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. 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).