Experimental Simulation And Analysis Of The Effect Of Temperature On The Formation And Distribution Of Sterane

Sterane is the general name of a class of asymmetric polycyclic alkanes,Sterol from membrane lipids of eukaryotic bacteria is a characteristic marker of eukaryotes.As a molecular fingerprint of important geological significance in sediments,sterane is chemically stable.Its distribution form and isomerization parameters are widely used to identify the source of organic matter,the type of organic matter,the degree of thermal evolution.^[1]At the same time,temperature,biological source and other factors also affect its formation and distribution.In order to verify the effect of temperature on the formation and distribution of sterane,the formation and distribution of sterane were studied by using low mature marine shale samples and prokaryotes-cyanobacteria from Xiamaling formation in Zhangjiakou,North China,and the influencing factors of sterane distribution were discussed by using C₂₉9 sterane standard sample.The thermal simulation and analysis experiments of the experimental samples were carried out by using DK-1 formation pore hot-pressing simulator?semi-open system?,vacuum glass tube?closed system?and Py-GC/MS?open system?system.The column chromatography was carried out for the extraction of the original sample,the liquid product at different temperatures and the residue extract.The saturated hydrocarbon fractions were analyzed by GC/MS and GC-MS/MS.The effect of temperature on the formation and distribution of sterane was discussed.The following results were obtained:No sterane compounds were detected in the saturated hydrocarbon fractions from the original samples of source rocks,indicating that the organic matter in the samples may be mainly derived from aquatic plankton and bacteria and other prokaryotic organisms.The thermal simulation experiments of source rock samples were carried out at 250?⁵50?by using DK-1 simulator.There is a complete series of routine steranes in saturated hydrocarbon fractions of experimental effluents,indicating that steranes are formed in the process of sterane formation.Temperature is an important factor that can not be ignored.Except for the complete sterane series detected at550?,no sterane with very low abundance was detected at the other temperature points,and only 327?,330?and 550?sterane series were found in the saturated hydrocarbon fractions of the eluate,except for the complete series of steranes detected at 550?,and no steranes with very low abundance were detected at other temperature points,and only 327?,330?and 550?respectively in the saturated hydrocarbon fractions.The relative abundance is higher and the other temperature points are very low.No sterane compounds were detected in saturated hydrocarbon fractions of cyanobacteria samples by GC/MS and Py-GC/MS analysis.The whole sterane was detected by vacuum glass tube in the range of 250?⁵00?,and the distribution of sterane relative content was significantly affected by temperature.At different temperatures,the distributions of the relative contents of C₂₇,C₂₈8 and C₂₉9 have three types:"V"type,"L"type and"L"type,and there are three types of sterane relative content distribution:"V"type,"L"type and"L"type.With the increase of temperature,the relative content of C₂₇7 sterane first decreased and then increased,while C₂₈8 sterane and C₂₉9 sterane showed the tendency of increasing first and then decreasing.The thermal simulation experiment of C₂₉9 standard sample was carried out with vacuum glass tube,the experimental temperature ranged from 250?to 500?,when the temperature was lower than 300?,C₂₈8 sterane was formed,and when the montmorillonite catalyst was added at 250?,no sterane was detected and a large number of short-chain alkanes were formed.In order to approach the real geological conditions,C₂₈8 and C₂₉9 steroids were detected at 250?by adding a small amount of water and catalyst under the same conditions.The thermal simulation experiments of source rock samples and cyanobacteria samples in different systems have proved that some organic molecules in prokaryotes can form steroids at appropriate temperatures?250?⁵00??,and the results show that some organic molecules in prokaryotes can form steroids at appropriate temperatures?250?⁵00??.That is,the formation and distribution of steranes are closely related not only to biological sources but also to temperature and other factors.Thermal simulation experiments were carried out with C₂₉9 sterane standard sample in closed system.Other possible factors?temperature,pressure,water,mineral,etc.?affecting sterane distribution were preliminarily discussed.When the simulated temperature was lower than 300?,Temperature is the main factor affecting the demethylation of C₂₉9 sterane.The change of the relative content of C₂₈8 and C₂₉9 sterane is due to the conversion of C₂₉sterane to C₂₈8 sterane.With the increase of temperature,the pressure of the system increases gradually,and the pressure can inhibit the demethylation of compounds.Therefore,when the temperature is higher than 300?,the pressure is the main factor affecting the demethylation of C₂₉9 sterane.