Paleoenvironment of the Lower Ordovician Meitan Formation in the Sichuan Basin and Adjacent Areas, China

Yang, Meihua; Zuo, Yinhui; Fu, Xiaodong; Qiu, Lei; Li, Wenzheng; Zhang, Jianyong; Zheng, Ziyun; Zhang, Jiazhen

doi:10.3390/min12010075

Open AccessArticle

Paleoenvironment of the Lower Ordovician Meitan Formation in the Sichuan Basin and Adjacent Areas, China

¹

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China

²

PetroChina Hangzhou Research Institute of Geology, Hangzhou 310023, China

³

4th Producting Plant, Southwest Oil & Gas Company, SINOPEC, Chongqing 402160, China

^*

Author to whom correspondence should be addressed.

Minerals 2022, 12(1), 75; https://doi.org/10.3390/min12010075

Submission received: 18 November 2021 / Revised: 3 January 2022 / Accepted: 5 January 2022 / Published: 8 January 2022

(This article belongs to the Special Issue Organic Matter in Sedimentary Systems: Insights from Organic Petrology and Organic Geochemistry)

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Abstract

:

The quality of hydrocarbon source rocks is affected by the sedimentary paleoenvironment. A paleoenvironment with anoxia and a high paleoproductivity is beneficial to source rocks. The paleoenvironment of the Lower Ordovician Meitan Formation of the Sichuan Basin and its adjacent areas is lacking, restricting the oil and gas exploration of the Ordovician in the Sichuan Basin and its adjacent areas. In this paper, the content of major and trace elements of 50 samples was tested to clarify the paleoenvironment of the Meitan Formation. The paleoclimate, paleosalinity, paleoredox, and paleoproductivity during the deposition of the Meitan Formation were analyzed. The control effect of the paleoenvironment on the development of source rocks was clarified, and the favorable paleoenvironment for source rock development was pointed out. The results show that the paleoenvironment of the Meitan Formation has the following characteristics: humidity, brackish water, oxygen depletion, anoxia environment, and high paleoproductivity. These characteristics are conducive to the development of poor and moderate source rocks. The source rocks of the Meitan Formation were developed in the north, west, and south of the Sichuan Basin and its adjacent areas. The organic matter of the source rocks is mainly typed II₁ kerogen, and the quality is evaluated as poor-medium source rocks having the potential of generating oil and gas. This study can provide fundamental parameters for the further exploration of Ordovician petroleum.

Keywords:

paleoenvironment; mudstone; source rock; Meitan Formation; Sichuan Basin and its adjacent areas

1. Introduction

The hydrocarbon generation and the expulsion intensity and amount are not only affected by tectonic thermal evolutions, but also by the quality of source rocks, including organic matter abundance, type, and maturity. These parameters are affected by the paleoclimate and sedimentary environment, such as paleosalinity, paleoredox, and paleoproductivity [1,2,3].

The Ordovian of the Sichuan Basin and its adjacent areas, in southwestern China (Figure 1), have been regarded as a concurrent layer for oil and gas exploration of the Silurian and the Cambrian, and no large oil and gas fields have been found. Moreover, from the Sinian to the Jurassic, the Ordovician is the only exploration strata that have not been commercially discovered [4]. At present, the oil and gas discoveries of the Ordovician in the Sichuan Basin and its adjacent areas are mainly concentrated in the Lower Ordovician Tongzi Formation and the Upper Ordovician Baota Formation, and the karst-fractured reservoirs are mainly found [5].

Previous studies have shown that the Lower Ordovician Meitan Formation has the most important source rock of the Ordovician in the Sichuan Basin and its adjacent areas [6,7]. However, there are few basic data for the study of the source rock of the Meitan Formation, only a small amount of outcrop sections and drilling data, and a lack of basic parameters such as seismic data and organic geochemical analyses. It is impossible to systematically analyze the paleoenvironment and the influences on source rocks, which seriously limits the oil and gas exploration of the Ordovician.

In this paper, the paleoclimate, paleosalinity, paleoredox, and paleoproductivity of the Meitan Formation were analyzed in combination with geological and geochemical parameters. This paper reveals the control of the paleoenvironment on the development of the source rocks and pointed out the favorable paleoenvironment of the source rock. It is expected to provide theoretical support for the assessment of the resource potential and exploration target optimization of the Meitan Formation.

2. Geological Settings

The Sichuan Basin and its adjacent areas are located in southwestern China (Figure 1a). The Songpan-Ganzi fold belt and the Longmenshan fault zone are in the west of the research area, the Chengkou-Fangxian fault is in the north, the Guizhou-Chongqing-Xiang-Hubei fold belt is in the east, and the Qianzhong paleo uplift is in the south. The Sichuan Basin is distributed in the study area in a NE direction, showing the characteristics of a multi-tectonic system under multi-stage structures (Figure 1b) [8].

From the Late Cambrian to the Early Silurian, several strata developed in the Sichuan Basin and its adjacent areas. The Lower Ordovician Tongzi Formation, Honghuayuan Formation, Meitan Formation, Middle Ordovician Shizipu Formation, Upper Ordovician Baota Formation, Linxiang Formation, and Wufeng Formation were deposited from the bottom to the top (Figure 2). However, the Middle-Upper Ordovician were missing in the north, west, and southwest of the Sichuan Basin under the influence of the Caledonian movement [6,9].

The Meitan Formation’s depositional period was affected by multiple sea-level fluctuations. During the early depositional period of the Meitan Formation, the basement rapidly subsided, and the sea level was rising under the affection of the Caledonian movement. The depositional characteristics were characterized by shallow cement shelf facies dominated by mudstones. During the middle Meitan Formation deposit, the sea level dropped, and the terrigenous debris was deposited. The lithology is characterized by siltstone, fine sandstone, and limestone, mainly through mixed shelf facies deposition. During the late depositional period of the Meitan Formation, the sea level rose again, marked by the end of deep-water shelf facies deposition, and limestone, bioclastic limestone, and sandy limestone were mainly developed [10,11].

3. Materials and Methods

In this paper, typical wells and geological sections of the Sichuan Basin and its adjacent areas were selected for geochemical experiments. The major and trace element data were taken from 10 mudstone samples from 2 wells and 42 mudstone samples from 4 geological sections (Table 1 and Table 2). Total organic carbon (TOC) data were taken from 486 mudstone samples from 23 wells and 7 geological sections, and kerogen carbon isotope data were taken from 20 mudstone samples from 3 wells and 2 geological sections.

Experiments were carried out in the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University. The major elements were tested by X-ray fluorescence spectrometry (ZTIX-1)(XRF). The trace elements were analyzed by an inductively coupled plasma optical emission spectrometer (VISTA MPX) (ICP-OES) and an inductively coupled plasma mass spectrometer (X II) (ICP-MS). The TOC was detected by a carbon-sulfur analyzer (LECO CS230), and the carbon isotope of kerogen was tested by a stable isotope mass spectrometer (DELTA PLUS V). These experiments were conducted at 25 °C. The experimental methods and instruments are in accordance with the National and Industry Standards.

4. Paleoenvironment and Paleoproductivity

4.1. Paleoclimate

The contents of the major and trace elements in sediments varied significantly under different climatic conditions, mainly affected by temperature and humidity. Therefore, chemical index alteration (CIA) would restore the paleoclimate (Equation (1)) [12].

CIA = Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O) × 100%

(1)

where the content of elements is the mole fraction, and CaO* represents CaO in silicate.

The CIA value of 80~100% reflects a hot and humid climate, 60~80% reflects a warm and humid climate, and 50~60% reflects a cold and dry climate [13,14]. The CIA value of the mudstones in the Meitan Formation is between 50.2% and 78.3%, with an average of 69%, showing a warm and humid climate as a whole (Figure 3a, Table 3).

In addition, the Sr/Cu ratio can also reflect the paleoclimate. Under a humid climate, the Sr/Cu ratio is less than 10, and under a dry climate, the Sr/Cu ratio is more than 10. The Sr/Cu ratio of the mudstones in the Meitan Formation is between 1.34 and 24.2, with an average of 5.03, reflecting the humid climate (Figure 3b). Combined with the CIA and the Sr/Cu ratio, the mudstones of the Meitan Formation were deposited under a warm and humid climate.

4.2. Paleosalinity

Paleosalinity is an important parameter reflecting the sea-level changes during geological histories. The Rb/K ratio has a good positive correlation with paleosalinity, which can discriminate the paleosalinity. Rb/K>0.006 indicates the saline water deposition, 0.004 < Rb/K < 0.006 indicates the brackish water deposition, and Rb/K < 0.004 indicates freshwater deposition [15,16]. The Rb/K ratio of mudstones in the Meitan Formation ranges from 0.0027 to 0.0069, with an average of 0.0052, indicating a brackish water sedimentary environment (Figure 4, Table 3).

4.3. Paleoredox

Paleoredox has a significant influence on the preservation of organic matter. The redox conditions control the content of some major and trace elements, such as V, U, Ni, and Th (Table 4). The Th/U ratio and the V/(V + Ni) ratio of the mudstones are 1.54~9.94 and 0.40~0.81, respectively, indicating the oxygen-depleted and anoxia environment (Figure 5a,b). However, the V/Cr ratio and the Ni/Co ratio are 0.47~1.96 and 1.15~3.98, respectively, indicating the oxygen-enriched environment (Figure 5c,d, Table 3). Combined with the characteristic of black and gray-black mudstone in core and geological samples, the mudstones of the Meitan Formation are mainly deposited in an oxygen-poor and anoxic environment.

4.4. Paleoproductivity

Paleoproductivity refers to the amount of organic carbon produced per time unit and volume unit of ancient marine organisms and is one of the controlling factors for source rocks [19,20]. The paleoproductivity can be qualitatively evaluated by major element P and trace elements Ba and Zn, reflecting the marine paleoproductivity [21,22]. However, as terrestrial debris enters the ocean, it will cause deviations in the element content. Therefore, the element Ti is used to remove the influence of sedimentary organic matter and authigenic minerals (Equation (2)) [23,24].

Xxs = X_total − Ti_total × (X/Ti)PAAS

(2)

where Xxs represents the corrected content of the element, X_total represents the measured content of the element from samples, Ti_total represents the measured content of Ti from samples, and (X/Ti)PAAS is a constant, representing the ratio of the average content of X and Ti in the Neoarchean Australian shale. The (Ba/Ti)PAAS and (Zn/Ti)PAAS are 0.1083 and 0.0083, respectively [25]. The corrected Xxs value is regular, indicating that the element in the sample is marine autogenic enrichment relative to PAAS, and negatively indicating that the element content is mainly contributed by terrestrial deposition [26]. The Baxs value of the mudstones in the Meitan Formation is between 8.0 and 13,053.5 ppm, with an average of 1227.4 ppm. The Znxs value is 4.8~363.3 ppm, with an average of 4233.7 ppm (Figure 6a,b, Table 3). The corrected Baxs and Znxs values indicate that the Meitan Formation has the characteristics of marine authigenic enrichment, and the higher element content indicated the higher productivity during the deposition period.

Moreover, the P/Ti ratio can also indicate paleoproductivity. P/Ti < 0.34 indicates low productivity, 0.34 < P/Ti < 0.79 indicates medium productivity, and P/Ti > 0.79 indicates high productivity. The P/Ti ratio of the mudstones in the Meitan Formation is between 0.09 and 0.52, with an average of 0.28, indicating that the Meitan Formation has low–medium productivity (Figure 6c, Table 3).

Combining the P/Ti ratio, Baxs, and Znxs, the mudstones of the Meitan Formation have low–medium paleoproductivity.

5. Discussion

5.1. Polysolution of the Paleoredox

The restoration of paleoredox is the most important part of the paleoenvironment. The Th/U, Ni/Co, V/(V + Ni), and V/Cr ratios are the traditional redox indexes, in which Th/U and V/(V+Ni) of the Meitan Formation indicate the oxygen-depleted and anoxic environment, while V/Cr and Ni/Co indicate the oxygen-enriched environment.

However, researchers have realized the limitations of element ratios to evaluate the paleoredox [27,28,29]. The threshold values corresponding to the same index are not uniform. For example, the threshold of U/Th to evaluate the paleoredox proposed by Jones and Manning [12] and Wignall and Hallam [17] are quite different. This is because the geological conditions of different research areas, such as geological ages, provenance, and paleoactivities, are quite different, and these thresholds do not have global applicability. Except for the redox environment, the organic matter types, depositional rate, and late diagenesis may affect the enrichment of trace elements. Thus, these indexes have multiple conclusions for the determination of redox. Therefore, trace element discrimination can only be used as a reference in the recovery of paleoredox, but also combined with the actual lithology and color of samples. In 50 mudstone samples, 90% of the samples are dark gray and black mudstones, indicating the oxygen-depleted and anoxic environments. Combined with the sample color and trace element ratios, the mudstones of the Meitan Formation were deposited under an oxygen-depleted and anoxic environment.

5.2. Control of Paleoenvironment on Source Rock Development

Paleoproductivity and preservation conditions are the main factors controlling the enrichment of organic matters [30,31,32].

Two shaly horizons at the Upper Ordovician Fjäcka and Mossen Formations are rich in organic matters and are one of the main source rocks in the central part of the Baltic Basin, Southwestern Lithuania. They were recognized as being formed in oxygen-depleted benthic settings, as indicated by their high TOC, with the average value of 3.28%. The source rocks were deposited in marine, non-carbonate settings and the paleoproductivity was high [33]. Ordovician black mudstones are the fair source rocks in the Paleozoic petroleum system in Iraq, and their average TOC is 0.9%. The source rocks were developed in a marine environment, the paleoredox indicators prove the oxygen-depleted and oxygen-enriched conditions, and the CIA is low, with the value of 52%~58% [34]. Dark mudstones of the Ordovician Tanjianshan Formation of the Qaidam Basin, China, are high-quality source rocks with a high hydrocarbon-production potential. The average TOC of the source rocks is 1.75%, the organic matter has reached a mature–overmature stage, and the kerogen was mainly typeⅡ₁. The major and trace elements indicate that the source rocks were deposited in a paleoenvironment with restricted water and paleoredox conditions, which played a significant role in the organic matter enrichment [35]. Compared with the paleoenvironment and the quality of the Ordovician source rocks in the above three basins, the quality of the Meitan Formation can be evaluated. The mudstones of the Meitan Formation were deposited in an oxygen-depleted and anoxic environment of brackish water. The retention environment of the anoxic bottom water and the slow deposition rate were beneficial to the preservation of organic matter in sedimentary rocks [36,37]. Meantime, a warm and humid climate was conducive to the reproduction of organisms during the depositional period of the Meitan Formation, with low–medium paleoproductivity. In general, the paleoenvironment is conducive to the development of poor and moderate source rocks.

The TOC content of the mudstone in the Meitan Formation is 0.03%~2.34%, with an average of 0.43% (Table 5 and Table S1). The source rocks only developed in the northern, western, and southern blocks, and the TOC content is 0.40~2.34%, with an average of 0.78%. According to the classification standard for Paleozoic marine source rocks [38], the evaluations of the source rocks of the northern and southern blocks are poor and moderate, and the ones in the western block are poor (Figure 1), which is consistent with the paleoenvironmental restoration. The kerogen carbon isotope of the Meitan Formation is between –31.99‰ and –26.6‰, with an average of –29.32‰, which has the characteristics of type II₁ kerogen and the potential of generating oil and gas (Figure 7).

In summary, the source rocks of the Meitan Formation are only developed in three blocks and have hydrocarbon generation potential. Therefore, the source rocks and adjacent reservoirs of the Meitan Formation can be the concurrent layers during the oil and gas exploration.

6. Conclusions

(1) The mudstones of the Meitan Formation were deposited in an oxygen-depleted and anoxic environment of brackish water. The paleoclimate is warm and humid, and the paleoproductivity is low to medium, which is beneficial to the development of poor and moderate source rocks.

(2) Controlled by the paleoenvironment, the kerogen type of the source rocks in the Meitan Formation is mainly type II₁. Poor and moderate source rocks were developed in the northern and southern blocks, and only poor source rocks were developed in the western block.

(3) In general, the source rocks of the Meitan Formation have hydrocarbon generation potential, and oil and gas can migrate and accumulate in the upper reservoirs. The Meitan Formation and its adjacent reservoirs can be de concurrent layers during the petroleum exploration in the Ordovician. In the next Ordovician petroleum exploration, gas and source rock correlation in the upper Baota Formation can be considered to reveal whether there is a contribution of the Meitan Formation. In addition, for the source rocks deposited in a similar paleoenvironment, the possibility of small-scaled oil and gas discovery can be considered.

Supplementary Materials

The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/min12010075/s1, Table S1: Total organic carbon content (TOC) of mudstone in the Meitan Formation.

Author Contributions

Conceptualization, M.Y., Y.Z. and X.F.; methodology, M.Y. and Y.Z.; experiment, M.Y. and L.Q.; writing—original draft, M.Y.; writing—review and editing, Y.Z., X.F. and W.L.; funding acquisition, X.F. W.L. and J.Z. (Jianyong Zhang); language and grammar revision, Z.Z. and J.Z. (Jiazhen Zhang). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Key Technology Projects of PetroChina Natural Gas Co., Ltd. (No. 2018A-0105) and the National Natural Science Foundation of China (No. 41772248).

Data Availability Statement

Data are contained in the tables of the article.

Acknowledgments

The authors thank the PetroChina Hangzhou Research Institute of Geology for their strong support in data use and funding. The authors also thank the reviewers for their constructive suggestions on the modification and improvement of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Petroliferous basins of China and position of the Sichuan Basin and its adjacent areas; (b) Structural division and TOC of the Sichuan Basin and its adjacent areas. TOC = Total organic carbon; Jun = Junnggar Basin; YE = Yingen-Ejinaqi Basin; EL = Erlian Basin; HL = Hailaer Basin; SL = Songliao Basin; QD = Qaidam Basin; BH = Bohai Bay Basin; SC = Sichuan Basin.

Figure 2. Stratigraphic column map of the Ordovician of the Sichuan Basin and its adjacent areas.

Figure 3. Paleoclimate discrimination of the mudstones in the Meitan Formation. TOC = Total organic carbon; CIA = Chemical index alteration. (a) relationship between CIA and TOC; (b) relationship between Sr/Cu and TOC.

Figure 4. Paleosalinity discrimination of the mudstones in the Meitan Formation. TOC = Total organic carbon.

Figure 5. Paleoredox discrimination of the mudstones in the Meitan Formation. TOC = Total organic carbon. (a) relationship between Th/U and TOC; (b) relationship between V/(V +Ni) and TOC (c): relationshio between V/Cr and TOC (d): relationship between Ni/Co and TOC.

Figure 6. Paleoproductivity discrimination of the mudstones in the Meitan Formation. TOC = Total organic carbon. (a) relationship between Baxs and TOC; (b) relationship between Znxs and TOC; (c) relationship between Pi/Ti and TOC.

Figure 7. Kerogen types of source rocks in the Meitan Formation (Kerogen type classification is based on [39]).

Table 1. Major elements of mudstones in the Meitan Formation.

Well and Section No.	Lithology	Na (%)	Mg (%)	Al (%)	Si (%)	Mn (%)	K (%)	Ca (%)	Ti (%)	P (%)	Fe (%)
JT1	Gray calcareous mudstone	0.28	1.18	6.99	16.48	0.05	3.68	14.46	0.28	0.05	3.68
JT1	Gray calcareous mudstone	0.34	1.01	5.78	14.93	0.03	2.94	17.95	0.24	0.05	2.93
JT1	Gray calcareous mudstone	0.36	0.97	7.83	19.3	0.02	4	9.25	0.28	0.06	4.62
JT1	Gray calcareous mudstone	0.43	1.34	9.12	23.26	0.08	4.59	4.18	0.42	0.1	4.68
JT1	Gray calcareous mudstone	1.37	0.3	10.66	22.53	0.02	4.6	4.26	0.33	0.06	4.42
WK1	Grayish black mudstone	0.73	1.54	10.84	24.94	0.06	4.29	0.93	0.43	0.06	5.96
WK1	Deep gray mudstone	0.67	1.58	11.33	25.16	0.05	4.39	0.56	0.44	0.06	5.75
WK1	Deep gray sandy mudstone	0.63	1.62	11.41	25.06	0.05	4.17	0.51	0.45	0.05	6.27
WK1	Deep gray mudstone	0.65	1.58	11.35	25.26	0.05	4.20	0.49	0.44	0.06	6.00
WK1	Grayish black mudstone	0.72	1.57	10.68	25.61	0.05	3.89	0.87	0.42	0.07	6.08
QJ	Deep gray mudstone	0.67	1.66	9.06	25.38	0.50	3.52	2.35	0.45	0.11	4.96
QJ	Black mudstone	0.63	1.61	9.39	25.08	0.50	3.70	2.15	0.46	0.11	5.07
QJ	Shallow black mudstone	0.64	1.39	10.18	25.86	0.32	3.87	0.60	0.51	0.13	5.30
QJ	Dark black mudstone	0.41	1.43	11.39	24.40	0.07	4.52	0.40	0.45	0.05	5.40
QJ	Deep gray mudstone	0.46	1.36	11.00	25.54	0.08	4.35	0.49	0.48	0.07	4.73
QJ	Deep gray mudstone	0.48	1.38	11.56	25.13	0.06	4.63	0.48	0.54	0.07	4.23
QJ	Deep gray mudstone	0.71	1.26	10.79	26.24	0.04	4.49	0.51	0.50	0.10	3.98
QJ	Grayish black mudstone	0.92	1.37	9.85	27.04	0.10	4.05	0.64	0.59	0.14	4.62
QJ	Grayish black mudstone	0.43	1.46	11.80	24.41	0.05	5.28	0.41	0.47	0.06	4.43
WXT	Dark black mudstone	0.56	1.62	8.73	25.73	0.03	3.29	3.17	0.44	0.05	4.06
WXT	Dark black mudstone	0.56	1.61	8.74	25.57	0.02	3.30	3.04	0.44	0.05	4.20
WXT	Dark black mudstone	0.57	1.63	8.66	25.27	0.03	3.22	3.54	0.44	0.05	4.32
WXT	Dark black mudstone	0.48	1.70	8.65	24.25	0.03	3.34	4.52	0.44	0.05	4.14
WXT	Dark black mudstone	0.42	1.84	9.42	25.59	0.02	3.66	1.81	0.43	0.04	4.41
WXT	Dark black mudstone	0.37	1.84	9.47	25.76	0.03	3.60	1.04	0.44	0.04	4.51
WXT	Dark black mudstone	0.41	1.71	8.16	23.17	0.04	3.14	6.28	0.39	0.04	3.92
WXT	Dark black mudstone	0.40	1.75	8.89	24.38	0.03	3.46	4.04	0.39	0.04	3.95
WXT	Dark black mudstone	0.39	1.76	8.95	24.44	0.03	3.50	4.01	0.41	0.04	4.11
WXT	Dark black mudstone	0.43	1.78	9.06	24.67	0.03	3.46	3.09	0.42	0.04	4.18
PD	Yellow-green mudstone	0.34	1.22	10.51	24.71	0.02	3.59	0.38	0.43	0.06	6.08
PD	Yellow-green mudstone	0.38	1.34	11.44	24.44	0.01	4.28	0.40	0.46	0.05	5.95
PD	Gray-green mudstone	0.23	1.22	11.48	24.34	0.04	4.62	0.47	0.46	0.06	5.61
PD	Yellow-green mudstone	0.32	1.26	11.56	24.75	0.05	4.51	0.51	0.48	0.05	4.70
BQ	Deep gray mudstone	0.24	1.42	11.51	25.57	0.01	4.89	0.26	0.46	0.06	6.11
BQ	Deep gray mudstone	0.16	1.38	11.34	25.8	0.02	4.66	0.08	0.48	0.06	6.16
BQ	Deep gray mudstone	0.6	1.68	10.81	26.08	0.03	4.24	0.78	0.47	0.08	5.47
BQ	Deep gray mudstone	1.56	0.79	6.1	23.47	0.08	2.14	11.17	0.36	0.06	2.36
BQ	Deep gray mudstone	1.23	1.63	11	27.32	0.01	4.57	0.47	0.53	0.09	4.01
BQ	Deep gray mudstone	0.73	2.18	11.12	24.39	0.02	4.62	0.34	0.5	0.06	7.14
BQ	Deep gray mudstone	0.44	2.16	12.4	23.59	0.02	5.67	0.29	0.5	0.05	5.71
BQ	Deep gray mudstone	1.57	0.92	7.09	27.65	0.07	2.79	5.57	0.32	0.12	3.12
BQ	Deep gray mudstone	0.81	1.66	10.56	25.83	0.02	4.86	0.5	0.44	0.08	5.91
BQ	Deep gray mudstone	0.83	1.6	9.86	25.8	0.08	4.42	2.01	0.48	0.09	5.07
BQ	Deep gray mudstone	0.52	0.61	2.83	10.96	0.23	1.16	24.33	0.14	0.76	4.03
BQ	Deep gray mudstone	0.82	1.81	9.81	27.38	0.02	4.59	0.67	0.52	0.08	4.63
BQ	Deep gray mudstone	1.24	1.02	7.6	28.49	0.06	3.86	4	0.56	0.21	2.89
BQ	Deep gray mudstone	1.1	1.58	9.61	27.7	0.02	4.59	0.78	0.61	0.22	4.18
BQ	Deep gray mudstone	0.94	1.65	9.35	26.58	0.04	4.53	2.03	0.59	0.24	3.91
BQ	Deep gray mudstone	1.18	1.38	7.65	26.55	0.09	3.56	4.51	0.62	0.32	3.85
BQ	Deep gray mudstone	0.5	0.8	3.64	14.94	0.08	1.6	21.98	0.19	0.03	2.02

Table 2. Trace elements of mudstones in the Meitan Formation.

Well and Section No.	Lithology	Li	Be	V	Co	Ni	Cu	Zn	Rb	Sr	Mo	Ba	Pb	Th	U	Cr
Well and Section No.	Lithology	ppm
JT1	Gray calcareous mudstone	36.9	2.24	93	13.8	33.2	30.2	53.2	162	460	1.56	5908	17.7	15.4	3.44	102
JT1	Gray calcareous mudstone	27.2	1.65	73.7	10.7	25.3	24.9	52.9	123	506	3.65	4909	15.5	13.5	3.91	100
JT1	Gray calcareous mudstone	41.9	2.31	108	15.2	42.7	37.3	70.7	173	451	3.26	12945	31.8	16.5	3.28	126
JT1	Gray calcareous mudstone	44.8	2.92	114	17.4	41.4	33.1	105	194	290	2.36	6411	30.4	20	3.43	129
JT1	Gray calcareous mudstone	35.6	3.04	129	17.9	47.6	48	55.9	173	312	9.26	13411	40.3	19.7	3.81	155
WK1	Grayish black mudstone	66.6	4.35	127	27	46.4	34.5	81	220	121.5	0.42	970	22	22.3	2.69	130
WK1	Deep gray mudstone	71.7	4.13	143	19	47.6	40.7	103	219	101.5	0.37	971	30.4	20.2	2.65	180
WK1	Deep gray sandy mudstone	83.3	3.82	147	19.8	47	33.8	98	204	114	0.27	974	51.2	20.3	2.91	140
WK1	Deep gray mudstone	81.9	4.17	147	17.9	48	41.6	113	209	93.3	0.21	933	10.7	20.3	2.53	140
WK1	Grayish black mudstone	77.5	3.92	130	23.5	46.5	36.2	107	193	146.5	0.36	895	15.1	21	2.61	130
QJ	Deep gray mudstone	55.6	4.51	111	18.6	43.3	34.3	115	206	157	1.28	631	19	17.6	2.62	97.6
QJ	Black mudstone	54.6	4.43	116	18.2	40.4	31.7	117	215	127	1.05	652	25.7	19.5	2.64	103
QJ	Shallow black mudstone	53.6	4.3	123	18.7	45	29.5	94.9	209	102	1.45	1017	18.9	18	2.45	115
QJ	Dark black mudstone	49.9	4.16	134	15.3	45.4	39.3	83	249	94.2	0.9	854	19.9	18.8	2.51	106
QJ	Deep gray mudstone	47	4.33	130	17.5	48.4	38	71.4	241	101	0.91	906	19	18.8	2.45	113
QJ	Deep gray mudstone	39.8	4.65	146	12.8	44.2	35.8	75.9	239	88.6	0.98	1109	12.5	20.2	2.64	130
QJ	Deep gray mudstone	39.2	4.28	124	14	41.6	37.2	71.3	222	109	2.49	1272	19.9	19.9	3.1	113
QJ	Grayish black mudstone	56.1	4.1	127	16.7	39.6	33.5	86.6	228	147	1.55	1342	18	21.8	3.49	111
QJ	Grayish black mudstone	41.4	4.31	147	12.3	48.6	47.7	84.8	252	73.1	1.76	1392	12.7	18.6	2.58	110
WXT	Dark black mudstone	40.4	2.74	150	15.6	42.5	42.6	95.3	209	147	3.42	793	27.5	18.8	3.3	92.9
WXT	Dark black mudstone	41.4	2.69	155	13.2	47.2	46.1	100	216	147	3.86	782	28.2	18.3	3.48	90.6
WXT	Dark black mudstone	41.6	2.82	147	18.4	49.9	45.4	103	210	167	4.56	783	28.1	18.8	3.61	97.6
WXT	Dark black mudstone	40.4	2.71	132	16.1	39.5	36.7	92.5	206	158	3	790	26.7	17.5	3.5	83
WXT	Dark black mudstone	46.9	3.03	181	14.1	53.6	43.7	120	236	96.1	2.11	992	20.1	17.7	3.39	92.6
WXT	Dark black mudstone	47.1	3.15	204	14.3	56.9	44.1	129	234	92	1.8	886	26.7	18.8	3.93	104
WXT	Dark black mudstone	39.6	2.47	131	15.6	47.5	37.5	105	207	231	3.07	1103	20.8	14.4	3.51	82.1
WXT	Dark black mudstone	40	2.52	144	14.1	44.8	35.2	101	218	134	2.8	989	21.6	16.3	3.49	90.1
WXT	Dark black mudstone	44.3	2.78	162	17	47.3	37.4	112	234	155	2.73	1191	23.7	16.2	3.33	91
WXT	Dark black mudstone	43.5	2.65	135	16	43.2	33.4	108	240	152	1.37	2236	19	16.6	3.15	86.8
PD	Yellow-green mudstone	53.2	2.92	130	14.6	37.7	50.1	61.2	215	67.2	0.84	380	8.14	12.8	1.58	90
PD	Yellow-green mudstone	39.4	2.46	131	15	37.7	30.5	65.4	213	68.4	0.84	356	9.17	12.9	1.53	94.1
PD	Gray-green mudstone	43.9	4.16	131	26.9	49.6	39.4	124	188	95.8	0.87	658	17.1	14.4	1.66	115
PD	Yellow-green mudstone	47	3.27	131	14.9	42.4	28.4	98.1	236	109	0.46	645	10	15	1.74	111
PD	Deep gray mudstone	39.5	3.54	142	19.6	42.2	42.7	99.4	267	91.4	<0.20	704	5.9	16.8	1.69	120
PD	Deep gray mudstone	39.2	3.55	155	15.4	35.2	38.9	94.9	247	93.6	0.45	670	13.6	17.3	2.22	120
BQ	Deep gray mudstone	36.2	4.37	125	13.5	44.8	27.1	91.1	207	52.3	0.52	317	7.06	14.4	1.96	124
BQ	Deep gray mudstone	35.9	4.51	124	15.2	44.8	37.4	102	250	51.2	0.5	389	6.39	18.3	2.35	119
BQ	Deep gray mudstone	57.1	4	117	17.6	38.2	27.8	74.1	184	73.1	0.53	368	21	17.5	2.75	114
BQ	Deep gray mudstone	30.2	1.67	56.1	8.6	20	16.5	65.4	102	212	2.22	398	12.1	16	3.46	95.1
BQ	Deep gray mudstone	58.8	4.14	106	16.7	43.8	27.1	58.1	214	94.8	0.5	655	17.9	15.2	3.55	108
BQ	Deep gray mudstone	79.8	3.76	123	23.7	53.5	38	62.5	233	88.8	0.75	554	24.4	18.7	3.46	120
BQ	Deep gray mudstone	61.7	5.75	158	16.9	46.7	30.5	62.3	293	74.7	0.33	575	7.2	19	3.17	138
BQ	Deep gray mudstone	36.9	1.68	49.8	8.8	24.5	46.2	65.1	129	201	0.61	555	16.6	15.5	2.71	105
BQ	Deep gray mudstone	56.3	3.86	117	23.8	46.1	77.7	400	246	114	0.92	628	18.5	20.3	3.52	122
BQ	Deep gray mudstone	57.8	3.82	125	17.7	34.7	22.7	55.8	222	145	0.77	599	14.9	20.7	3.15	135
BQ	Deep gray mudstone	25.8	1.01	32.3	41.4	47.8	41.7	37.4	60.9	579	5.7	264	35.2	7.78	5.05	43.5
BQ	Deep gray mudstone	59.1	3.72	120	15.7	34	30.9	49.7	230	96.2	0.76	678	18.8	20.6	3.42	107
BQ	Deep gray mudstone	33.7	2.41	67.4	11.3	21.6	25.1	36.8	162	199	1.34	714	22.1	36.6	5.51	114
BQ	Deep gray mudstone	52.5	3.4	103	15.9	35.7	29.4	55.6	211	111	0.97	793	31.1	21.9	5.12	147
BQ	Deep gray mudstone	52.3	3.98	103	15.2	30.3	29	54.7	218	128	0.84	663	26.2	24.4	5.05	145
BQ	Deep gray mudstone	49.4	2.57	95.8	16.5	32.6	33.3	50.5	151	200	0.87	619	33.2	26.8	5.09	169
BQ	Deep gray mudstone	24.7	1.21	36.9	6.43	10.4	13.3	26.6	81.3	322	0.8	315	7.66	10.6	1.95	44.5

Table 3. Paleoenvironmental indicators of shale in the Meitan Formation.

Well and Section No.	Lithology	Paleoclimate		Paleosalinity	Paleoredox				Paleoproductivity
Well and Section No.	Lithology	CIA (%)	Sr/Cu	Rb/K	Th/U	V/Cr	V/(V + Ni)	Ni/Co	P/Ti	Baxs	Znxs
JT1	Gray calcareous mudstone	34.53	15.23	0.0044	4.48	0.91	0.74	2.41	0.18	5604.67	29.87
JT1	Gray calcareous mudstone	27.28	20.32	0.0042	3.45	0.74	0.74	2.36	0.21	4649.00	32.90
JT1	Gray calcareous mudstone	44.78	12.09	0.0043	5.03	0.86	0.72	2.81	0.21	12,641.67	47.37
JT1	Gray calcareous mudstone	59.04	8.76	0.0042	5.83	0.88	0.73	2.38	0.24	5956.00	70.00
JT1	Gray calcareous mudstone	60.14	6.50	0.0038	5.17	0.83	0.73	2.66	0.18	13,053.50	28.40
WK1	Grayish black mudstone	73.32	3.52	0.0051	8.29	0.98	0.73	1.72	0.14	504.17	45.17
WK1	Deep gray mudstone	75.43	2.49	0.0050	7.62	0.79	0.75	2.51	0.14	494.33	66.33
WK1	Deep gray sandy mudstone	76.60	3.37	0.0049	6.98	1.05	0.76	2.37	0.11	486.50	60.50
WK1	Deep gray mudstone	76.41	2.24	0.0050	8.02	1.05	0.75	2.68	0.14	456.33	76.33
WK1	Grayish black mudstone	74.59	4.05	0.0050	8.05	1.00	0.74	1.98	0.17	440.00	72.00
QJ	Deep gray mudstone	67.00	4.58	0.0059	6.72	1.14	0.72	2.33	0.24	143.50	77.50
QJ	Black mudstone	68.09	4.01	0.0058	7.39	1.13	0.74	2.22	0.24	153.67	78.67
QJ	Shallow black mudstone	75.14	3.46	0.0054	7.35	1.07	0.73	2.41	0.25	464.50	52.40
QJ	Dark black mudstone	76.65	2.40	0.0055	7.49	1.26	0.75	2.97	0.11	366.50	45.50
QJ	Deep gray mudstone	76.05	2.66	0.0055	7.67	1.15	0.73	2.77	0.15	386.00	31.40
QJ	Deep gray mudstone	76.00	2.47	0.0052	7.65	1.12	0.77	3.45	0.13	524.00	30.90
QJ	Deep gray mudstone	74.23	2.93	0.0049	6.42	1.10	0.75	2.97	0.20	730.33	29.63
QJ	Grayish black mudstone	72.63	4.39	0.0056	6.25	1.14	0.76	2.37	0.24	702.83	37.43
QJ	Grayish black mudstone	74.80	1.53	0.0048	7.21	1.34	0.75	3.95	0.13	882.83	45.63
WXT	Dark black mudstone	64.31	3.45	0.0064	5.70	1.61	0.78	2.72	0.11	316.33	58.63
WXT	Dark black mudstone	64.77	3.19	0.0065	5.26	1.71	0.77	3.58	0.11	305.33	63.33
WXT	Dark black mudstone	63.02	3.68	0.0065	5.21	1.51	0.75	2.71	0.11	306.33	66.33
WXT	Dark black mudstone	59.78	4.31	0.0062	5.00	1.59	0.77	2.45	0.11	313.33	55.83
WXT	Dark black mudstone	70.33	2.20	0.0064	5.22	1.95	0.77	3.80	0.09	526.17	84.17
WXT	Dark black mudstone	73.99	2.09	0.0065	4.78	1.96	0.78	3.98	0.09	409.33	92.33
WXT	Dark black mudstone	54.02	6.16	0.0066	4.10	1.60	0.73	3.04	0.10	680.50	72.50
WXT	Dark black mudstone	61.85	3.81	0.0063	4.67	1.60	0.76	3.18	0.10	566.50	68.50
WXT	Dark black mudstone	62.03	4.14	0.0067	4.86	1.78	0.77	2.78	0.10	746.83	77.83
WXT	Dark black mudstone	65.36	4.55	0.0069	5.27	1.56	0.76	2.70	0.10	1781.00	73.00
PD	Yellow-green mudstone	78.89	2.43	0.0026	8.67	1.14	0.73	1.84	0.14	203.00	89.00
PD	Yellow-green mudstone	77.63	3.84	0.0055	8.62	1.18	0.76	2.85	0.11	146.67	59.77
PD	Gray-green mudstone	76.85	2.14	0.0058	9.94	1.18	0.77	2.15	0.13	205.67	61.07
PD	Yellow-green mudstone	76.86	2.41	0.0055	7.79	1.29	0.81	2.29	0.10	150.00	54.90
BQ	Deep gray mudstone	76.78	1.93	0.0042	7.35	1.01	0.74	3.32	0.13	–181.33	52.77
BQ	Deep gray mudstone	78.29	1.37	0.0054	7.79	1.04	0.73	2.95	0.13	–131.00	62.00
BQ	Deep gray mudstone	74.46	2.63	0.0043	6.36	1.03	0.75	2.17	0.17	–141.17	34.93
BQ	Deep gray mudstone	36.21	12.85	0.0048	4.62	0.59	0.74	2.33	0.17	8.00	35.40
BQ	Deep gray mudstone	72.66	3.50	0.0047	4.28	0.98	0.71	2.62	0.17	80.83	13.93
BQ	Deep gray mudstone	74.94	2.34	0.0050	5.40	1.03	0.70	2.26	0.12	12.33	20.83
BQ	Deep gray mudstone	74.95	2.45	0.0052	5.99	1.14	0.77	2.76	0.10	33.33	20.63
BQ	Deep gray mudstone	50.24	4.35	0.0046	5.72	0.47	0.67	2.78	0.38	208.33	38.43
BQ	Deep gray mudstone	72.30	1.47	0.0051	5.77	0.96	0.72	1.94	0.18	151.33	363.33
BQ	Deep gray mudstone	66.81	6.39	0.0050	6.57	0.93	0.78	1.96	0.19	79.00	15.80
BQ	Deep gray mudstone	12.89	13.88	0.0053	1.54	0.74	0.40	1.15	5.43	112.33	25.73
BQ	Deep gray mudstone	71.00	3.11	0.0050	6.02	1.12	0.78	2.17	0.15	114.67	6.37
BQ	Deep gray mudstone	54.65	7.93	0.0042	6.64	0.59	0.76	1.91	0.38	107.33	–9.87
BQ	Deep gray mudstone	69.14	3.78	0.0046	4.28	0.70	0.74	2.25	0.36	132.17	4.77
BQ	Deep gray mudstone	64.88	4.41	0.0048	4.83	0.71	0.77	1.99	0.41	23.83	5.53
BQ	Deep gray mudstone	54.25	6.01	0.0042	5.27	0.57	0.75	1.98	0.52	–52.67	–1.17
BQ	Deep gray mudstone	17.09	24.21	0.0051	5.44	0.83	0.78	1.62	0.16	109.17	10.77

Table 4. Paleoredox evaluation index.

Index	Oxygen-Enriched	Oxygen-Depleted	Anoxia	References
Th/U	>7.0	2.0~7.0	<2	[17]
Ni/Co	<5.0	5.0~7.0	>7.0	[12]
V/(V + Ni)	<0.45	0.45~0.60	>0.60	[18]
V/Cr	<2.0	2.0~4.25	>4.25	[12]

Table 5. Total organic carbon content of mudstone in the Meitan Formation.

Well No.	TOC (%)	Well No.	TOC (%)	Well No.	TOC (%)	Well No.	TOC (%)	Well No.	TOC (%)
JT1	$\frac{0.27 - 0.51}{0.35 (14)}$	LL1	$\frac{0.10 - 0.61}{0.29 (9)}$	Z3	$\frac{0.15}{0.15 (1)}$	YS1	$\frac{0.03 - 0.09}{0.06 (9)}$	QJ	$\frac{0.14 - 0.31}{0.22 (22)}$
MS1	$\frac{0.41 - 2.34}{1.07 (59)}$	WK1	$\frac{0.04 - 0.20}{0.14 (9)}$	C7	$\frac{0.12 - 0.46}{0.21 (6)}$	YS2	$\frac{0.03 - 0.25}{0.10 (37)}$	WXT	$\frac{0.51 - 0.88}{0.66 (20)}$
TT1	$\frac{0.13 - 0.63}{0.28 (33)}$	L32	$\frac{0.07 - 0.48}{0.22 (11)}$	W2	$\frac{0.13 - 0.84}{0.30 (16)}$	GT2	$\frac{0.30 - 0.74}{0.47 (8)}$	PD	$\frac{0.049 - 0.17}{0.08 (15)}$
LS1	$\frac{0.23 - 0.49}{0.41 (7)}$	LT1	$\frac{0.29 - 0.60}{0.45 (8)}$	W15	$\frac{0.14 - 1.04}{0.40 (19)}$	HS1	$\frac{0.22 - 1.59}{0.49 (18)}$	LZY	$\frac{0.08 - 0.37}{0.18 (12)}$
ZS1	$\frac{0.21 - 0.69}{0.36 (9)}$	WS1	$\frac{0.11 - 2.29}{0.36 (14)}$	WH103	$\frac{0.08 - 0.42}{0.21 (19)}$	NC1	$\frac{0.31 - 0.74}{0.50 (36)}$	SQ	$\frac{0.17 - 0.84}{0.50 (16)}$
A8	$\frac{0.16 - 0.28}{0.21 (6)}$	G2	$\frac{0.18 - 0.61}{0.37 (11)}$	WH104	$\frac{0.08 - 0.25}{0.15 (20)}$	HT	$\frac{0.17 - 0.87}{0.51 (13)}$	HHY	$\frac{0.76 - 1.54}{1.17 (9)}$

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Yang, M.; Zuo, Y.; Fu, X.; Qiu, L.; Li, W.; Zhang, J.; Zheng, Z.; Zhang, J. Paleoenvironment of the Lower Ordovician Meitan Formation in the Sichuan Basin and Adjacent Areas, China. Minerals 2022, 12, 75. https://doi.org/10.3390/min12010075

AMA Style

Yang M, Zuo Y, Fu X, Qiu L, Li W, Zhang J, Zheng Z, Zhang J. Paleoenvironment of the Lower Ordovician Meitan Formation in the Sichuan Basin and Adjacent Areas, China. Minerals. 2022; 12(1):75. https://doi.org/10.3390/min12010075

Chicago/Turabian Style

Yang, Meihua, Yinhui Zuo, Xiaodong Fu, Lei Qiu, Wenzheng Li, Jianyong Zhang, Ziyun Zheng, and Jiazhen Zhang. 2022. "Paleoenvironment of the Lower Ordovician Meitan Formation in the Sichuan Basin and Adjacent Areas, China" Minerals 12, no. 1: 75. https://doi.org/10.3390/min12010075

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Paleoenvironment of the Lower Ordovician Meitan Formation in the Sichuan Basin and Adjacent Areas, China

Abstract

1. Introduction

2. Geological Settings

3. Materials and Methods

4. Paleoenvironment and Paleoproductivity

4.1. Paleoclimate

4.2. Paleosalinity

4.3. Paleoredox

4.4. Paleoproductivity

5. Discussion

5.1. Polysolution of the Paleoredox

5.2. Control of Paleoenvironment on Source Rock Development

6. Conclusions

Supplementary Materials

Author Contributions

Funding

Data Availability Statement

Acknowledgments

Conflicts of Interest

References

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