Sedimentary environment controls on the accumulation of organic matter in the Upper Ordovician Wufeng–Lower Silurian Longmaxi mudstones in the Southeastern Sichuan Basin of China
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The Upper Ordovician–Lower Silurian mudstones (including the Wufeng, Guanyinqiao and Longmaxi Formations) in the Sichuan Basin are some of the most important shale gas plays in China. In order to enhance our understanding of the process of formation of organic carbon up to 10%, optical, microscopy and geochemical methods have been used to investigate the petrographic and geochemical characteristics of the formation. Firstly, three mudstone lithofacies were identified based on a wide variety of mudstone laminations. These are: (a) indistinctly laminated mudstone; (b) parallel-laminated mudstone; and (c) nonparallel-laminated mudstone. Then, combining with the evidence from depocenter migration, Th/U ratios and total organic carbon, the abundant organo-minerallic fabrics suggest that organic carbon was preferentially deposited and preserved in anoxic, low energy and stagnant water conditions during deposition of the Wufeng and Longmaxi Formations. On the contrary, the Guanyinqiao Formation with poor organic carbon was deposited in oxic and high-energy water conditions.
KeywordsGuanyinqiao Formation Sichuan Basin Mudstone Laminated mudstone Water condition
Very fine-grained mudstones typically consist of various materials, including clays, quartz, and organic matter (Macquaker and Bohacs 2007). In petroleum source rocks and shale gas reservoirs, organic matter is one of the most important components of black mudstones, so organic matter accumulation is a research focus (e.g., Potter et al. 2005; Aplin and Macquaker 2011). The Upper Ordovician–Lower Silurian marine mudstones in the southeastern Sichuan Basin are one of the most important source rocks and potential shale gas plays in China (e.g., Chen et al. 2011; Liu et al. 2013; Guo and Zhang 2014; Liang et al. 2016), but the variable mechanisms of organic matter accumulation restricts understanding about Chinese marine source rocks and shale gas development. In former studies, there were four different points for the mechanism of organic matter accumulation: (1) upwelling and suspension (Liang et al. 2016); (2) low preservation, high dilution, and high organic production (Li et al. 2015); (3) a limited oxygen and anoxic environment (Liu et al. 2013; Yang et al. 2016); (4) high paleoproductivity and dysoxic/anoxic conditions (Zhao et al. 2016). These former studies were only based on geochemical and lithofacies data without constraints from mudstone lamination and the regional depositional environment. As a result, we have undertaken an investigation of the organic matter, lithofacies, mineralogy, and trace element composition of the mudstone using data from outcrops at different locations in the southeastern Sichuan Basin in this study.
2 Geological background
The Wufeng–Longmaxi mudstones of southeast Sichuan Basin are underlain by the Upper Ordovician Linxiang Formation and overlain by the Lower Silurian Shiniulan Formation. Across much of the southeast Sichuan Basin, the Wufeng–Longmaxi mudstones are subdivided into three different Formations: (1) the lower part, the Wufeng Formation is composed of black mudstone rich in graptolites and radiolaria; (2) the middle part, the Guanyinqiao Formation, is composed of gray or dark gray carbonate rocks rich in rugose corals, gastropods, bryozoa and bivalves; (3) the upper part, the Longmaxi Formation, is composed of black mudstone and dark gray siltstone rich in graptolites (Chen et al. 2004; Rong et al. 2011; Yan et al. 2009).
3 Sampling and analytical methods
3.1 Sampling and experimental protocol
Mineralogy: X-ray diffraction
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, CDUT
Experimental Center of Exploration and Development Research Institute, PetroChina Southwest Oil and Gas Field Company, Chengdu
The Institute of Tibetan Plateau Research, CAS, Beijing
Key Laboratory of Tectonic Controlled Mineralization and Oil Reservoir, Ministry of Land and Resources, CDUT
3.2 Analytical procedures
Detailed description of these outcrop intervals included macroscopic sedimentary structures, grain size, color, and bounding surfaces. Petrographic examination of 36 thin sections was completed in order to identify the microsedimentary structures in the black mudstone. Each section was examined with a stereomicroscope (Nikon AZ100M) in the Key Laboratory of Tectonic Controlled Mineralization and Oil Reservoir, Ministry of Land and Resources, Chengdu University of Technology (CDUT).
Forty-eight geochemical samples from the Wufeng and Longmaxi Formations at the Guanyinqiao section of Qijiang Country, Daijiagou section of Tongzi Country, and Rongxi section of Xiushan Country were analyzed. Prior to geochemical analysis, all fresh samples were crushed in a steel vessel and further ground to powder in an agate mill (200 mesh). The sample splits (100 mg) for element analysis were heated in an oven at 105° for 1–2 h and cooled to room temperature, then digested in a tightly sealed Teflon screw-cap vessel with 0.5 mL ultra-pure HNO3 + 2.5 mL HF + 0.5 mL HClO4, then dried. The dried sample was digested again with 1 mL HNO3 + 3 mL H2O until a clear solution was obtained. The solution was diluted to 1:1000 by mass and analyzed on a VG PQ2 Turbo inductively coupled plasma source mass spectrometer (ICP-MS) at the Institute of Tibetan Plateau Research, Chinese Academy of Sciences (CAS). Analytical precision for elemental analysis is generally greater than 4% of absolute concentrations.
4.1 Regional sedimentary characteristics
Because of the paleoclimatic and paleoceanic importance of the Ordovician–Silurian transition, most sections near the Qianzhong uplift have been studied in terms of biostratigraphy (e.g., Rong et al. 2011). In this study, the Guanyinqiao Formation at every section was used to accurately distinguish between the two series of black mudstone due to the similarity between the Wufeng and Longmaxi Formations.
4.2 Paleobiology characteristics
4.3 Microsedimentary features
Lamination in mudstones is an important feature useful in providing clues to ancient sedimentary environments and processes. Various unique macro- and microfabrics associated with lamination type offer a useful frame of reference for interpretation of conditions of mudstone formation (O'Brien 1990). The Wufeng and Longmaxi Formations contain a variety of intergradational lithofacies dominated by fine-grained (clay- to silt-size) sediment. Based on vertical facies transitions, bed types, and sedimentary textures and structures, four main general lithofacies were identified: (1) indistinctly laminated mudstone; (2) parallel-laminated mudstone; (3) nonparallel-laminated mudstone; (4) lenticular mudstone.
Lenticular layers are lenses of granule-sized particles of quartz, feldspar, and rock fragments in fine- to very fine silt-sized sandstone. The long axes of lenticular layers are aligned parallel to the bedding plane (Figs. 7, 8, samples TZ-22B and TZ-18B-4). These lithofacies were deposited by multiple episodes of plastic flows and bottom currents in a shallow marine environment (e.g., Shanmugam 2002).
Low-angled laminae appear in the Longmaxi Formation (Figs. 7, 8, samples TZ-22B and TZ-13B-3). Laminae are composed of very-fine siltstone to claystone. Low-angle and micro-cross-stratification are the diagnostic characteristics with sharp-based laminae. This lithofacies is interpreted to have formed in a shallow marine environment associated with storms or bottom currents (Abouelresh and Slatt 2011).
4.4 Total organic carbon content in the region
4.5 Trace element geochemistry
In the Wufeng Formation, Th/U ratios of these samples with higher TOC values (average > 2.0%) are mostly lower than 2.0. The opposite phenomenon occurs in Guanyinqiao samples showing higher Th/U ratios (average > 2.0) and lower TOC values (average < 2.0%). However, in the Longmaxi Formation, there is a complex change from lower Th/U ratios (average < 2.0) and higher TOC values (average > 2.0%) in the lower part of the Longmaxi Formation to higher Th/U ratios (average > 2.0) and lower TOC values (average < 2.0%) in the upper part of the Longmaxi Formation (Fig. 11).
Although the formation of organic matter is controlled by many factors, there is good agreement between Th/U and TOC indicating anoxic conditions being the primary factor based on lower Th/U ratios (< 2.0) pointing to anoxic conditions (Kimura and Watanabe 2001; Guo et al. 2007; Yan et al. 2009).
4.6 Mineral content
Detailed petrographic studies show traces of bioturbation by micro-organisms in fauna and event beds with sharp-based, graded beds and organo-minerallic aggregates. These are common in many organic-rich sediments at a micron to submillimeter scale (e.g., Macquaker and Gawthorpe 1993; Schieber 1999; Macquaker et al. 2007; Ghadeer and Macquaker 2012). This does not support the idea that long-term bottom water stagnation and anoxia is a prerequisite for the formation of organic carbon-rich sediments (e.g. Arthur and Sageman 1994; Sageman et al. 2003; Yan et al. 2009). Study of thin sections from the organic carbon-rich part (TOC > 2.0%) of the Wufeng and Longmaxi Formations during our work not only revealed evidence for bioturbation, plankton, graptolites and deep water radiolaria, but also indicated indistinctly laminated mudstone was the main lithofacies (Figs. 7, 8). In addition, the relationship between the Th/U ratio and TOC shows good correlation that most Th/U ratios of organic carbon-rich parts (TOC > 2.0%) were lower than 2.0 (Fig. 11). All this data provides evidence that the organic-rich black mudstone of the Wufeng and Longmaxi Formations has been deposited into stagnant and anoxic bottom waters and preserved.
However, there are clear differences in the depositional environments for the Wufeng and Longmaxi Formations. First, radiolaria are limited to the Wufeng Formation in the Upper Yangtze (Huang et al. 1991; Liu et al. 2010; Ran et al. 2015), but graptolites are common in both the Wufeng and Longmaxi Formations (Su et al. 2002; Chen et al. 2004). Second, indistinctly laminated mudstones are the only lithofacies in the Wufeng Formation. On the other hand, in the Longmaxi Formation, the main lithofacies show a complicated mix from indistinctly laminated mudstone at the bottom to rhythmic mudstone in the middle to lenticular and nonparallel-laminated mudstone with erosive contacts (Figs. 7, 8, 9) at the top. Third, the relationship between TOC and Th/U ratio shows a poor negative correlation (linear relationship, R2 = 0.2782) in the Wufeng Formation, but a moderate positive correlation (linear relationship, R2 = 0.5133) in the Longmaxi Formation (Fig. 11).
Above all, the evidence shows that the depositional environments for the Wufeng and Longmaxi Formations are not the same. In the Wufengian stage, the average 3.76% TOC, and the Th/U ratios lower than 2.0 in most sections show that the organic-rich mudstones may have formed under mostly anoxic stagnant conditions with (Figs. 10, 11). Former studies have shown most anoxic intervals occur predominantly during sea-level high stands, because the water depth over a basin sill changes with sea level and can have profound effects on the nutrient regime of a basin (e.g., Werne et al. 2002; Arthur and Sageman 2005; Meyer and Kump 2008). Therefore, the fact that the sea level in the Katian stage was the highest in the Ordovician and Silurian in the region suggests favorable marine environments for organic enrichment (Chen et al. 2004; Haq and Schutter 2008). At the same time, the fact that siliceous radiolaria are limited in the Wufeng Formation provides evidence of a deep water origin (Fig. 5; Liu et al. 2010), and this is also supported by the increased content of quartz (Fig. 12). After the oxic and bioturbated Guanyinqiao Formation deposited in the Hirnantian low sea level stage (Fig. 5; Chen et al. 2004; Li et al. 2005; Yan et al. 2008; 2009), the depositional environment changed in the Longmaxi Formation. Following the sea level rising again (e.g., Haq and Schutter 2008), the lower part of the Longmaxi Formation deposited into mostly anoxic conditions forming the organic-rich mudstone. However, the organic-rich mudstone (TOC > 2.0%) was limited to the Qijiang depocenter, including the Qilongcun and Guanyinqiao sections, but TOC of the black mudstone deposited into the Tongzi depocenter, including Nanbazi and Daijiagou sections, in the former stage was significantly lower than 2.0%. This indicates the clear northward migration of the organic-rich mudstone deposited area and depocenter (Figs. 4, 10). Simultaneously, in the Longmaxi Formation, the lithofacies changed from bottom massive and rhythmic to top lenticular and nonparallel-laminated mudstones, indicating increased hydrodynamic conditions from a deep water environment to a shallow water setting (Figs. 7, 8), consistent with the fluctuation of the global sea level and the resulting sea level change in Yangtze block (Feng et al. 2000; Chen et al. 2004; Haq and Schutter 2008). The obvious change of the lithofacies shows the bottom anoxic conditions were disrupted by distal turbidites and dropping sea levels (Chen et al. 2004; Guo et al. 2011; Ran et al. 2016). As a result, the preferred condition for organic matter preservation had been destroyed, directly causing the decrease of TOC and quartz content and the increase of Th/U ratio (Figs. 11, 12).
The main mechanisms responsible for sediment dispersal, delivery, and subsequent reworking that might influence organic carbon preservation in source rocks and gas mudstone were investigated in the Wufeng and Longmaxi Formations exposed in the Sichuan Basin. This succession is mainly dominated by four types of lithofacies: (a) indistinctly laminated mudstones (average 4.17, TOC, ranging from 2.14% to 10.24%, with relatively high quartz content) which are the dominant facies in the Wufeng Formation and lower part of the Longmaxi Formation; (b) rhythmic mudstones (c) lenticular mudstones and (d) nonparallel-laminated mudstones (average 1.15% TOC, ranging from 0.20% to 8.45%, with relatively low quartz content) which are the dominant facies in the upper part of the Longmaxi Formation. The data presented here suggest that organic matter preservation was controlled by low energy and anoxic conditions. Based on the obvious differences between the Wufeng and Longmaxi Formations, the two series of mudstones should be divided into independent stratigraphic units.
This study was supported by the Science and Technology Support Program of Sichuan Province (No. 15ZC1390), National Natural Science Foundation of China (No. 41102064). The authors appreciate the reviewers’ comments which improved the quality of the manuscript, and the welcome help of Zhifeng Zhang.
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