Geochemistry Evidence For The Paleoenvironment Evolution Of Lacustrine Sedimens Of Nenjiang Formation In Songliao Basin

Songliao Basin has been developed a set of cmpletely Cretaceous stratigraphy in China. The high resolution in the geological record has a contributed to the study for paleoenvironment. This paper is aimed to discuss the palaeolacustrine environment evolution of Nenjiang Formation. Sulfur element as the active element of redox property, it’s involved in the process of microbial cycle in nature. Nitrogen as a characteristis of nutrients, can be characterized by the deposition of input sources, and associated with the oxidation reduction process. C/N ratio or total organic nitrogen is regarded as an important indicator to determine the organic matter source, which is usually used to measure the sedimentary characteristics of the lake environment.Pyrite is a characteristic mineral rich in organic matter, and it is an important index to restore sedimentary environment.In this study through the observation of mudstone samples in Nenjiang Formation from the south well of SK-1 with microscope, got a framboid pyrite diameter curve, and combined with rock sulfur and organic nitrigen geochemistry analysis data, suggest that the evolution of the palaeoenvironment is complex. The stage of large-scale transgression occurred during the geological time, and the provenance supply input has changed. Based on the study of sulfur isotopic,suggested that the large scale transgression occurreed during the deposition of the Unit1 of Nenjiang Formation. The ratio of carbon to nitrogen data reveal sedimentary source change of input supply. Based on the pyrite morphology and sulfur geochemistry, it’s reveal that the whole Nenjiang Formation changed form oxic-dysoxic to euxinic-dysoxic to oxic-dysoxic. During the deposition period, the water body change from shallow to deep to shallow. It is concluded that the anoxic environments is favorable for the formation and preservation of hydrocarbon source rocks.