Origin and paleoenvironment of organic matter in the Wufeng–Longmaxi shales in the northeastern Sichuan basin

Abstract

The formation environment and preservation conditions of sedimentary organic matter (OM) play an important role in the accumulation of shale gas. In the present study, inorganic and organic geochemical data were analyzed to determine the origin and preservation environment of sedimentary OM in the Wc-1 well of the Wufeng–Longmaxi (WF–LMX) Formation in northeastern Chongqing, China. In a biomarkers analysis, the numerical characteristics of n-alkanes ( n-C17/ n-C31>4.0), tricyclic terpenes (C23TT/C30H>1.0), and steranes (C27/C29St>1.0) suggested that the main origin of OM in the black shale was planktonic algae. High values of P/Ti and BaXS in the paleoproductivity indices suggested that primary productivity in the WF–LMX Formation was relatively high, peaking in the lower LMX Formation. Relative enrichment in U, V, and Mo, and the changing trends in V/(V+Ni) and Ni/Co suggested that the redox conditions for the bottom water, which changed from the WF Formation to the lower and upper LMX Formation, were oxic/dysoxic to anoxic and dysoxic, respectively. The relationship between total organic carbon and the above indexes indicates that different key factors controlled OM enrichment in the WF–LMX Formation. In the WF Formation, oxic bottom water was not conducive to the preservation of sedimentary OM. In the lower LMX Formation, the highest paleoproductivity and anoxic bottom water conditions promoted the enrichment and preservation of sedimentary OM. In the upper LMX Formation, excessive terrigenous inputs and relatively low paleoproductivity limited the enrichment of sedimentary OM.

Spatiotemporal Evolution and Genesis of the Late Ordovician—Early Silurian Marine Euxinia in Northeastern Upper Yangtze Basin, South China

“…In this study, the middle-upper interval of the WF formation (approximately 4-10 m thick) and the GYQ and lower part of the LMX formations (approximately 2-10 m thick) were analyzed to determine the characteristics of marine redox conditions during this period. According to previous research which performed, vitrine reflectance measurement and clay mineral analysis, the four sections experienced a similar thermal evolution history, and the maximum burial temperature was approximately 210-250 °C (Su et al, 2007: Nie et al, 2018Wang et al, 2021;Xi et al, 2021).…”

“…Based on paleogeographic reconstruction, the four study areas were concluded to be deposited in different water depth environments. The Datianba section, X3 well, and Shaba section (Qianjiang County, Chongqing Municipality, 29 °20′32.72″N, 108 °51′20.93″E) are located on the flank of the submarine highland of the Xiang'e or the structural saddle connecting it with the Qianzhong uplift, belonging to the shallow sea area, Wc-1 well (located in Chengkou City, Chongqing Municipality, 31 °54′26.67″N, 108 °32′58.21″E; Wang et al, 2021) is situated near the Qinling Ocean, and the water depth gradually deepens in this section (Figure 1B; Liu et al, 2016;Xi et al, 2021).…”

“…The reason for this phenomenon may be related to the higher biogenic silica content in the region. In different sediment types, the content of REEs is affected by the dilution of biogenic silica, with the content of REES being lowest in mudstone with more biogenic silica (Lu and Jiang, 1999;Wang et al, 2021).…”

“…To restore the change in redox conditions of the water column in the northeastern Upper Yangtze Basin during the O-S period, the redox condition data of Datianba section and X3 well in the same basin during this period were compared with those of the study samples (Figure 7; Liu et al, 2016;Wang et al, 2021;Xi et al, 2021). The Datianba section was located close to the Qianzhong uplift, and the X3 well and Shaba section were located on the northern and southern sides of the Xiang'e Submarine High (the paleomagnetic regions were on the east and west sides), respectively, and Wc-1 well was close to the Qinling Ocean (Figure 1B).…”

“…The paleo-ocean during the transition from the Late Ordovician to the Early Silurian (O-S) in the Upper Yangtze Basin, South China, has been widely studied with respect to the sedimentary environment (Wang et al, 2016;Li et al, 2017;Liang et al, 2017;Wu et al, 2019;Wang et al, 2021), redox conditions (Li et al, 2019;Liu et al, 2019;Wang et al, 2019), and sea level change (Wang et al, 2017;Jin et al, 2020). During this period, a set of organic-rich black shale was deposited, including the Wufeng Formation (WF, Katian-Hirnantian) and Longmaxi Formation (LMX, Hirnantian-Rhuddanian), which have become the main targets for shale gas exploration in China (Zou et al, 2016;Zou et al, 2019).…”