814.5820.7967.955.7926.26
SD03-1981.0758.274.3718.4271.885.422.72
SD03-2080.7457.014.8118.9270.615.9523.44
SZ01-265.9926.86.632.5840.6210.0149.37
SZ01-458.2526.536.0125.7145.5410.3244.14
SZ01-655.2326.994.323.9448.877.7943.34
SZ02-152.315.665.5731.0629.9510.6559.4
SZ02-250.2117.314.7428.1634.479.4456.09
SZ02-350.314.885.230.2229.5810.3560.08
Table
Table 7. Table of differentiation criterion for redox conditions.
Table 7. Table of differentiation criterion for redox conditions.
The ProxiesReducing EnvironmentOxic
AnoxicDysoxic
V/Cr>4.252–4.25<2
δU>1 <1
Ni/Co>75–7<5
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Zhang, Y.; Li, R.; Xi, S.; Gao, J.; Chen, L.; Huang, H.; Zhao, B.; Khaled, A. Quartz Origins and Paleoenvironmental Controls on Organic Matter Accumulation of Marine Shale in the Ordovician Wulalike Formation, Northwestern Ordos Basin, China: Significance for Shale Gas Exploration. Energies 2023, 16, 5278. https://doi.org/10.3390/en16145278

AMA Style

Zhang Y, Li R, Xi S, Gao J, Chen L, Huang H, Zhao B, Khaled A. Quartz Origins and Paleoenvironmental Controls on Organic Matter Accumulation of Marine Shale in the Ordovician Wulalike Formation, Northwestern Ordos Basin, China: Significance for Shale Gas Exploration. Energies. 2023; 16(14):5278. https://doi.org/10.3390/en16145278

Chicago/Turabian Style

Zhang, Yanni, Rongxi Li, Shengli Xi, Jianwen Gao, Lei Chen, Hexin Huang, Bangsheng Zhao, and Ahmed Khaled. 2023. "Quartz Origins and Paleoenvironmental Controls on Organic Matter Accumulation of Marine Shale in the Ordovician Wulalike Formation, Northwestern Ordos Basin, China: Significance for Shale Gas Exploration" Energies 16, no. 14: 5278. https://doi.org/10.3390/en16145278

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