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

Chaoyong Wang,Zaitian Dong,Xin Hu,Zhen Li,Xuehai Fu

doi:10.1177/0144598720978007

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.

Highlights

As one of the largest shale gas basins in China, the Sichuan Basin has undergone substantial exploration and development in recent years (Bao et al, 2020; Chen, 2016; Chen et al, 2014; Gao et al, 2012; Guo and Zeng, 2015; Liu et al, 2019)
Northeastern Chongqing lies within the Dabashan fault zone at the northeastern margin of the Sichuan Basin, where the geological structure is complicated by a multistage geological history (Li et al, 2007)
Among the Wc-1 well samples, shales from the WF Formation have low TOC content ranging from 0.94% to 2.18%, shales from the lower LMX Formation have relatively high TOC contents ranging from 3.26% to 6.89%, and shales from the upper LMX Formation have TOC contents ranging from 1.15% to 4.45%

Summary

Introduction

As one of the largest shale gas basins in China, the Sichuan Basin has undergone substantial exploration and development in recent years (Bao et al, 2020; Chen, 2016; Chen et al, 2014; Gao et al, 2012; Guo and Zeng, 2015; Liu et al, 2019). Some exploratory wells in this area have produced shale gas, such as the Wuxi-2 well in the Ordovician–Silurian Wufeng–Longmaxi (WF–LMX) Formation and the Chatang-1 well in the lower Cambrian Shuijingtuo Formation. These formations have shale field desorption gas contents of $8 m3/t, which indicate considerable shale gas exploration potential in this area (Liang et al, 2016; Wang et al, 2015). Research on the black shale of the WF-LMX Formation in northeastern Chongqing has focused on determining its sedimentary structure, pore structure characteristics, and shale gas resource (Chen et al, 2016; Dong et al, 2011; Wang et al, 2015). The origin of sedimentary organic matter (OM), controlling factors, and the characteristics of the sedimentary environment in this area require further study

Methods

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Discussion

Conclusion