Geochemical Characteristics And Geological Significance Of Crude Oil Thermal Simulation Products

Deep and ultra-deep oil-gas reservoirs are becoming more and more important targets for oil and gas exploration.Oil cracking is an important evolution process in deep and ultra-deep reservoirs.Therefore,it is of great significance to strengthen the understanding of geochemical characteristics of products during thermal cracking of crude oil to study the formation and evolution of deep oil-gas reservoirs in China.In addition,carbon isotope inversion of natural gas occurs in some areas of Sichuan Basin,and its genetic mechanism is not clear yet.In this paper,the variation of the yield of different products and the amount of residual liquid hydrocarbon,the variation of carbon isotope fractionation and the variation of biomarker compound parameters during the pyrolysis of crude oil and group components were studied by means of the gold tube thermal simulation experiment.The main conclusions of this study are as follows:（1）According to different product yields and residual liquid hydrocarbons,the cracking stages of crude oil and family components are divided into five stages:（1）Crude oil/group component stabilization stage（Easy R_o≤1.08%）;（2）Solid asphalt rapid production stage（Easy R_o:1.08%～1.75%）;（3）Condensate production stage（Easy R_o:1.75%-2.19%）;（4）Heavy hydrocarbon gas production stage（Easy R_o:2.19%-2.67%）;（5）Methane production stage（Easy R_o≥2.67%）.Due to the difference in bond energy,cracking reaction and element composition,the alkane gas generated by the cracking of aromatic hydrocarbons,nonhydrocarbons+asphaltenes has a higher proportion and higher humidity than the alkane gas methane generated by the cracking of saturated hydrocarbons,saturated hydrocarbons+aromatic hydrocarbons.The yields of hydrogen and carbon dioxide produced by the cracking of nonhydrocarbons and asphaltenes are significantly higher than those of other components.（2）The phenomenon of carbon isotope inversion appeared in the later stage of the thermal simulation experiment,which may be due to the formation of polycyclic aromatic hydrocarbons and other macromolecular substances that are easy to capture linear hydrocarbons in the early stage of thermal cracking of crude oil and group components.The cracking of polycyclic aromatic hydrocarbons and captured linear hydrocarbons in the later stage of thermal cracking produces relatively ¹³C-depleted alkane gases,which mix with residual gases resulting in carbon isotope inversions.（3）The theoretical and measured values of the yields of various products,the amount of residual liquid hydrocarbons and the carbon isotope values calculated according to the proportions of the group components are not much different from the measured values.The interaction between components has little effect on the yields of various products and the amount of residual liquid hydrocarbons and carbon isotope values（4）The higher the thermal maturity,the less significant the carbon isotope fractionation effect among the products.The carbon isotope fractionation effect is different in the pyrolysis process of different components,and the carbon isotope fractionation of saturated hydrocarbons is more obvious than that of other components.（5）During the cracking process of crude oil and group components,the carbon isotope value of methane generated in the early stage is lighter than that generated in the later stage.When the gas stored in the gas reservoir is dominated by methane generated in the later stage,the carbon isotope value of methane may be abnormally heavy,resulting in the heavy carbon isotope of natural gas.In addition,because of the different carbon isotope fractionation effect during the pyrolysis of different components,the composition content of crude oil may have a certain effect on the degree of carbon isotope bias of methane in natural gas.（6）Most of the parameters of biomarker compounds in the high-over-mature stage have failed,but benzo[b]fluoranthene/benzo[e]pyrene,perylene/benzo[e]pyrene benzo[b]fluoranthene/chrysene and chrysene/benzo[e]pyrene can still be used as a good crude oil maturity parameter.