J. Cent. South Univ. (2020) 27: 3779-3792

DOI: https://doi.org/10.1007/s11771-020-4507-7

Sedimentary facies characteristics and organic matter enrichment mechanism of lower Cambrian Niutitang Formation in South China

QIN Ming-yang(秦明阳)^{1, 2, 3}, GUO Jian-hua(郭建华)³, TAN Hui(谭慧)³, WU Shi-qing(吴诗情)³, BIAN Rui-kang(边瑞康)¹

1. State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China;

2. Editorial Office of Journal of Central South University (Science and Technology), Central South University, Changsha 410083, China;

3. School of Geosciences and Info-Physics Engineering, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: The purpose of this study was to examine the sedimentary facies characteristics of lower Cambrian Niutitang Formation (∈₁n) in South China, to reveal the mechanism of organic matter enrichment, and to guide exploration of shale gas. Macro investigation and experimental analyses were used to assess the lithology in detail, total organic matter mass fraction w(TOC), mineral composition, and trace element characteristics of ∈₁n. The influencing factors of organic matter enrichment were discussed extensively, and a sedimentary facies mode was suggested. In the early stage of ∈₁n, the locations of Well E’yangye 1, Well Ciye 1, Well Changye 1, and Well Anye 1 respectively develop, platform inner sag, outer shelf, Jiangnan slope belt, and South China detention basin. In the late stage of ∈₁n, the sedimentary facies evolve with decreasing sea level. The study area presents a complete three-step basin in the Early Cambrian. In the early stage of ∈₁n, the first step is the Yangtze carbonate platform, the second step is the outer shelf and slope, and the third step is the deep-water basin. From the Yangtze carbonate platform to the deep-water basin, w(TOC) and the mass fraction of quartz gradually increase, the mass fraction of carbonate mineral decreases, and the mass fraction of clay mineral is higher in the second step. The sea level fluctuation results in a higher w(TOC) vertically in the lower ∈₁n shale, and the paleogeographic (provenance) conditions lead to better horizontal development of organic matter in the outer shelf, slope and detention basin. Trace elements are abundant in the lower ∈₁n, and w(TOC) is correlated positively with many trace elements. In the outer shelf, slope, and adjacent areas, hydrothermal activity and upwelling current bring nutrient-rich material and promote organic matter enrichment under a strong reducing condition. Deep-shelf, slope and deep-water basin are the best facies for the formation and preservation of organic matter, especially deep-water basin facies. It remains necessary to strengthen the exploration of shale gas in the deep-water basin of ∈₁n in central Hunan, China.

Key words: Niutitang formation (∈₁n); organic matter; sedimentary facies; enrichment mechanism; hydrothermal activity; upwelling current; exploration target

Cite this article as: QIN Ming-yang, GUO Jian-hua, TAN Hui, WU Shi-qing, BIAN Rui-kang. Sedimentary facies characteristics and organic matter enrichment mechanism of lower Cambrian Niutitang Formation in South China [J]. Journal of Central South University, 2020, 27(12): 3779-3792. DOI: https://doi.org/10.1007/s11771-020-4507-7.

1 Introduction

The exploration and development of shale gas has become an important measure for China to cope with resource shortage and energy security. The “Shale gas revolution” has been greatly successful. The annual output of marine shale gas in South China has reached 100×10⁸ m³[1, 2]. By exploration and theoretical research, scholars put forward the concepts of “two-factor enrichment”[3], “three- element enrichment”[4], “source-cap hydrocarbon- controlling”[5], “shale gas sweet spot”[6], and “five properties in one position”[7]. They emphasized that “favorable sedimentary facies determine the hydrocarbon generation basis and reservoir characteristics of shale gas enrichment”. The main shale systems in the United States formed in foreland basins, but two sets of hot exploration shale formed in the Yangtze craton basin, i.e., the lower Cambrian Niutitang Formation (∈₁n) and the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi formation (O₃w-S₁l)[8-11]. Therefore, the study of the sedimentary facies characteristics and spatial distribution of ∈₁n shale, especially the mechanism and influencing factors of organic matter enrichment, has become indispensable for shale oil and gas exploration and evaluation.

Investigators have studied extensively the sedimentary facies, enrichment mechanism of organic matter and non-ferrous elements (e.g., V, Ni, Mo), and the origin of stone coal and siliceous matter of ∈₁n in South China [12-20]. Regarding sedimentary facies, LIU et al [14] suggested that the lower Cambrian Qiongzhusi Formation (∈₁q) in the Upper Yangtze area belonged to the sedimentary system of shore, shelf, slope and deep-water basin. However, QIU et al [15] proposed that the shale of the lower Cambrian Shuijigntuo Formation (∈₁s) in the middle Yangtze developed mainly the sedimentary system of carbonate gentle slope, shallow shelf, and deep- shelf basin.

For organic matter enrichment, CHEN et al [13] emphasized that “the depression in the platform is the foundation and the area with organic matter accumulation is the favorable facies belt” on the basis of breakthrough of shale gas of Shuijingtuo Formation (∈₁s) in Yichang, Hubei Province, China. XU et al [16], LIU et al [17] and JIA et al [18] pointed out that the organic rich shale of ∈₁n in the Yangtze block may be due to upwelling current, anoxic events, and reductive hydrothermal solution with eutrophic elements along the extensional faults. ZHOU et al [19] and LI et al [20] showed that hydrocarbon-generating organisms of ∈₁n in Northwest Hunan were mainly phytoplankton, benthic macroalgae, zooplankton and benthic sponge.

An industrial shale gas breakthrough was achieved in Yichang, Hubei Province, China, the oldest stratum (∈₁s) in the world, which can serve as a leading role in the exploration of ∈₁n shale gas in southern China [21]. Therefore, it is urgent to study the sedimentary facies characteristics and enrichment mechanism of organic matter of ∈₁n in the middle Yangtze areas. Previous research has been limited by a large scope and few drilling data. These studies have five shortcomings: 1) too few comparisons of drilling data in large-scale horizontal area; 2) lack of attention to systematic research on multiple factors that affect sedimentary facies and organic matter enrichment; 3) only geochemistry data (trace elements or rare earth elements) used to study the enrichment mechanism of organic matter; 4) lack of attention to the key influence of structural sedimentary background [22]; 5) too little study of prototype basin characteristics and sedimentary evolution of the entire Yangtze region.

On the basis of many years of shale gas investigation and research, investigators have studied the typical wells of ∈₁n. By a combination of macroscopic core/outcrop observation and laboratory experiments, geologists compared the development characteristics of organic shale in the study area, analyzed the spatial distribution and evolution of ∈₁n sedimentary facies, and studied the vertical and horizontal change laws of organic matter mass fraction w(TOC). The mechanistic model of organic matter enrichment and future exploration target of shale gas were proposed.

2 Regional geological background

Figure 1 shows the geological background and location map of the study area. As shown in Figure 1, in the Early Cambrian, the breakup of Pangaea, the aggravation of structural subsidence, and the rise of global sea level lead to the overall location of the Yangtze region below sea level [23, 24]. The paleogeographic pattern is high in the north and low in the south. Figure 1(b) shows that the study area is from Yichang, Hubei Province, China, and it covers the entire area of Hunan Province, China. Moving from northwest to southeast, the area belongs to Yangtze platform, Jiangnan slope, South China Basin, and Cathay uplift. Different structural backgrounds determine the sedimentary facies characteristics of the lower Cambrian and further affect the enrichment of organic matter [25]. Niutitang Formation (∈₁n) in Northwest Hunan, Shuijingtuo Formation (∈₁s) in west Hubei, and Qiongzhusi Formation (∈₁q) in the east of Sichuan Province all develop organic shale which are of regional contrast and obviously different from the shale and dolomitic limestone of Sinian Dengying Formation (Z₂d).

Figure 1 Geological background of the study area:

3 Characteristics of sedimentary facies

3.1 Lithology and organic matter richment

Typical wells and outcrops were selected to compare the spatial variation characteristics of ∈₁n lithology at a large scale. Figure 2 and Table 1 show the characteristics of organic-rich shale of the lower Cambrian in the study area. The lower ∈₁s in Well E’yangye 1 is composed of mainly black mudstone with a small amount of limestone and siltstone. The w(TOC) is 1.89%-2.43%, with an average of 2.34%. The upper ∈₁s is composed of mainly light yellow siltstone and light gray argillaceous limestone [13].

The lithology of the lower ∈₁n in Well Ciye 1 is mainly black shale, carbonaceous shale, stone coal and siliceous shale, which is rich in carbonaceous matter, siliceous matter and pyrite. The w(TOC) is 1.18%-5.36%, with an average of 2.87%. The lithology of the upper ∈₁n is mainly black-gray and celadon mudstone, with a layer of celadon argillaceous limestone. The w(TOC) is less than 2.0%, and the mass fraction of pyrite is low [26].

The lithology of ∈₁n in Well Changye 1 is relatively stable. The lower ∈₁n is dominated by black shale and carbonaceous shale, and the w(TOC) of shale with depth of 1100-1250 m is 2.16%-17.60%, with an average of 10.14%. The upper ∈₁n develops stable gray shale and w(TOC) is generally lower than 2.00% [27].

The lower ∈₁n in Well Anye 1 is mainly composed of black siliceous shale and carbonaceous shale, and the organic matter well develops. The w(TOC) of shale with depth of 790-857 m is 4.03%-31.50%, with an average of 15.0%. The upper ∈₁n is composed of mainly black argillaceous shale, and w(TOC) is generally 2.61%-5.85%, with an average of 4.20%. The w(TOC) of shale in the upper ∈₁n is significantly lower than w(TOC) in the lower ∈₁n.

Figure 2 Characteristics of organic-rich shale of lower Cambrian in the study area:

Table 1 Sedimentary characteristics of ∈₁ in typical wells/sections

The lithology of Leijiadong Section in Zixing is mainly gray feldspar quartz, greywacke with thickness of more than 400 m, and a small amount of black carbon shale develops.

3.2 Mineral composition

The regular changes of mineral composition of shale in the study area can reflect the characteristics of sedimentary environment. Figure 3 shows the characteristics of mineral composition change of ∈₁n in the study area. The mass fractions of quartz in Well E’yangye 1, Well Ciye 1, Well Changye 1 and Well Anye 1 are 39.3%, 41.4%, 52.3% and 65.0%, respectively. The mass fractions of clay are 15.5%, 26.4%, 27.9% and 16.9%, respectively. The mass fractions of carbonate are 17.7%, 8.6, 8.7% and 2.7%, respectively. These data indicate that, with the increase in distance from the Yangtze platform, the water body deepens gradually, the mass fraction of carbonate gradually decreases, and the mass fraction of quartz gradually increases. The mass fraction of clay is more than 25.0% in the outer shelf and slope, whereas the mass fraction of clay mineral is just about 16.0% in the platform and detention basin.

Figure 3 Mineral composition characteristics of ∈₁n organic-rich shale

3.3 Sedimentary facies correlation

In the direction perpendicular to the structure, Well E’yangye 1, Well Ciye 1, Well Changye 1, Well Anye 1 and Leijiadong outcrop in Zixing were selected from the northwest to the southeast. A sedimentary facies contrast map (Figure 4) was constructed according to the regional sedimentary background, the characteristics of lithology, w(TOC) and mineral composition.

1) In the early stage of ∈₁n deposition, Well E’yangye 1 is located in the Yangtze platform, adjacent to the continent in the middle Hubei Province. The lithology is mainly black shale, with a small amount of limestone and siltstone shale. The mass fraction of carbonate and clay are 17.7% and 15.5% on average, respectively. The w(TOC) is low with only 2.43% on average, and the vertical development is unstable. It is inferred that the facies of Well E’yangye 1 is platform inner sag. Well Ciye 1 is in a deeper water body, adjacent to Baojing-Cili Fault (F1). The mass fractions of clay and carbonate of organic shale are 26.4% and 8.6% on average, respectively. The w(TOC) increases with an average of 2.87%. Thus, Well Ciye 1 belongs to outer shelf facies. Well Changye 1 is in Jiangnan Slope Belt, where the body of water is deepened further, and the development of carbonaceous shale is stable. The mass fraction of quartz and clay are 52.3% and 27.9% on average, respectively. The w(TOC) is 10.14% on average, and the thickness of organic-rich shale is large, which reflects the characteristic of slope facies. Well Anye 1 is in South China Basin, near Anhua-Yiyang Fault (F2). The mass fractions of quartz is 65.0%, the mass fraction of carbonate is only 2.7%, and the w(TOC) is 15.00% on average, which indicates the characteristic of detention basin facies. The lithology of Xiangnan Formation (∈₁x) in Leijiadong, Zixing is a large set of turbidite greywacke, with a small amount of carbonaceous shale, which reflects the characteristics of turbidite basin facies.

2) In the late stage of ∈₁n deposition, the sedimentary facies in the study area change greatly with a drop in sea level. The lithology of Well E’yangye 1 is mainly siltstone and limestone, and the sedimentary facies gradually evolves from platform inner sag to platform. Mudstone mainly develops in Well Ciye 1 and Well Changye 1, but organic matter poorly develops, which is characteristic of shallow shelf facies. A large amount of shale still develops in Well Anye 1, but w(TOC) is obviously lower than that of lower ∈₁n. Thus the facies of Well Anye 1 are still in detention basin facies, but the water depth is shallower than that of the early stage of ∈₁n deposition. The lithology of Xiangnan Formation in Leijiadong Section, Zixing is relatively stable, and is similar to that of the lower ∈₁n.

4 Factors affecting sedimentary facies and organic matter enrichment

According to the field geological survey, drilling geological results and experimental data, we considered that the factors that affect the sedimentary facies and organic matter enrichment in the study area include mainly sea level change, geographic (provenance) conditions, hydrothermal activities, upwelling current and redox conditions [16, 18, 28-31].

Figure 4 Sedimentary correlation of connected wells/sections of ∈₁n

4.1 Sea level change

The periodic change of sea level determines the characteristics of sedimentary facies in a large area [32]. The long-term, medium-term and short- term influences of sea level change on the development of ∈₁n shale were analyzed, especially for the longitudinal characteristic change of shale and w(TOC). The long-term, medium-term and short-term sea-level changes control the second-order, third-order and fourth-order sequences, respectively. Referring to previous research results [33] , we divided the lower ∈₁ into one second-order sequence and six third-order sequences. Therefore, the lower ∈₁n was divided into one third-order sequence (SQ1) and the upper ∈₁n was divided into three third-order sequences (SQ2, SQ3 and SQ4), as shown in Figure 4.

1) Long-term changes in sea level

Five sets of source rocks in South China, including ∈₁, D, C₁ and P₁, develop in the period of long-term sea level rise. There are obvious coupling relationships between source rocks and long-term sea level rise. In the Early Cambrian, the global sea level raises rapidly, and reaches the most extensive surface in the middle of ∈₁n deposition(SQ1). A large range of organic shale of ∈₁n develops in the Southern Yangtze platform, Jiangnan slope, and South China basin. With the decrease in sea level, the facies of Yangtze area evolves into platform in the middle-late Cambrian, depositing large sets of limestone, dolomitic limestone, and dolomite.

2) Medium-term change in sea level

The sea level is highest in the middle period of ∈₁n deposition (SQ1), and then sea level decreases slowly, which leads to an overall decrease in water depth in the study area. In the latter period of ∈₁n deposition (SQ2, SQ3 and SQ4), shale does not develop in the west of Hubei Province, and the facies evolve from platform inner sag to platform. In the west of Hunan Province, the facies evolve from outer shelf to shallow shelf and a large set of shale continue to develop. However, w(TOC) of shale is low, and the rock is no longer effective source rock. In the middle of Hunan Province (such as Well Anye 1), which is located in a deep-water detention basin, organic-rich shale continues to develop, but w(TOC) is significantly lower than that of the lower ∈₁n. After the drop in sea level, the clastic from the Cathaysian continent enters on a large scale into the South China Basin (South Hunan Province), resulting in the development and distribution of turbidites.

3) Short-term changes in sea level

A small amplitude of sea level fluctuation results in local enrichment (w(TOC)＞2%) in the lower ∈₁n (SQ1) in the longitudinal direction, as clearly presented in Well E’yangye 1, Well Ciye 1, Well Changye 1 and Well Anye 1 (Figure 4).

4.2 Paleogeographic (provenance) conditions

In the Early Cambrian, the entire Yangtze area is in a sedimentary environment of passive continental margin [34]. The western Hubei Province belongs to the Yangtze platform, which is dominated by chemical deposition. With lowering sea level, the adjacent middle ancient Hubei continent provides terrigenous clastic. From the lithology columnar section of Well E’yangye 1, when the sea level is high, the lower ∈₁s is dominated by mudstone, and a small amount of siltstone and argillaceous siltstone develops occasionally. With falling sea level, a large amount of terrigenous clastic material increases, resulting in development of siltstone over 70 m in the upper ∈₁s. It is inferred that the quartz of Well E’yangye 1 is mainly terrigenous clastic. From the northwest to the southeast, with increasing distance from the continent, the depth of the water body is gradually deepened. The terrigenous clastic deposition diminishes, and the mass fraction of carbonate minerals becomes smaller. Even in the period of sea level reduction, the lithology of the upper ∈₁n such as Well Ciye 1, Well Changye 1 and Well Anye 1 is composed of mainly mudstone with flocculation and biogenic deposition. South Hunan belongs to South China Basin, and Cathaysian continent provides a large amount of coarse clastic directly into the adjacent South China Basin, resulting in the deposition of a large set of turbidites and the occasional development of a small amount of thin-layer carbonaceous shale.

4.3 Hydrothermal activity

In areas such as Guizhou, Hunan, and Hubei, the widely developed black source rock series of lower Cambrian are closely related with frequent and active hydrothermal activity. There is a good space-time coupling relationship between source rock and hydrothermal activity [18, 35]. F1 is the boundary between the Yangtze and Jiangnan regions. F1 forms in the Proterozoic and has characteristics of multi-stage activity. It is the channel for the deep hydrothermal overflow, which brings trace elements and rare earth elements, provides heat for the water body and promotes biological productivity. The w(TOC) in the areas adjacent to F1 is generally higher than that of other areas in Northwest Hunan [36]. Figure 5 shows the enrichment coefficient of trace elements in ∈₁n shale samples of Well Anye 1. The enrichment coefficient of trace elements in the lower ∈₁n is significantly higher than that in the upper ∈₁n. The higher coefficient may exist because the F1 and hydrothermal activity became weak in the late ∈₁n. Compared with the upper continental crust, V, Ni, Zn, Cu, Mo, Ag, Ba, U, Sb and B are obviously abundant, which reflects the characteristics of hydrothermal activity.

Figure 6 shows the correlation between w(TOC) and trace elements in Well Anye 1. Generally, w(Ni) and w(Sb) reflect the strength of hydrothermal activity. As shown in Figures 6(a) and (b), w(TOC) increases with the increase of w(Ni) and w(Sb), which indicates that hydrothermal activity has a positive effect on the enrichment of organic matter in ∈₁n black rock series. However, when w(TOC) is greater than 20.0%, the correlation between w(TOC) and w(Ni)/w(Sb) becomes worse, which indicates that other factors affect w(TOC).

4.4 Upwelling current

In the Early Cambrian, North Guizhou and Northwest Hunan are located in the Jiangnan slope zone and its vicinity. The products of upwelling current widely develop [37], such as 1) assemblage of phosphorite and phosphorous shale, Zhongjiapu Town, Changde, shown in Figure 2(a); 2) assemblage of silicon-carbon shale, Well Huaye 1, shown in Figure 2(f); 3) assemblage of stone coal, siliceous rock, metal sulfide enrichment layer and black shale, Well Changye 1 and Well Ciye 1, shown in Figures 2(g) and 2(h); 4) assemblage of carbon-siliceous shale and metal sulfidation enrichment layer, Well Anye 1, shown in Figure 2(e). Upwelling current is an important sedimentary processes in continental shelf and slope. By bringing nutrients such as phosphorus, silicon, and iron that are beneficial to biological development, upwelling current promotes the growth of bacteria, algae and other organisms, causes the water body to be in a strong reducing environment, and promotes formation of organic source rocks. The substances brought by upwelling current lead to a higher mass fraction of clay in the Well Changye 1 and Well Ciye 1(＞25.0%) [14, 38].

Phosphorous and molybdenum are essential elements for organisms [39-41]. Their presence can be used to evaluate the paleoproductivity of water bodies. As shown in Figures 6(c) and (d), w(TOC) increases with the increase of w(P) and w(Mo). Both w(P) and w(Mo) show significant anomalies for the three samples with w(TOC) more than 20%, which indicates that paleoproductivity has a significant effect on the enrichment of organic matter in ∈₁n black rock.

Figure 5 Enrichment coefficient of trace element of ∈₁n shale sample in Well Anye 1

4.5 Strong reduction conditions

The above four factors together lead to high biological productivity of a water body; however, the final preservation of organic matter formed by organisms depends on the reducing strength of the water body [42]. In anoxic conditions, U and V are easily reduced to low valence ions and transferred to sediments by catalysis and adsorption of organic matter and microorganisms [43]. The mass fraction of radioactive U of ∈₁n is about 2.5 times that of O₃w-S₁l in Sichuan Basin [9]. In addition, the GR curve of organic-rich shale of ∈₁n presents high value characteristics, such as Changye 1 well. At present, the w(V)/w(Cr) value and the w(U)/w(Th) value are used widely to distinguish the redox environment of sedimentary water. Figures 6(e) and (f) show that, with the increase in w(V)/w(Cr) value and w(U)/w(Th) value, w(TOC) gradually increases. When w(TOC) is greater than 5.0%, the correlation between w(TOC) and w(V)/w(Cr) (or w(U)/w(Th)) is good, which indicates that high reduction potential is an important factor for the enrichment of organic matter, especially for the formation of high w(TOC).

Vertically, the w(V)/w(Cr) value in the lower ∈₁n of Well Anye 1 is 1.1-28.7, with an average of 10.6, and the w(U)/w(Th) value is 4.3-391.2, with an average of 63.9. Generally, in an anaerobic environment, w(V)/w(Cr) is greater than 4.25, and w(U)/w(Th) is greater than 1.25. It is infer red that the lower ∈₁n belongs to anaerobic deposition. In the upper ∈₁n of Well Anye 1, w(V)/w(Cr) is 1.7-5.5, with an average of 2.8, and w(U)/w(Th) is 1.0-2.3, with an average of 1.5. Generally, in an oxygen-deficient environment, w(V)/w(Cr) is 2.00-4.25 and w(U)/w(Th) is 0.75-1.25. Therefore, the upper ∈₁n gradually transforms into an oxygen deficient environment. These data are consistent with the conclusion that the sea level raises in the early stage of ∈₁n, reaches the maximum flooding surface in the middle stage of ∈₁n, and declines in the late stage of ∈₁n.

5 Discussion

5.1 Sedimentary facies model

Figure 7 and Table 2 describe the sedimentary facies model of the Early Cambrian in the study area. The distributions of facies belts are different in different periods of the Early Cambrian. Although the division is obvious, the facies belts show continuous changes without obvious discontinuity or abrupt contact. Thus, the study area is a complete three-step basin in ∈₁n.

In the early stage of ∈₁n deposition (SQ1), the first step is the Yangtze carbonate platform, which is dominated by chemical deposition. Carbonate matter mainly develops and organic shale develops only in platform inner sag. The second step is outer shelf and slope with deep-water body. In the second step, hydrothermal activity and upwelling current have a great influence on the deposition characteristic, and the sedimentation is controlled mainly by flocculation, biology, machinery and hydrothermal or mixed deposition. The high mass fraction of siliceous matter comes from biological and hydrothermal activity [44]. The high mass fraction of clay and lower mass fraction of carbonate results from upwelling current. In the second step, carbonaceous shale, siliceous shale and stone coal seams develop with high w(TOC). The third step is a deep-water basin, which is deep and less affected by sea level change. In the north of the basin far away from the continent, biological deposition mainly develops, followed by vertical fine-grained sediments. Therefore, the mass fraction of clay and carbonate is low, the mass fraction of silicon from biological sources is high, w(TOC) is the highest, and siliceous rocks and black shale develop. In the south of the basin adjacent to the Cathaysian, mechanical deposition mainly develops and a large set of turbidites develops [45, 46].

Figure 6 Correlation between w(TOC) and trace elements in Well Anye 1:

Figure 7 Sedimentary facies and organic matter enrichment model of ∈₁n:

Table 2 Sedimentary facies model of the early stage of ∈₁n (SQ1)

In the late stage of ∈₁n deposition (SQ2, SQ3 and SQ4), the sea level decreases obviously. The facies in the first step evolve into platform with mainly chemical sedimentation. The facies in the second step evolve into shallow shelf, with weakened hydrothermal activity and upwelling current. The organic matter is no longer rich, and flocculation sedimentation mainly develops. The facies in the third step is still deep-water basin, but the water depth decreases. The siliceous rock and black shale develop in the north of the basin. Flocculation sedimentation mainly develops; however, w(TOC) declines. The clastic from Cathaysian continent pushes towards the basin interior, and a large set of turbidites develops with mainly mechanical sedimentation in the south of the basin.

5.2 Enrichment mechanism of organic matter

Sea level change and paleogeographic (provenance) conditions control the spatial distribution of regional sedimentary facies, which are the main factors for the large scale horizontal and vertical abundance of organic matter. Hydrothermal activities and upwelling current develop in the outer shelf, slope and adjacent areas. They bring rich mud and trace elements and lead to biological prosperity, which in turn leads to a high reducing potential of the water environment and further improves the abundance of organic matter. A strong reducing condition is necessary for the preservation of organic matter, leading to the highest w(TOC) in the deep-water detention basin. In Figure 6, three dots of samples with w(TOC) greater than 20.0% in Well Anye 1 are abnormal and usually above the fitting trend line, which reflects a variety of factors that lead to high quality organic matter.

The value of w(Al)/[w(Al)+w(Fe)+w(Mn)] is employed to judge the genesis of siliceous rocks, which can also reflect the genesis of organic matter enrichment. The value is less than 0.01 for pure hydrothermal genesis. The value increases gradually with the increasing influence of non-hydrothermal factors. If siliceous rock is affected mainly by terrigenous materials, the value is greater than 0.40. If the siliceous rock is biogenetic genesis, the value is greater than 0.6.

This value in the shale of Well Anye 1 is generally greater than 0.40, which indicates that the siliceous matter is controlled mainly by terrigenous materials, and it may be related to the paleogeographic location of Well Anye 1. The value for the sample with w(TOC) of 31.5% is only 0.38, which indicates that the hydrothermal activity is strong and greatly promotes the accumulation of organic matter and the formation of siliceous matter. The average value is 0.58 in the lower ∈₁n, which reflects that the siliceous and organic matter are mainly affected by the terrigenous materials brought by upwelling current. The average value is 0.68 in the upper of ∈₁n, which indicates that the siliceous and organic matter are mainly affected by biological factors.

5.3 Shale gas exploration target

Organic matter is not only the source material of shale gas, but also affects the characteristics of shale reservoir. One single organic body constitutes one micro-gas reservoir, and the directional arrangement of multiple organic bodies also affects the lateral seepage capacity and the mechanical properties of rocks. Therefore, organic matter is the basis and core of hydrocarbon generation, reservoir and accumulation of shale gas. The w(TOC) is the key index of “geological sweet” and “engineering sweet” in shale gas evaluation [47].

The w(TOC) is higher and the mass fraction of siliceous matter is lower in platform inner sag. However, the distribution of horizontal and vertical organic shale is not stable. The turbidite basin is not conducive to the enrichment of organic matter, and it is not the main exploration target. Outer shelf, slope and deep-water basin are beneficial to formation and preservation of organic matter. They have the characteristics of wide distribution, abundant organic matter, relatively stable thickness, high mass fraction of siliceous matter, and low mass fraction of clay matter, especially deep-water basin facies.

At present, there is more exploration in the early Cambrian platform inner sag (such as Yichang, Hubei) and the outer shelf and slope (Northwest Hunan). There is relatively less exploration in the detention basin. Thus, it is necessary to boost the exploration of shale gas in the detention basin, that is, the Niutitang Formation in the central Hunan.

6 Conclusions

1) In the early stage of ∈₁n deposition, the locations of Well E’yangye 1, Well Ciye 1, Well Changye 1, Well Anye 1 and Leijiadong Sections in Zixing develop, respectively, Yangtze platform inner sag, outer shelf, Jiangnan slope belt, South China detention basin and turbidite basin. With the sea level declining in the late stage of ∈₁n deposition, the location of Well E'yangye 1 evolves into platform, and the locations of Well Ciye 1 and Well Changye 1 evolve into shallow shelf.

2) The study area presents a complete three-step basin in the stage of ∈₁n. In the early stage of ∈₁n deposition, the first step is Yangtze carbonate platform with mainly chemical deposition. The second step is outer shelf and slope with mainly flocculation, biological, mechanical, hydrothermal or mixed deposition. The third step is deep-water basin, far away from the source area in the north of the basin, with mainly biological deposition. However, in the south of the basin adjacent to Cathaysia, the third step is mainly mechanical deposition.

3) Sea level change and paleogeographic (provenance) conditions are the main factors that determine the large-scale horizontal and vertical abundance of organic matter. Hydrothermal activities and upwelling currents relatively develop in the outer shelf, slope and adjacent areas, promoting the further improvement of w(TOC). Strong reduction conditions lead to the highest mass fraction of organic matterin in the deep detention basin.

4) Outer shelf, slope and deep-water basin are beneficial for the formation and preservation of organic matter, especially deep-water basin facies. It is necessary to strengthen the exploration of Niutitang Formation in central Hunan.

Contributors

QIN Ming-yang was responsible for writing the whole article. GUO Jian-hua provided academic guidance. TAN Hui was involved in field geological survey and sampling. WU Shi-qing was responsible for drawing figure, and BIAN Rui-kang edited the article.

Conflict of interest

The content of the article does not involve state secrets and trade secrets. QIN Ming-yang, GUO Jian-hua, TAN Hui, WU Shi-qing and BIAN Rui-kang declare that all authors have no objection to the order of the contribution statement and signature.

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(Edited by FANG Jing-hua)

中文导读

中国南方下寒武系牛蹄塘组沉积相特征与有机质富集机理

摘要：为了研究中国南方下寒武统牛蹄塘组沉积相特征，揭示有机质富集机理，指导页岩气实践勘探采用宏观调查与实验分析相结合的手段，详细分析研究区内寒武系牛蹄塘组的岩性、有机质质量分数(w(TOC))、矿物组成及微量元素特征，提出有机质富集影响因素以及沉积相模式，并指出未来勘探方向。研究结果表明：∈₁n沉积早期，鄂阳页1、慈页1、常页1井和安页1井所在位置分别发育为台内凹陷、外陆棚、江南斜坡带、华南滞留盆地。伴随着海平面下降，∈₁n沉积晚期，沉积相发生演化；研究区早寒武世呈现一个完整的三阶式海盆。∈₁n沉积早期，第1阶为扬子碳酸盐台地，第2阶为深水陆棚-斜坡，第3阶为深水盆地；自扬子碳酸盐台地至深水盆地，w(TOC)逐渐增大，碳酸盐岩矿物质量分数逐渐降低，石英质量分数逐渐升高，而黏土矿物质量分数在第2阶较高。海平面变化导致纵向上∈₁n下部页岩发育的w(TOC)较高，而古地理(物源)条件导致横向的有机质在深水陆棚、斜坡和滞留盆地发育较好；∈₁n下部页岩富集大量微量元素，且w(TOC)与页岩中多种微量元素呈现较好的正相关性；在深水陆棚和斜坡及其邻近区域，热液活动和上升洋流作用带来了丰富的营养物质，并使水体处于强还原环境，促进页岩有机质丰度进一步提升。深水陆棚、斜坡和深水盆地是有利于有机质形成和保存的沉积相，尤其是深水盆地相最佳，未来需要加强滞留盆地，即湘中地区的牛蹄塘组勘探。

关键词：牛蹄塘组；有机质；沉积相；富集机理；热液活动；上升洋流；勘探目标

Foundation item: Project(2017GK2233) supported by the Science and Technology Innovation Program of Hunan Provine, China; Project(2017JJ1034) supported by the Natural Science Foundation of Hunan Province, China

Received date: 2020-04-26; Accepted date: 2020-08-31

Corresponding author: QIN Ming-yang, PhD, Engineer; Tel: +86-731-88879765; E-mail: qinmingyang503@csu.edu.cn; ORCID: https://orcid.org/0000-0001-9443-259X