NMR Study Of Kerogens In Source Roks And Crude Oils Of The Eocene Dongpu Sag,Bohai Bay Basin

Nuclear magnetic resonance(NMR)approach has unique advantages in the geochemical analysis of kerogen and crude oil because it can provide information on chemical functional groups.Previous studies have focused on marine kerogen and crude oils and investigated the chemical meaning of chemical shifts and interrelationship between the ~1H spectra and physical properties of the oil.However,the organic geochemical significance of the comprehensive NMR information and its effectiveness for studying lacustrine kerogen crude oil are still not clear.Here we present such an investigation of kerogens from the Paleogene Shahejie Formation in the Dongpu Sag,Bohai Bay Basin,eastern China.Results show that the NMR spectra and their parameters are sensitive to the type of kerogen.The NMR spectra of type I–II kerogens are similar,indicating the chemical structure comprises mainly the aliphatic functional group,followed by the aromatic functional group,with low contents of the heteroatomic functional group.In contrast,type III kerogen is completely different,having a much lower aliphatic than aromatic functional group content and higher degree of aromaticity.Based on this,a formula is proposed to determine the source rock hydrogen index and hydrogen generation potential,and thereby pointing to the source rock potential in the study area.Moreover,we investigated crude oil from the Shahejie Formation in the lacustrine basin of the Dongpu Sag,Bohai Bay Basin,China,with NMR techniques.Our results show that the NMR spectra of these crude oils provide important information on organic matter functional groups,including aliphatic hydrocarbons,cycloalkanes,aromatic hydrocarbons,and other structural types.The NMR parameters of the studied crude oils reliably reflect the source rock sedimentary environment,and organic matter source and thermal maturity.It was found that the organic matter that produced crude oil in the northern part of the basin is of better quality,and deposited in a higher-salinity and more reducing sedimentary environment,than in the south.The sedimentary environment in the central part of the basin reflects transitional characteristics with large variation.This complements our understanding of traditional organic(biomarker)geochemistry.Our data suggest that the structural and hydrocarbon characteristics of saline lacustrine facies kerogens can be effectively studied by NMR,and the NMR spectra has a good response to different types,maturity and sedimentary environment of kerogen and crude oil.NMR technique has a promising future in the field of geochemistry.