Distribution And Application Of Organic Sulfur Compounds And Their Isotopes In Geological Body

In order to reveal the composition,distribution,main controlling factors,geochemical significance and application value of organic sulfur compounds and their isotopes in different genetic types of crude oils,the marine crude oil in the Tarim Basin and lacustrine oils/rocks from the Dongying Depression（Bohai Bay Basin）were characterized by Chromatographic-mass spectrometry（GC-MS）,positive-ion electrospray ionization（ESI）Fourier transform ion cyclotron resonance mass spectrometry（FT-ICR MS）,single atom sulfur isotope and whole oil sulfur isotope analysis.The relevant results are of great significance for improving the theory of organic non-hydrocarbon geochemistry and revealing the genesis and accumulation mechanism of hydrocarbon.Seven types of organic sulfur compounds were detected from marine and terrestrial crude oils by ESI FT-ICR MS,including S1,O1S1,O2S1,N1S1,N1O1S1,S2 and O1S2.There are differences in the composition and distribution of organic sulfur compounds in different genetic types of crude oils.Marine crude oils are rich in S1 and O1S1 compounds.S1 compounds are dominant in saltwater lacustrine oils.Freshwater lacustrine oils are rich in S1,O1S1,N1S1 compounds.The Tarim marine oils are rich in S1 compounds with high double bond equivalent（DBE）（DBE=3,6,9 as the main peak）.The saltwater lacustrine immature oils in the eastern are rich in S1 compounds with low DBE（DBE=1 as the main peak）.The compounds of C20-DBE1-S1,C37-DBE1-S1,C30-DBE1-S1,C40-S1 and（C27-C30）-DBE5-S1 are more enriched in the low-mature oil of saltwater lacustrine oils.Organic sulfur compounds are controlled by biogenic/sedimentary environment and maturity.The high abundance of C20-DBE1-S1,C37-DBE1-S1,C30-DBE1-S1,C40-S1 and（C27-C30）-DBE5-S1 compounds in saltwater lacustrine oils are similar to the skeletons of phytane,n C37,squalene,carotenoid and sterane,respectively,which are genetically related and controlled by the biogenic/sedimentary environment.The C19/C20-DBE1-S1and（C27-C30）/（C31-C34）-DBE3-S1 indexes have a good correlation with the hydrocarbon sedimentary environment indexes Pr/Ph and gammacerane/C30-hopane,which further indicates the control of sedimentary environment.With the increase of maturity,the carbon number and DBE of S1 compounds changed regularly.From low mature oil to mature oil,the distribution characteristics of DBE are as follows:the main peak gradually increases from 1 to 6,9,12,the dominant peak gradually shifts backward,and the abundance of compounds with high DBEs increases,reflecting the condensation/aromatization of sulfur-containing compounds.With the increase of maturity,the distribution range of carbon number gradually becomes narrower and smaller,reflecting that the carbon number decreases due to the fracture of side chain or long chain alkanes under thermal evolution.Based on the distribution characteristics of organic sulfur compounds and their controlling factors,a number of ESI FT-ICR MS indicators for evaluating biogenic/sedimentary environments and maturity were developed.The maturity evaluation indexes include:NMI-S1（the parameters are based on the carbon number and DBE distribution of S1 compounds,suitable for marine maturity evaluation）,C35⁺/C34^--DBE1-17-S1（suitable for terrestrial maturity identification）,DBE10-S1（%）,DBE1+2+3+5-S1（%）.Based on the analysis of continental oils and source rocks,the relationship between vitrinite reflectance and ESI FT-ICR MS index was established.The conversion formula is Ro=0.0576×（DBE12-S1）+0.2929 and Ro=0.0491×（DBE15-S1）+0.4404.Combined with GC-MS maturity index,C40-DBE3+4/DBE5+6-S1>1.75,DBE1/DBE9-S1>10 and DBE1/DBE12-S1>50 can be used as the identification index of low mature oil.The maturity evaluation index based on ESI FT-ICR MS makes up for the lack of vitrinite reflectance and overcomes the failure of biomarker maturity index of high-over mature samples.Compared with biomarker compound parameters,it has the advantage of being less affected by other factors.Sedimentary environment characterization parameters:（C27-C30）/（C31-C34）-DBE5-S1 and C40/C39-S1 can be used for continental oil-rock correlation.Based on the distribution characteristics of carbon number,C19-/C20-DBE1-S1>0.6 and<0.6,C30/C29-DBE1-S1<1.2 and>1.2 are divided into salt water and freshwater crude oil.Based on the difference of DBE,DBE5-/DBE6,9,12-S1<0.6 and DBE1,2/DBE4-O1S1<2.0,indicating freshwater environment.The biogenic/sedimentary environment indicators based on ESI FT-ICR MS have advantages in the case of biomarker failure.The above ESI FT-ICR MS parameters have achieved good application results in the oil-rock correlation of source rocks and related crude oils in the Es3 and Es4 members of the Dongying Depression.The comparison results are consistent with the comparison results of biomarker compounds.The sulfur isotope values of different genetic types of crude oil are significantly different.The sulfur isotope(δ³⁴S)of the Tertiary freshwater/saline phase（Dongying Depression）is 20.02‰～27.04‰,and the Paleozoic marine normal oil in the Tarim Basin is 10.92‰～26.92‰,which is close to theδ³⁴S value of the contemporary seawater sulfate.Sulfur isotopic of individual organosulfur compounds of alkyl dibenzothiophene series has an excellent negative correlation with Pr/Ph,and sulfur isotopic of individual organosulfur compounds has a positive correlation with gammacerane/C30hopane.Theδ³⁴S value of marine and terrestrial oils is related to the sedimentary environment parameters in biomarkers,which is controlled by biogenic/sedimentary environment.Theδ³⁴S value of the total oil is positively correlated with the maturity parameters,indicating the fractionation effect of thermal maturity.Based on the distribution and controlling factors of organic sulfur isotope,it is proposed that organic sulfur isotope has the following geochemical significance and application value.1)The oxidation-reduction potential of the parent rock is divided byδ³⁴S_DBTs value.Theδ³⁴S_DBTs>20‰～22.5‰is an ultra-salinity reducing environment.δ³⁴S_DBTs=16‰～22.5‰is marine-salt water reduction environment.δ³⁴S_DBTs<15‰is a weak oxygen-oxidation environment in the lake.This index has significant advantages for the failure of biomarker compounds caused by secondary modification such as biodegradation.2)Applied to oil-source correlation.The oil-source correlation of crude oil and source rock in Tarim Basin was carried out by using sulfur isotope value.The results show that the crude oil in the study area comes from Cambrian source rock.3)Applied to the identification of thermochemical sulfate reduction（TSR）.Theδ³⁴S value of crude oil in Tazhong uplift experienced TSR is heavier than that of normal crude oil.4)Organic sulfur isotope is used to identify transgression.The sulfur isotope of Paleogene crude oil in the Dongying Depression is significantly heavier than that of Liaohe crude oil.Combined with biomarker identification indicators,it is considered that the Paleogene source rocks in the Dongying Depression have been subjected to transgression during the formation process.