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.