Research paperGeochemistry and sedimentology of the Lower Silurian Longmaxi mudstone in southwestern China: Implications for depositional controls on organic matter accumulation
Introduction
Mudstones with high TOC (>1.0 wt%) and high hydrocarbon yield upon pyrolysis are good source rocks (e.g., Bissada, 1982, Katz, 2005). Although such source rocks have very low porosity and permeability, they are in certain cases prolific unconventional reservoirs because hydraulic fracturing and horizontal drilling have made commercial hydrocarbon production from such rocks feasible (e.g., Hao et al., 2013, Tan et al., 2014, Camp et al., 2016). Rock texture, composition, organic carbon type and amount are often heterogeneously distributed throughout each mudstone unit, making geologic characterization of unconventional reservoirs a requirement that has to be part of each evaluation of gas capacity, optimization of well designs, and stimulation strategy (Bowker, 2007, Hickey and Henk, 2007, Ross and Bustin, 2008, Chen et al., 2011, Guo, 2013, Hart et al., 2013, Romero-Sarmiento et al., 2013, Sondergeld et al., 2013, Tan et al., 2014).
Depositional processes, productivity, and bottom water redox conditions have a profound control on the basin-wide distribution of lithofacies as well as the amount and quality of organic matter (e.g., Pedersen, 1985, Wignall, 1989, Macquaker, 1994, Macquaker et al., 2010, Bowker, 2007, Hickey and Henk, 2007, Loucks and Ruppel, 2007, Schieber et al., 2007, Könitzer et al., 2014). Recent advances using microscopic techniques have allowed significant progress in understanding mudstone heterogeneity and the underlying depositional processes that form fine-grained sedimentary rocks. For example, studies about major gas shale plays in North America, including the Barnett Shale in the Fort Worth Basin and the Marcellus Shale in the Appalachian Basin, have demonstrated that mudstone lithofacies can be well characterized based on mineralogy composition, rock texture, and organic matter content, and are crucial for effective placement of lateral wells and choice of proppant material (e.g., Jarvie et al., 2007, Hickey and Henk, 2007, Loucks and Ruppel, 2007, Abouelresh and Slatt, 2012, Wang and Carr, 2013). The combination of geochemical and sedimentologic approaches not only brings understanding to paleoproductivity and water chemistry conditions, but also unravels the relationships among organic matter accumulation, depositional processes and water column chemistry (e.g., Jiang et al., 2013, Könitzer et al., 2014).
The Lower Silurian black mudstone of the Longmaxi Formation is encountered throughout the upper Yangtze craton in southwestern China (Fig. 1). The Longmaxi Formation has long been known to be the principal source rock for conventional carbonate petroleum reservoirs of the Carboniferous Huanglong Formation and the Early Permian Qixia and Maokou formations (Zhang et al., 2011). The formation has recently become a target for exploration and development of unconventional shale gas. In the Sichuan Basin, the formation is 229–673 m thick (Chen et al., 2011), and the formation is thermally mature with a measured vitrinite reflectance (R0%) varying from 1.5% in most parts of the basin to 3.5% in the Fulin-Shizhu area, the eastern Sichuan Basin and the Dazhou-Wanxian area in the northeastern Sichuan Basin (Wang et al., 2009, Liu et al., 2013, Wu et al., 2013). The lower Longmaxi Formation is the most prolific shale interval with a stable gas production rate greater than 60,000 m3 per day in some wells in Fulin County. Previous studies of the Longmaxi Formation focused mainly on understanding the mechanisms of gas maturation and accumulation by characterizing the regional structure, sedimentation patterns, and the micro-pore structures, with the aim of making quantitative gas content predictions (e.g., Su et al., 2007, Yan et al., 2008, Zhang et al., 2011, Guo, 2013, Guo and Liu, 2013, Jiang et al., 2013, Liu et al., 2013, Chen, 2014, Guo et al., 2014, Guo and Zhang, 2014, Yan et al., 2014). A paleoenvironmental description and lithologic framework of the Lower Silurian strata within the Sichuan Basin has been presented elsewhere (e.g., Zhao, 1984, Chen et al., 1998, Wen et al., 2002, Wan and Xu, 2003, Guo et al., 2004, Wang et al., 2008, Jiang et al., 2013). A recent study of the Longmaxi Formation in the southeastern Sichuan Basin has documented sequence stratigraphic control on mudstone heterogeneity (Chen et al., 2015). However, as one of the most important source rocks and unconventional reservoirs in China and a prime example for a Lower Paleozoic deep shelf succession deposited during a worldwide marine transgression after the Early Silurian melting of Gondwana glaciation (Mu et al., 1981, Wang and Mo, 1995, Chen et al., 2004, Guo, 2013, Guo and Zhang, 2014, Yan et al., 2015, Yang et al., 2016), a detailed depositional model that explains the influences of depositional processes and water column conditions on organic matter accumulation in the prolific lower Longmaxi Formation is still lacking.
In this study, we characterize the mudstone-dominated lower Longmaxi Formation in an 83 m long, continuous vertical core from the JY1 Well in Fulin County, which is located in the center of the upper Yangtze craton (Fig. 1). We combine geochemical and sedimentological data collected from closely-spaced shale samples. Our geochemical analyses include TOC weight percentage, mineralogy compositions based on X-ray diffraction (XRD) analysis, and trace element index. Our sedimentologic study is based on microscopic observations of mineralogy, rock texture and structure, biota, and degree of bioturbation. The new data and lithofacies interpretation are used to develop a depositional evolution model of the lower Longmaxi Formation and understand the depositional controls on organic matter abundance.
Section snippets
Geologic setting and stratigraphy
The Sichuan Basin developed on the Precambrian metamorphic basement of the upper Yangtze craton (Zhang et al., 2012) (Fig. 1A). The accumulation of basinal, Lower Paleozoic strata was significantly influenced by several important tectonic events during the Sinian to Silurian, including the late Sinian Tongwan movements, the Early Cambrian Xingkai movements, the Late Cambrian Yunan movements, and the Late Ordovician to Late Silurian Kwangsian movements (Tong, 1985, Chen et al., 2013). By the
Methods
This study integrates the observations of sedimentary structure and texture, and chemical and biogenic features in core and thin sections of an 83 m long drill core from the JY1 Well, which was drilled by China Petroleum and Chemical Corporation in November 2012 (Guo and Zhang, 2014). XRD analysis of 80 samples, TOC analysis of 93 samples, and trace element analysis of 88 samples were included to characterize the mudstone lithofacies and assist with the interpretations of depositional
Principles of trace element proxies
Redox and productivity proxies were adopted to infer the water redox condition and marine productivity during the deposition of the Long-1 Member. This research particularly focuses on uranium (U), vanadium (V), molybdenum (Mo), nickel (Ni) and copper (Cu). U, V and Mo are good paleoredox proxies and Ni and Cu are good paleoproductivity proxies (Algeo and Maynard, 2004, Algeo and Lyons, 2006, Algeo and Rowe, 2012). In oxidizing environments, uranium is converted to soluble uranyl (U6+)
Analytical results
Thin-section examination and XRD analysis show that all the lithologies of the Long-1 Member are primarily composed of quartz, feldspar, carbonate, and clay minerals (Fig. 2, Fig. 3; Table 1; Appendix A). Pyrite is commonly observed in thin sections as framboid, authigenic crystal, and cement. Quartz grains are dominantly of silt size and the diameters are mostly of 63–80 μm. Quartz abundances range from 18 wt% to 57 wt%, while clay mineral contents vary between 20 wt% and 63 wt% (Fig. 2). Clay
Lithofacies and lithofacies assemblages
We subdivide the Long-1 Member of the JY 1 core into six lithofacies based on sedimentary structure and texture, mineralogy, biota, TOC content, and trace element proxies. The average mineral composition and TOC content of each lithofacies are summarized in Table 1.
Siliceous mudstone (lithofacies 1)
The presence of good lamination and fine sediment grain size suggest that the siliceous mudstone was deposited predominantly by suspension sedimentation (Loucks and Ruppel, 2007, Abouelresh and Slatt, 2012). Microcrystalline quartz within the shale is most likely formed as a diagenetic product of opal from radiolarians and sponges (Bowker, 2003, Papazis, 2005, Fishman et al., 2015, Camp et al., 2016). Angular-subrounded monocrystalline quartz is generally from terrigenous sources (Zuffa et al.,
Conclusions
The prolific mudstones of the Lower Silurian Long-1 Member of the Longmaxi Formation in the Sichuan Basin, southwestern China have been studied to understand lithofacies variability and underlying reasons for enhanced organic matter accumulation. We identified six lithofacies that were deposited at different water depths and proximity to shoreline and under different degree of basin stratification. Our results show that biogenic quartz is abundant, and TOC are well correlated to quartz content
Acknowledgements
This research was funded by National Key Basic Research Program of China (973 program) (2014CB239102), China Geological Survey Major Project (12120114046601), National Science Foundation of China (41202086, 41472122, and 41302089). Many thanks go to China Scholarship Council. We thank SINOPEC Exploration Southern Company for providing the core for this study, Drs. Ruobing Liu, Hanrong Zhang and Xiangfeng Wei for guiding core descriptions, and Drs. Lanyu Wu, Hao Lin and Hui Jian for preparing
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