Insight into Geochemical Significance of NO Compounds in Lacustrine Shale Source Rocks by FT-ICR MS

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.