Abstract
Nitrogen and oxygen (NO) compounds are important compositions in shale source rocks, and they carry an abundance of geochemical information for hydrocarbon generation. Due to technical limitations, the significance of NO compounds has not been paid enough attention. In this paper, the NO compounds from shale rocks of the Dongpu Depression are analyzed to explore the compositional characteristics and geochemical significance of using geological and organic geochemical ways of rock-eval, gas chromatography-mass spectrometry (GC/MS), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results show that shale rocks are rich in NO compounds, with twelve types of compounds that were detected: N1, N1O1, N1O2, N1O3, N1S1, N1S2, N2O1, O1, O2, O3, O3S1, and O4. Of these compounds, O2 and O3 predominated, followed by N1 and N1O1. Of the N1 species, the most abundant classes are DBE of 9, 12, 15, and 18, which changed with maturity. Of the O2 species, compounds of DBE of 1 (fatty acids) are the predominant class. Classes of DBE 5 and 6 in the O2 species are naphthenic acids with special biological skeleton structures, which are usually appear in immature and low-mature oils. N1, O2, and N1O1 compounds are affected by their maturity and they often run to polarization with enhanced DBE species and a shorter carbon chain as their maturity increases. The parameters of DBE18–25/DBE9–18-N1 and DBE12–20/DBE5–12-O2 increase with the increase of buried depth and maturity. The NO compounds that were revealed by FT-ICR MS may have a promising application in distinguishing between the different depositional environments. Source rocks of saline lacustrine are rich in O2 and N1O1, but less N1 and O1 compounds. The research results are of vital importance for expanding the application of the NO molecular compounds in petroleum exploration.
Highlights
Nitrogen (N) and oxygen (O) compounds are important heteroatomic polar compounds in shale oil
Understanding the composition and distribution of Nitrogen and oxygen (NO) compounds in shale rocks is of vital importance for studying the physical properties of shale rocks, as well as the mobility and the source of shale oil [1–3]
The generation type and abundance of polar compounds in NO compounds directly affects the wettability and occurrence state of shale oil and organic acid compounds that directly participate in improving the microscopic pore structure of the rock minerals [4,5], which are the decisive factors for shale oil mobility
Summary
Nitrogen (N) and oxygen (O) compounds are important heteroatomic polar compounds in shale oil. Understanding the composition and distribution of NO compounds in shale rocks is of vital importance for studying the physical properties of shale rocks, as well as the mobility and the source of shale oil [1–3]. NO compounds carry abundant geochemical information for hydrocarbon exploration, and the application of low molecular weight NO compounds in oil exploration is under constant research. Some research on the hydrocarbon compounds that were generated from the mud shale in the Dongpu Depression has been carried out [18,19]. Taking shale rocks of the northern part of the Dongpu Depression as examples, this study uses rock-eval, GC-MS, and FT-ICR MS methods to reveal the detailed compositional characteristics of hydrocarbons and non-hydrocarbons, especially the NO compounds, of the mud shale and explore their geochemical significance in petroleum exploration. The results of this work can be used for other basins with similar geologic backgrounds, providing both regional and comprehensive expectations for the findings of this study
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