Geochemical Characteristics And Application Of NSO Compounds In Saline Lacustrine Oils

The Dongpu Depression is an important hydrocarbon-generating depression in the Bohai Bay Basin in eastern of China.The crude oils are rich in NSO compounds,which mainly from saline lacustrine source rocks.There are few studies on the distribution and the geochemical information about NSO compounds,because of the limitations of traditional geochemical experiments.Therefore,in this paper,the main research objects are the saline lacustrine oils in the northern Dongpu Depression,using advanced gas chromatography with dual plasma sulfur chemiluminescence detector（GC-SCD）,compound-specific stable sulfur isotope analyses（CSSIA）and Fourier transform ion cyclotron resonance mass spectrometry（FT-ICR MS）analysis combined with conventional gas chromatography-mass spectrometry（GC-MS）to reveal the composition and distribution characteristics of NSO compounds and sulfur isotopes in the saline lacustrine oils of the Dongpu Depression,analyze the main factors controlling the composition and distribution of NSO compounds and sulfur isotopes,learn the geochemical significance of them,and try to apply them to the study the properties of source rocks,genetic types of oils and correlation of oil/gas-source rocks.The experimental results show there are eight kinds of heteroatom compounds detected by（+）ESI FT-ICR MS in positive ion mode,including S2,S1,O2S2,O2S1,O1S2,O1S1,N1S1 and N1O1S1,of which O1S1 and S1 are the main heteroatom compounds.The S1 species are compounds with DBE=6（mainly benzothiophenes）and DBE=9（mainly dibenzothiophenes）,while the C40S1 compounds are detectedβ-carotane structural compounds in most samples.Five kinds of heteroatom compounds are detected in negative ion mode,including O3,O2,O1,N1O1 and N1,among which N1,O1 and O2 were the main heteroatom compounds in negative ion mode.N1 species mainly include compounds with DBE=9（mainly carbazoles）,DBE=12（mainly benzocarbazoles）and DBE=15（mainly dibenzocarbazoles）.Benzothiophene and dibenzothiophene compounds are detected in all oil samples by GC-SCD,it is common in oil samples.But there are some differences in the relative abundances of these compounds in different areas of the Dongpu Depression.There are also some differences in CSSIA analysis of different oilfields.Generally speaking,the sulfur isotope values show a trend of gradually decreasing from north to south:oils in Pucheng have the highest sulfur isotope values;Huqing and mature oils in Wenliu take the second place;the Weicheng-Wenmingzhai oils and immature oils in Wenliu have the similar sulfur isotopes;the sulfur isotope values of freshwater lacustrine oils in the south are the lowest in the Dongpu Depression.It is found that the values 2C41/（C40+C42）in FT-ICR MS analysis have the similar trend like the values ofδ³⁴SOSCs with Weicheng-Wenmingzhai>Pucheng>Wenliu>Huqing>Southern oils.This trend is also consistent with the gradual decrease of salinity in the Dongpu Depression from north to south,indicating the sulfur isotopes and NSO compounds are affected by sedimentary environment.It shows a good linear relationship between theδ³⁴SDBT values and trace element parameters(Sr/Ba,V/Cr,Ni/Co,δU,Rb/Sr andδ¹³C)of source rocks,indicating the higher salinity of the water body is conducive to the formation of a strong reducing sedimentary environment and increases the values ofδ³⁴SDBT.It also found that the sedimentary environment indicated by NSO compounds is consistent with that indicated by trace elements and biomarkers.Based on the characteristics of NSO compounds,five maturity parameters of heteroatomic compounds are proposed:DBE4-7/DBE0-7 values of O1S1,DBE7-13/DBE0-13 values of S1,DBE13-25/DBE0-25 values of N1,DBE7-13/DBE0-13 values of O1 and the sulfur isotope values ofΔδ³⁴S1-m DBT–4-m DBT（the sulfur isotopes between1-methyldibenzothiophene and 4-methyldibenzothiophene）,which have good linear relationship with the maturity parameters of biomarkers.It indicates that NSO compounds and sulfur isotopes can be used as auxiliary parameters to evaluate the maturity of oils/source rocks.The different mineral composition of source rocks in the Dongpu Depression can lead to significant differences in the composition and distribution of NSO compounds,indicating the control of source rocks.Based on the distribution of low thermal stability and high thermal stability compounds by FT ICR-MS,the sulfur isotope differences of different organic sulfur compounds defined by the values of Δδ³⁴SBTs–DBTs,Δδ³⁴S1-m DBT–4-m DBT and Δδ³⁴STDs–DBTs can indicate whether thermochemical sulfate reduction has occurred in oils.The results of ESI FT-ICR MS and CSSIA can be used to identify immature-low mature oils in the Dongpu Depression,and provide a new method for the study of hydrocarbon generation mechanism of immature-low mature oils.Using the composition and distribution of NSO compounds and the sulfur isotope values of organic sulfur compounds can effectively divide the groups of different oils.It is found that the differences of these geochemical information are more obvious in saline lacustrine oils of the Dongpu Depression,which can highlight the differences between the oils of various groups.At the same time,it is also found that the results of oil-source correlation using sulfur isotopes are consistent with those of previous biomarkers,indicating that sulfur isotopes of organic sulfur compounds can also effectively analyze oil-source correlation.