Multiple controlling factors of lower Palaeozoic organic-rich marine shales in the Sichuan Basin, China: Evidence from minerals and trace elements

Abstract

Lower Palaeozoic organic-rich shales are widely distributed in southern China. In this study, the organic matter contents of Lower Cambrian, Upper Ordovician, and Lower Silurian shales were analysed, and a high degree of vertical heterogeneity was found. In the studied sections, the total organic carbon concentrations are high at the bottom and gradually decrease upward. The total sulphur contents of the studied shales display a similar trend in the vertical direction, and they are high in the lower part of the section. The carbon isotopes of organic matter (d 13 C org) present a distinct trend that is indicative of 13 C enrichment, which indicates that the total organic carbon variations between the different shale units in the Sichuan Basin may be related to changes in ocean biomass and the sedimentary environment. The mineral composition (specifically, the pyrite, quartz, and clay abundances) of these shales suggests that the deep-water sedimentary environment was conducive to the preservation of organic matter. Total organic carbon is positively correlated with quartz contents but negatively correlated with clay contents, suggesting an input of biogenic silica. The majority of the quartz was deposited via the slow settling and accumulation of recrystallised biogenic opal and silica-rich plankton. This study of the trace element geochemistry of the Lower Palaeozoic shales indicates that productivity, burial efficiency, redox environment, and hydrothermal activity all affected the accumulation of organic matter. The Cambrian Niutitang shale is rich in nutrient elements. The Longmaxi and Wufeng shales feature high Mo contents and total organic carbon correlation coefficients of 0.93 and 0.89, respectively. Efficient burial was essential for the enrichment of organic matter in the Wufeng and Longmaxi shales. The redox parameters suggest that the organic-rich shales (total organic carbon > 2.0%)

Geochemistry, Paleoenvironment and Mechanism of Organic‐Matter Enrichment in the Lower Silurian Longmaxi Formation Shale in the Sichuan Basin, China

“…All the evidence discussed here, combined with reported hydrothermal sedimentary phenomenon in the Longmaxi Formation shale (Wu et al, 2017;Zhang et al, 2018), indicates that hydrothermal intrusion might have occurred during the deposition of the LLF in the Sichuan Basin, and the impact of these hydrothermal fluids varied in different regions at different periods. The geochemical indexes of hydrothermal activity (Fe, Mn, and Y concentrations and Fe/Ti ratio) indicated that the LLF in the Weiyuan area was more affected by hydrothermal activity than the LLF in the Jiaoshiba and Zhaotong areas.…”

“…Co is considered to be associated with a common seawater source, while Zn and Ni are primarily considered to be from hydrothermal origins. Therefore, a Ni-Zn-Co ternary diagram was established and is used to distinguish hydrogenic and hydrothermal sediments (Choi and Hariya, 1992;Wu et al, 2017). Fig.…”

“…9. Ni-Zn-Co ternary diagram for the Lower Longmaxi Formation in the Jiaoshiba, Zhaotong and Weiyuan areas, Sichuan Basin, distinguishing hydrothermal and hydrogenic sediments (modified from Choi and Hariya, 1992;Wu et al, 2017). This might have been because of the paleogeography of the Weiyuan area, which placed it near the Chuanzhong uplift, where enhanced tectonic activity took place (Xu et al, 2012;Li et al, 2017). The Caledonian orogeny movement not only extended the Chuanzhong uplift but also activated basement faults around the uplift, providing vents for deep-hydrothermal-fluid upwelling.…”

“…Although the Longmaxi Formation shale is generally considered to have been deposited in an oxygendeprived deep-sea environment, the abundance of organic matter in the Longmaxi Formation shale shows strong vertical and lateral heterogeneity (Zhang et al, 2012;Guo, 2013;Guo and Zhang, 2014;Li et al, 2016). Therefore, numerous studies have been conducted on the mechanism of the organic-matter enrichment in the Longmaxi Formation shale in the Sichuan Basin and its peripheral areas Wang et al, 2015;Yan et al, 2015;Zhao et al, 2016;Wu et al, 2017;Zhang et al, 2018). These studies have generally suggested that the lower part of the shale, characterized by high organic-matter abundance, was deposited in an anoxic or dysoxic environment, while the upper shale, characterized by low organic-matter content, was deposited in an oxic environment.…”

Determining of an oceanic anoxic event and paleoenvironmental conditions of the Gulneri Formation Kirkuk Oilfield, Northern Iraq

“…The anomaly of uranium, or the so-called uranium coefficient, depends on the relationship between uranium and thorium to determine the redox environment. To determine the anomaly coefficient, the following equation (Wignall, 1994 in Wu et al, 2017) is used:…”