Geochemical Characteristics and Significance of Source Rocks of Upper Permian Nayixiong Formation in QD18 Well, Qiangtang Basin

To further clarify the conditions of Paleozoic source rocks and the prospect of oil and gas exploration in Qiangtang Basin, the black mud shale of Upper Permian Paleozoic Nayixiong Formation, which was drilled into QD18 Well, was taken as the research object. Through rock pyrolysis, elemental analysis, kerogen identification, combined with saturated hydrocarbon chromatography-mass spectrometry, the source quality of Nayixiong Formation was evaluated, and the development environment and evolution process of source rocks were studied. The exploration prospect of Paleozoic oil and gas in Qiangtang Basin is discussed. The results show that: (1) The TOC of the black shale of Nayixiong Formation in QD18 Well is between 0.58% and 6.74%, with an average value of 1.5%. The organic matter composition is mainly vitrinite, some samples are mainly sapropelic, H/C atomic ratio is between 0.42～0.81, O/C atomic ratio is between 0.08～0.28, kerogen type index TI is between －75.25～38.50, kerogen type belongs to type II₂-Ⅲ, mainly type Ⅲ. The R_o was between 1.44% and 2.01%, and the T_max was between 455 and 544℃. Most of them entered the stage of high maturity evolution, and some reached the stage of over-maturity, mainly condensate and moisture. (2) Biomarkers revealed that the water body of Nayixiong Formation was a reducing environment with the characteristics of mixture source. (3) Effective source rocks are developed in the Permian of Qiangtang Basin, and the abundance of organic matter in the eastern part of the basin is significantly higher than that in the western part of the basin. The thickness of effective source rocks in QD18 Well is the largest, reaching 212 m. There are four high-quality source rocks developed in total, and the accumulated thickness of high-quality source rocks is 51.9 m, which has very good hydrocarbon generation potential and exploration prospect. It can be used as target strata for subsequent Qiangtang oil and gas exploration.