Sedimentary Facies Characteristics and Organic Matter Enrichment Mechanism of Lower Cambrian Niutitang Formation in South China

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

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.

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: [email protected]; ORCID: https://orcid.org/0000-0001-9443-259X 3780 J. Cent. South Univ. (2020) 27: 3779−3792

pointed out that the organic rich shale of 1n in the 1 Introduction Yangtze block may be due to upwelling current, anoxic events, and reductive hydrothermal solution The exploration and development of shale gas with eutrophic elements along the extensional faults. has become an important measure for China to cope ZHOU et al [19] and LI et al [20] showed that with resource shortage and energy security. The hydrocarbon-generating organisms of 1n in “Shale gas revolution” has been greatly successful. Northwest Hunan were mainly phytoplankton, The annual output of marine shale gas in South benthic macroalgae, zooplankton and benthic China has reached 100×108 m3[1, 2]. By exploration sponge. and theoretical research, scholars put forward the An industrial shale gas breakthrough was concepts of “two-factor enrichment”[3], “three- achieved in Yichang, Hubei Province, China, the element enrichment”[4], “source-cap hydrocarbon- oldest stratum (1s) in the world, which can serve controlling”[5], “shale gas sweet spot”[6], and “five as a leading role in the exploration of 1n shale gas properties in one position”[7]. They emphasized in southern China [21]. Therefore, it is urgent to that “favorable sedimentary facies determine the study the sedimentary facies characteristics and hydrocarbon generation basis and reservoir enrichment mechanism of organic matter of 1n in characteristics of shale gas enrichment”. The main the middle Yangtze areas. Previous research has shale systems in the United States formed in been limited by a large scope and few drilling data. foreland basins, but two sets of hot exploration These studies have five shortcomings: 1) too few shale formed in the Yangtze craton basin, i.e., the comparisons of drilling data in large-scale lower Cambrian Niutitang Formation (1n) and the horizontal area; 2) lack of attention to systematic Upper Ordovician Wufeng Formation and Lower research on multiple factors that affect sedimentary

Silurian Longmaxi formation (O3w-S1l)[8−11]. facies and organic matter enrichment; 3) only Therefore, the study of the sedimentary facies geochemistry data (trace elements or rare earth characteristics and spatial distribution of 1n shale, elements) used to study the enrichment mechanism especially the mechanism and influencing factors of of organic matter; 4) lack of attention to the key organic matter enrichment, has become influence of structural sedimentary background [22]; indispensable for shale oil and gas exploration and 5) too little study of prototype basin characteristics evaluation. and sedimentary evolution of the entire Yangtze Investigators have studied extensively the region. sedimentary facies, enrichment mechanism of On the basis of many years of shale gas organic matter and non-ferrous elements (e.g., V, Ni, investigation and research, investigators have

Mo), and the origin of stone coal and siliceous studied the typical wells of 1n. By a combination matter of 1n in South China [12−20]. Regarding of macroscopic core/outcrop observation and sedimentary facies, LIU et al [14] suggested that the laboratory experiments, geologists compared the lower Cambrian Qiongzhusi Formation (1q) in the development characteristics of organic shale in the Upper Yangtze area belonged to the sedimentary study area, analyzed the spatial distribution and system of shore, shelf, slope and deep-water basin. evolution of 1n sedimentary facies, and studied the However, QIU et al [15] proposed that the shale of vertical and horizontal change laws of organic the lower Cambrian Shuijigntuo Formation (1s) in matter mass fraction w(TOC). The mechanistic the middle Yangtze developed mainly the model of organic matter enrichment and future sedimentary system of carbonate gentle slope, exploration target of shale gas were proposed. shallow shelf, and deep- shelf basin. For organic matter enrichment, CHEN et al [13] 2 Regional geological background emphasized that “the depression in the platform is the foundation and the area with organic matter Figure 1 shows the geological background and accumulation is the favorable facies belt” on the location map of the study area. As shown in Figure 1, basis of breakthrough of shale gas of Shuijingtuo in the Early Cambrian, the breakup of Pangaea, the

Formation (1s) in Yichang, Hubei Province, China. aggravation of structural subsidence, and the rise of XU et al [16], LIU et al [17] and JIA et al [18] global sea level lead to the overall location of the

J. Cent. South Univ. (2020) 27: 3779−3792 3781

Figure 1 Geological background of the study area: (a) Location of Early Cambrian paleocontinent [23, 24]; (b) Ediacaran- Cambrian paleogeography and Location of Well/Section

Yangtze region below sea level [23, 24]. The w(TOC) is 1.89%−2.43%, with an average of paleogeographic pattern is high in the north and low 2.34%. The upper 1s is composed of mainly light in the south. Figure 1(b) shows that the study area is yellow siltstone and light gray argillaceous from Yichang, Hubei Province, China, and it covers limestone [13]. the entire area of Hunan Province, China. Moving The lithology of the lower 1n in Well Ciye 1 from northwest to southeast, the area belongs to is mainly black shale, carbonaceous shale, stone Yangtze platform, Jiangnan slope, South China coal and siliceous shale, which is rich in Basin, and Cathay uplift. Different structural carbonaceous matter, siliceous matter and pyrite. backgrounds determine the sedimentary facies The w(TOC) is 1.18%−5.36%, with an average of characteristics of the lower Cambrian and further 2.87%. The lithology of the upper 1n is mainly affect the enrichment of organic matter [25]. black-gray and celadon mudstone, with a layer of

Niutitang Formation (1n) in Northwest Hunan, celadon argillaceous limestone. The w(TOC) is less Shuijingtuo Formation (1s) in west Hubei, and than 2.0%, and the mass fraction of pyrite is low

Qiongzhusi Formation (1q) in the east of Sichuan [26]. Province all develop organic shale which are of The lithology of 1n in Well Changye 1 is regional contrast and obviously different from the relatively stable. The lower 1n is dominated by shale and dolomitic limestone of Sinian Dengying black shale and carbonaceous shale, and the w(TOC) Formation (Z2d). of shale with depth of 1100−1250 m is 2.16%−17.60%, with an average of 10.14%. The 3 Characteristics of sedimentary facies upper 1n develops stable gray shale and w(TOC) is generally lower than 2.00% [27].

3.1 Lithology and organic matter richment The lower 1n in Well Anye 1 is mainly Typical wells and outcrops were selected to composed of black siliceous shale and compare the spatial variation characteristics of 1n carbonaceous shale, and the organic matter well lithology at a large scale. Figure 2 and Table 1 show develops. The w(TOC) of shale with depth of the characteristics of organic-rich shale of the lower 790−857 m is 4.03%−31.50%, with an average of

Cambrian in the study area. The lower 1s in Well 15.0%. The upper 1n is composed of mainly black E’yangye 1 is composed of mainly black mudstone argillaceous shale, and w(TOC) is generally with a small amount of limestone and siltstone. The 2.61%−5.85%, with an average of 4.20%. The

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Figure 2 Characteristics of organic-rich shale of lower Cambrian in the study area: (a) Black siliceous shale with phosphorus nodule, Zhongjiapu Town, Changde; (b) Stone coal mine, Nanmupu, Yuanling; (c) Siliceous shale and carbonaceous shale, Anhua; (d) Greywacke, Dong’an section, Xiangnan Formation; (e) Carbonaceous shale, Well Anye 1; (f) Silicon-carbon shale, bio-fossil, Well Huaye 1; (g) Black carbonaceous shale, Well Ciye 1; (h) Vermicular barite; Well Ciye 1; (i) Phosphorus nodule, Well Anye 1

Table 1 Sedimentary characteristics of 1 in typical wells/sections w/% Well/ Section Paleogeographic location Strata Facies w(TOC)/% Quartz Clay Carbonate

Yangtze platform, adjacent to the Upper 1n Platform — — — — E’yangye 1 continent in the central Hubei Platform inner Lower 1n 1.89−2.43(2.34) 39.3 15.5 17.7 Procince sag—platform

Upper 1n Shallow shelf <2.00 — — — Ciye 1 Outer shelf belt, near F1 Lower 1n Outer shelf 1.18−5.36(2.87) 41.4 26.4 8.6

Upper 1n Shallow shelf <2.00 — — — Changye 1 Jiangnan slope belt, near F2 Lower 1n Slope 2.16−17.60(10.14) 52.3 27.9 8.7

Huanan basin, far away from Upper 1n Detention basin 2.61−5.85(4.20) — — — Anye 1 continent and fault Lower 1n Detention basin 4.03−31.50(15.0) 65.0 16.9 2.7

Leijiadong Huanan Basin, near Cathaysian 1x Turbidite basin — — — —

w(TOC) of shale in the upper 1n is significantly 3.2 Mineral composition lower than w(TOC) in the lower 1n. The regular changes of mineral composition of The lithology of Leijiadong Section in Zixing shale in the study area can reflect the characteristics is mainly gray feldspar quartz, greywacke with of sedimentary environment. Figure 3 shows the thickness of more than 400 m, and a small amount characteristics of mineral composition change of of black carbon shale develops. 1n in the study area. The mass fractions of quartz

J. Cent. South Univ. (2020) 27: 3779−3792 3783 in Well E’yangye 1, Well Ciye 1, Well Changye 1 of organic shale are 26.4% and 8.6% on average, and Well Anye 1 are 39.3%, 41.4%, 52.3% and respectively. The w(TOC) increases with an average 65.0%, respectively. The mass fractions of clay are of 2.87%. Thus, Well Ciye 1 belongs to outer shelf 15.5%, 26.4%, 27.9% and 16.9%, respectively. The facies. Well Changye 1 is in Jiangnan Slope Belt, mass fractions of carbonate are 17.7%, 8.6, 8.7% where the body of water is deepened further, and and 2.7%, respectively. These data indicate that, the development of carbonaceous shale is stable. with the increase in distance from the Yangtze The mass fraction of quartz and clay are 52.3% and platform, the water body deepens gradually, the 27.9% on average, respectively. The w(TOC) is mass fraction of carbonate gradually decreases, and 10.14% on average, and the thickness of the mass fraction of quartz gradually increases. The organic-rich shale is large, which reflects the mass fraction of clay is more than 25.0% in the characteristic of slope facies. Well Anye 1 is in outer shelf and slope, whereas the mass fraction of South China Basin, near Anhua-Yiyang Fault (F2). clay mineral is just about 16.0% in the platform and The mass fractions of quartz is 65.0%, the mass detention basin. 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 Figure 3 Mineral composition characteristics of 1n platform inner sag to platform. Mudstone mainly organic-rich shale develops in Well Ciye 1 and Well Changye 1, but organic matter poorly develops, which is 3.3 Sedimentary facies correlation characteristic of shallow shelf facies. A large In the direction perpendicular to the structure, amount of shale still develops in Well Anye 1, but Well E’yangye 1, Well Ciye 1, Well Changye 1, w(TOC) is obviously lower than that of lower 1n. Well Anye 1 and Leijiadong outcrop in Zixing were Thus the facies of Well Anye 1 are still in detention selected from the northwest to the southeast. A basin facies, but the water depth is shallower than sedimentary facies contrast map (Figure 4) was that of the early stage of 1n deposition. The constructed according to the regional sedimentary lithology of Xiangnan Formation in Leijiadong background, the characteristics of lithology, w(TOC) Section, Zixing is relatively stable, and is similar to and mineral composition. that of the lower 1n.

1) In the early stage of 1n deposition, Well E’yangye 1 is located in the Yangtze platform, 4 Factors affecting sedimentary facies adjacent to the continent in the middle Hubei and organic matter enrichment Province. The lithology is mainly black shale, with a small amount of limestone and siltstone shale. According to the field geological survey, The mass fraction of carbonate and clay are 17.7% drilling geological results and experimental data, and 15.5% on average, respectively. The w(TOC) is we considered that the factors that affect the low with only 2.43% on average, and the vertical sedimentary facies and organic matter enrichment development is unstable. It is inferred that the facies in the study area include mainly sea level change, of Well E’yangye 1 is platform inner sag. Well Ciye 1 geographic (provenance) conditions, hydrothermal is in a deeper water body, adjacent to Baojing-Cili activities, upwelling current and redox conditions Fault (F1). The mass fractions of clay and carbonate [16, 18, 28−31].

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

4.1 Sea level change sea level rise. In the Early Cambrian, the global sea The periodic change of sea level determines level raises rapidly, and reaches the most extensive the characteristics of sedimentary facies in a large surface in the middle of 1n deposition(SQ1). A area [32]. The long-term, medium-term and short- large range of organic shale of 1n develops in the term influences of sea level change on the Southern Yangtze platform, Jiangnan slope, and development of 1n shale were analyzed, especially South China basin. With the decrease in sea level, for the longitudinal characteristic change of shale the facies of Yangtze area evolves into platform in and w(TOC). The long-term, medium-term and the middle-late Cambrian, depositing large sets of short-term sea-level changes control the limestone, dolomitic limestone, and dolomite. second-order, third-order and fourth-order 2) Medium-term change in sea level sequences, respectively. Referring to previous The sea level is highest in the middle period of research results [33] , we divided the lower 1 into 1n deposition (SQ1), and then sea level decreases one second-order sequence and six third-order slowly, which leads to an overall decrease in water sequences. Therefore, the lower 1n was divided depth in the study area. In the latter period of 1n into one third-order sequence (SQ1) and the upper deposition (SQ2, SQ3 and SQ4), shale does not

1n was divided into three third-order sequences develop in the west of Hubei Province, and the (SQ2, SQ3 and SQ4), as shown in Figure 4. facies evolve from platform inner sag to platform. 1) Long-term changes in sea level In the west of Hunan Province, the facies evolve Five sets of source rocks in South China, from outer shelf to shallow shelf and a large set of including 1, D, C1 and P1, develop in the period of shale continue to develop. However, w(TOC) of long-term sea level rise. There are obvious coupling shale is low, and the rock is no longer effective relationships between source rocks and long-term source rock. In the middle of Hunan Province (such

J. Cent. South Univ. (2020) 27: 3779−3792 3785 as Well Anye 1), which is located in a deep-water the widely developed black source rock series of detention basin, organic-rich shale continues to lower Cambrian are closely related with frequent develop, but w(TOC) is significantly lower than that and active hydrothermal activity. There is a good of the lower 1n. After the drop in sea level, the space-time coupling relationship between source clastic from the Cathaysian continent enters on a rock and hydrothermal activity [18, 35]. F1 is the large scale into the South China Basin (South boundary between the Yangtze and Jiangnan Hunan Province), resulting in the development and regions. F1 forms in the Proterozoic and has distribution of turbidites. characteristics of multi-stage activity. It is the 3) Short-term changes in sea level channel for the deep hydrothermal overflow, which A small amplitude of sea level fluctuation brings trace elements and rare earth elements, results in local enrichment (w(TOC)＞2%) in the provides heat for the water body and promotes lower 1n (SQ1) in the longitudinal direction, as biological productivity. The w(TOC) in the areas clearly presented in Well E’yangye 1, Well Ciye 1, adjacent to F1 is generally higher than that of other Well Changye 1 and Well Anye 1 (Figure 4). areas in Northwest Hunan [36]. Figure 5 shows the

enrichment coefficient of trace elements in 1n 4.2 Paleogeographic (provenance) conditions shale samples of Well Anye 1. The enrichment

In the Early Cambrian, the entire Yangtze area coefficient of trace elements in the lower 1n is is in a sedimentary environment of passive significantly higher than that in the upper 1n. The continental margin [34]. The western Hubei higher coefficient may exist because the F1 and

Province belongs to the Yangtze platform, which is hydrothermal activity became weak in the late 1n. dominated by chemical deposition. With lowering Compared with the upper continental crust, V, Ni, sea level, the adjacent middle ancient Hubei Zn, Cu, Mo, Ag, Ba, U, Sb and B are obviously continent provides terrigenous clastic. From the abundant, which reflects the characteristics of lithology columnar section of Well E’yangye 1, hydrothermal activity. when the sea level is high, the lower 1s is Figure 6 shows the correlation between dominated by mudstone, and a small amount of w(TOC) and trace elements in Well Anye 1. siltstone and argillaceous siltstone develops Generally, w(Ni) and w(Sb) reflect the strength of occasionally. With falling sea level, a large amount hydrothermal activity. As shown in Figures 6(a) and of terrigenous clastic material increases, resulting in (b), w(TOC) increases with the increase of w(Ni) development of siltstone over 70 m in the upper 1s. and w(Sb), which indicates that hydrothermal It is inferred that the quartz of Well E’yangye 1 is activity has a positive effect on the enrichment of mainly terrigenous clastic. From the northwest to organic matter in 1n black rock series. However, the southeast, with increasing distance from the when w(TOC) is greater than 20.0%, the correlation continent, the depth of the water body is gradually between w(TOC) and w(Ni)/w(Sb) becomes worse, deepened. The terrigenous clastic deposition which indicates that other factors affect w(TOC). diminishes, and the mass fraction of carbonate minerals becomes smaller. Even in the period of sea 4.4 Upwelling current level reduction, the lithology of the upper 1n such In the Early Cambrian, North Guizhou and as Well Ciye 1, Well Changye 1 and Well Anye 1 is Northwest Hunan are located in the Jiangnan slope composed of mainly mudstone with flocculation zone and its vicinity. The products of upwelling and biogenic deposition. South Hunan belongs to current widely develop [37], such as 1) assemblage South China Basin, and Cathaysian continent of phosphorite and phosphorous shale, Zhongjiapu provides a large amount of coarse clastic directly Town, Changde, shown in Figure 2(a); 2) into the adjacent South China Basin, resulting in the assemblage of silicon-carbon shale, Well Huaye 1, deposition of a large set of turbidites and the shown in Figure 2(f); 3) assemblage of stone coal, occasional development of a small amount of siliceous rock, metal sulfide enrichment layer and thin-layer carbonaceous shale. black shale, Well Changye 1 and Well Ciye 1, shown in Figures 2(g) and 2(h); 4) assemblage of 4.3 Hydrothermal activity carbon-siliceous shale and metal sulfidation In areas such as Guizhou, Hunan, and Hubei, enrichment layer, Well Anye 1, shown in Figure

3786 J. Cent. South Univ. (2020) 27: 3779−3792 2(e). Upwelling current is an important sedimentary value are used widely to distinguish the redox processes in continental shelf and slope. By environment of sedimentary water. Figures 6(e) and bringing nutrients such as phosphorus, silicon, and (f) show that, with the increase in w(V)/w(Cr) value iron that are beneficial to biological development, and w(U)/w(Th) value, w(TOC) gradually increases. upwelling current promotes the growth of bacteria, When w(TOC) is greater than 5.0%, the correlation algae and other organisms, causes the water body to between w(TOC) and w(V)/w(Cr) (or w(U)/w(Th)) be in a strong reducing environment, and promotes is good, which indicates that high reduction formation of organic source rocks. The substances potential is an important factor for the enrichment brought by upwelling current lead to a higher mass of organic matter, especially for the formation of fraction of clay in the Well Changye 1 and Well high w(TOC). Ciye 1(＞25.0%) [14, 38]. Vertically, the w(V)/w(Cr) value in the lower

Phosphorous and molybdenum are essential 1n of Well Anye 1 is 1.1−28.7, with an average of elements for organisms [39−41]. Their presence can 10.6, and the w(U)/w(Th) value is 4.3−391.2, with be used to evaluate the paleoproductivity of water an average of 63.9. Generally, in an anaerobic bodies. As shown in Figures 6(c) and (d), w(TOC) environment, w(V)/w(Cr) is greater than 4.25, and increases with the increase of w(P) and w(Mo). w(U)/w(Th) is greater than 1.25. It is infer red that

Both w(P) and w(Mo) show significant anomalies the lower 1n belongs to anaerobic deposition. In for the three samples with w(TOC) more than 20%, the upper 1n of Well Anye 1, w(V)/w(Cr) is which indicates that paleoproductivity has a 1.7−5.5, with an average of 2.8, and w(U)/w(Th) is significant effect on the enrichment of organic 1.0−2.3, with an average of 1.5. Generally, in an matter in 1n black rock. 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 Figure 5 Enrichment coefficient of trace element of facies model of the Early Cambrian in the study

1n shale sample in Well Anye 1 area. The distributions of facies belts are different in different periods of the Early Cambrian. Although 4.5 Strong reduction conditions the division is obvious, the facies belts show The above four factors together lead to high continuous changes without obvious discontinuity biological productivity of a water body; however, or abrupt contact. Thus, the study area is a complete the final preservation of organic matter formed by three-step basin in 1n. organisms depends on the reducing strength of the In the early stage of 1n deposition (SQ1), the water body [42]. In anoxic conditions, U and V are first step is the Yangtze carbonate platform, which easily reduced to low valence ions and transferred is dominated by chemical deposition. Carbonate to sediments by catalysis and adsorption of organic matter mainly develops and organic shale develops matter and microorganisms [43]. The mass fraction only in platform inner sag. The second step is outer of radioactive U of 1n is about 2.5 times that of shelf and slope with deep-water body. In the second

O3w-S1l in Sichuan Basin [9]. In addition, the GR step, hydrothermal activity and upwelling current curve of organic-rich shale of 1n presents high have a great influence on the deposition value characteristics, such as Changye 1 well. At characteristic, and the sedimentation is controlled present, the w(V)/w(Cr) value and the w(U)/w(Th) mainly by flocculation, biology, machinery and

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Figure 6 Correlation between w(TOC) and trace elements in Well Anye 1: (a) w(Ni)；(b) w(Sb)；(c) w(P)；(d) w(Mo)； (e) w(V)/w(Cr)；(f) w(U)/w(Th) hydrothermal or mixed deposition. The high mass basin far away from the continent, biological fraction of siliceous matter comes from biological deposition mainly develops, followed by vertical and hydrothermal activity [44]. The high mass fine-grained sediments. Therefore, the mass fraction fraction of clay and lower mass fraction of of clay and carbonate is low, the mass fraction of carbonate results from upwelling current. In the silicon from biological sources is high, w(TOC) is second step, carbonaceous shale, siliceous shale and the highest, and siliceous rocks and black shale stone coal seams develop with high w(TOC). The develop. In the south of the basin adjacent to the third step is a deep-water basin, which is deep and Cathaysian, mechanical deposition mainly develops less affected by sea level change. In the north of the and a large set of turbidites develops [45, 46].

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Figure 7 Sedimentary facies and organic matter enrichment model of 1n: (a) Early stage of 1n (SQ1)；(b) Late stage of 1n (SQ2, SQ3 and SQ4)

Table 2 Sedimentary facies model of the early stage of black shale develop in the north of the basin.

1n (SQ1) Flocculation sedimentation mainly develops; Geologic however, w(TOC) declines. The clastic from Step Facies Sedimentary Sample background Cathaysian continent pushes towards the basin Yangze Yangtze Chemical 1st E’yangye 1 interior, and a large set of turbidites develops with plateform platform deposition mainly mechanical sedimentation in the south of the Outer shelf Flocculation, Ciye 1 basin. Jiangnan biology, machinery, 2nd zone Slope hydrothermal or Changye 1 mixed deposition 5.2 Enrichment mechanism of organic matter Detention Sea level change and paleogeographic Biology Anye 1 Huanan basin (provenance) conditions control the spatial 3rd basin Turbidite distribution of regional sedimentary facies, which Machinery Leijiadong basin are the main factors for the large scale horizontal and vertical abundance of organic matter. In the late stage of 1n deposition (SQ2, SQ3 Hydrothermal activities and upwelling current and SQ4), the sea level decreases obviously. The develop in the outer shelf, slope and adjacent areas. facies in the first step evolve into platform with They bring rich mud and trace elements and lead to mainly chemical sedimentation. The facies in the biological prosperity, which in turn leads to a high second step evolve into shallow shelf, with reducing potential of the water environment and weakened hydrothermal activity and upwelling further improves the abundance of organic matter. A current. The organic matter is no longer rich, and strong reducing condition is necessary for the flocculation sedimentation mainly develops. The preservation of organic matter, leading to the facies in the third step is still deep-water basin, but highest w(TOC) in the deep-water detention basin. the water depth decreases. The siliceous rock and In Figure 6, three dots of samples with w(TOC)

J. Cent. South Univ. (2020) 27: 3779−3792 3789 greater than 20.0% in Well Anye 1 are abnormal formation and preservation of organic matter. They and usually above the fitting trend line, which have the characteristics of wide distribution, reflects a variety of factors that lead to high quality abundant organic matter, relatively stable thickness, organic matter. high mass fraction of siliceous matter, and low mass The value of w(Al)/[w(Al)+w(Fe)+w(Mn)] is fraction of clay matter, especially deep-water basin employed to judge the genesis of siliceous rocks, facies. which can also reflect the genesis of organic matter At present, there is more exploration in the enrichment. The value is less than 0.01 for pure early Cambrian platform inner sag (such as Yichang, hydrothermal genesis. The value increases Hubei) and the outer shelf and slope (Northwest gradually with the increasing influence of Hunan). There is relatively less exploration in the non-hydrothermal factors. If siliceous rock is detention basin. Thus, it is necessary to boost the affected mainly by terrigenous materials, the value exploration of shale gas in the detention basin, that is greater than 0.40. If the siliceous rock is is, the Niutitang Formation in the central Hunan. biogenetic genesis, the value is greater than 0.6. This value in the shale of Well Anye 1 is 6 Conclusions generally greater than 0.40, which indicates that the siliceous matter is controlled mainly by terrigenous 1) In the early stage of 1n deposition, the materials, and it may be related to the locations of Well E’yangye 1, Well Ciye 1, Well paleogeographic location of Well Anye 1. The value Changye 1, Well Anye 1 and Leijiadong Sections in for the sample with w(TOC) of 31.5% is only 0.38, Zixing develop, respectively, Yangtze platform which indicates that the hydrothermal activity is inner sag, outer shelf, Jiangnan slope belt, South strong and greatly promotes the accumulation of China detention basin and turbidite basin. With the organic matter and the formation of siliceous matter. sea level declining in the late stage of 1n The average value is 0.58 in the lower 1n, which deposition, the location of Well E'yangye 1 evolves reflects that the siliceous and organic matter are into platform, and the locations of Well Ciye 1 and mainly affected by the terrigenous materials Well Changye 1 evolve into shallow shelf. brought by upwelling current. The average value is 2) The study area presents a complete

0.68 in the upper of 1n, which indicates that the three-step basin in the stage of 1n. In the early siliceous and organic matter are mainly affected by stage of 1n deposition, the first step is Yangtze biological factors. carbonate platform with mainly chemical deposition. The second step is outer shelf and slope with 5.3 Shale gas exploration target mainly flocculation, biological, mechanical, Organic matter is not only the source material hydrothermal or mixed deposition. The third step is of shale gas, but also affects the characteristics of deep-water basin, far away from the source area in shale reservoir. One single organic body constitutes the north of the basin, with mainly biological one micro-gas reservoir, and the directional deposition. However, in the south of the basin arrangement of multiple organic bodies also affects adjacent to Cathaysia, the third step is mainly the lateral seepage capacity and the mechanical mechanical deposition. properties of rocks. Therefore, organic matter is the 3) Sea level change and paleogeographic basis and core of hydrocarbon generation, reservoir (provenance) conditions are the main factors that and accumulation of shale gas. The w(TOC) is the determine the large-scale horizontal and vertical key index of “geological sweet” and “engineering abundance of organic matter. Hydrothermal sweet” in shale gas evaluation [47]. activities and upwelling currents relatively develop The w(TOC) is higher and the mass fraction of in the outer shelf, slope and adjacent areas, siliceous matter is lower in platform inner sag. promoting the further improvement of w(TOC). However, the distribution of horizontal and vertical Strong reduction conditions lead to the highest organic shale is not stable. The turbidite basin is not mass fraction of organic matterin in the deep conducive to the enrichment of organic matter, and detention basin. it is not the main exploration target. Outer shelf, 4) Outer shelf, slope and deep-water basin are slope and deep-water basin are beneficial to beneficial for the formation and preservation of

3790 J. Cent. South Univ. (2020) 27: 3779−3792 organic matter, especially deep-water basin facies. Chen-sheng, ZHENG Zhen-hua, ZHANG Liang-pin, GUO It is necessary to strengthen the exploration of Jun, CAO Zheng. “Sweet spots zone” optimization of marine shale gas in complex geological conditions area out of Niutitang Formation in central Hunan. Sichuan basin: A case of Paleozoic in Northwestern Hunan, China [J]. Journal of Central South University: Science and Contributors Technology), 2019, 50(3): 596−606. DOI: 10.11817/j.issn. QIN Ming-yang was responsible for writing 1672-7207.2019.03.013. (in Chinese) [7] JIN Zhi-jun, HU Zong-quan, GAO Bo, ZHAO Jian-hua. the whole article. GUO Jian-hua provided academic Controlling factors on the enrichment and high productivity guidance. TAN Hui was involved in field geological of shale gas in the Wufeng-Longmaxi Formations, survey and sampling. WU Shi-qing was responsible southeastern Sichuan Basin [J]. 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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