Even and Odd Carbon Predominance Coexisting N-Alkanes in Paleogene Dark Shale in Jinhu Sag Of Subei Basin

摘要

Gas chromatograms of saturated hydrocarbons of shale rock extracts show three patterns of n-alkanets in the second member of Funning formation (E_1f_2) in Jinhu Sag of Subei basin, eastern China. Three patterns of n-alkanets are the odd-to-even carbon predominance, the even-to-odd carbon predominance, and the even and odd carbon predominance coexisting, respectively. In this paper, the characteristics of n-alkanet coexisting even and odd carbon predominance and its origin and sedimentary environment are investigated only. The short-chain n-alkanets and long-chain n-alkanets are characterized by pronounced odd carbon predominance, with the dominant carbon of Nc_(15), nC_(17) and of nC_(25), nC_(27), nC_(29) and nC_(31). The mid-chain n-alkanets are dominated by even carbon predominance, with the dominant carbon of nC_(20) and nC_(22). The composition of biomarkers are characterized by phytane preference, high gamma cerate abundance, the presence of β-carotene and C_(24)-tetracyclic terrapin, which indicate that the organic matter in source rocks deposited in an anoxic, hyper saline lacus trine environment. The biomarkers are no homogeneous, which shows that salinity and redo is not constant in pale-lacus trine. The n-alkanets are mainly derived from algae in the ancient lake and macro pyres on land or around the edges of the ancient lake. The n-alkanets distribution patterns of the dark shale's studied are believed not to be dependent on the redo in the depositional environments or the salinity of the water body, but to be associated with original organic matter composition.