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Source analysis of siliceous minerals and uranium in Early Cambrian shales, South China: Significance for shale gas exploration
Accepted Manuscript Source analysis of siliceous minerals and uranium in Early Cambrian shales, South China: Significance for shale gas exploration Tao Jiang, Zhijun Jin, Guangxiang Liu, Quanyou Liu, Bo Gao, Zhongbao Liu, Haikuan Nie, Jianhua Zhao, Ruyue Wang, Tong Zhu, Tao Yang PII:
S0264-8172(18)30472-0
DOI:
https://doi.org/10.1016/j.marpetgeo.2018.11.002
Reference:
JMPG 3569
To appear in:
Marine and Petroleum Geology
Received Date: 1 June 2018 Revised Date:
30 September 2018
Accepted Date: 3 November 2018
Please cite this article as: Jiang, T., Jin, Z., Liu, G., Liu, Q., Gao, B., Liu, Z., Nie, H., Zhao, J., Wang, R., Zhu, T., Yang, T., Source analysis of siliceous minerals and uranium in Early Cambrian shales, South China: Significance for shale gas exploration, Marine and Petroleum Geology (2018), doi: https:// doi.org/10.1016/j.marpetgeo.2018.11.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Source analysis of siliceous minerals and uranium in Early Cambrian shales,
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South China: Significance for shale gas exploration
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Tao Jianga,b,c,d,e , Zhijun Jina,b,c,d,e, *, Guangxiang Liu c,d,e, Quanyou Liu c,d,e,
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Bo Gao c,d,e, Zhongbao Liu c,d,e, Haikuan Nie c,d,e, Jianhua Zhao c,d,e,f, Ruyue Wang c,d,e,
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Tong Zhu c,d,e, Tao Yang c,d,e,g
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a
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School of Earth and Space Sciences, Peking University, Beijing 100871, China
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b
Institute of Oil & Gas, Peking University, Beijing 100871, China
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c
State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective
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Key Laboratory of Orogenic Belt and Crustal Evolution, Ministry of Education,
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Development, Beijing 100083, China
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d
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100083, China
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e
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100083, China
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f
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266580, China
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g
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China
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*
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Mechanisms and Effective Development, Beijing 100083, China
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E-mail addresses: [email protected] (Z, Jin)
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Key Laboratory of Shale Oil/Gas Exploration and Production, SINOPEC, Beijing
Petroleum Exploration and Production Research Institute, SINOPEC, Beijing
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School of Geosciences, China University of Petroleum (East China), Qingdao
College of Geosciences, China University of Petroleum (Beijing), Beijing 102249,
Corresponding author. State Key Laboratory of Shale Oil and Gas Enrichment
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Abstract Efficient shale gas exploration in the Early Cambrian shales of southern China
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requires the identification of favorable target areas. This study used logs for U, Si, Al,
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and total organic carbon (TOC) from three typical wells to assess a new framework
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for gas exploration in this area. The excess siliceous mineral content (from a
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non-terrigenous clastic source) ranged from 20–30% in most layers, reaching 50%.
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Analysis of these excess siliceous minerals using an Al-Fe-Mn ternary plot showed
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that most were probably derived from hydrothermal fluids within the boundary of the
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Yangtze and Cathaysian plates. Excess siliceous mineral content and uranium content
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had a good positive correlation, both tending to decrease away from the plate
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boundary, indicating that both were derived from hydrothermal fluids. Radioactive
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uranium in hydrothermal fluids can enhance biological productivity, which is
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conducive to the increased accumulation of sedimentary organic matter. Therefore,
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shale gas exploration in the Early Cambrian layers of South China should focus on
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target areas near the plate boundary that have increased uranium levels, moderate
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maturity, and good preservation conditions.
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Keywords: Silicon source; uranium source; hydrothermal solution; plate junction;
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Yangtze plate
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1. Introduction
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Since 2000, shale gas production has achieved great success in North America
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due to theoretical and technical advances in exploration and development methods
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have also been found recently in China. In 2010, the China National Petroleum
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Corporation (CNPC) made a new industrial breakthrough in shale gas development
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with the Wei 201 well, drilled in the southern Sichuan Basin. In 2012, the China
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Petroleum & Chemical Corporation (Sinopec) achieved a breakthrough in marine
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shale gas exploration in the Jiaoshiba block in the southern part of the Fuling shale
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gas field. In 2015, the production capacity of this field reached 50×108 m3, becoming
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the second largest shale gas field in the world after North America. Subsequently,
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successful commercial shale gas development has been carried out in the Weiyuan,
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Changning, Zhaotong, Fushun-Yongchuan, Fuling, and Ding Shan gas fields (Dong et
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al., 2016; Guo, 2016a; Guo et al., 2016b; Wei et al., 2017).
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However, exploration for shale gas in Lower Cambrian formations has resulted
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in a number of low-production or failed wells, primarily due to a the lack of material
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basis in the shale gas reservoir process. Therefore, locating zones containing
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organic-rich shale (i.e., total organic content (TOC) content > 2%) is an important
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challenge to solve (Jarvie et al., 2007; Ross et al., 2009; Zhao et al., 2016a). Previous
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studies have proposed a series of methods to find organic-rich shale development
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areas, such as using seismic interpretation to calculate the thickness of organic-rich
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shale in a given block (Goodway et al., 2010; Zhu et al., 2011), using organic-rich
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shale intervals drilled in wildcat wells to determine the connecting-well section and
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study its distribution (Montgomery et al., 2005; Bowker, 2007), and utilizing regional
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sedimentary characteristics during shale deposition to predict the distribution of
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organic-rich shale (Loucks et al., 2007; Zhao et al., 2016b; Zhang et al., 2017a; Zhang
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et al., 2018a). However, these approaches have limitations when implemented. The seismic
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interpretation method assumes that a certain block has adequate seismic data, but such
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data may be incomplete in a new block. Drilling a new well in a new block faces a
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higher failure risk due to the high cost of drilling. The sedimentary characteristics of
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shale during deposition may be inaccurately defined when based on analyses of
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adjacent well drilling data. However, previous analyses of log data have shown a good
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positive correlation between elemental uranium and shale TOC content (Beers, 1945;
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Schmoker, 1981; Fisher et al., 2001; Chen et al., 2004; Lu et al., 2006; Zhao et al.,
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2016c).
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Shale contains various elements including silicon, calcium, aluminum, uranium,
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thorium, potassium, iron, and manganese. Previous researchers studying the sources
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of siliceous minerals, such as Holdaway and Clayton (1982), defined the concept of
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excess silicon (i.e., siliceous minerals beyond the normal terrigenous clastic sources)
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and proposed a method to quantitatively calculate excess siliceous content.
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Furthermore, Wedepohl (1971), Adachi et al. (1986) and Yamamoto (1987) proposed
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a method for determining whether silicate minerals were derived from a hydrothermal
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origin or biogenesis using an Al-Fe-Mn ternary plot.
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This study tested the prediction of organic-rich shale development areas by
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investigating the origin and distribution of uranium elements in Lower Cambrian
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shales, providing another effective method for shale gas exploration in southern China.
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minerals and excess siliceous mineral content in shale siliceous minerals while
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determining the origin of the excess silicon. Further analysis of the relationship
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between excess silicon and uranium clarified the source of uranium in Lower
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Cambrian shales, the relevant tectonic setting, and the relationship with enrichment of
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organic matter. This approach allowed the distribution of organic-rich shales in the
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Chinese Lower Cambrian to be summarized, providing the basis for prediction of
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favorable areas for shale gas exploration.
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2. Geologic setting
During the Early Cambrian, the continental crust in southern China was divided
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by an extensional boundary between the cratonic Yangtze plate and the Cathaysian
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plate (Li et al., 1995; Li et al., 2002; Wang et al., 2003). Oceanic transgression
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resulted in the deposition of organic-rich shales covering most of the area, followed
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by regression that deposited fine-grained shale and silty shale, siltstone, sandstone,
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and other coarse-grained clastic rocks. During the Ordovician, the sea level remained
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low due to the collision of these two plates, and the sedimentary system was
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transformed from clastic to carbonate. In the Silurian, transgression re-occurred and
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the depositional setting changed back to clastic. The oceanic basin between the two
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plates gradually subducted during ongoing collision, eventually integrating the
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Yangtze and Cathaysian plates into the unified South China plate at the end of the
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Silurian.
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ACCEPTED MANUSCRIPT The Early Cambrian sedimentary environments of these plates mirrored one
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another (Fig. 1), consisting of ancient lands farthest from the plate boundary followed
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by a gradual transition into deeper water toward the plate junction through shallow
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shelf, deep shelf, continental slope, and ocean basin settings (Zhu et al., 2003; Zhu et
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al., 2006). Due to the vast area involved, the successional strata were named
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differently in different regions, being called the Wangyinpu Formation in the Lower
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Yangtze region, the Qiongzhusi Formation in the northwestern Yangtze plate, and the
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Niutitang Formation in the southeastern Yangtze plate. All of these formations consist
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of black to dark gray organic-rich siliceous shale deposited in the Early Cambrian and
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are among the key exploration targets for shale gas in China; these were the focus of
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this study.
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3. Materials and experimental methodology
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3.1 Data sources
Log data for U, Si, and Al were provided by the Schlumberger Corp. from three
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wells in the Lower Cambrian: Jiangyi-1 in the Lower Yangtze Xiuwu basin,
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Tianxing-1 in the southeastern Upper Yangtze region, and Wei-201 in northwestern
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Upper Yangtze region (Fig. 1). Elemental analyses for 84 cuttings from Jiangye-1
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were performed using an X-ray fluorescence model (Axios-MAX). A total organic
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carbon analyzer (OG-2000 V) was used to test the TOC of 65 cores from Wei-201, 75
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cores from Tianxing-1, and 26 cores from Jiangye-1.
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3.2 Source analysis of excess siliceous minerals
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ACCEPTED MANUSCRIPT Siliceous sources are grouped into three types: normal terrestrial deposits,
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hydrothermal silicon in special cases, and biogenic silicon (Bostrom et al., 1973;
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Murray et al., 1991; Liu and Zheng., 1993; Yang et al., 1999; Liu et al., 2017; Zhao et
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al., 2017a). Excess siliceous mineral content (Siex), referring to siliceous minerals
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excluding normal terrigenous clastic deposits, can be calculated as：
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Si
= Si − (Si/Al)
× Al
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(1)
where Sis is the elemental silicon content in the sample, Als is the elemental
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aluminum content in the sample, and (Si/Al) bg is 3.11 (the average content of the
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shale) (Holdaway et al., 1982). When this formula was applied to Jiangye-1 in the
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Lower Cambrian Wangyinpu Formation, excess siliceous minerals were found in most
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layers. When present, this excess siliceous mineral content was mostly between 20–
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30%, reaching 40–50% in some cases.
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The element test values of Al, Fe, and Mn in layers with excess siliceous
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minerals from Jiangye-1 in the Wangyinpu Formation were plotted on an Al-Fe-Mn
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ternary diagram (Fig. 3). All were located in or very near the hydrothermal origin area,
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indicating that these excess siliceous minerals were derived from hydrothermal fluids,
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supporting the siliceous sources shown in Fig. 2. This paper does not present the Al,
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Fe and Mn element test values from the Tianxing-1 and Wei-201 wells.
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The excess siliceous mineral content from all three wells were calculated as
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shown in Fig. 4. As these were in the same sedimentary system during the Early
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Cambrian (Fig. 1), the influencing factors for sediment deposition should also have
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been the same, meaning that the source of the excess siliceous minerals should have
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been hydrothermal in all cases. Insert Figure2-4
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4. Results and discussion
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4.1 Uranium source analysis
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There are five primary sources of uranium in rocks: hydrothermal fluids rich in
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uranium from the crust (Bostrom et al., 1973; Blatt, et al., 1987; Liu and Zheng.,
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1993), uranium-derived solutions that precipitated during the cooling crystallization of
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magma after volcanic eruptions (Smith et al., 1982; Murray et al., 1991; Khalil et al.,
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2007; Schmitt et al., 2011), weathering and denudation of magmatic rocks during
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sedimentation periods and diagenesis (Liu, et al., 1992; Lu, 2000; Dosseto et al., 2006;
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Polito et al., 2006), groundwater leached from magmatic rocks (Cuney, 1978; Leroy,
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1978; Pecher et al., 1985; Negga, 1986), and mixed genesis controlled by two or more
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of these factors (Min et al., 1999; Derome, 2005; Min et al., 2005).
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Previous analyses of tectonic and sedimentary features during the Early
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Cambrian in southern China have shown no evidence of large-scale volcanic activity,
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indicating that the uranium content did not come from volcanic eruptions (Li et al.,
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1995; Li et al., 2002; Wang et al., 2003; Zhu et al., 2003; Wei et al., 2006; Zhu et al.,
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2006). If the uranium originated from terrestrial sources, its content should gradually
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decrease with distance from the ancient land and toward the plate boundary; however,
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both excess silica and uranium levels actually increase in this direction (Fig. 5),
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eliminating a terrestrial source.
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There was a positive correlation between uranium content and excess siliceous
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derived from hydrothermal fluids, this indicates that the uranium was also derived
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from this source. Extension activities between the Yangtze and Cathaysian plates were
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intense during this time, developing faults reaching the deep crust. It is well-known
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that hydrothermal fluids in the deep crust can carry various minerals toward the ocean
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(i.e., silicon, uranium, phosphorus, and other metal/non-metal elements) (Zhang et al.,
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2017a; Zhang et al., 2018a; Zhang et al., 2018b), so this is a likely source for both the
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excess hydrothermal silica and the uranium present in this study’s well cores.
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Insert Figure5-6
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4.2 Influence of hydrothermal activity and uranium on organic matter
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enrichment
Bio-productivity is related closely to hydrothermal activity, the original source of
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the uranium present in the cored sediments (Liang et al., 2014a). A study in the Fiji
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basin found that the quantity and activity intensity of organisms near a hydrothermal
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area was 1–3 orders of magnitude higher than in normal marine surface water
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(Koschinsky et al., 2002). Many dissolved elements transported upward by
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hydrothermal activity are rare at the crustal surface despite being necessary nutrients
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for phytoplankton growth, such as nitrogen, phosphorus, and potassium (Korzhinsky,
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et al., 1994). The introduction of these hydrothermal nutrients into the water column
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promotes plankton production and thus enhances bio-productivity. These abundant
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organisms eventually return to the ocean bottom, producing organic matter
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enrichment in source rocks (Marchig et al., 1982; Sun et al., 2004; Zhang et al., 2010;
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He et al., 2011). On the other hand, the inflow of hydrothermal fluids strengthens the water
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reducibility (Sun et al., 2003). Increased biological respiration and organic matter
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decomposition resulting from intense bio-productivity in hydrothermal areas can
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consume large amounts of oxygen in the water. This leads to stratification, in which
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the surface contains more oxygen and the bottom less, conditions favorable to the
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preservation of sedimentary organic matter (McKibben et al., 1990; Korzhinsky et al.,
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1994; Halbach et al., 2001; Liang et al., 2014b).
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Uranium’s inherent radioactivity can also enhance biological productivity, as
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medical and nuclear studies have shown this can lead to anomalous organism growth
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(Ryabokon et al., 2005; Geras'kin et al., 2008; Buesseler et al., 2011; Cerne et al.,
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2012). Although uranium radiation from Lower Cambrian shale is lower than artificial
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nuclear radiation, low-level radiation can also cause DNA to mutate with prolonged
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exposure, causing abnormal growth in organisms (Lin et al., 2006). As a result,
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organisms become larger in body size and increase in number. The TOC and uranium
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levels in this study’s three wells were positively correlated with each other and
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distance from the plate boundary, suggesting that hydrothermally sourced radioactive
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uranium may have increased the TOC content in these shales (Figure 7).
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Insert Figure7
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4.3 Sedimentary model for siliceous mineral and uranium distribution and their
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geological significance for shale gas
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Zhang et al. (2017b, 2017c) and Zhu et al. (2003, 2006) focused on sedimentary
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Yangtze area changed respectively from ancient land, shoreland, shallow-water shelf,
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deep-water shelf, slope and deep-water basin, to slope, deep-water shelf,
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shallow-water shelf, shoreland and ancient land towards the Cathaysian plate.
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According to this study and previous sedimentary context of study area, it
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summarized a sedimentary model of origins of siliceous minerals and uranium for the
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Lower Cambrian in South China. (Fig. 8). During this time, hydrothermal fluids rich
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in silicon, uranium, and other elements from the deep crust entered the oceanic basin
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due to extension between the Cathaysian Yangtze plates and were distributed into the
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deep and shallow shelf areas (far from the plate boundary) by upwelling. With
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increasing distance from the boundary, levels of excess silicon and uranium gradually
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decreased. In this manner, hydrothermal activity rich in silicon and uranium increased
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the anoxic nature of bottom water and promoted plankton production by increasing
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nutrient levels via upwelling. In addition, natural uranium radiation could have
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encouraged abnormal organism growth in size and number (Lin et al., 2006; Zhao et
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al., 2016). This would improve biological productivity and contribute to the
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enrichment of sedimentary organic matter, resulting in a higher TOC content closer to
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the plate boundary.
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Insert Figure8
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The formation of shale gas reservoirs requires a certain material basis; high TOC
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content in shale plays an important role in controlling shale gas enrichment. Organic
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matter in the mature and over-mature stages can provide a steady supply for shale gas
ACCEPTED MANUSCRIPT enrichment (Kent, 2007; Guo, 2013). This high TOC content is conducive to the
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development of organic pores, which provide an important storage space for shale gas
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(Ji et al., 2014; Tang et al., 2015; Ji et al., 2016; Tang et al., 2016; Tang et al., 2017;
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Zhao et al., 2017b). Due to their good connectivity, organic pores can also act as
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channels for shale gas seepage with bedding planes (Wang et al., 2016a; Wang et al.,
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2016b). The adsorption capacity of organic matter is relatively high, such that a high
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TOC content can increase the content of the adsorbed gas. Natural gas molecules can
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block channels of shale gas loss and enhance the vertical and lateral self-sealing
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ability of the shale.
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The study area in southern China, composed of the Cathaysian plate and the
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Yangtze plate during the Early Cambrian, has undergone a complex and multi-stage
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thermal evolution and structural evolution (Hong et al., 2005; Mei et al., 2012; Li et
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al., 2015). The evolution of organic pores can be divided into 3 stages: formation
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(0.60%＜Ro ≤ 2.00%), development (2.00%＜Ro ≤ 3.50%) and destruction (Ro >
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3.50%) (Chen et al., 2014). When organic matter begins to convert into oil and gas
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(Ro > 0.60%), organic pores develop and grow significantly in the pyrolysis stage of
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liquid hydrocarbon and kerogen (Reed et al., 2007; Slatt et al., 2011 Curtis et al.,
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2012). However, when this organic matter evolves into the over-mature stage (Ro >
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3.50%), it becomes graphitized and organic pores collapse, decreasing in number or
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disappearing (Wang et al., 2017), reducing the adsorption capacity of organic matter
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to natural gas (Wang et al., 2013; Wang et al., 2014).
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Multi-stage and violent tectonic movements deform formations through folding,
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Generally, a gentle anticline or syncline is conducive to the preservation of shale gas,
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while more developed faults will accelerate the gas loss (Guo, 2013; Guo, 2015; Jin et
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al., 2016; He et al., 2017). Therefore, when selecting favorable areas for gas
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exploration, the degree of thermal evolution and regional structural characteristics
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should be considered along with the material basis. This study’s results suggest that
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shale gas exploration in Early Cambrian formations in South China should be focused
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on areas with a high uranium and organic matter content with moderately matured and
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well-preserved blocks near the junction of the Yangtze and Cathaysian plates.
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5. Conclusions
Based on log data for U, Si, Al, and TOC from three typical shale-gas-producing
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wells in South China and further analysis of the lithofacies and elements in the
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Jiangye-1 well, the following can be concluded:
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(1) In addition to siliceous minerals of terrigenous origin, Early Cambrian shale
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in this region contains significant siliceous mineral and elemental uranium content of
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hydrothermal origin, both of which tend to increase towards the boundary between the
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Yangtze and Cathaysian plates.
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(2) Hydrothermal activity enriched with silicon and uranium is favorable for the
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enrichment of sedimentary organic matter, resulting in an increase in shale TOC
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content. Shale gas exploration in the Early Cambrian shales within the Yangtze plate
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should target areas with high uranium content, moderate thermal maturity, and good
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preservation potential near the boundary of the Yangtze and Cathaysian plates.
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Acknowledgments
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This study was supported by the fund from Sinopec Key Laboratory of Shale Oil
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and
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5800-17-ZS-ZZGY002) and the National Science and Technology Major Project (No.
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2017ZX05036-002-001,2016ZX05060, 2016ZX05061). We sincerely appreciate all
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anonymous reviewers and the handling editor for their critical comments and
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constructive suggestions.
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Fig. 1 Early Cambrian depositional settings over the Yangtze and Cathaysian plates in
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South China, with locations of the Jiangye-1, Tianxing-1, and Wei-201 wells used in
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this study. Modified from Zhu et al. (2003), Zhu et al. (2006), and Liu et al. (2017).
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Fig. 2 Abundance and sources of Si and Al, excess Si, and siliceous mineral origin
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from the Jiangye-1 well in the Lower Cambrian Wangyinpu Formation (location given
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in Fig. 1).
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Fig. 3 Identification of Si origin using an Al-Fe-Mn ternary plot modified from
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Wedepohl (1971), Adachi et al. (1986), and Yamamoto (1987). Excess siliceous
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minerals from the Jiangye-1 well (location given in Fig. 1) in the Lower Cambrian
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Wangyinpu Formation clearly plot in or near the hydrothermal origin zone.
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Fig. 4 Levels of Si, Al, excess siliceous minerals, U, and TOC content by depth in
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comparable formations within the Wei-201, Tianxing-1, and Jiangyi-1 wells (locations
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Fig. 5 Comparison of (A) distance from the plate boundary, (B) excess silica content,
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Fig. 6 Comparison of excess silica and uranium content in the (A) Wei-201, (B)
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Tianxing-1, and (C) Jiangye-1 wells, showing a clear positive correlation between the
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two (well locations given in Fig. 1).
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Fig. 7 Comparison of TOC and uranium levels from the Wei-201, Tianxing-1, and
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Jiangye-1 wells, showing a positive correlation (well locations are given in Fig. 1).
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Fig. 8 Proposed model for the source and distribution of siliceous minerals and
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elemental uranium within the ocean basin between the Yangtze and Cathaysian plates
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in the Early Cambrian of South China. Both elements are sourced from the deep crust
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and enter the ocean at the plate boundary via hydrothermal fluids, with further
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Fig. 1 Early Cambrian depositional settings over the Yangtze and Cathaysian plates in
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Fig. 2 Abundance and sources of Si and Al, excess Si, and siliceous mineral origin from the Jiangye-1 well in the Lower Cambrian Wangyinpu Formation (location given in Fig. 1).
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Fig. 4 Levels of Si, Al, excess siliceous minerals, U, and TOC content by depth in comparable formations within the Wei-201, Tianxing-1, and Jiangyi-1 wells (locations given in Fig. 1).
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Fig. 5 Comparison of (A) distance from the plate boundary, (B) excess silica content, and (C) uranium content from the Wei-201, Tianxing-1, and Jiangye-1 wells (locations given in Fig. 1).
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Fig. 6 Comparison of excess silica and uranium content in the (A) Wei-201, (B) Tianxing-1, and (C) Jiangye-1 wells, showing a clear positive correlation between the two (well locations given in Fig. 1).
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Fig. 7 Comparison of TOC and uranium levels from the Wei-201, Tianxing-1, and Jiangye-1 wells, showing a positive correlation (well locations are given in Fig. 1).
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Fig. 8 Proposed model for the source and distribution of siliceous minerals and elemental uranium within the ocean basin between the Yangtze and Cathaysian plates in the Early Cambrian of South China. Both elements are sourced from the deep crust and enter the ocean at the plate boundary via hydrothermal fluids, with further distribution by upwelling.
ACCEPTED MANUSCRIPT Highlights (1) Early Cambrian shales within the Yangtze plate are enriched in excess silicon and uranium.
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(2) These enriched elements are of hydrothermal origin related to the local plate boundary.
(3) A clear relationship exists between uranium and marine organic matter
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enrichment.
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moderately mature areas.
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(4) Shale gas exploration in this region should thus focus on high-uranium,
S0264-8172(18)30472-0
DOI:
https://doi.org/10.1016/j.marpetgeo.2018.11.002
Reference:
JMPG 3569
To appear in:
Marine and Petroleum Geology
Received Date: 1 June 2018 Revised Date:
30 September 2018
Accepted Date: 3 November 2018
Please cite this article as: Jiang, T., Jin, Z., Liu, G., Liu, Q., Gao, B., Liu, Z., Nie, H., Zhao, J., Wang, R., Zhu, T., Yang, T., Source analysis of siliceous minerals and uranium in Early Cambrian shales, South China: Significance for shale gas exploration, Marine and Petroleum Geology (2018), doi: https:// doi.org/10.1016/j.marpetgeo.2018.11.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Source analysis of siliceous minerals and uranium in Early Cambrian shales,
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South China: Significance for shale gas exploration
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Tao Jianga,b,c,d,e , Zhijun Jina,b,c,d,e, *, Guangxiang Liu c,d,e, Quanyou Liu c,d,e,
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Bo Gao c,d,e, Zhongbao Liu c,d,e, Haikuan Nie c,d,e, Jianhua Zhao c,d,e,f, Ruyue Wang c,d,e,
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Tong Zhu c,d,e, Tao Yang c,d,e,g
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a
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School of Earth and Space Sciences, Peking University, Beijing 100871, China
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b
Institute of Oil & Gas, Peking University, Beijing 100871, China
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c
State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective
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Key Laboratory of Orogenic Belt and Crustal Evolution, Ministry of Education,
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Development, Beijing 100083, China
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d
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100083, China
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e
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100083, China
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f
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266580, China
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g
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China
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*
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Mechanisms and Effective Development, Beijing 100083, China
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E-mail addresses: [email protected] (Z, Jin)
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Key Laboratory of Shale Oil/Gas Exploration and Production, SINOPEC, Beijing
Petroleum Exploration and Production Research Institute, SINOPEC, Beijing
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School of Geosciences, China University of Petroleum (East China), Qingdao
College of Geosciences, China University of Petroleum (Beijing), Beijing 102249,
Corresponding author. State Key Laboratory of Shale Oil and Gas Enrichment
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Abstract Efficient shale gas exploration in the Early Cambrian shales of southern China
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requires the identification of favorable target areas. This study used logs for U, Si, Al,
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and total organic carbon (TOC) from three typical wells to assess a new framework
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for gas exploration in this area. The excess siliceous mineral content (from a
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non-terrigenous clastic source) ranged from 20–30% in most layers, reaching 50%.
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Analysis of these excess siliceous minerals using an Al-Fe-Mn ternary plot showed
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that most were probably derived from hydrothermal fluids within the boundary of the
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Yangtze and Cathaysian plates. Excess siliceous mineral content and uranium content
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had a good positive correlation, both tending to decrease away from the plate
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boundary, indicating that both were derived from hydrothermal fluids. Radioactive
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uranium in hydrothermal fluids can enhance biological productivity, which is
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conducive to the increased accumulation of sedimentary organic matter. Therefore,
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shale gas exploration in the Early Cambrian layers of South China should focus on
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target areas near the plate boundary that have increased uranium levels, moderate
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maturity, and good preservation conditions.
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Keywords: Silicon source; uranium source; hydrothermal solution; plate junction;
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Yangtze plate
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1. Introduction
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Since 2000, shale gas production has achieved great success in North America
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due to theoretical and technical advances in exploration and development methods
ACCEPTED MANUSCRIPT (Curtis, 2002; Montgomery et al., 2005; Warlick, 2006). Huge shale gas resources
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have also been found recently in China. In 2010, the China National Petroleum
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Corporation (CNPC) made a new industrial breakthrough in shale gas development
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with the Wei 201 well, drilled in the southern Sichuan Basin. In 2012, the China
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Petroleum & Chemical Corporation (Sinopec) achieved a breakthrough in marine
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shale gas exploration in the Jiaoshiba block in the southern part of the Fuling shale
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gas field. In 2015, the production capacity of this field reached 50×108 m3, becoming
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the second largest shale gas field in the world after North America. Subsequently,
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successful commercial shale gas development has been carried out in the Weiyuan,
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Changning, Zhaotong, Fushun-Yongchuan, Fuling, and Ding Shan gas fields (Dong et
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al., 2016; Guo, 2016a; Guo et al., 2016b; Wei et al., 2017).
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However, exploration for shale gas in Lower Cambrian formations has resulted
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in a number of low-production or failed wells, primarily due to a the lack of material
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basis in the shale gas reservoir process. Therefore, locating zones containing
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organic-rich shale (i.e., total organic content (TOC) content > 2%) is an important
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challenge to solve (Jarvie et al., 2007; Ross et al., 2009; Zhao et al., 2016a). Previous
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studies have proposed a series of methods to find organic-rich shale development
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areas, such as using seismic interpretation to calculate the thickness of organic-rich
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shale in a given block (Goodway et al., 2010; Zhu et al., 2011), using organic-rich
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shale intervals drilled in wildcat wells to determine the connecting-well section and
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study its distribution (Montgomery et al., 2005; Bowker, 2007), and utilizing regional
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sedimentary characteristics during shale deposition to predict the distribution of
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organic-rich shale (Loucks et al., 2007; Zhao et al., 2016b; Zhang et al., 2017a; Zhang
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et al., 2018a). However, these approaches have limitations when implemented. The seismic
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interpretation method assumes that a certain block has adequate seismic data, but such
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data may be incomplete in a new block. Drilling a new well in a new block faces a
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higher failure risk due to the high cost of drilling. The sedimentary characteristics of
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shale during deposition may be inaccurately defined when based on analyses of
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adjacent well drilling data. However, previous analyses of log data have shown a good
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positive correlation between elemental uranium and shale TOC content (Beers, 1945;
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Schmoker, 1981; Fisher et al., 2001; Chen et al., 2004; Lu et al., 2006; Zhao et al.,
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2016c).
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Shale contains various elements including silicon, calcium, aluminum, uranium,
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thorium, potassium, iron, and manganese. Previous researchers studying the sources
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of siliceous minerals, such as Holdaway and Clayton (1982), defined the concept of
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excess silicon (i.e., siliceous minerals beyond the normal terrigenous clastic sources)
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and proposed a method to quantitatively calculate excess siliceous content.
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Furthermore, Wedepohl (1971), Adachi et al. (1986) and Yamamoto (1987) proposed
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a method for determining whether silicate minerals were derived from a hydrothermal
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origin or biogenesis using an Al-Fe-Mn ternary plot.
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This study tested the prediction of organic-rich shale development areas by
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investigating the origin and distribution of uranium elements in Lower Cambrian
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shales, providing another effective method for shale gas exploration in southern China.
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minerals and excess siliceous mineral content in shale siliceous minerals while
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determining the origin of the excess silicon. Further analysis of the relationship
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between excess silicon and uranium clarified the source of uranium in Lower
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Cambrian shales, the relevant tectonic setting, and the relationship with enrichment of
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organic matter. This approach allowed the distribution of organic-rich shales in the
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Chinese Lower Cambrian to be summarized, providing the basis for prediction of
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favorable areas for shale gas exploration.
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2. Geologic setting
During the Early Cambrian, the continental crust in southern China was divided
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by an extensional boundary between the cratonic Yangtze plate and the Cathaysian
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plate (Li et al., 1995; Li et al., 2002; Wang et al., 2003). Oceanic transgression
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resulted in the deposition of organic-rich shales covering most of the area, followed
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by regression that deposited fine-grained shale and silty shale, siltstone, sandstone,
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and other coarse-grained clastic rocks. During the Ordovician, the sea level remained
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low due to the collision of these two plates, and the sedimentary system was
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transformed from clastic to carbonate. In the Silurian, transgression re-occurred and
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the depositional setting changed back to clastic. The oceanic basin between the two
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plates gradually subducted during ongoing collision, eventually integrating the
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Yangtze and Cathaysian plates into the unified South China plate at the end of the
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Silurian.
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ACCEPTED MANUSCRIPT The Early Cambrian sedimentary environments of these plates mirrored one
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another (Fig. 1), consisting of ancient lands farthest from the plate boundary followed
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by a gradual transition into deeper water toward the plate junction through shallow
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shelf, deep shelf, continental slope, and ocean basin settings (Zhu et al., 2003; Zhu et
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al., 2006). Due to the vast area involved, the successional strata were named
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differently in different regions, being called the Wangyinpu Formation in the Lower
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Yangtze region, the Qiongzhusi Formation in the northwestern Yangtze plate, and the
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Niutitang Formation in the southeastern Yangtze plate. All of these formations consist
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of black to dark gray organic-rich siliceous shale deposited in the Early Cambrian and
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are among the key exploration targets for shale gas in China; these were the focus of
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this study.
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3. Materials and experimental methodology
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3.1 Data sources
Log data for U, Si, and Al were provided by the Schlumberger Corp. from three
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wells in the Lower Cambrian: Jiangyi-1 in the Lower Yangtze Xiuwu basin,
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Tianxing-1 in the southeastern Upper Yangtze region, and Wei-201 in northwestern
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Upper Yangtze region (Fig. 1). Elemental analyses for 84 cuttings from Jiangye-1
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were performed using an X-ray fluorescence model (Axios-MAX). A total organic
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carbon analyzer (OG-2000 V) was used to test the TOC of 65 cores from Wei-201, 75
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cores from Tianxing-1, and 26 cores from Jiangye-1.
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3.2 Source analysis of excess siliceous minerals
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ACCEPTED MANUSCRIPT Siliceous sources are grouped into three types: normal terrestrial deposits,
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hydrothermal silicon in special cases, and biogenic silicon (Bostrom et al., 1973;
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Murray et al., 1991; Liu and Zheng., 1993; Yang et al., 1999; Liu et al., 2017; Zhao et
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al., 2017a). Excess siliceous mineral content (Siex), referring to siliceous minerals
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excluding normal terrigenous clastic deposits, can be calculated as：
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Si
= Si − (Si/Al)
× Al
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(1)
where Sis is the elemental silicon content in the sample, Als is the elemental
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aluminum content in the sample, and (Si/Al) bg is 3.11 (the average content of the
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shale) (Holdaway et al., 1982). When this formula was applied to Jiangye-1 in the
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Lower Cambrian Wangyinpu Formation, excess siliceous minerals were found in most
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layers. When present, this excess siliceous mineral content was mostly between 20–
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30%, reaching 40–50% in some cases.
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The element test values of Al, Fe, and Mn in layers with excess siliceous
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minerals from Jiangye-1 in the Wangyinpu Formation were plotted on an Al-Fe-Mn
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ternary diagram (Fig. 3). All were located in or very near the hydrothermal origin area,
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indicating that these excess siliceous minerals were derived from hydrothermal fluids,
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supporting the siliceous sources shown in Fig. 2. This paper does not present the Al,
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Fe and Mn element test values from the Tianxing-1 and Wei-201 wells.
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The excess siliceous mineral content from all three wells were calculated as
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shown in Fig. 4. As these were in the same sedimentary system during the Early
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Cambrian (Fig. 1), the influencing factors for sediment deposition should also have
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been the same, meaning that the source of the excess siliceous minerals should have
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been hydrothermal in all cases. Insert Figure2-4
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4. Results and discussion
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4.1 Uranium source analysis
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There are five primary sources of uranium in rocks: hydrothermal fluids rich in
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uranium from the crust (Bostrom et al., 1973; Blatt, et al., 1987; Liu and Zheng.,
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1993), uranium-derived solutions that precipitated during the cooling crystallization of
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magma after volcanic eruptions (Smith et al., 1982; Murray et al., 1991; Khalil et al.,
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2007; Schmitt et al., 2011), weathering and denudation of magmatic rocks during
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sedimentation periods and diagenesis (Liu, et al., 1992; Lu, 2000; Dosseto et al., 2006;
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Polito et al., 2006), groundwater leached from magmatic rocks (Cuney, 1978; Leroy,
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1978; Pecher et al., 1985; Negga, 1986), and mixed genesis controlled by two or more
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of these factors (Min et al., 1999; Derome, 2005; Min et al., 2005).
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Previous analyses of tectonic and sedimentary features during the Early
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Cambrian in southern China have shown no evidence of large-scale volcanic activity,
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indicating that the uranium content did not come from volcanic eruptions (Li et al.,
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1995; Li et al., 2002; Wang et al., 2003; Zhu et al., 2003; Wei et al., 2006; Zhu et al.,
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2006). If the uranium originated from terrestrial sources, its content should gradually
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decrease with distance from the ancient land and toward the plate boundary; however,
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both excess silica and uranium levels actually increase in this direction (Fig. 5),
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eliminating a terrestrial source.
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There was a positive correlation between uranium content and excess siliceous
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derived from hydrothermal fluids, this indicates that the uranium was also derived
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from this source. Extension activities between the Yangtze and Cathaysian plates were
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intense during this time, developing faults reaching the deep crust. It is well-known
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that hydrothermal fluids in the deep crust can carry various minerals toward the ocean
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(i.e., silicon, uranium, phosphorus, and other metal/non-metal elements) (Zhang et al.,
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2017a; Zhang et al., 2018a; Zhang et al., 2018b), so this is a likely source for both the
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excess hydrothermal silica and the uranium present in this study’s well cores.
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Insert Figure5-6
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4.2 Influence of hydrothermal activity and uranium on organic matter
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enrichment
Bio-productivity is related closely to hydrothermal activity, the original source of
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the uranium present in the cored sediments (Liang et al., 2014a). A study in the Fiji
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basin found that the quantity and activity intensity of organisms near a hydrothermal
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area was 1–3 orders of magnitude higher than in normal marine surface water
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(Koschinsky et al., 2002). Many dissolved elements transported upward by
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hydrothermal activity are rare at the crustal surface despite being necessary nutrients
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for phytoplankton growth, such as nitrogen, phosphorus, and potassium (Korzhinsky,
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et al., 1994). The introduction of these hydrothermal nutrients into the water column
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promotes plankton production and thus enhances bio-productivity. These abundant
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organisms eventually return to the ocean bottom, producing organic matter
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enrichment in source rocks (Marchig et al., 1982; Sun et al., 2004; Zhang et al., 2010;
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He et al., 2011). On the other hand, the inflow of hydrothermal fluids strengthens the water
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reducibility (Sun et al., 2003). Increased biological respiration and organic matter
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decomposition resulting from intense bio-productivity in hydrothermal areas can
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consume large amounts of oxygen in the water. This leads to stratification, in which
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the surface contains more oxygen and the bottom less, conditions favorable to the
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preservation of sedimentary organic matter (McKibben et al., 1990; Korzhinsky et al.,
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1994; Halbach et al., 2001; Liang et al., 2014b).
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Uranium’s inherent radioactivity can also enhance biological productivity, as
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medical and nuclear studies have shown this can lead to anomalous organism growth
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(Ryabokon et al., 2005; Geras'kin et al., 2008; Buesseler et al., 2011; Cerne et al.,
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2012). Although uranium radiation from Lower Cambrian shale is lower than artificial
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nuclear radiation, low-level radiation can also cause DNA to mutate with prolonged
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exposure, causing abnormal growth in organisms (Lin et al., 2006). As a result,
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organisms become larger in body size and increase in number. The TOC and uranium
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levels in this study’s three wells were positively correlated with each other and
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distance from the plate boundary, suggesting that hydrothermally sourced radioactive
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uranium may have increased the TOC content in these shales (Figure 7).
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Insert Figure7
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4.3 Sedimentary model for siliceous mineral and uranium distribution and their
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geological significance for shale gas
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Zhang et al. (2017b, 2017c) and Zhu et al. (2003, 2006) focused on sedimentary
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Yangtze area changed respectively from ancient land, shoreland, shallow-water shelf,
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deep-water shelf, slope and deep-water basin, to slope, deep-water shelf,
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shallow-water shelf, shoreland and ancient land towards the Cathaysian plate.
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According to this study and previous sedimentary context of study area, it
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summarized a sedimentary model of origins of siliceous minerals and uranium for the
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Lower Cambrian in South China. (Fig. 8). During this time, hydrothermal fluids rich
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in silicon, uranium, and other elements from the deep crust entered the oceanic basin
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due to extension between the Cathaysian Yangtze plates and were distributed into the
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deep and shallow shelf areas (far from the plate boundary) by upwelling. With
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increasing distance from the boundary, levels of excess silicon and uranium gradually
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decreased. In this manner, hydrothermal activity rich in silicon and uranium increased
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the anoxic nature of bottom water and promoted plankton production by increasing
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nutrient levels via upwelling. In addition, natural uranium radiation could have
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encouraged abnormal organism growth in size and number (Lin et al., 2006; Zhao et
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al., 2016). This would improve biological productivity and contribute to the
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enrichment of sedimentary organic matter, resulting in a higher TOC content closer to
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the plate boundary.
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Insert Figure8
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The formation of shale gas reservoirs requires a certain material basis; high TOC
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content in shale plays an important role in controlling shale gas enrichment. Organic
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matter in the mature and over-mature stages can provide a steady supply for shale gas
ACCEPTED MANUSCRIPT enrichment (Kent, 2007; Guo, 2013). This high TOC content is conducive to the
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development of organic pores, which provide an important storage space for shale gas
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(Ji et al., 2014; Tang et al., 2015; Ji et al., 2016; Tang et al., 2016; Tang et al., 2017;
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Zhao et al., 2017b). Due to their good connectivity, organic pores can also act as
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channels for shale gas seepage with bedding planes (Wang et al., 2016a; Wang et al.,
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2016b). The adsorption capacity of organic matter is relatively high, such that a high
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TOC content can increase the content of the adsorbed gas. Natural gas molecules can
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block channels of shale gas loss and enhance the vertical and lateral self-sealing
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ability of the shale.
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The study area in southern China, composed of the Cathaysian plate and the
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Yangtze plate during the Early Cambrian, has undergone a complex and multi-stage
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thermal evolution and structural evolution (Hong et al., 2005; Mei et al., 2012; Li et
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al., 2015). The evolution of organic pores can be divided into 3 stages: formation
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(0.60%＜Ro ≤ 2.00%), development (2.00%＜Ro ≤ 3.50%) and destruction (Ro >
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3.50%) (Chen et al., 2014). When organic matter begins to convert into oil and gas
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(Ro > 0.60%), organic pores develop and grow significantly in the pyrolysis stage of
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liquid hydrocarbon and kerogen (Reed et al., 2007; Slatt et al., 2011 Curtis et al.,
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2012). However, when this organic matter evolves into the over-mature stage (Ro >
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3.50%), it becomes graphitized and organic pores collapse, decreasing in number or
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disappearing (Wang et al., 2017), reducing the adsorption capacity of organic matter
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to natural gas (Wang et al., 2013; Wang et al., 2014).
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Multi-stage and violent tectonic movements deform formations through folding,
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Generally, a gentle anticline or syncline is conducive to the preservation of shale gas,
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while more developed faults will accelerate the gas loss (Guo, 2013; Guo, 2015; Jin et
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al., 2016; He et al., 2017). Therefore, when selecting favorable areas for gas
269
exploration, the degree of thermal evolution and regional structural characteristics
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should be considered along with the material basis. This study’s results suggest that
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shale gas exploration in Early Cambrian formations in South China should be focused
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on areas with a high uranium and organic matter content with moderately matured and
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well-preserved blocks near the junction of the Yangtze and Cathaysian plates.
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5. Conclusions
Based on log data for U, Si, Al, and TOC from three typical shale-gas-producing
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wells in South China and further analysis of the lithofacies and elements in the
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Jiangye-1 well, the following can be concluded:
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(1) In addition to siliceous minerals of terrigenous origin, Early Cambrian shale
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in this region contains significant siliceous mineral and elemental uranium content of
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hydrothermal origin, both of which tend to increase towards the boundary between the
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Yangtze and Cathaysian plates.
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(2) Hydrothermal activity enriched with silicon and uranium is favorable for the
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enrichment of sedimentary organic matter, resulting in an increase in shale TOC
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content. Shale gas exploration in the Early Cambrian shales within the Yangtze plate
286
should target areas with high uranium content, moderate thermal maturity, and good
ACCEPTED MANUSCRIPT 287
preservation potential near the boundary of the Yangtze and Cathaysian plates.
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Acknowledgments
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This study was supported by the fund from Sinopec Key Laboratory of Shale Oil
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and
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5800-17-ZS-ZZGY002) and the National Science and Technology Major Project (No.
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2017ZX05036-002-001,2016ZX05060, 2016ZX05061). We sincerely appreciate all
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anonymous reviewers and the handling editor for their critical comments and
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constructive suggestions.
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Fig. 1 Early Cambrian depositional settings over the Yangtze and Cathaysian plates in
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South China, with locations of the Jiangye-1, Tianxing-1, and Wei-201 wells used in
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this study. Modified from Zhu et al. (2003), Zhu et al. (2006), and Liu et al. (2017).
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Fig. 2 Abundance and sources of Si and Al, excess Si, and siliceous mineral origin
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from the Jiangye-1 well in the Lower Cambrian Wangyinpu Formation (location given
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in Fig. 1).
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Fig. 3 Identification of Si origin using an Al-Fe-Mn ternary plot modified from
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Wedepohl (1971), Adachi et al. (1986), and Yamamoto (1987). Excess siliceous
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minerals from the Jiangye-1 well (location given in Fig. 1) in the Lower Cambrian
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Wangyinpu Formation clearly plot in or near the hydrothermal origin zone.
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Fig. 4 Levels of Si, Al, excess siliceous minerals, U, and TOC content by depth in
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comparable formations within the Wei-201, Tianxing-1, and Jiangyi-1 wells (locations
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Fig. 5 Comparison of (A) distance from the plate boundary, (B) excess silica content,
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Fig. 6 Comparison of excess silica and uranium content in the (A) Wei-201, (B)
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Tianxing-1, and (C) Jiangye-1 wells, showing a clear positive correlation between the
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two (well locations given in Fig. 1).
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Fig. 7 Comparison of TOC and uranium levels from the Wei-201, Tianxing-1, and
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Jiangye-1 wells, showing a positive correlation (well locations are given in Fig. 1).
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Fig. 8 Proposed model for the source and distribution of siliceous minerals and
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elemental uranium within the ocean basin between the Yangtze and Cathaysian plates
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in the Early Cambrian of South China. Both elements are sourced from the deep crust
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and enter the ocean at the plate boundary via hydrothermal fluids, with further
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Fig. 1 Early Cambrian depositional settings over the Yangtze and Cathaysian plates in
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Fig. 2 Abundance and sources of Si and Al, excess Si, and siliceous mineral origin from the Jiangye-1 well in the Lower Cambrian Wangyinpu Formation (location given in Fig. 1).
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Fig. 4 Levels of Si, Al, excess siliceous minerals, U, and TOC content by depth in comparable formations within the Wei-201, Tianxing-1, and Jiangyi-1 wells (locations given in Fig. 1).
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Fig. 5 Comparison of (A) distance from the plate boundary, (B) excess silica content, and (C) uranium content from the Wei-201, Tianxing-1, and Jiangye-1 wells (locations given in Fig. 1).
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Fig. 6 Comparison of excess silica and uranium content in the (A) Wei-201, (B) Tianxing-1, and (C) Jiangye-1 wells, showing a clear positive correlation between the two (well locations given in Fig. 1).
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Fig. 7 Comparison of TOC and uranium levels from the Wei-201, Tianxing-1, and Jiangye-1 wells, showing a positive correlation (well locations are given in Fig. 1).
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Fig. 8 Proposed model for the source and distribution of siliceous minerals and elemental uranium within the ocean basin between the Yangtze and Cathaysian plates in the Early Cambrian of South China. Both elements are sourced from the deep crust and enter the ocean at the plate boundary via hydrothermal fluids, with further distribution by upwelling.
ACCEPTED MANUSCRIPT Highlights (1) Early Cambrian shales within the Yangtze plate are enriched in excess silicon and uranium.
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(2) These enriched elements are of hydrothermal origin related to the local plate boundary.
(3) A clear relationship exists between uranium and marine organic matter
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enrichment.
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moderately mature areas.
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(4) Shale gas exploration in this region should thus focus on high-uranium,